JP2013025253A - Polarized quantum entangled photon pair generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarized quantum entangled photon pair generator which is capable of selecting and achieving all of four bell states.SOLUTION: A polarized quantum entangled photon pair generator includes: a loop optical path 20 of a Sagnac interferometer configured to include a polarization splitting and combining device 101 and its two input/output ends; and an optical branch input/output part 10 which inputs excitation light to the loop optical path and spatially separates and outputs quantum entangled photo pair wavelength components outputted from the loop optical path. In the loop optical path, a nonlinear optical medium 102, a polarization plane 90-degree rotating part 103, and a polarization plane and optical phase adjustment part 104 are disposed in series and are connected by an optical fiber. The optical branch input/output part is configured to include a band pass optical filter 105, a low pass optical filter 106, and a WDM filter 107. A half wave plate 108 is disposed in a succeeding stage of the optical branch input/output part.

Description

  The present invention relates to a quantum entangled photon pair generator required in quantum information communication technologies such as quantum cryptography and quantum computers, and in particular, selects any one of the four bell-state polarization entangled photon pairs. The present invention relates to a polarization entangled photon pair generator that can output in the same manner.

  Quantum entangled photon pair generation has become an important elemental technology for realizing advanced quantum information communication technology that applies the nonlocality of quantum mechanical particles (photons) such as quantum cryptography and quantum computers.

As a means for generating entangled photon pairs, a method using natural parametric fluorescence in a second-order or third-order nonlinear optical medium has been studied. When an excitation photon with wavelength λ _p , wave number k _p , and angular frequency ω _p is input into a second-order or third-order nonlinear optical medium, a signal photon with wavelength λ _s , wave number k _s , angular frequency ω _s , and wavelength λ _It is natural parametric fluorescence that idler photons of _i , wave number k _i , and angular frequency ω _i are output. At that time, the signal photon and the idler photon are always generated simultaneously as a pair.

  Hereinafter, excitation photons, signal photons, and idler photons are referred to as excitation light, signal light, and idler light, respectively.

  The wave number and angular frequency of excitation light, signal light, and idler light have correlations corresponding to the momentum conservation law and energy conservation law, respectively.

When a second-order nonlinear optical medium is used, the natural parametric fluorescence phenomenon is also called natural parametric down conversion (SPDC). At this time, the wave number and angular frequency of each photon satisfy the following equations (1) and (2), respectively.
k _p = k _s + k _i + K (1)
ω _p = ω _s + ω _i (2)
On the other hand, when a third-order nonlinear optical medium is used, the natural parametric fluorescence phenomenon is also called natural four-wave mixing (SFWM). At this time, the wave number and angular frequency of each photon satisfy the following equations (3) and (4), respectively.
2k _p = k _s + k _i + K (3)
2ω _p = ω _s + ω _i (4)
Here, K included in the expressions (1) and (3) is a parameter corresponding to the period of the periodic modulation structure of the nonlinear optical constant, and when the period of the periodic modulation structure is Λ, K = 2π / Is given by Λ. A periodic modulation structure having a nonlinear optical constant is frequently used at present, for example, when a LiNbO ₃ crystal is used as a nonlinear optical medium, for the purpose of improving the efficiency of the nonlinear optical effect by quasi phase matching.

  The inventors of the present invention, as a process of generating natural parametric fluorescence, generate second harmonic generation (SHG) and SPDC, which are seed light in the SPDC process, in a single second-order nonlinear optical medium. The cascade SHG / SPCD process has already been reported (see Non-Patent Document 1).

According to this method, when excitation light is input to a LiNbO ₃ crystal (PPLN: Periodically Poled Lithium Niobate, hereinafter also referred to as a PPLN crystal) in which a LiNbO3 crystal has a periodically modulated structure with a nonlinear optical constant, First, SHG light is generated, and then SPDC is generated using this SHG light as seed light. This process can be regarded as a pseudo third-order nonlinear optical effect, and the wave number and angular frequency of the input excitation light and the generated signal light, idler light are related to the above equations (3) and (4), respectively. Satisfied.

  In addition to the above-described correlation between the wave number, the angular frequency, and the generation time, there is also a correlation with respect to polarization between the signal light and the idler light. Hereinafter, a correlated photon pair of signal light and idler light generated by such natural parametric fluorescence may be referred to as a correlated photon pair.

  Further, by using a correlated photon pair generated in the above-described SPDC process, SFWM process, or cascade SHG / SPDC process, a polarization entangled photon pair using polarization correlation can be generated. As an example of this, a 1550 nm band entangled photon pair generator using a Sagnac interferometer using a polarization beam splitter (PBS) and a single PPLN crystal has been disclosed in the prior art. (For example, see Non-Patent Document 2). In Non-Patent Document 2, a 90 ° optical axis conversion part is provided at one place in a polarization-maintaining optical fiber connecting PBS and a PPLN crystal. This 90-degree optical axis conversion portion can be formed by an optical fiber fusion technique or the like.

  In the entangled photon pair generator disclosed in Non-Patent Document 2, when 45-degree polarized excitation light having a wavelength of 776 nm is input to a PPLN crystal via PBS, the wavelength is 1542 nm by natural parametric down-conversion in the PPLN crystal. Signal light and 1562 nm idler light are generated. In the entangled photon pair generator, the intensity of the excitation light is sufficiently weak, so these correlated photon pairs are either the excitation light propagating clockwise in the Sagnac interferometer loop or the excitation light propagating counterclockwise in the loop. It occurs only from either one. In this case, the state of the correlated photon pair generated from the quantum entangled photon pair generator is such that the correlated photon pair propagating clockwise through the loop and the correlated photon pair propagating counterclockwise and orthogonally polarized with the clockwise component. It will be in the superposition state. In other words, a polarization entangled photon pair is generated from this entangled photon pair generator.

  On the other hand, a technique for generating a polarization entangled photon pair using SPDC in a second-order nonlinear optical medium using phase matching or quasi phase matching called type II is disclosed (for example, see Non-Patent Document 3). ). According to this technique, the correlated photon pair of the signal light and idler light is generated in a polarization state orthogonal to each other.

Shin Arahira, Naoshi Kishimoto, "Generating Parametric Down-Conversion Light Using Cascade Chi (2) Method Using PPLN Ridge Waveguide Devices", 21st Quantum Information Technology Meeting, IEICE, Quantum Information Technology Timed Research Specialist Society, pp.184-187, (2009) HCLim, A. Yoshizawa, H. Tsuchida, and K. Kikuchi, "Stable source of high quality telecom-band polarization-entangled photon-pairs based on a single, pulse-pumped, short PPLN waveguide," Optics Express. Vol. 16, No.17, pp.12460-12468, (2008) Keigo Hasegawa, Masafumi Ohinata, Naoto Nogata, Nao Kurimura, Shuichiro Inoue, "Generation of 1550 nm narrow-band orthogonally polarized entangled photon pairs using Type-II type periodically poled lithium niobate waveguide", 19th Quantum Information Technology Research Materials, IEICE, Quantum Information Technology Timed Research Committee, pp.137-140, (2008)

Regarding the polarization entangled photon pair, it is known that the following four states exist as quantum states for realizing the maximum entangled state. These four states are expressed by the following equations (5-a) to (5-d).
| H> _s | H> _i + | V> _s | V> _i (5-a)
| H> _s | H> _i- | V> _s | V> _i (5-b)
| H> _s | V> _i + | V> _s | H> _i (5-c)
| H> _s | V> _i- | V> _s | H> _i (5-d)
Here, the state vectors | H> and | V> are ket vectors indicating states of photons of transverse polarization and longitudinal polarization, respectively. Further, subscripts s and i attached to these ket vectors are codes for identifying signal light and idler light, respectively.

  The quantum states represented by the formula (5-a) and the formula (5-b) are both states in which signal light and idler light having the same polarization state are generated in pairs, while the formula (5-c) And (5-d) are states in which orthogonally polarized signal light and idler light are generated in pairs. In addition, in the state represented by Formula (5-a) and Formula (5-b), and the state represented by Formula (5-c) and Formula (5-d), the phases of the superimposed states are reversed. It shows the state. The states represented by the formula (5-a), the formula (5-b), the formula (5-c), and the formula (5-d) are also called “bell states”. The development of wave entangled light sources is an important issue in the field of quantum information communications.

  In the polarization quantum entangled light source disclosed in Non-Patent Document 2 described above, the signal light and idler light are generated as a pair of the same polarization state, so the equations (5-a) and (5-b ) Can be realized.

  On the other hand, according to the method of generating the polarization entangled photon pair disclosed in Non-Patent Document 3 described above, the correlation photon pair of the signal light and idler light is generated in a polarization state orthogonal to each other. Any of the states represented by 5-c) and (5-d) can be realized.

  However, any one of the four bell states represented by the formula (5-a), the formula (5-b), the formula (5-c), and the formula (5-d) can be selected by the same apparatus so far. A polarization entangled photon pair output light source that can be realized, that is, a polarization entangled photon pair generator has not been developed.

  The inventor of this application inputs excitation light into a loop optical path including a nonlinear optical medium, generates a correlation photon pair clockwise and counterclockwise, and outputs a clockwise correlation photon pair output from the loop optical path. Then, a configuration in which the correlated photon pair and the counterclockwise correlated photon pair whose polarization planes are orthogonal to each other is extracted by separating the wavelength components of the photons of the correlated photon pair with a wavelength separation filter. By adopting such a configuration, any one of the states represented by the above formulas (5-a) and (5-b) can be realized, and any one of the wavelength-separated wavelength components can be realized. It has been thought that either state represented by the formula (5-c) or the formula (5-d) can be realized by adopting a configuration in which is taken out through a half-wave plate. That is, the wavelength separation filter is configured to separate and extract the wavelength components of each photon of the correlated photon pair, and by installing a half-wave plate in the optical path of one wavelength component, all four bell states I realized that it is possible to realize a polarization entangled photon pair generator that can be realized by selecting.

  As a result of the earnest research by the inventors of this application, in order to specifically realize all four bell states, the plane of polarization is orthogonal by the plane of polarization and the optical phase adjustment unit installed in the loop optical path. The configuration of the functional part that adjusts the optical phase difference between the correlated photon pairs and that is arranged outside this loop optical path and that inputs the excitation light into this loop optical path and outputs the correlated photon pair generated in this loop optical path And it was confirmed that it can be realized by devising the arrangement method.

  In addition, a birefringent medium that adjusts the optical phase difference between the functional part that rotates the plane of polarization in the loop optical path and the correlated photon pair in which the plane of polarization is orthogonal is devised and the plane of polarization is orthogonalized by this birefringent medium. The configuration of the functional part that adjusts the optical phase difference between the correlated photon pairs, is arranged outside the loop optical path, inputs the excitation light to the loop optical path, and outputs the correlated photon pair generated in the loop optical path, and It was confirmed that it is possible to select and realize all four bell states by devising the arrangement method.

  In addition, as described later, the optical phase difference between the orthogonal polarization planes of the 45-degree polarized excitation light is adjusted to the optical path outside the loop optical path until the 45-degree polarized excitation light is input to the loop optical path. The birefringent medium is devised and arranged, and the optical phase difference between the pumping lights whose polarization planes are orthogonal is adjusted by this birefringent medium. The pumping light placed outside this loop optical path is input to this loop optical path. It was confirmed that all four bell states can be selected and realized by devising the configuration and arrangement method of the functional part that outputs the correlated photon pair generated in the loop optical path.

  Accordingly, an object of the present invention is to provide a polarization entangled photon pair generating device that is an apparatus having a simple configuration and can be realized by selecting all four bell states.

  Therefore, according to the gist of the present invention, there is provided a polarization entangled photon pair generating device having the following configuration.

  A first polarization entangled photon pair generating device according to the present invention comprises a polarization quantum entangled photon comprising a loop optical path, an optical branching input / output unit, and a half-wave plate disposed downstream of the optical branching input / output unit. Pair generator.

  A loop optical path is generated in the nonlinear optical medium based on a nonlinear optical medium and excitation light that is input to the loop optical path so that the loop optical path propagates clockwise and counterclockwise. A polarization plane and an optical phase adjuster for adjusting the optical phase difference between the light component that rotates clockwise and the light component that rotates counterclockwise in the loop optical path of the signal light and idler light propagating through the light are disposed.

  The optical branching input / output unit inputs excitation light to the loop optical path, and correlates the correlated photon pair consisting of signal light and idler light propagating clockwise and counterclockwise along the loop optical path into the signal light wavelength component and idler light wavelength component. Are output in a state of being separated into spaces.

  A half-wave plate is disposed in one of the optical paths from which the signal light wavelength component and the idler light wavelength component that are separated and output from the light branching input / output unit are output.

  The first polarization entangled photon pair generating device of the present invention is preferably configured as follows.

  The loop optical path is connected to the second and third input / output terminals of the polarization beam splitter / combiner including the first to third input / output terminals by arranging the nonlinear optical medium, the polarization plane, and the optical phase adjusting unit in series. Connected to both ends of the optical fiber.

  The output terminal of the optical branching input / output unit is connected to the first input / output terminal of the polarization beam splitter / combiner, and the pumping light is input from the input terminal of the optical branching input / output unit and passes through the optical branching input / output unit. The signal is input to the loop optical path from the first input / output end of the polarization beam splitter, and the signal light wavelength component and the idler light wavelength component are output in a state of being spatially separated from the output end of the optical branching input / output unit. .

  The second polarization quantum entangled photon pair generation device of the present invention is a polarization quantum entangled photon pair generation device including a loop optical path, an input unit, and an optical branching input / output unit.

  In the loop optical path, a first nonreciprocal polarization plane conversion unit, a nonlinear optical medium, a second nonreciprocal polarization plane conversion unit, and a birefringence medium are arranged in series.

  The input unit is a part for inputting excitation light. The optical branching input / output unit then rotates the loop optical path generated in the nonlinear optical medium clockwise and counterclockwise based on the excitation light input so as to propagate in the loop optical path clockwise and counterclockwise. A correlated photon pair consisting of signal light and idler light propagating around is output in a state of being spatially separated into a signal light wavelength component and an idler light wavelength component.

  The first half-wave plate is disposed in the optical path through which either the signal light wavelength component or the idler light wavelength component output after being spatially separated from the light branching input / output unit passes.

  The second polarization entangled photon pair generating device of the present invention is preferably configured as follows.

  The loop optical path includes a second input / output end and a third input / output end of a polarization beam splitter / combiner having first to fourth input / output ends, a first nonreciprocal polarization plane conversion unit, a nonlinear optical medium, and a second non-optical medium. A reciprocal polarization plane converter and a birefringent medium are connected to both ends of an optical fiber that is arranged and connected in series. The input unit is the first input / output terminal of the polarization beam splitter / combiner including the first to fourth input / output terminals. The fourth input / output terminal of the polarization beam splitter / combiner is connected to the input terminal of the optical branching input / output unit.

  The first non-reciprocal polarization plane conversion unit includes a first Faraday rotator that rotates the polarization plane by 45 degrees and a second half-wave plate, and the second non-reciprocal polarization plane conversion unit It comprises a second Faraday rotator that rotates 45 degrees and a third half-wave plate.

  Also, the pumping light is input from the first input / output end of the polarization beam splitter / combiner and is input to the loop optical path from the first input / output port of the polarization beam splitter / combiner via this polarization beam splitter / combiner. The component and the idler wavelength component are output in a state of being spatially separated from the output end of the optical branching input / output unit.

  A third polarization entangled photon pair generating device according to the present invention comprises a polarization entangled photon comprising a loop optical path, an optical branching input / output unit, and a half-wave plate disposed downstream of the optical branching input / output unit. Pair generator.

  In the loop optical path, a nonlinear optical medium, one of excitation light input to propagate clockwise and counterclockwise in the loop optical path, and the nonlinear optical medium based on the excitation light. A 90-degree polarization plane rotation unit that rotates the polarization plane of either the signal light that propagates through this loop optical path and the optical component that rotates clockwise in the loop path of idler light or the light component that rotates counterclockwise by 90 degrees is arranged Has been.

  The optical branching input / output unit inputs excitation light to the loop optical path, and correlates the correlated photon pair consisting of signal light and idler light propagating clockwise and counterclockwise along the loop optical path into the signal light wavelength component and idler light wavelength component. Are output in a state of being separated into spaces. Also, a birefringence medium for adjusting the optical phase difference between orthogonal polarization planes of 45-degree polarized excitation light, which will be described later, is disposed in any optical path until the excitation light is input to the loop optical path.

  A half-wave plate is disposed in one of the optical paths from which the signal light wavelength component and the idler light wavelength component that are separated and output from the light branching input / output unit are output.

  The third polarization entangled photon pair generator of the present invention is preferably configured as follows.

  An optical fiber that connects the second and third input / output ends of a polarization beam splitter / combiner having first to third input / output ends by connecting a nonlinear optical medium and a 90-degree polarization plane rotation unit in series to the loop optical path. Connected to both ends.

  The output terminal of the optical branching input / output unit is connected to the first input / output terminal of the polarization beam splitter / combiner, and the pumping light is input from the input terminal of the optical branching input / output unit and passes through the optical branching input / output unit. The signal is input to the loop optical path from the first input / output end of the polarization beam splitter, and the signal light wavelength component and the idler light wavelength component are output in a state of being spatially separated from the output end of the optical branching input / output unit. . The above-described birefringent medium that adjusts the optical phase difference between orthogonal polarization planes of 45-degree polarized excitation light is disposed at the input end of the above-described optical branching input / output unit. Further, it may be arranged at a location where the output terminal of the optical branching input / output unit and the first input / output terminal of the polarization beam splitter / combiner are connected. In any case, it is arranged in any optical path that passes after the excitation light becomes 45-degree polarized excitation light and is input to the loop optical path.

  According to the first polarization entangled photon pair generating device of the present invention, the loop optical path is changed to the clockwise direction based on the excitation light that is input to the loop optical path so as to propagate the loop optical path clockwise and counterclockwise. Signal light and idler light propagating around and counterclockwise are generated by a nonlinear optical medium disposed in the loop optical path.

  As will be described later, the polarization plane of the signal light and idler light propagating clockwise and the polarization plane of the signal light and idler light propagating counterclockwise are orthogonal to each other by the polarization plane and the optical phase adjustment unit. Further, the phase difference between the light component that rotates clockwise in the loop optical path and the light component that rotates counterclockwise is adjusted by the polarization plane and the optical phase adjustment unit. Adjusting this phase difference, and placing a half-wave plate in one of the optical paths that output the signal light wavelength component and idler light wavelength component that are separated and output from the optical branching input / output unit By doing so, it is realized by selecting any of the bell states represented by the equations (5-a) to (5-d) as described later.

  According to the second polarization entangled photon pair generating apparatus of the present invention, the loop optical path is changed to the clockwise direction based on the excitation light that is input to the loop optical path so as to propagate the loop optical path clockwise and counterclockwise. Signal light and idler light propagating around and counterclockwise are generated by a nonlinear optical medium disposed in the loop optical path.

  Signal light and idler light propagating clockwise and signal light and idler light propagating counterclockwise are orthogonal to each other by the first non-reciprocal polarization conversion unit and the second non-reciprocal polarization conversion unit It becomes a state to do. Further, the birefringent medium adjusts the phase difference between the light component that rotates clockwise in the loop optical path and the light component that rotates counterclockwise, as will be described later. Adjusting this phase difference and placing a half-wave plate in the optical path where either the signal light wavelength component or idler light wavelength component that is output after being spatially separated from the optical branching input / output unit is output By doing so, it is realized by selecting any of the bell states represented by the equations (5-a) to (5-d) as described later.

  According to the third polarization entangled photon pair generating device of the present invention, the loop optical path is changed to the clockwise direction based on the excitation light that is input to propagate in the loop optical path clockwise and counterclockwise. Signal light and idler light propagating around and counterclockwise are generated by a nonlinear optical medium disposed in the loop optical path.

  In the third polarization entangled photon pair generating device, a birefringent medium is disposed in the optical path through which the excitation light is input to the loop optical path after passing through the optical branching input / output unit. Therefore, unlike the first polarization entangled photon pair generator, the photon pair does not pass through the birefringent medium. On the other hand, the optical phase of the excitation light that generates the cross-polarized correlation photon pair and the optical phase of the excitation light that generates the longitudinally-polarized correlation photon pair differ depending on the retardation generated in the birefringent medium. This retardation adjustment is performed, and a half-wave plate is disposed in the optical path where either one of the signal light wavelength component and the idler light wavelength component output after being spatially separated from the light branching input / output unit is output. As described later, this is realized by selecting any of the bell states represented by the equations (5-a) to (5-d).

1 is a schematic block configuration diagram of a first polarization entangled photon pair generation device according to an embodiment of the present invention. FIG. It is a figure where it uses for description about the structure of a polarization plane and an optical phase adjustment part, and its function. FIG. 5 is a diagram for explaining the configuration and function of a polarization plane and an optical phase adjustment unit configured to use a third Faraday rotator that rotates a polarization plane of linearly polarized light by −45 degrees instead of the first Faraday rotator. is there. It is a diagram for explaining how to set the direction of the optical axis of the half-wave plate and arrange the half-wave plate, (A) is a formula (5-a) and formula (5 (B) shows the arrangement method for generating the bell-state photon pair given by -b), and (B) shows the case for generating the bell-state photon pair given by equations (5-c) and (5-d). It is a figure where it uses for description of the arrangement method. FIG. 6 is a schematic block configuration diagram of a first modification of the first polarization entangled photon pair generation device according to the embodiment of the present invention. FIG. 6 is a schematic block configuration diagram of a second modification of the first polarization entangled photon pair generation device according to the embodiment of the present invention. FIG. 3 is a schematic block configuration diagram of a second polarization entangled photon pair generation device according to an embodiment of the present invention. FIG. 5 is a diagram for explaining a schematic configuration and operation of a first nonreciprocal polarization plane converter. FIG. 5 is a diagram for explaining a schematic configuration and operation of a second nonreciprocal polarization plane conversion unit. FIG. 5 is a schematic block configuration diagram of a third polarization entangled photon pair generation device according to an embodiment of the present invention. It is a figure where it uses for description of the relationship between the optical axis of the birefringent medium inserted in any one optical path until excitation light is input into a loop optical path, and the polarization direction of excitation light.

  Hereinafter, embodiments of the present invention will be described with reference to FIGS. In the following description, specific elements and operating conditions may be taken up. However, these elements and operating conditions are only one of preferred examples, and thus are not limited to these. FIG. 1 and FIGS. 5 to 7 illustrate an example of the configuration according to the present invention, and merely schematically show the arrangement relationship of each component to the extent that the present invention can be understood. However, the present invention is not limited to the illustrated example.

≪First polarization entangled photon pair generator≫
<Description of configuration>
A basic configuration of the first polarization entangled photon pair generating device of the embodiment of the present invention will be described with reference to FIG.

  The first polarization entangled photon pair generator includes a Sagnac interferometer loop optical path 20 including a polarization splitting synthesizer 101 and its two input / output terminals, and pump light into the loop optical path 20. An optical branching input / output unit 10 is provided for spatially separating and outputting the polarization entangled photon pair wavelength component output from the loop optical path 20.

  In the loop optical path 20, a nonlinear optical medium 102, a polarization plane and optical phase adjustment unit 104, and a 90-degree polarization plane rotation unit 103 are connected in series with an optical fiber, and both ends of the optical fiber are connected to the first input / output end 101. -1, connected to the second input / output terminal 101-2 and the third input / output terminal 10-3 of the polarization beam splitter / combiner 101 having the second input / output terminal 101-2 and the third input / output terminal 101-3. Is made up of.

The optical branching input / output unit 10 includes a bandpass optical filter 105, a lowpass optical filter 106, a WDM (wavelength) including a first input / output terminal 105-1, a second input / output terminal 105-2, and a third input / output terminal 105-3. (Division Multiplexer / Demultiplexer) filter 107 is provided. The pumping light having the wavelength λ _p is input to the bandpass optical filter 105 from the first input / output terminal 105-1, and the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 passes through the bandpass optical filter 105. To the loop optical path 20.

  When excitation light is input to the loop optical path 20 via the optical branching input / output unit 10 so as to propagate clockwise and counterclockwise in the loop optical path 20, nonlinearity is generated in the nonlinear optical medium 102 based on the excitation light. Due to the optical effect, a correlated photon pair consisting of signal light and idler light propagating clockwise and counterclockwise in the loop optical path 20 is generated. This correlated photon pair is output from the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 constituting the loop optical path 20, and is then bandpassed from the second input / output terminal 105-2 of the bandpass optical filter 105. The light is input to the optical filter 105 and is input from the third input / output terminal 105-3 to the WDM filter 107 via the low-pass optical filter 106 provided in the optical branching input / output unit 10. The correlated photon pair consisting of signal light and idler light input to the WDM filter 107 is output in a state of being spatially separated into a signal light wavelength component and an idler wavelength component by the WDM filter 107.

As will be described later, the low-pass optical filter 106 functions to remove the wavelength component (λ _p / 2) of the SHG light generated in the nonlinear optical medium 102.

  A half-wave plate 108 that rotates the polarization direction of the passing photon by 90 degrees in the optical path in which either the signal light wavelength component or the idler light wavelength component that is output after being spatially separated from the WDM filter 107 is output. Has been placed. In the apparatus shown in FIG. 1, the half-wave plate 108 is disposed in the optical path from which the idler wavelength component is output, but the half-wave plate 108 is disposed in the optical path from which the signal light wavelength component is output. It is good also as a structure to be made.

  The first polarization entangled photon pair generating device is preferably composed of a polarization plane preserving optical system. In addition, the relationship between the optical axes of the polarization splitting / combining device 101, the nonlinear optical medium 102, the 90-degree polarization plane rotating unit 103, the polarization plane and the optical phase adjusting unit 104, etc., constituting the loop optical path 20, is described below with special considerations. It is prescribed with.

  In addition, the first polarization entangled photon pair generation device may be configured to spatially couple each optical component with an optical coupling lens. Moreover, it is good also as a structure which couple | bonds each optical component with a polarization-maintaining optical fiber. In any case, it is desirable that this apparatus is composed of a polarization plane preserving optical system until reaching the output end where the signal light and idler light are finally spatially separated and output.

  However, even in the case of using a normal optical fiber that is not compensated for preserving the polarization plane in place of the polarization plane-maintaining optical fiber, it can be simulated by using additional optical components such as a polarization plane controller as appropriate. In other words, it is possible to construct a polarization plane preserving optical system.

Let the excitation light wavelength be λ _p , which is the wavelength near the wavelength of the polarization entangled photon pair to be generated. That is, if the wavelength band of the generated polarization entangled photon pair is 1550 nm, the value of the excitation light wavelength λ _p is set to a value in the vicinity of 1550 nm.

  The nonlinear optical medium 102 is a nonlinear optical medium that effectively expresses a second-order nonlinear optical effect such as PPLN or a nonlinear optical medium that effectively expresses a third-order nonlinear optical effect such as an optical fiber. In any case, a nonlinear optical medium that generates parametric fluorescence by excitation light is employed.

When a second-order nonlinear optical medium is employed as the nonlinear optical medium 102, correlated photon pairs can be generated by a cascade SHG / SPDC process (see Non-Patent Document 1). That is, when excitation light having a wavelength λ _p is used, SHG light having a wavelength λ _p / 2 is generated in a second-order nonlinear optical medium, and signal light having a wavelength λ _s is generated by an SPDC process using the SHG light as seed light. Correlated photon pairs of idler light of wavelength λ _i are simultaneously generated at the same spatial location, that is, simultaneously spatially.

On the other hand, when a third-order nonlinear optical medium is adopted as the nonlinear optical medium 102, a correlated photon pair is generated by the SFWM process. That is, the excitation light having the wavelength λ _p simultaneously generates a correlated photon pair of the signal light having the wavelength λ _s and the idler light having the wavelength λ _i through the SFWM process.

Regardless of the parametric fluorescence process of the second-order nonlinear optical medium or the third-order nonlinear optical medium, the wavelengths of the excitation light, signal light, and idler light λ _p , λ _s , and λ _i are expressed by the following equation (6) Satisfies the relation corresponding to the law of conservation of energy given by. Here, λ _p , λ _s , and λ _i are all wavelengths in vacuum.

2 / λ _p = 1 / λ _s + 1 / λ _i (6)
However, it is assumed that λ _s ≠ λ _p and λ _i ≠ λ _p .

In the band pass optical filter 105, the input light (excitation light of wavelength λ _p ) input from the first input / output terminal 105-1 transmits the component of wavelength λ _p that is the transmission wavelength of the band pass optical filter 105. Are output from the second input / output terminal 105-2 and input to the first input / output terminal 101-1 of the polarization beam splitter / combiner 101.

On the other hand, the input light output from the loop optical path 20, that is, output from the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 and input to the second input / output terminal 105-2 of the bandpass optical filter 105 is input. Of these, only the wavelength component (excitation light component) in the vicinity of λ _p is selectively transmitted through the bandpass optical filter 105 and output from the first input / output terminal 105-1. Among the wavelength components other than the transmitted wavelength component, that is, the reflected wavelength component of the bandpass filter 105, the wavelength component including the signal wavelength component and the idler wavelength component is output from the third input / output terminal 105-3. The light is input to the WDM filter 107 via the low-pass optical filter 106.

  The band-pass optical filter 105 is a kind of optical filter using a dielectric multilayer film, and an optical filter currently available as a band-pass optical filter can be appropriately used. Also, a so-called AWG (Arrayed waveguide grating) type WDM filter can be used as appropriate.

  The polarization beam splitter / combiner 101 includes a first input / output terminal 101-1 coupled to the second input / output terminal 105-2 of the bandpass optical filter 105, and a polarization signal obtained by orthogonalizing the input light from the first input / output terminal 101-1. A second input / output terminal 101-2 and a third input / output terminal 101-3 for separating and outputting the wave component, that is, the p-polarized wave component and the s-polarized wave component are provided. For such a polarization beam splitter / combiner 101, for example, a commercially available polarizing beam splitter can be appropriately selected and used. Alternatively, a so-called polarizing prism using a birefringent crystal can be used as appropriate.

  The p-polarization component input from the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 is output from the second input / output terminal 101-2 and s input from the first input / output terminal 101-1. The polarization component is output from the third input / output terminal 101-3. The p-polarized component input from the second input / output terminal 101-2 is output from the first input / output terminal 101-1, and the s-polarized component input from the third input / output terminal 101-3 is the first 1 Output from the input / output terminal 101-1.

  A nonlinear optical medium is connected to the second input / output terminal 101-2 of the polarization beam splitter / combiner 101 by an optical fiber. On the other hand, a 90-degree polarization plane rotation unit 103 is connected to the third input / output terminal 101-3 by an optical fiber.

The excitation light having the wavelength λ _p is input from the first input / output terminal 105-1 of the bandpass optical filter 105 and output from the second input / output terminal 105-2. Thereafter, the pumping light of wavelength λ _p is input to the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 and separated into the p polarization component and the s polarization component, respectively. 101 is output from the second input / output terminal 101-2 and the third input / output terminal 101-3.

  The p-polarized component and the s-polarized component of the polarization-separated pumping light must have the same intensity. For this reason, the excitation light when input to the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 is polarized so that the intensity ratio of the p-polarized component and the s-polarized component is 1: 1. Adjust it.

  In order to adjust the polarization splitting / combining device 101 so that the linear polarization is inclined by 45 degrees from the p-polarization direction or the s-polarization direction, for example, the first input / output terminal 105-1 of the bandpass optical filter 105 A polarization plane controller is prepared in front, and the polarization state of the pumping light when input to the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 is changed to the p polarization direction in the polarization beam splitter / combiner 101 or What is necessary is just to adjust so that it may become a linear polarization inclined 45 degree | times from the s polarization direction. The pumping light whose intensity ratio is adjusted in this way may be referred to as 45-degree polarized pumping light hereinafter.

The 90-degree polarization plane rotation unit 103 has a function of rotating the polarization plane by 90 degrees with respect to linearly polarized light. That is, the planes of polarization of excitation light, signal light, and idler light that are linearly polarized light are rotated by 90 degrees. An example of an element suitable for use in the 90-degree polarization plane rotating unit 103 is a half-wave plate. Hereinafter, unless otherwise specified, the term “half-wave plate” means a half-wave plate for light of wavelength λ _p .

  Further, as a preferable configuration example of the 90-degree polarization plane rotating unit 103, a fusion splicing unit formed by the following method may be used. That is, as the loop optical path 20, the second input / output end 101-2 and the third input / output end 101-3 of the polarization beam splitter / combiner 101 having the first to third input / output ends are connected to the nonlinear optical medium 102 and the polarization plane. And a third input / output terminal 101 of the polarization beam splitter / combiner 101, which employs an optical module component configured by connecting both ends of a polarization-maintaining optical fiber that is connected in series with the optical phase adjustment unit 104. -3 and the polarization plane preserving optical fiber connecting the polarization plane and the optical phase adjustment unit 104 are fused by rotating the optical axes (fast axis and slow axis) perpendicular to each other by 90 degrees. By connecting, the 90-degree polarization plane rotation unit 103 can be formed.

  The configuration and function of the polarization plane and optical phase adjustment unit 104 will be described with reference to FIG. The polarization plane and optical phase adjustment unit 104 is installed between the 90-degree polarization plane rotation unit 103 and the nonlinear optical medium 102 as shown in FIG.

  The polarization plane and optical phase adjustment unit 104 includes a first Faraday rotator 278 that rotates the polarization plane of linearly polarized light by +45 degrees, a birefringent medium 282 having optical axes X and Y, and a polarization plane of linearly polarized light of −45. The second Faraday rotator 280 rotating in degrees is arranged in series in this order.

  The operation of the polarization plane and optical phase adjustment unit 104 is as follows.

  When light propagates in the direction indicated as the clockwise component in FIG. 2, that is, p-polarized light is input from the second Faraday rotator 280 side of the polarization plane and optical phase adjustment unit 104, and the first Faraday rotator The operation when output from the 278 side will be described. The p-polarized light passes through the second Faraday rotator 280 and the polarization direction is rotated by −45 degrees. The birefringent medium 282 is arranged so that the polarization direction after the polarization rotation coincides with one of the optical axis directions of the birefringent medium 282 (the Y axis in FIG. 2). This light passes through the birefringent medium 282 as a linearly polarized wave parallel to the Y axis of the birefringent medium 282 and is then input to the first Faraday rotator 278. Then, the first Faraday rotator 278 is rotated by +45 degrees of polarization. As a result, this light returns to p-polarized light again and is output from the polarization plane and the optical phase adjusting unit 104.

  Next, when light propagates in the direction indicated as the counterclockwise component in FIG. 2, that is, p-polarized light is input from the first Faraday rotator 278 side of the polarization plane and optical phase adjustment unit 104, and the first The operation when output from the 2 Faraday rotator 280 side will be described. The p-polarized light passes through the first Faraday rotator 278 and the polarization direction is rotated by +45 degrees. At this time, the polarization direction of the polarization-rotated light coincides with the X-axis direction of the birefringent medium 282. Therefore, this light passes through the birefringent medium 282 as a linearly polarized wave parallel to the X axis of the birefringent medium 282. Thereafter, the signal is input to the second Faraday rotator 280. Then, the second Faraday rotator 280 is rotated by −45 degrees of polarization. As a result, the light returns to the p-polarized light again and is output from the polarization plane and the optical phase adjustment unit 104.

  That is, the p-polarized light input from the left and right of the polarization plane and optical phase adjustment unit 104 shown in FIG. 2 is the polarization plane and optical phase adjustment as p-polarized light regardless of the presence of the polarization plane and optical phase adjustment unit 104. Output from the unit 104. On the other hand, the p-polarized light input from the left and right shown in FIG. 2 to the polarization plane and optical phase adjustment unit 104 is transmitted through the birefringent medium 282 disposed in the polarization plane and optical phase adjustment unit 104 with optical axes orthogonal to each other ( Passes in a state of linear polarization parallel to the X and Y axes). For this reason, an optical phase difference based on the birefringence of the birefringent medium 282 is generated between both lights input from the left and right shown in FIG.

  That is, by inserting the polarization plane and the optical phase adjusting unit 104 into the loop optical path 20, the birefringence of the birefringent medium 282 is based on the light component propagating clockwise and counterclockwise in the loop optical path 20. An optical phase difference can be generated.

  As the birefringent medium 282, a polarization-preserving optical fiber can be used. Further, an optical crystal having uniaxial or biaxial birefringence may be used. In either case, the amount of optical phase difference generated by adjusting the length of the birefringent medium can be adjusted. Alternatively, a birefringent medium adjusted to be approximately equal to the length required to produce the amount of optical phase difference to be produced is provided and precisely generated by controlling the temperature of the birefringent medium. A control method may be adopted so that a power optical phase difference can be generated.

  On the other hand, as a suitable example of the birefringent medium, a Babinet Soleil compensator can also be used. In this case, the optical phase difference amount can be changed in a wide range, and the performance of the first polarization entangled photon pair generating device of the present invention can be realized to the maximum.

  Further, in the polarization plane and optical phase adjustment unit 104, a polarization plane and optical phase adjustment unit configured to use a third Faraday rotator that rotates the polarization plane of linearly polarized light by −45 degrees instead of the first Faraday rotator. By using this, it is possible to integrate the functions of the polarization plane and optical phase adjustment unit 104 and the 90-degree polarization plane rotation unit 103. That is, by using the polarization plane and the optical phase adjustment unit configured as described above instead of the polarization plane and the optical phase adjustment unit 104, the 90-degree polarization plane rotation unit 103 becomes unnecessary. Hereinafter, the polarization plane and optical phase adjustment unit configured to use the third Faraday rotator will be referred to as polarization plane and optical phase adjustment unit 109.

  Both of the polarization plane and optical phase adjusting units 104 and 109 rotate clockwise in the loop optical path of the signal light and idler light generated in the nonlinear optical medium based on the excitation light input to the loop optical path and propagating through the loop optical path. And a function of adjusting the optical phase difference between the light component that rotates and the light component that rotates counterclockwise.

  Referring to FIG. 3, the configuration and function of the polarization plane and the optical phase adjustment unit configured to use a third Faraday rotator that rotates the polarization plane of linearly polarized light by −45 degrees instead of the first Faraday rotator Will be described.

  As shown in FIG. 3, the p-polarized light input from the second Faraday rotator 280 side of the polarization plane and optical phase adjustment unit 109 is converted into s-polarized light when output from the polarization plane and optical phase adjustment unit 109. Is done. On the other hand, the s-polarized light input from the third Faraday rotator 284 side is converted into p-polarized light when output from the polarization plane and optical phase adjustment unit 109. 3 passes through the birefringent medium 282 in a state of linearly polarized light parallel to the optical axes (X axis, Y axis) perpendicular to each other of the birefringent medium 282. For this reason, an optical phase difference based on the birefringence of the birefringent medium 282 occurs between the two lights.

  The effect obtained by the polarization plane and optical phase adjustment unit 109 is equivalent to the effect realized by the two functional parts of the polarization plane and optical phase adjustment unit 104 and the 90-degree polarization plane rotation unit 103 described above. Therefore, instead of using the polarization plane and optical phase adjustment unit 104 and the 90-degree polarization plane rotation unit 103, even if only the polarization plane and optical phase adjustment unit 109 are used, the polarization entangled photon pair having the same function is used. It is possible to form a generator.

  In the embodiment of the first polarization entangled photon pair generation device described above, the excitation light, SHG light, signal light, and idler light input to and output from the nonlinear optical medium 102 are in the same polarization direction. It was defined as linearly polarized light. To make the excitation light, SHG light, signal light and idler light input / output to / from the nonlinear optical medium 102 become linearly polarized light in the same polarization direction, for example, using a PPLN crystal as the nonlinear optical medium 102, PPLN The d33 component of the tensor component of the second-order nonlinear optical constant of the crystal is used for SHG expression and SPDC expression, and the excitation light polarized in the z-axis direction of the nonlinear optical crystal constituting the PPLN crystal is input. It is realized by doing.

The low-pass optical filter 106 removes the wavelength component (λ _p / 2) of the SHG light generated in the nonlinear optical medium 102 from the output light output from the third input / output terminal 105-3 of the band-pass optical filter 105. Work. The wavelength components of the transmitted light of the low-pass light filter 106 are the residual light of the excitation light wavelength component (λ _p ), the signal light wavelength component (λ _s ), and the idler light wavelength component (λ _i ). Note that when a third-order nonlinear optical medium is used as the nonlinear optical medium 102, no SHG light is generated, so that the low-pass optical filter 106 is unnecessary.

The WDM filter 107 cuts at least the signal light wavelength component (λ _s ) and the idler light wavelength component (λ _i ) out of the transmitted light of the low-pass optical filter 106 into separate optical paths, and the signal light wavelength component (λ _s ) It is output in a state of being spatially separated into the idler wavelength component (λ _i ). As such a WDM filter 107, an AWG type WDM filter having at least a signal light wavelength component (λ _s ) and an idler light wavelength component (λ _i ) as transmission wavelength components can be appropriately used.

  A half-wave plate 108 is arranged in one of the optical paths through which signal light or idler light output through the WDM filter 107 passes. In FIG. 1, the half-wave plate 108 is disposed in the optical path through which idler light passes, but the half-wave plate 108 may be disposed in the optical path through which signal light passes.

  With reference to FIGS. 4A and 4B, how to set the direction of the optical axis of the half-wave plate 108 and arrange the half-wave plate 108 will be described. FIG. 4 (A) shows an arrangement method for generating a bell-state photon pair given by the equations (5-a) and (5-b), and FIG. 4 (B) shows the equations (5-c) and It is a figure with which it uses for description of the arrangement | positioning method in the case of producing | generating the bell state photon pair given by (5-d).

  When generating a bell-state photon pair given by Equation (5-a) and Equation (5-b), as shown in FIG.4 (A), the two optical axes of the half-wave plate 108 are: Arrange them so that they match the p-polarization direction and the s-polarization direction. Further, when generating the bell-state photon pair given by the equations (5-c) and (5-d), as shown in FIG.4 (B), the two optical axes of the half-wave plate 108 However, they are arranged to make an angle of 45 degrees with each of the p polarization direction and the s polarization direction. Both arrangements shown in FIGS. 4 (A) and 4 (B) can be arbitrarily selected. Therefore, a suitable example of the half-wave plate 108 is a half-wave plate module that can be set by arbitrarily rotating the optical axis direction of the half-wave plate 108.

  The signal light wavelength component and the idler light wavelength component that have passed through the WDM filter 107 and the half-wave plate 108 propagate through an optical transmission line such as an optical fiber communication network, and are transmitted to the receiver A and the receiver B, respectively. . The receiver A and the receiver B execute simultaneous measurement, thereby executing information transmission based on the quantum information communication technology.

The polarization beam splitter / combiner 101, the 90-degree polarization plane rotation unit 103, the polarization plane / optical phase adjustment unit 104, and the polarization plane / optical phase adjustment unit 109 are near the wavelength λ _p (including λ _s and λ _i ), respectively. An element or a device that can guarantee the operation may be used. That is, at the wavelength λ _p / 2, it is not necessary to ensure the operation of these elements or devices. If λ _p is 1550 nm, it is possible to appropriately use commercially available optical components such as a polarization demultiplexer / synthesizer, a Faraday rotator, and a wave plate for the 1550 nm band. It is not necessary to use a special polarization beam splitter / combiner that operates normally at both wavelengths λ _p and λ _p / 2, as described.

  In order to produce the first polarization entangled photon pair generating device, the loop light path 20, the bandpass optical filter 105, the lowpass optical filter 106, the WDM filter 107 and the signal light that has passed through the half-wave plate 108, It is desirable to preserve the polarization of idler light. Therefore, as the band pass optical filter 105, the low pass optical filter 106, the WDM filter 107, and the half-wave plate 108, it is desirable to use an optical module using a spatial coupling system or a polarization maintaining optical fiber.

<Description of operation>
Here, in order to select and generate the bell-state polarization entangled photon pairs represented by the equations (5-a) to (5-d), the first polarization entangled photon pair generator is How it works will be described in detail.

Using a PPLN crystal that is a second-order nonlinear optical medium as the nonlinear optical medium 102, using the cascade SHG / SPDC process disclosed in Non-Patent Document 1 as a parametric fluorescence generation process, and the tensor component of the second-order nonlinear optical constant SHG expression by d _33, as utilizing the SPDC expression, the operation of the first polarization entangled photon pair generating device.

Excitation light having a wavelength λ _p is input to the first input / output terminal 105-1 of the bandpass optical filter 105, and this excitation light is then input to the first input / output terminal 101-1 of the polarization beam splitter / combiner 101. At this time, the excitation light is appropriately polarized so as to be in a 45-degree polarization state, so that the second input / output terminal 101-2 and the third input / output terminal 101-3 of the polarization beam splitter / combiner 101 are provided. Therefore, pump light of the same intensity is output with p polarization and s polarization, respectively. Then, the PPLN crystal is arranged so that the p-polarization direction matches the z-axis direction of the PPLN crystal.

  The pump light propagating clockwise in the loop optical path 20 and entering the PPLN crystal is p-polarized light. At this time, as disclosed in Non-Patent Document 1, SHG light is generated in the PPLN crystal. Furthermore, a correlated photon pair of signal light and idler light is generated in the PPLN crystal by the SPDC process using this SHG light as seed light. All of these lights are p-polarized.

Next, the excitation light, SHG light, signal light, and idler light (all these lights are in the same polarization state) output from the PPLN crystal pass through the polarization plane and the optical phase adjustment unit 104. At this time, the excitation light, signal light, and idler light having a wavelength in the vicinity of λ _p are output from the polarization plane and the optical phase adjustment unit 104 with p polarization. Then, the birefringent crystal 282 passes as a linearly polarized wave in the Y-axis direction of the birefringent crystal 282, and an optical phase is generated with respect to the polarization component in the Y-axis direction. On the other hand, for the SHG light, the polarization plane and the optical system of the optical phase adjustment unit 104 are not optimized for the wavelength of the SHG light, so it is uncertain what kind of polarization state is output. However, this is not a problem because the SHG light component is finally removed by the low-pass optical filter 106.

  Next, the pump light, the signal light, and the idler light pass through the 90-degree polarization plane rotating unit 103 and the polarization direction is rotated by 90 degrees to become s-polarized light, which is the third input / output of the polarization beam splitter / combiner 101. As a result, the signal is output from the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 as s polarized wave.

  In other words, the excitation light propagating clockwise in the loop optical path 20 manifests both the SHG process and the SPDC process in the PPLN crystal, so that the correlated photon pair of s-polarized signal light and idler light is polarized. The signal is output from the first input / output terminal 101-1 of the synthesizer 101.

On the other hand, the pump light propagating counterclockwise in the loop optical path 20 is s-polarized when it is output from the third input / output terminal 101-3 of the polarization beam splitter / combiner 101, but rotated by 90 degrees Passing through the section 103, the polarization direction is rotated 90 degrees to become p-polarized light. Thereafter, the excitation light passes through the polarization plane and optical phase adjustment unit 104. At this time, the pumping light having a wavelength in the vicinity of λ _p is output from the polarization plane and the optical phase adjusting unit 104 with p polarization. However, at this time, the birefringent crystal 282 passes through the birefringent crystal 282 as a linearly polarized wave in the X-axis direction, and an optical phase is generated with respect to the polarization component in the X-axis direction.

  Thereafter, when this excitation light is input to the PPLN crystal, the polarization of the excitation light is p-polarization that matches the z-axis method of the PPLN crystal. Then, SHG light is generated in the PPLN crystal, and a correlated photon pair of signal light and idler light is generated in the PPLN crystal by the SPDC process using this SHG light as seed light. These are all p-polarized waves.

  Next, the pumping light, the signal light, and the idler light are input as p-polarized light to the second input / output terminal 101-2 of the polarization beam splitter / combiner 101, and the first input / output terminal 101- 1 is output as p-polarization. That is, the p-polarized signal light and the idler light correlated photon pair is output from the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 by the pump light propagating counterclockwise in the loop optical path.

  Neglecting the optical loss of optical components such as the polarization demultiplexer / synthesizer 101, the bandpass optical filter 105, etc., the pumping light that rotates in the loop optical path 20 and the pumping light that rotates counterclockwise in the loop optical path 20 are input to the PPLN crystal. Are equal and the polarization directions are also equal, so if the PPLN crystal has a centrally symmetric structure, the generation probability of SPDC correlated photons generated in the PPLN crystal is equal to the clockwise and counterclockwise excitation light. .

  Accordingly, since the excitation light intensity is sufficiently weak, the correlated photon pair of the signal light and idler light output from the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 propagates clockwise in the loop optical path 20. This is either an s-polarized correlated photon pair generated based on the pumping light or a p-polarized correlated photon pair generated based on the pumping light propagating counterclockwise in the loop optical path 20. In other words, the state of the correlated photon pair output from the polarization entangled photon pair generator is such that the correlated photon pair propagating clockwise in the loop optical path 20 and the counterclockwise component are polarized orthogonally to the clockwise component. It becomes the state of superposition with the correlated photon pair. That is, a polarization quantum entangled photon pair is generated from the polarization quantum entangled photon pair generator.

  The pumping light, signal light, and idler light output from the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 are then input to the second input / output terminal 105-2 of the bandpass optical filter 105 to be signaled. Light and idler light are output from the third input / output terminal 105-3. Further, most of the excitation light (ideally all) is output from the first input / output terminal 105-1. A part of the remaining light component is output from the third input / output terminal 105-3.

The output light output from the third input / output terminal 105-3 of the bandpass optical filter 105 passes through the lowpass optical filter 106, so that the SHG light component having the wavelength λ _p / 2 is removed.

Next, the signal light wavelength component (λ _s ) and the idler light wavelength component (λ i) are separated into separate optical paths and output by the WDM filter 107. The WDM filter 107 needs to have sufficient wavelength separation capability so that the remaining excitation light wavelength component (λ _p ) is not mixed in each optical path. As such a WDM filter, a so-called AWG type WDM filter having at least a signal light wavelength component and an idler wavelength wavelength component as a transmission wavelength component can be appropriately used. In addition, a structure capable of sufficiently removing the excitation light wavelength component can be obtained in combination with a diffraction grating type filter having a Bragg reflection wavelength of λ _p , such as a fiber Bragg grating.

  As already explained, a half-wave plate is arranged in one of the optical paths through which the signal light and idler light that have passed through the WDM filter 107 pass. The direction of the optical axis of this half-wave plate is shown in FIG. They are arranged as shown in (A) and (B).

  That is, as shown in FIG. 4A, the two optical axes of the half-wave plate 108 are arranged so as to coincide with the p-polarization direction and the s-polarization direction, respectively. Alternatively, as shown in FIG. 4B, the two optical axes of the half-wave plate 108 are arranged to form an angle of 45 degrees with each of the p-polarization direction and the s-polarization direction. One of these arrangements (one of the arrangement shown in FIG. 4A or the arrangement shown in FIG. 4B) can be arbitrarily selected.

  The signal light wavelength component and idler light wavelength component that have passed through the WDM filter 107 and the half-wave plate 108 are transmitted to the receiver A and the receiver B, respectively, after passing through an optical transmission line such as an optical fiber communication network. The receivers A and B execute information transmission based on the quantum information communication technology by performing simultaneous measurement and the like.

  In order to allow the polarization quantum entangled photon pair generating device of this invention to operate suitably, the signal that has passed through the loop optical path 20, the bandpass optical filter 105, the lowpass optical filter 106, the WDM filter 107, and the half-wave plate 108 It is necessary to guarantee polarization preservation of light and idler light. Therefore, as described above, as the band-pass optical filter 105, the low-pass optical filter 106, the WDM filter 107, and the half-wave plate 108, an optical module using a spatial optical system or a polarization-maintaining optical fiber is used. Is desirable.

  Next, how the polarization quantum entangled photon pair generating apparatus of the present invention can realize four bell states will be described.

  First, for simplicity, the case where the half-wave plate 108 is not provided will be described.

  In the polarization quantum entangled photon pair generating device of the present invention, in order to generate a photon pair of signal light and idler light in the same polarization direction at the two output ends of the signal light and idler light of the WDM filter 107, The bell state that can be realized is either given by the formula (5-a) or (5-b).

The general state of the polarization entangled photon pair at this time is given by the following equation (7).
α | H> _s | H> _i + βe ^jφ | V> _s | V> _i (7)
However, j is an imaginary unit, and α and β satisfy | α | ² + | β | ² = 1.

| Α | and | β | represent the probability of occurrence of a pair of photons of transverse polarization (H) and longitudinal polarization (V), respectively. The maximum entanglement state is realized when | α | = | β | = ^{2−1 /} 2 and φ = 0 or π. Hereinafter, horizontal polarization (H) may be written as H polarization, and vertical polarization (V) as V polarization.

  The first polarization entangled photon pair generating device is formed using an optical module using a spatial optical system or a polarization-maintaining optical fiber. Therefore, the respective polarization directions of the transverse polarization (H) and the longitudinal polarization (V) coincide with one of the polarization directions of the p polarization and the s polarization.

Here, it is assumed that the polarization directions of p-polarization and transverse polarization (H) and s-polarization and longitudinal polarization (V) are matched. That is, from the pumping light propagating clockwise in the loop optical path 20, the V-polarized correlated photon pair | V> _s | V> _i is H-polarized from the pumping light propagating counterclockwise in the loop optical path 20. Wave correlated photon pairs | H> _s | H> _i are output from different output terminals of the WDM filter 107, respectively.

Here, when the 45-degree polarized excitation light is input to the first input / output terminal 101-1 of the polarization beam splitter / combiner 101, the second input / output terminal 101-2 and the third input / output terminal 101-3 The polarization separation combiner 101 is adjusted in polarization so that the intensity of the p-polarization component and the s-polarization component output is equal (intensity ratio is 1: 1), and the loop optical path 20 If the optical loss is adjusted so as to be negligible, the generation probability of H-polarized and V-polarized photon pairs is equal, and | α | = | β | = ^{2−1 / 2} is realized.

  φ is a parameter representing the optical phase difference between a photon pair of longitudinal polarization (V) and a photon pair of transverse polarization (H). In order to realize the maximum entanglement, the optical path length of the light may be set so that φ = 0 or φ = π. In order to do so, it reaches the two output ends of the WDM filter 107 (the output end from which the V-polarized correlated photon pair is output and the output end from which the H-polarized correlated photon pair is output). The optical path lengths through which the excitation light, signal light, and idler light propagating clockwise in the loop optical path need to be adjusted with an accuracy equal to or less than the wavelength.

  In order to achieve this, it is necessary to adjust and control the spatial optical path length of the spatial optical system and the length of the polarization-preserving optical fiber used to construct the apparatus with sub-wavelength accuracy. Adjustment and control are very difficult.

  Therefore, in the first polarization entangled photon pair generator, the optical path length for the excitation light, the signal light, and the idler light is adjusted with sub-wavelength accuracy by using the polarization plane and the optical phase adjustment unit 104 or 109.・ It can be controlled. Hereinafter, the above-described adjustment and control of the optical path length will be described using the polarization plane and optical phase adjustment unit 104.

When V-polarized correlated photon pair | V> _s | V> _i is generated (generated by pumping light propagating clockwise in loop optical path 20), optical phase change by polarization plane and optical phase adjustment unit 104 Can be considered as the sum of phase changes with respect to the p-polarized signal light and idler light that are input from the second Faraday rotator 280 and pass through the polarization plane and optical phase adjustment unit 104. At this time, the signal light and idler light pass through the birefringent medium 282 as linearly polarized waves in the Y-axis direction.

On the other hand, when a correlated photon pair of H polarization | H> _s | H> _i is generated (generated by excitation light propagating counterclockwise in the loop optical path 20), the polarization plane and the optical phase adjustment unit 104 The optical phase change may be considered by doubling the phase change for the p-polarized pumping light that is input from the first Faraday rotator 278 and passes through the polarization plane and optical phase adjustment unit 104. At this time, the excitation light passes through the birefringent medium 282 as a linearly polarized wave in the X-axis direction.

Therefore, it can be considered that φ including the effect of the polarization plane and the optical phase adjustment unit 104 is generally given by the following equation (8).
φ = φ ₀ +2 [(2πn _X / λ _p ) L] − [(2πn _Y / λ _s ) L] − [(2πn _Y / λ _i ) L]
= Φ ₀ +2 [(2πΔn / λ _p ) L] (8)
Here, n _X and n _Y are the refractive indices of the birefringent medium 282 with respect to linear polarization in the X-axis direction and the Y-axis direction, L is the thickness of the birefringent medium 282, φ ₀ is the polarization plane and optical phase adjustment This is the total amount of optical phase difference between the longitudinally polarized photon pair and the transversely polarized photon pair generated in the optical path other than the unit 104. This is the total amount of optical phase difference caused by the birefringence and optical path length difference of the optical components such as the polarization beam splitter / combiner 101, the polarization plane and the optical phase adjustment unit 104, the spatial optical system connecting them, or the polarization plane maintaining optical fiber. is there.

2 [(2πn _X / λ _p ) L] in Equation (8) gives twice the optical phase change for the p-polarized pump light passing through the polarization plane and the optical phase adjusting unit 104, and [(2πn _Y / λ _s ) L] and [(2πn _Y / λ _i ) L] give the optical phase change for the p-polarized signal light and idler light passing through the polarization plane and the optical phase adjusting unit 104, respectively. In equation (8), Δn≡n _X −n _Y is the retardation of the birefringent medium 282, and corresponds to the difference in the thickness of the two stacked crystals when a Babinet Soleil compensator is used.

If L is set so that φ given by formula (8) becomes 0, the bell state given by formula (5-a) can be realized, and if L is set so that φ = π, formula (5 The bell state given by -b) can be realized. As the birefringent medium 282, if a variable Babinet Soleil compensator capable of continuously realizing any value of 0 to π as the phase change amount [(2πΔn / λ _p ) L] is arbitrarily given φ _{For 0} , both φ = 0 and φ = π can be realized. In other words, it is not necessary to adjust and control the spatial optical path length of the spatial optical system and the fiber length of the polarization-maintaining optical fiber used to configure the device with high accuracy, and the equations (5-a) and (5 Any bell state given by -b) can be realized.

  Next, the function of the half-wave plate 108 arranged at the subsequent stage of the WDM filter 107 will be described. Now consider the polarization relationship and phase relationship between the signal light output from the WDM filter 107 and the idler light output from the half-wave plate 108. In FIG. 1, the half-wave plate 108 is disposed in the optical path through which idler light passes, but may be disposed in the optical path through which signal light passes.

  As shown in FIG. 4A, when the two optical axes of the half-wave plate 108 coincide with the p polarization direction (H polarization direction) and the s polarization direction (V polarization direction), The polarization state of idler light does not change at all. Therefore, the optical path length difference between the optical path through which the clockwise, counterclockwise excitation light, signal light, and idler light pass from the signal light output end of the WDM filter 107 to the idler light output end of the half-wave plate 108. If the corresponding optical phase difference (φ0) is canceled and an optical phase difference obtained by adding an optical phase difference of 0 or π is given to the polarization plane and the optical phase adjustment unit 104, it is given by equation (5-a) And the bell state given by equation (5-b) can be realized.

On the other hand, as shown in FIG. 4B, the two optical axes of the half-wave plate 108 are 45 degrees each in the p polarization direction (H polarization direction) and the s polarization direction (V polarization direction). If they are arranged so as to form the angle, the plane of polarization of idler light is rotated by 90 degrees. That is, the states of the signal light output from the WDM filter 107 and the idler light output from the half-wave plate 108 are polarization states | H> _s | V> _i and | V> _s | The polarization entangled state is given by the superposition of H> _i . At this time, the optical path length difference between the optical path through which the clockwise, counterclockwise excitation light, signal light, and idler light propagates from the signal light output end of the WDM filter 107 to the idler light output end of the half-wave plate 108. If the optical phase difference (φ0) corresponding to is canceled and the optical phase difference obtained by adding the optical phase difference of 0 or π is given by the polarization plane and the optical phase adjustment unit 104, the equation (5-c) The bell state given and the bell state given by equation (5-d) can be realized.

  In other words, the optical axis method can be arbitrarily rotated (at least the two states shown in FIGS. 4A and 4B can be used), and the half-wave plate 108 is used, and the polarization plane and the optical phase adjustment unit If optical phase compensation is performed appropriately by 104, a polarization quantum entangled photon pair generation measure that can realize any of the four bell states (bell states given by equations (5-a) to (5-d)) can be realized. .

  The first polarization entangled photon pair generating device has a modification based on the difference in the configuration of the input / output portions of the excitation light and the polarization entangled photon pair. Hereinafter, two modified examples will be described as a first modified example and a second modified example.

≪First modification of first polarization entangled photon pair generator≫
With reference to FIG. 5, a first modification of the first polarization entangled photon pair generation device will be described. In the first modification shown in FIG. 5, the optical branching input / output unit is the optical branching input / output unit 12 because its configuration is slightly different from that of the first polarization entangled photon pair generating device shown in FIG.

The first modification is a configuration example in which the roles of the bandpass optical filter 105 and the WDM filter 107 of the polarization entangled photon pair generating device shown in FIG. Here, an AWG type WDM filter is used as the WDM filter 107. As the AWG type filter, an AWG type filter having three wavelengths of a signal light wavelength (λ _s ), an idler light wavelength (λ _i ), and an excitation light wavelength (λ _p ) as transmission wavelength components is used.

  The AWG type filter has a plurality of transmitted light input / output ports for inputting light having a wavelength corresponding to the transmitted wavelength, and a common input / output port for combining and outputting the light input to each transmitted light input / output port. . Conversely, the light input to the common input / output port is demultiplexed according to the respective wavelengths, and is demultiplexed from the transmitted light input / output ports corresponding to the respective transmitted light wavelengths.

  Using the above-mentioned characteristics of the AWG filter, the following configuration is adopted. That is, the pump light is input to the transmitted light input / output port corresponding to the pump light wavelength, and the output light from the common input / output port is input to the first input / output terminal 101-1 of the polarization beam splitter / combiner 101. Similar to the apparatus shown in FIG. 1, bidirectional input is made to the nonlinear optical medium 102.

  The output light from the loop optical path 20 output from the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 is input to the common input / output port of the AWG type filter, and the signal light and idler light respectively correspond To the transmitted light input / output port, and the excitation light wavelength component is removed at this stage. Furthermore, in order to remove the SHG optical wavelength component, a low-pass optical filter 106 is connected between the common input / output port of the WDM filter 107 and the first input / output terminal 101-1 of the polarization beam splitter / combiner 101, if necessary. Then, the SHG light wavelength component is removed.

≪Second modification of first polarization entangled photon pair generator≫
With reference to FIG. 6, a second modification of the first polarization entangled photon pair generating device will be described. In the second modification shown in FIG. 6, the optical branching input / output unit is the optical branching input / output unit 14 because its configuration is slightly different from that of the first polarization quantum entangled photon pair generating device shown in FIG.

  The second modification uses an optical circulator 110 instead of using the bandpass optical filter 105. The pumping light input from the first input / output terminal 110-1 of the optical circulator 110 is output from the second input / output terminal 110-2 and input to the first input / output terminal 101-1 of the polarization beam splitter / combiner 101. In the same way as in the apparatus shown in FIG. Also, the output light from the loop optical path 20 output from the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 is input to the second input / output terminal 110-2 of the optical circulator 110, and the third input / output terminal Output from the end 110-3. Thereafter, similarly to the apparatus shown in FIG. 1, the SHG light wavelength component is removed and the signal light and idler light are separated using the low-pass light filter 106 and the WDM filter 107.

≪Second polarization entangled photon pair generator≫
<Description of configuration>
A basic configuration of the second polarization entangled photon pair generation device of the embodiment of the present invention will be described with reference to FIG.

  The second polarization quantum entangled photon pair generation device is based on the operation of the first polarization quantum entangled photon pair generation device described above. Input / output is configured via different input / output terminals. Therefore, instead of the 90-degree polarization plane rotation unit 103, the polarization plane and the optical phase adjustment unit 104 used in the first polarization quantum entangled photon pair generation device described above, a Faraday rotator and a half-wave plate are used. Two pairs of optical components (non-reciprocal polarization plane conversion unit) and a birefringent medium are used. Here, as the birefringent medium, an element equivalent to the birefringent medium 282 used in the first polarization entangled photon pair generating device described above is used. In addition, a polarization beam splitter / combiner having four input / output terminals is used as the polarization beam splitter / combiner.

  That is, the second polarization quantum entangled photon pair generating device includes a loop optical path 40 and an input unit for inputting pumping light, and an optical branching input / output that outputs the signal light wavelength component and the idler light wavelength component in a spatially separated state. Part 30.

  The loop optical path 40 connects the second input / output end 201-2 and the third input / output end 201-3 of the polarization beam splitter / combiner 201 having the first to fourth input / output ends to the first nonreciprocal polarization plane conversion unit. 211, the nonlinear optical medium 102, the second nonreciprocal polarization plane conversion unit 212, and the birefringent medium 213 are connected to both ends of an optical fiber that is arranged and connected in series. The input unit is the first input / output terminal 201-1 of the polarization beam splitter / combiner 201.

  The fourth input / output end 201-4 of the polarization beam splitter / combiner 201 is connected to the input end of the optical branching input / output unit 30, and the first nonreciprocal polarization plane converting unit 211 rotates the polarization plane by 45 degrees. The first Faraday rotator 207 and the second half-wave plate 208 are provided, and the second non-reciprocal polarization plane conversion unit 212 includes a second Faraday rotator 209 that rotates the polarization plane by 45 degrees, and a third 1 A / 2 wavelength plate 210 is provided.

  The excitation light is input from the first input / output terminal 201-1 of the polarization beam splitter / combiner 201, and is input to the loop optical path 40 from the first input / output terminal 201-1 via the polarization beam splitter / combiner 201. The wavelength component and the idler wavelength component are output in a state of being spatially separated from the output end of the optical branching input / output unit 30.

  The first half-wave plate 214 is disposed in the optical path through which either the signal light wavelength component or the idler light wavelength component that is spatially separated and output from the light branching input / output unit 30 passes.

  A pair of the first Faraday rotator 207 and the second half-wave plate 208 constitutes a first nonreciprocal polarization plane conversion unit 211. Similarly, the second nonreciprocal polarization plane conversion unit 212 is configured by a pair of the second Faraday rotator 209 and the third half-wave plate 210.

The first Faraday rotator 207 and the second Faraday rotator 209, the pumping light wavelength is in the vicinity of lambda _p, the signal light and the polarization plane -45 degrees with respect to the idler light (45 degrees counterclockwise) rotating . Further, a second half-wave plate 208 third half-wave plate 210, the excitation light wavelength is near the lambda _p, the signal light and functions as a half wavelength plate with respect to the idler light .

  The polarization beam splitter / combiner 201 has four input / output terminals 201-1 to 201-4. The functions and roles of the first input / output terminal 201-1 to the third input / output terminal 201-3 are the three input / output terminals 101 of the polarization beam splitter / combiner 101 of the first polarization entangled photon pair generator described above. -1 to 101-3 have the same functions and roles.

  That is, the p polarization component input from the first input / output terminal 201-1 is output from the second input / output terminal 201-2, and the s polarization component input from the first input / output terminal 201-1 is Output from the third input / output terminal 201-3. The p-polarized component input from the second input / output terminal 201-2 is output from the first input / output terminal 201-1, and the s-polarized component input from the third input / output terminal 201-3 is the first Output from the input / output terminal 201-1. Further, in the polarization beam splitter / combiner 201, the p polarization component input from the second input / output terminal 201-2 is output from the fourth input / output terminal 201-4 and input from the third input / output terminal 201-3. The s-polarized component is output from the fourth input / output terminal 201-4.

  As the polarization beam splitter / combiner 201 satisfying such a function, for example, a commercially available polarization beam splitter can be appropriately selected and used.

  The second polarization quantum entangled photon pair generation device receives 45 degree polarized excitation light from the first input / output terminal 201-1 of the polarization beam splitter / combiner 201. In addition, overlapping description of optical components used in the second polarization entangled photon pair generator having the same function as the optical components used in the first polarization entangled photon pair generator described above Is omitted.

<Description of operation>
Here, in order to select and generate the bell-state polarization entangled photon pairs represented by the equations (5-a) to (5-d), the second polarization entangled photon pair generator is How it works will be described in detail.

Similar to the first polarization entangled photon pair generating device described above, the wavelength is λ _p and the 45-degree polarized excitation light is input from the first input / output terminal 201-1 of the polarization beam splitter / combiner 201 and is the same The signals are separated into the p-polarized component and the s-polarized component of intensity and output from the second input / output terminal 201-2 and the third input / output terminal 201-3, respectively.

  Here, the directions of the optical axes of the first Faraday rotator 207, the second half-wave plate 208, the second Faraday rotator 209, and the third half-wave plate 210, and the first nonreciprocal polarization plane Operations of the conversion unit 211 and the second non-reciprocal polarization plane conversion unit 212 will be described with reference to FIGS. FIG. 8 is a diagram illustrating a schematic configuration of the first nonreciprocal polarization plane conversion unit 211, and is a diagram for explaining a change in the polarization plane of light passing through the first nonreciprocal polarization plane conversion unit 211 in both the left and right directions. . FIG. 9 shows a schematic configuration of the second non-reciprocal polarization plane conversion unit 212, and is a diagram for explaining a change in the polarization plane of light passing through the second non-reciprocal polarization plane conversion unit 212 in both the left and right directions. It is.

  As shown in FIG. 8, in the first nonreciprocal polarization plane conversion unit 211, excitation of linear polarization in a specific polarization direction from the first Faraday rotator 207 side or the second half-wave plate 208 side Light is input. This specific polarization direction coincides with either the p polarization direction or the s polarization direction in the polarization beam splitter / combiner 201. The optical axis of the second half-wave plate 208 is adjusted to form an angle of 22.5 degrees with a specific polarization direction (shown as the s polarization direction in FIG. 8). As shown in FIG. 9, the third half-wave plate 210 in the second non-reciprocal polarization plane converter 212 is similarly adjusted to form an angle of 22.5 degrees.

  The s-polarized excitation light that is input to the loop optical path 40 from the third input / output terminal 201-3 of the polarization beam splitter / combiner 201 and propagates counterclockwise through the loop optical path 40 is connected to the first Faraday rotator 207 and the second Faraday rotator 207. The first nonreciprocal polarization plane conversion unit 211 made up of the half-wave plate 208 is passed through. A change in the polarization state at this time will be described with reference to FIG.

  In FIG. 8, the s-polarized pumping light propagating counterclockwise output from the third input / output terminal 201-3 of the polarization beam splitter / combiner 201 is indicated by an upward arrow in the figure. This light passes through the first Faraday rotator 207. In the first Faraday rotator 207, assuming that 45 degrees of polarization rotation occurs counterclockwise in the figure, the polarization direction of the output light of the first Faraday rotator 207 is represented by an arrow pointing to the upper left 45 degrees. Is done. Next, this light passes through the second half-wave plate 208. At this time, since the polarization direction forms an angle of 22.5 degrees with the optical axis of the second half-wave plate 208, the polarization direction of the excitation light output from the second half-wave plate 208 is rightward. It can be expressed in the direction of the arrow.

  This means that the polarization direction of the excitation light output through the second half-wave plate 208 is the p-polarization direction. That is, the polarization of the excitation light that has passed through the first nonreciprocal polarization plane converter 211 in the order of the first Faraday rotator 207 and the second half-wave plate 208 is rotated by 90 degrees. This excitation light is input to the nonlinear optical medium 102, and a correlated photon pair of SHG light, signal light and idler light having the same polarization as that of the excitation light (p polarization) is generated.

  The excitation light, the SHG light, and the correlated photon pair propagating counterclockwise in the loop optical path 40 are then input to the second nonreciprocal polarization plane conversion unit 212. A change in the polarization state at this time will be described with reference to FIG.

  As shown in FIG. 9, the excitation light, which is p-polarized light propagating counterclockwise, and the correlated photon pair pass through the second Faraday rotator 209, and then rotate 45 degrees counterclockwise in the figure. This results in a polarization state represented by an arrow pointing 45 degrees diagonally upward to the right. Next, as a result of this light passing through the third half-wave plate 210, the polarization direction is converted into an upward arrow, that is, s-polarized light.

  After that, the excitation light and the correlated photon pair propagated counterclockwise in the loop optical path 40 are input to the second input / output terminal 201-2 of the polarization beam splitter / combiner 201, and the polarization state is s polarization. And output from the fourth input / output terminal 201-4. That is, the excitation light and the correlated photon pair are output from an input / output terminal different from the input / output terminal to which the first excitation light is input.

  Next, the p-polarized pump light propagating clockwise through the loop optical path 40 and the loop optical path 40 generated by the pump light output from the second input / output terminal 201-2 of the polarization beam splitter / combiner 201 are rotated clockwise. The correlation photon pair propagating to the above will be described.

  The excitation light passes through the second nonreciprocal polarization plane converter 212. The state of polarization at this time will be described with reference to FIG. As shown in FIG. 9, the excitation light input with p-polarized light propagating counterclockwise passes through the third half-wave plate 210, and is shown by an arrow pointing 45 degrees diagonally to the upper left in the figure. It is converted to the state of polarization to represent. Next, the excitation light passes through the second Faraday rotator 209, and the polarization rotation of 45 degrees occurs counterclockwise in the figure. As a result, it is output from the second non-reciprocal polarization plane conversion unit 212 as a polarized wave indicated by a left arrow in the drawing, that is, a p-polarized wave.

  This excitation light is input as p-polarized light to the nonlinear optical medium 102, and generates SHG light and SPDC correlated photon pairs having the same polarization as the polarization of the excitation light (p-polarization). The SHG light and SPDC correlation photon pair propagating clockwise in the loop optical path 40 is then input to the first nonreciprocal polarization plane converter 211. A change in the polarization state at this time will be described with reference to FIG.

  As shown in FIG. 8, the excitation light and the SPDC correlation photon pair, which are p-polarized waves propagating counterclockwise, first pass through the second half-wave plate 08, and as a result, an arrow pointing to the upper left 45 degrees It is converted into the polarization expressed by Next, after passing through the first Faraday rotator 207, a 45-degree polarization rotation occurs in the counterclockwise direction in the figure, and is converted into a p-polarized wave represented by a left-pointing arrow in the figure.

  After that, the excitation light and the correlated photon pair propagating clockwise in the loop optical path 40 are input to the third input / output terminal 201-3 of the polarization beam splitter / combiner 201, and the polarization state is p polarization. And output from the fourth input / output terminal 201-4. That is, the excitation light and the correlated photon pair are output from an input / output terminal different from the input / output terminal to which the first excitation light is input. The pumping light and correlated photon pairs that are output are the same as the input / output ends from which the pumping light and correlated photon pairs that have propagated counterclockwise in the loop optical path 40 are output, and the loop optical path 40 is counteracted. The excitation light and the correlated photon pair propagating clockwise are in a relationship of orthogonal polarization. That is, the polarization quantum entangled photon pair is output from the fourth input / output terminal 201-4 of the polarization beam splitter / combiner 201.

  After that, the polarization entangled photon pair output from the fourth input / output terminal 201-4 of the polarization beam splitter / combiner 201 has an extra SHG wavelength component and pump wavelength component in the low-pass optical filter 106 and the WDM filter 107. After being removed, the signal light wavelength component and idler light wavelength component are output in a spatially separated state.

  On the other hand, in order to obtain the maximum entangled polarization entangled photon pair in the second polarization entangled photon pair generator, the optical phase compensator for setting φ to 0 or π is provided with this polarization entangled photon pair. It must be inserted somewhere on the photon pair generator.

As described as the operation of the polarization plane and optical phase adjustment unit 104 of the first polarization quantum entangled photon pair generator, the optical phase compensation by the polarization plane and optical phase adjustment unit 104 is given by the above equation (8). . In the excitation light and correlated photon pair (signal light and idler light) that are mutually reverse in the loop optical path, if a birefringent medium is placed at a location where these polarizations are orthogonal to each other, an amount corresponding to the birefringence of the birefringent medium Optical phase compensation of [(2πΔn / λ _p ) L] can be performed.

  In the second polarization quantum entangled photon pair generation device, the location where the excitation light traveling in the loop optical path 40 and the polarization of the correlated photon pair are orthogonal to each other is the third input / output terminal 201-3 of the polarization separation combiner 201 And the first non-reciprocal polarization plane conversion unit 211 (the location indicated by a in FIG. 7), the second input / output terminal 201-2 of the polarization beam splitter / combiner 201 and the second non-reciprocal polarization plane conversion unit 212 A location (b shown in FIG. 7) to be coupled, a location between the second Faraday rotator 209 and the third half-wave plate 210 in the second nonreciprocal polarization plane converter 212 (c and FIG. 7) (Location shown).

  Therefore, if the birefringent medium 213 is disposed at any location indicated by a to c in FIG. 7, φ can be set to 0 or π, and the maximum entangled state can be realized.

  In consideration of the polarization directions of the excitation light and the correlated photon pair, when the birefringent medium 213 is disposed at the position indicated by a or b in FIG. What is necessary is just to arrange | position so that it may correspond with a polarization direction and s polarization direction. On the other hand, when the birefringent medium 213 is disposed at the position indicated by c in FIG. 7, the direction of the optical axis of the birefringent medium 213 coincides with the direction of the p-polarization direction and the angle of 45 degrees with the s-polarization direction. What is necessary is just to arrange | position.

  Also, the polarization of idler light (or signal light) can be rotated 90 degrees by placing the first half-wave plate 214 in the optical path of idler light (or signal light) that passes through the WDM filter 107. Therefore, as with the first polarization entangled photon pair generator, the second polarization entangled photon pair generator can be any of the equations given by the equations (5-a) to (5-d). It is also possible to realize the bell state.

  However, the first polarization quantum entangled photon pair generation device and the second polarization quantum entangled photon pair generation device have the following differences.

  According to the second polarization entangled photon pair generation device, the excitation light does not return to the original excitation light input end and return. Therefore, there is little concern about unstable operation of the apparatus due to reflected return light or the like. An optical component such as an optical isolator is required to eliminate the concern of the reflected return light. However, the second polarization entangled photon pair generation device does not have to consider this. If an optical isolator or the like is used, the device manufacturing cost increases, and inserting an optical element such as an optical isolator causes optical loss in the optical element and reduces the performance of the device. Such a problem does not occur in the entangled photon pair generator.

≪Third polarization entangled photon pair generator≫
<Description of configuration>
The basic configuration of the third polarization entangled photon pair generation device according to the embodiment of the present invention will be described with reference to FIG.

  The third polarization entangled photon pair generator differs from the configuration of the first polarization entangled photon pair generator in the following points. That is, unlike the configuration of the first polarization entangled photon pair generation device, the loop optical path 50 includes the 90-degree polarization plane rotation unit 103 and the nonlinear optical medium 102, and does not include the polarization plane and the optical phase adjustment unit 104. .

  On the other hand, the birefringent medium 216 is inserted into any one of the optical paths until the 45-degree polarized excitation light is input to the loop optical path 50. In FIG. 10, the birefringent medium 216 is connected to the first input / output terminal 105-1 of the bandpass optical filter 105 (the input terminal of the optical branching input / output unit 10). The insertion location of the birefringent medium 216 is not limited to the example shown in FIG. 10, for example, the first input / output terminal 101-1 of the polarization beam splitter / combiner 101 and the second input / output terminal 105-2 of the bandpass optical filter 105 May be disposed on the path connecting the two (output end of the optical branching input / output unit 10). In any case, it is arranged in any optical path that passes after the excitation light becomes 45-degree polarized excitation light and is input to the loop optical path 50.

  Further, the optical axis of the birefringent medium 216 is arranged as shown in FIG. That is, the optical axes X and Y are arranged in a direction that forms an angle of 45 degrees with the polarization direction of the 45-degree polarized excitation light.

  Since other configurations are the same as those of the first polarization entangled photon pair generating device, the description thereof is omitted here.

<Description of operation>
Here, a third polarization entangled photon pair generator is used to select and generate the bell-state polarization entangled photon pairs represented by the equations (5-a) to (5-d). How it works will be described.

  In the third polarization quantum entangled photon pair generation device, unlike the first polarization quantum entangled photon pair generation device, the photon pair does not pass through the birefringent medium 216. On the other hand, the optical phase of the pumping light that generates the correlated photon pair of H polarization and the optical phase of the pumping light that generates the correlated photon pair of V polarization differ according to the retardation generated in the birefringent medium 216.

Therefore, in the third polarization entangled photon pair generator, the optical phase difference φ between the longitudinally polarized (V) photon pair and the transversely polarized (H) photon pair is expressed by the following equation (9): expressed.
φ = φ ₀ +2 [(2πΔn / λ _p ) L] (9)
Here, Δn≡n _X −n _Y , where n _X and n _Y are the refractive indices of the birefringent medium 216 with respect to linear polarization in the X-axis direction and the Y-axis direction, and L is the thickness of the birefringent medium 216. . φ ₀ is the total amount of optical phase difference between the longitudinally polarized photon pair and the transversely polarized photon pair generated in the optical path other than the birefringent medium 216.

  Therefore, as in the case of the first polarization entangled photon pair generator, using a variable Babinet Soleil compensator as the birefringent medium 216, L is set so that φ given by Equation (9) becomes 0. If set, the bell state given by equation (5-a) can be realized, and if L is set so that φ = π, the bell state given by equation (5-b) can be realized. Further, by rotating the half-wave plate 108, the bell state given by the equations (5-c) and (5-d) can also be realized.

  However, the first polarization quantum entangled photon pair generating device and the third polarization quantum entangled photon pair generating device have the following differences.

  That is, according to the third polarization entangled photon pair generation device, the birefringent medium 216 does not exist in the optical path through which the polarization quantum entangled photon pair passes. Also, the plane of polarization and optical phase adjustment unit 104 are not used. Therefore, there is no insertion loss of these optical components. That is, the polarization quantum entangled photon pair can be generated with lower loss.

<< Variation of embodiment >>
In the first, second, and third polarization entangled photon pair generation devices of the above-described embodiments, the PPLN crystal is used as the nonlinear optical medium, but a second-order nonlinear optical medium other than the PPLN crystal can also be used. . In addition, a third-order nonlinear optical medium such as an optical fiber can be used as the nonlinear optical medium. Furthermore, a bulk crystal, a waveguide type element such as a PPLN waveguide element in which an optical waveguide is formed in a PPLN crystal, and various other forms of nonlinear optical media can be used as the nonlinear optical medium.

It is a design matter that the 90-degree polarization plane rotating unit 103 and the polarization plane and optical phase adjusting units 104 and 109 are arranged with respect to the nonlinear optical medium 102 in the loop optical path. That is, for example, when using the d ₁₁ component that is a tensor component of the nonlinear optical coefficient of the second-order nonlinear optical medium, the 90-degree polarization plane rotating unit 103 is connected to the second input / output terminal 101-2 of the polarization beam splitter / combiner 101. And the nonlinear optical medium 102 may be arranged in an optical path. Further, it is possible to adopt a configuration in which the positions of the arrangement of the 90-degree polarization plane rotation unit 103 and the polarization plane and optical phase adjustment unit 104 are interchanged. That is, the position of the arrangement of the 90-degree polarization plane rotation unit 103 and the polarization plane and optical phase adjustment unit 104 is the direction of the optical axis used by the nonlinear optical medium and the polarization of the excitation light input to the nonlinear optical medium. It can be set flexibly as appropriate based on the direction.

10, 12, 14, 30: Optical branching input / output unit
20, 40, 50: Loop optical path
101, 201: Polarization demultiplexer
102: Nonlinear optical medium
103: 90 degree polarization plane rotation part
104, 109: Polarization plane and optical phase adjustment unit
105: Bandpass optical filter
106: Low-pass optical filter
107: WDM filter
108: 1/2 wave plate
110: Optical circulator
207, 278: First Faraday rotator
208: Second half-wave plate
209, 280: 2nd Faraday rotator
210: Third half-wave plate
211: First nonreciprocal polarization plane converter
212: Second nonreciprocal polarization plane converter
213, 216, 282: Birefringent medium
214: First half-wave plate
284: Third Faraday rotator

Claims (18)

It has a loop optical path and an optical branching input / output unit.
A non-linear optical medium in the loop optical path and a signal photon and an idler photon generated in the non-linear optical medium based on the excitation photon input to the loop optical path and propagating in the loop optical path are rotated clockwise in the loop optical path. A polarization plane and an optical phase adjustment unit for adjusting the optical phase difference between the light component and the counterclockwise light component are arranged,
The optical branching input / output unit inputs the excitation photons to the loop optical path so as to propagate the loop optical path clockwise and counterclockwise, and the signal propagates clockwise and counterclockwise on the loop optical path. A correlated photon pair consisting of a photon and the idler photon is output in a state of being spatially separated into the signal photon wavelength component and the idler photon wavelength component,
A half-wave plate is disposed in one of the optical paths from which the signal photon wavelength component and the idler photon wavelength component output from the optical branching input / output unit are separated and output. Polarization entangled photon pair generator. In the loop optical path, the second and third input / output ends of a polarization beam splitter / combiner having first to third input / output ends are arranged in series with the nonlinear optical medium, the polarization plane, and the optical phase adjustment unit. Connected to both ends of the optical fiber to be connected,
An output terminal of the optical branching input / output unit is connected to a first input / output terminal of the polarization beam splitter / combiner, and the excitation photon is input from an input terminal of the optical branching input / output unit. Is input to the loop optical path from the first input / output end of the polarization beam splitter / combiner, and the signal photon wavelength component and the idler photon wavelength component are spatially separated from the output end of the optical branching input / output unit. 2. The polarization entangled photon pair generating device according to claim 1, wherein the device is configured to output in a state. 90 degree polarization plane rotation for rotating the polarization plane by 90 degrees between the position where the polarization plane and the optical phase adjustment unit of the loop optical path are located and the position where the polarization separation / combination combiner is located Part is further arranged,
The polarization plane and the optical phase adjustment unit, a first Faraday rotator that rotates the polarization plane +45 degrees, a second Faraday rotator that rotates the polarization plane +45 degrees in the same direction as the first Faraday rotator, 3. The birefringent medium having a variable optical phase difference between optical axes orthogonal to each other between the first Faraday rotator and the second Faraday rotator. Polarized quantum entangled photon pair generator described in 1.   The polarization plane and the optical phase adjustment unit include a third Faraday rotator that rotates the polarization plane by −45 degrees, and a fourth Faraday rotator that rotates the polarization plane by +45 degrees in the same direction as the third Faraday rotator. And a birefringent medium having a variable optical phase difference between optical axes orthogonal to each other between the third Faraday rotator and the fourth Faraday rotator. Item 3. The polarization entangled photon pair generation device according to item 2.   The optical branching input / output unit includes a bandpass optical filter having a wavelength of the excitation photon in a transmission wavelength band and a wavelength of the signal photon and the idler photon in a reflection wavelength band, a wavelength of the signal photon, and the idler photon. 4. A WDM (Wavelength Division Multiplexer / Demultiplexer) filter that has a wavelength of a transmission wavelength band and spatially separates and outputs the wavelength component of the signal photon and the wavelength component of the idler photon. Or the polarization entangled photon pair generating device described in 4;   The optical branching input / output unit has the wavelength of the excitation photon, the wavelength of the signal photon, and the wavelength of the idler photon in a transmission wavelength band, inputs the excitation photon, and the wavelength component of the signal photon and the idler photon 5. The polarization entangled photon pair generating device according to claim 3, further comprising a WDM (Wavelength Division Multiplexer / Demultiplexer) filter that spatially separates and outputs wavelength components.   The optical branching input / output unit guides the excitation photon to the loop optical path, and an optical circulator having an input / output terminal for taking out the signal photon and the idler photon output from the loop optical path, and the signal photon And a wavelength division multiplexer / demultiplexer (WDM) filter that spatially separates and outputs the wavelength component of the signal photon and the wavelength component of the idler photon. 5. The polarization quantum entangled photon pair generating device according to claim 3 or 4,   8. The optical branching input / output unit further includes a low-pass optical filter that removes an optical second harmonic component generated based on the excitation photons. Polarized quantum entangled photon pair generator. A loop optical path in which a first nonreciprocal polarization plane conversion unit, a nonlinear optical medium, a second nonreciprocal polarization plane conversion unit, and a birefringence medium are arranged in series;
An input unit for inputting excitation photons;
Signal photons and idler photons that propagate clockwise and counterclockwise along the loop optical path generated in the nonlinear optical medium based on the excitation photons input so as to propagate clockwise and counterclockwise through the loop optical path An optical branching input / output unit that outputs a correlated photon pair consisting of a signal photon wavelength component and an idler photon wavelength component in a spatially separated state,
A first half-wave plate is disposed in one of the optical paths from which the signal photon wavelength component and idler photon wavelength component that are output after being spatially separated from the optical branching input / output unit are output. Characteristic polarization entangled photon pair generator. The loop optical path includes a second input / output end and a third input / output end of a polarization beam splitter / combiner including first to fourth input / output ends, the first nonreciprocal polarization plane converting unit, and the nonlinear optical medium. The second nonreciprocal polarization plane converter and the birefringent medium are arranged in series and connected with both ends of an optical fiber,
The input unit is a first input / output end of the polarization beam splitter / combiner including first to fourth input / output ends,
The fourth input / output end of the polarization beam splitter / combiner and the input end of the optical branching input / output unit are connected,
The first nonreciprocal polarization plane conversion unit includes a first Faraday rotator that rotates the polarization plane by 45 degrees and a second half-wave plate,
The second non-reciprocal polarization plane conversion unit includes a second Faraday rotator that rotates the polarization plane by 45 degrees and a third half-wave plate,
Excitation photons are input from the first input / output end of the polarization beam splitter / combiner and are input to the loop optical path from the first input / output port of the polarization beam splitter / combiner via the polarization beam splitter / combiner. 10. The polarization entanglement according to claim 9, wherein the photon wavelength component and the idler photon wavelength component are output in a state of being spatially separated from an output end of the optical branching input / output unit. Photon pair generator.   The optical branching input / output unit has a wavelength of the signal photon and the wavelength of the idler photon in a transmission wavelength band, and WDM (Wavelength Division) that spatially separates the wavelength component of the signal photon and the wavelength component of the idler photon. 11. The polarization entangled photon pair generation device according to claim 10, further comprising a Multiplexer / Demultiplexer) filter.   12. The polarization entangled photon pair according to claim 11, wherein the optical branching input / output unit further includes a low-pass optical filter that removes an optical second harmonic component generated based on the excitation photons. Generator. A loop optical path and an optical branching input / output unit
In the nonlinear optical medium, based on the nonlinear optical medium, any one of the excitation photons input so as to propagate clockwise and counterclockwise in the loop optical path, and the excitation photon. A 90-degree polarization plane rotating unit that rotates the polarization plane of either one of the light component rotating clockwise and the counterclockwise light component of the signal photon and idler photon generated and propagating through the loop optical path by 90 degrees is disposed. And
A birefringent medium is disposed in an optical path through which the excitation light is input to the loop optical path through the optical branching input / output unit;
The optical branching input / output unit inputs the excitation photons to the loop optical path so as to propagate the loop optical path clockwise and counterclockwise, and the signal propagates clockwise and counterclockwise on the loop optical path. A correlated photon pair consisting of a photon and the idler photon is output in a state of being spatially separated into the signal photon wavelength component and the idler photon wavelength component,
A half-wave plate is disposed in one of the optical paths from which the signal photon wavelength component and the idler photon wavelength component output from the optical branching input / output unit are separated and output. Polarization entangled photon pair generator. In the loop optical path, the second and third input / output ends of a polarization beam splitter / combiner including first to third input / output ends are arranged in series with the nonlinear optical medium and the 90-degree polarization plane rotation unit. Consists of connecting both ends of the optical fiber to be connected,
The birefringent medium is disposed at an input end or an output end of the optical branching input / output unit;
An output terminal of the optical branching input / output unit is connected to a first input / output terminal of the polarization beam splitter / combiner, and the excitation photon is input from an input terminal of the optical branching input / output unit. Is input to the loop optical path from the first input / output end of the polarization beam splitter / combiner, and the signal photon wavelength component and the idler photon wavelength component are spatially separated from the output end of the optical branching input / output unit. 14. The polarization quantum entangled photon pair generating device according to claim 13, wherein the device is configured to output in a state.   The optical branching input / output unit includes a bandpass optical filter having a wavelength of the excitation photon in a transmission wavelength band and a wavelength of the signal photon and the idler photon in a reflection wavelength band, a wavelength of the signal photon, and the idler photon. 15. A WDM (Wavelength Division Multiplexer / Demultiplexer) filter that spatially separates and outputs the wavelength component of the signal photon and the wavelength component of the idler photon in a transmission wavelength band. Polarized quantum entangled photon pair generator described in 1.   The optical branching input / output unit has the wavelength of the excitation photon, the wavelength of the signal photon, and the wavelength of the idler photon in a transmission wavelength band, inputs the excitation photon, and the wavelength component of the signal photon and the idler photon 15. The polarization entangled photon pair generation device according to claim 14, further comprising a WDM (Wavelength Division Multiplexer / Demultiplexer) filter that spatially separates and outputs wavelength components.   The optical branching input / output unit guides the excitation photon to the loop optical path, and an optical circulator having an input / output terminal for taking out the signal photon and the idler photon output from the loop optical path, and the signal photon And a wavelength division multiplexer / demultiplexer (WDM) filter that spatially separates and outputs the wavelength component of the signal photon and the wavelength component of the idler photon. 15. The polarization quantum entangled photon pair generation device according to claim 14,   18. The optical branching input / output unit further includes a low-pass optical filter that removes an optical second harmonic component generated based on the excitation photons. Polarized quantum entangled photon pair generator.