JP2014067061A - Quantum entangled photon pair generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable generation of highly pure quantum entangled photon pairs without inducing unnecessary excessive loss and the like using common and simple optical components.SOLUTION: A quantum entangled photon pair generator includes a polarization preserving loop optical path which has both ends connected to a polarization splitting/combining module and has first and second non-linear optical media and a 1/2 wavelength plate for rotating a polarization plane by 90° located between the non-linear optical media. The first non-linear optical medium generates first SHG light from split excitation light propagating anti-clockwise, and generates correlated photon pairs through SPDC conversion of second SHG light propagating clockwise generated by the second non-linear optical medium. The second second-order non-linear optical medium performs a process symmetrical to this. Two correlated photon pairs propagating in opposite directions are combined by the polarization splitting/combining module, after which signal light and idler light are separated and extracted. The loop optical path includes polarization plane manipulation means which manipulates polarization planes of some light components to combine the light as mentioned above.

Description

  The present invention relates to a quantum entangled photon pair generating apparatus, and can be applied to a quantum information communication system using a quantum mechanical correlation of photons, such as a quantum cryptography and a quantum computer.

  In recent years, quantum information communication technologies using physical phenomena at the quantum mechanical level, such as quantum cryptography and quantum computers, have attracted attention. Among them, the quantum cryptography is attracting attention as an ultimate secure cryptographic communication system in which the security of the encryption key is guaranteed by the principle of quantum mechanics, and is particularly actively researched and developed.

  A photon pair having a quantum mechanical correlation, that is, a entangled photon pair is an important element for realizing an advanced quantum information communication system that applies the nonlocality of photons.

  One of the methods for generating a pair of entangled photons that has been conventionally used is a method using natural parametric down-conversion in a second-order nonlinear optical medium.

Patent Document 1 describes a entangled photon pair generator using a BBO (β-BaB ₂ O ₄ ) crystal. Here, two BBO crystals are arranged in series, and a half-wave plate is arranged at the center. Pump light (pump light) having a linear polarization and a wavelength of 351.1 nm is input thereto. Through the natural parametric down-conversion process, a correlated photon pair (referred to as signal light (signal light) and idler light) having a wavelength twice the excitation light wavelength (702.2 nm) is output from each BBO crystal. When the excitation light intensity is sufficiently weak and the probability that these correlated photon pairs will be generated simultaneously from both of the two BBO crystals is low, the state of the correlated photon pair generated from this device is that of one BBO. It is given by superposition of the state of the correlated photon pair generated from the crystal and the state of the correlated photon pair generated from the other BBO crystal. The correlated photon pair output from these two BBO crystals has a half-wave plate disposed between the two BBO crystals, and the polarization of the correlated photon pair generated from the preceding BBO crystal close to the excitation light input end is Since it is configured to rotate by 90 ° at the device output end, it is orthogonal to the polarization. As a result, polarization entangled photon pairs are generated from this system.

  Many other examples have been reported in which quantum entangled photon pairs in the wavelength range of 700 nm to 800 nm are generated with a configuration similar to the above.

  On the other hand, if a quantum entangled photon pair with a wavelength of 1550 nm, which is the minimum absorption loss wavelength band of an optical fiber, can be generated, it can be expected that the quantum information communication system will be extended using an existing commercial optical fiber. Useful.

Patent Document 2 discloses a entangled photon in a wavelength band of 1550 nm using a periodically poled LiNbO ₃ (Periodically Poled LiNbO ₃ , hereinafter abbreviated as PPLN) waveguide, which is also a kind of second-order nonlinear optical medium. A pair generator is described. Here, it has a fiber loop structure using two PPLN waveguides and a polarizing beam splitter (PBS). The two PPLN waveguides are arranged so that their optical axes are orthogonal. Excitation light (femtosecond pulse) polarized by 45 ° at a wavelength of 775 nm is input via PBS. Similar to the case of Patent Document 1, a correlated photon pair of signal light and idler light having a wavelength of 1550 nm is generated from two PPLN waveguides by natural parametric down conversion. Furthermore, as in the case of Patent Document 1, the excitation light intensity is sufficiently weak, and these correlated photon pairs are not generated from both PPLN waveguides at the same time. A photon pair is generated.

  Non-Patent Document 1 describes an apparatus for generating entangled photon pairs having a wavelength of 1550 nm using PBS and a single PPLN element. Here, in the polarization maintaining optical fiber connecting the PBS and the PPLN element, a 90 ° optical axis conversion portion is provided at one location (this can be easily realized by fiber fusion or the like). Excitation light polarized at 45 ° at a wavelength of 776 nm is input via PBS. A correlated photon pair of signal light and idler light with wavelengths of 1542 nm and 1562 nm is generated from the PPLN waveguide by natural parametric down conversion. If the excitation light intensity is weak enough, these correlated photon pairs are generated from either the excitation light propagating clockwise in the loop or the excitation light propagating counterclockwise in the loop. The state of the photon pair is a superposition state of the correlated photon pair propagating clockwise through the loop and the correlated photon pair propagating counterclockwise and orthogonal to the clockwise component. That is, a polarization entangled photon pair is generated from this device.

JP 2003-228091 A JP 2005-258232 A

"Stable source for high quality telecom-band polarization-entangled photo pairs based on a single, pulse-pumped, short PPLN waveguide." C. Lim, A.D. Yoshizawa, H .; Tsuchida, and K.K. Kikuchi, Optics Express, vol. 16, No, 17, pp. 12460-12468, 2008.

  In the prior art, signal light based on conversion in a nonlinear medium (for example, a second-order nonlinear optical medium) (for example, natural parametric down conversion (hereinafter, also abbreviated as SPDC)), When a correlated photon pair of idler light is generated and an attempt is made to construct a quantum entangled photon pair generation device based on the pair, the quantum entangled photon pair generated along with the nonlinear medium that causes the conversion process on the device configuration. An excitation light source having a wavelength approximately half the wavelength was an essential component. That is, for example, when trying to realize a quantum entangled photon pair generating device having a wavelength band of 1550 nm, which is a wavelength band suitable for use in optical fiber communication, an excitation light source having a wavelength of 775 nm band is essential as a component. there were.

  In this case, there are the following problems.

  In the methods described in Patent Document 2 and Non-Patent Document 1, when realizing a quantum entangled photon pair generating device, a polarization beam splitter (PBS) has a wavelength of 775 nm and a wavelength of 1550 nm which is twice that wavelength. It needs to be “specially designed” optical components that work together in the band.

  Furthermore, optical coupling components such as optical lenses are required for optical coupling of various optical components such as PBS, PPLN element, and optical fiber for optical output to the outside of the apparatus. In addition, special component design is required, such as design of a focal length so that optical coupling can be obtained in both the 775 nm band and the 1550 nm band. Moreover, it is necessary to deal with the non-reflective coating for preventing reflection at both wavelengths.

  That is, in the prior art, it is necessary to prepare special optical components that operate with both excitation light and correlated photon pairs whose wavelengths are approximately twice different. This contributes to an increase in manufacturing cost of the device.

  Furthermore, generally speaking, a design capable of obtaining an equivalent characteristic with respect to light having a large wavelength cannot be expected to be as good as a design optimized for only one of them. Specifically, for example, the polarization extinction ratio in PBS, the coupling efficiency of the lens system, and the like are characteristic compared to a design prepared so that optimum characteristics can be obtained at one wavelength (for example, 1550 nm band). It will be inferior to. That is, the device design sacrifices an increase in excess loss for the excitation light input to the PPLN element and the output of the correlated photon pair.

  On the other hand, quantum information communication systems using entangled photon pairs handle very weak light (single photon (pair) or near it) with an average number of photons (pairs) per time slot of 1 or less. Therefore, a system configuration in which such excess loss and the like are likely to occur is hindered by system performance and is not preferable.

  Furthermore, for example, in providing an apparatus for generating a entangled photon pair in the 1550 nm band, not only the passive optical components operating in the 1550 nm band described above, but also active optical components such as an excitation light source used therein are 1550 nm. If it can be handled by a band laser or an optical amplifier, it can be used practically because it can provide equipment using optical components that are used in existing commercial optical communication systems with high reliability, low cost, and excellent controllability. is there.

  The present invention has been made in view of the above points, and the purpose of the present invention is to use a general and simple optical component without using a special optical component, without causing excessive excess loss, etc. An object of the present invention is to provide a quantum entangled photon pair generator capable of generating a highly pure quantum entangled photon pair.

The entangled photon pair generating apparatus according to the first aspect of the present invention is a first polarization component in which (1) the polarization maintaining loop optical path and (2) the input excitation light from the first out-of-loop optical path are orthogonal to each other. Are branched into a first and second demultiplexed pump light, the first demultiplexed pump light is input clockwise to the loop optical path, and the second demultiplexed pump light is input to the loop optical path. An excitation light demultiplexing input means for inputting counterclockwise, and (3) a first nonlinear optical medium, a second half-wave plate, and a second nonlinear optical medium are connected in series in this order counterclockwise. A first natural parametric down-converted light propagating clockwise in the loop optical path, and a second natural parametric down-converted light propagating counterclockwise in the loop optical path. And (4) a clockwise rotation of the loop optical path. The propagating first natural parametric down-converted light and the second natural parametric down-converted light propagating counterclockwise in the loop optical path are combined, and the combined light is out of the second loop. Converted light combining output means for outputting to the optical path; and (5) the first demultiplexed pump light, the second demultiplexed pump light, the first natural parametric down converted light, and the second natural parametric down converted. (6) the first nonlinear optical medium propagates the loop optical path in a counterclockwise direction; and a polarization plane manipulating means for manipulating a polarization direction of at least one of the lights. A first optical second harmonic wave for the second demultiplexed pumping light, and a second optical second harmonic wave propagating clockwise in the loop optical path generated by the second nonlinear optical medium. Clockwise by conversion Generating the first natural parametric down-converted light to be seeded; and (7) the second nonlinear optical medium is the second light with respect to the first demultiplexed pump light propagating clockwise in the loop optical path. Generating the second harmonic wave and propagating counterclockwise by the conversion to the first optical second harmonic wave propagating counterclockwise in the loop optical path generated by the first nonlinear optical medium. And (8) the second half-wave plate is output from one of the first or second nonlinear optical medium, and the first or second nonlinear optical is generated. The polarization plane of the first or second demultiplexed excitation light input to the other of the medium is rotated by 90 ° and output, (9) the first and second optical second harmonics, and the first The first and second natural parametric down-converted lights, Serial and generating the loop optical path arranged the first or second nonlinear optical medium while.

The quantum entangled photon pair generation device according to the second aspect of the present invention includes: (1) a first optical demultiplexing that is a polarization component orthogonal to the input optical beams from the loop optical path and (2) the first optical path outside the loop. Branching into light and second demultiplexed pump light, inputting the first demultiplexed pump light clockwise into the loop optical path and counterclockwise feeding the second demultiplexed pump light into the loop optical path An excitation light demultiplexing input means for inputting, and (3) a first nonlinear optical medium, a second half-wave plate, and a second nonlinear optical medium are connected in series in this order counterclockwise. Light that generates first natural parametric down-converted light that propagates clockwise in the loop optical path, and second natural parametric down-converted light that propagates counterclockwise in the loop optical path, whose polarization planes are orthogonal (4) Propagating clockwise in the loop optical path, The first natural parametric down-converted light and the second natural parametric down-converted light propagating counterclockwise through the loop optical path are combined to pass the combined light to the second out-of-loop optical path. And (5) aligning the plane of polarization of the first demultiplexed pumping light and the plane of polarization of the second demultiplexed pumping light when input to the nonlinear optical medium, Alternatively, a combined output in which the planes of polarization of the first natural parametric down-converted light and the second natural parametric down-converted light are orthogonal to the second optical path outside the loop in the converted light combining output means is executed. At least one of the first demultiplexed pump light, the second demultiplexed pump light, the first natural parametric down-converted light, and the second natural parametric down-converted light. (6) the first nonlinear optical medium propagates the loop optical path in the counterclockwise direction. (6) The second nonlinear optical medium has a polarization plane manipulating means for manipulating the polarization direction of one or more lights. A first optical second harmonic for the wave excitation light, and a clockwise rotation by a conversion to the second optical second harmonic propagating clockwise in the loop optical path generated by the second nonlinear optical medium. (7) the second nonlinear optical medium is configured to generate the second natural parametric down-converted light propagating in the loop optical path in the clockwise direction; Producing an optical second harmonic and propagating counterclockwise by conversion to the first optical second harmonic propagating counterclockwise through the loop optical path generated by the first nonlinear optical medium Second natural paramet (8) the second half-wave plate is output from one of the first or second nonlinear optical medium and the other of the first or second nonlinear optical medium. The polarization plane of the first or second demultiplexed pumping light input to is rotated by 90 ° and output, (9) the first and second optical second harmonics, and the first and second The second natural parametric down-converted light is generated in the first or second nonlinear optical medium disposed in the loop optical path.

  According to the quantum entangled photon pair generating apparatus of the present invention, a high-purity quantum entangled photon pair can be generated without generating excessive excess loss by using general and simple optical components without using special optical components. can do.

FIG. 3 is an arrangement diagram of components showing the configuration of the polarization entangled photon pair generating device of the first embodiment. It is explanatory drawing which shows the relationship between the polarization direction of the excitation light from the 3rd input / output end of the polarization separation / combination module of FIG. 1, and the optical axis direction of the 1st 1/2 wavelength plate. It is explanatory drawing of the optical axis direction in each component of the light conversion production | generation part of FIG. It is explanatory drawing which showed typically each input-output relationship of each input / output terminal of the polarization-separation combining module of FIG. 1, and each light input / output. It is an arrangement drawing of the component which shows the composition of modification (1) concerning a 1st embodiment. It is an arrangement drawing of the component which shows the composition of modification (2) concerning a 1st embodiment. It is explanatory drawing which shows the structural example of the other light conversion production | generation part replaced with the light conversion production | generation part of FIG. It is an arrangement diagram of components showing the configuration of the polarization entangled photon pair generation device of the second embodiment. It is explanatory drawing which shows the relationship of the optical axis direction of the component in the nonreciprocal polarization plane conversion part of FIG. Explanatory drawing which shows the input / output end of the polarization splitting / combining module for inputting / outputting excitation light, transient signal light, idler light, intermediate SHG light, and correlated photon pairs (signal light, idler light) in the second embodiment It is. It is an arrangement diagram of components showing the configuration of the polarization entangled photon pair generation device of the third embodiment. It is explanatory drawing of the optical axis direction of the 4th 1/2 wavelength plate of FIG. 11, a 1/4 wavelength plate, and the 5th 1/2 wavelength plate. Explanatory drawing which shows the input / output end of the polarization splitting / combining module for inputting / outputting excitation light, transient signal light, idler light, intermediate SHG light, and correlated photon pairs (signal light, idler light) in the third embodiment It is. It is an arrangement drawing of the components showing the configuration of the polarization entangled photon pair generation device of the fourth embodiment. Explanatory drawing which shows the input / output end of the polarization splitting / synthesizing module which inputs / outputs excitation light, transient signal light, idler light, intermediate SHG light, and correlated photon pairs (signal light, idler light) in the fourth embodiment It is.

(A) First Embodiment Hereinafter, a first embodiment of a entangled photon pair generating apparatus according to the present invention will be described with reference to the drawings.

(A-1) Configuration of the First Embodiment FIG. 1 is an arrangement diagram of components showing the configuration of the polarization entangled photon pair generation device of the first embodiment.

  In FIG. 1, a polarization quantum entangled photon pair generation device 10 according to the first embodiment includes a polarization splitting / combining module 101 and two input / output terminals of a Sagnac interferometer type optical loop LP. And at least a first half-wave plate 104 and a light conversion generation unit 100 installed on the optical path of the optical loop LP. The light conversion generation unit 100 includes a first second-order nonlinear optical medium 102, a second half-wave plate 105, and a second second-order nonlinear optical medium 103. The first half-wave plate 104 is installed on an optical path that connects one input / output end (the third input / output end 101-3 in the example of FIG. 1) of the light conversion generation unit 100 and the polarization splitting / combining module 101. Has been. The second half-wave plate 105 is installed between the first and second second-order nonlinear optical media 102 and 103.

  Furthermore, the polarization entangled photon pair generation device 10 of the first embodiment inputs the pumping light into the optical path of the optical loop LP, and outputs the desired entangled photon output from the optical loop LP. An optical circulator 106, an optical low-pass filter 107, and a WDM (wavelength division multiplexing) filter 108 are provided as optical components for optical input / output for selectively extracting and outputting only the wavelength component.

  The optical loop LP is preferably composed of a polarization maintaining optical system. For this purpose, the relationship between the optical axes of the polarization separation / combination module 101, the first and second second-order nonlinear optical media 102 and 103, and the first and second half-wave plates 104 and 105 constituting the optical loop LP. Is constructed with special consideration as described below.

  The optical loop LP may be realized by spatially coupling each optical component using only an optical coupling lens (that is, it may be realized without using an optical fiber). You may comprise using what was combined with the wave holding optical fiber and was provided as an optical module. In addition, even when using an optical module in which an ordinary optical fiber that does not have polarization maintaining properties is used instead of a polarization maintaining optical fiber, by using additional optical components such as a polarization plane controller as appropriate, It can also be realized by configuring a polarization maintaining optical system.

  Here, it is assumed that the wavelength of the input excitation light is λp. This wavelength λp is a wavelength in the vicinity of the desired entangled photon pair wavelength. That is, if the desired entangled photon pair wavelength band is 1550 nm, λp is a value near 1550 nm.

When λs and λi are the wavelengths of the desired entangled photon pair of signal light and idler light, the wavelengths λp, λs, and λi of the input excitation light, signal light, and idler light correspond to the energy conservation law of equation (1). (Where λp, λs, and λi are all wavelengths in a vacuum).

  The polarization separation / combination module 101 includes a first input / output end 101-1 coupled to the second output end 106-2 of the optical circulator 106, and a first input / output end 101-1, which is provided on the side facing the first input / output end 101-1. A second input / output end 101-2 that couples one end of the second-order nonlinear optical medium 103, a third input / output end 101-3 that couples one end of the first half-wave plate 104, and a third input A fourth input / output terminal 104-4 is provided on the side facing the output terminal 101-3. The fourth input / output end 104-4 does not input / output light from / in the first embodiment and the third embodiment to be described later. For example, an optical fiber pigtail, an optical connector, etc. There is no need to connect optical components for an optical signal input / output interface. In that case, the fourth input / output terminal 104-4 is merely provided for convenience of explanation of the detailed operation principle described in detail later, and itself is the same as in the first and third embodiments. It is not a required component. However, in the second and fourth embodiments to be described later, it is an essential component as will be described in detail later.

  The optical circulator 106 outputs a first input terminal 106-1 for inputting the input pumping light having the wavelength λp, and an input light from the first input terminal 106-1 and a first input / output of the polarization beam splitting / combining module 101. A second input / output terminal 106-2 coupled to the terminal 101-1 and a third input / output terminal 106-3 for outputting input light from the second input / output terminal 106-2 are provided.

  As the polarization beam splitting / combining module 101 and the optical circulator 106, modules that can guarantee a desired operation near the wavelength λp (including λs and λi) may be used. That is, the operation at the half wavelength (λp / 2) need not be guaranteed. In other words, if the wavelength λp is in the vicinity of 1550 nm, an optical component generally commercially available as a polarization demultiplexing / synthesizing module or optical circulator for the 1550 nm band may be used, as described in Patent Document 2 and Non-Patent Document 1. , Λp, and λp / 2, it is not necessary to be a “special” polarization splitting / combining module or the like.

  For convenience of the following description, when light having a wavelength λp is incident on the polarization separation / combination module 101, it corresponds to the vibration direction of the electric field vector with respect to the polarization plane selective reflection surface of the polarization separation / combination module of the incident light. Ingredients are defined as follows: That is, a component in which the electric field vector oscillates in a direction parallel to the incident surface of the incident light incident on the polarization plane selective reflection surface is a p component, and a component in which the electric field vector oscillates in a direction perpendicular to the incident surface of the incident light is an s component. I will call it.

  In the polarization separation / combination module 101, the p-polarization component input from the first input / output terminal 101-1 is output to the second input / output terminal 101-2, and the first input / output terminal 101-1. The input s-polarized component is output to the third input / output terminal 101-3. Further, the p-polarized wave component input from the second input / output terminal 101-2 is output to the first input / output terminal 101-1, and the s-polarized wave component input from the third input / output terminal 101-3 is The data is output to the first input / output terminal 101-1.

  As the polarization separation / combination module 101, for example, a suitable one from commercially available polarization beam splitters can be selected and used. Alternatively, the polarization beam splitter is not limited to the type of polarization beam splitter assumed in the above description, and the polarization separation / combination module 101 may be a so-called polarization prism using a birefringent crystal. good.

  The input pump light having the wavelength λp is input to the first input / output terminal 106-1 of the optical circulator 106, output from the second input / output terminal 106-2, and then the first input / output terminal of the polarization beam splitting / combining module 101. 10-1, separated into a p-polarization component and an s-polarization component, and output to the second input / output terminal 101-2 and the third input / output terminal 101-3 of the polarization beam splitting / combining module 101, respectively. The

  In the first embodiment, the p-polarized component and the s-polarized component of the polarization-separated input pump light must have the same light intensity for the reason described later. Therefore, the excitation light when input to the first input / output terminal 101-1 of the polarization beam splitting / combining module 101 is such that the intensity ratio of the p polarization component intensity and the s polarization component intensity is 1: 1. The polarization must be adjusted.

The input pumping light prepared in this way is referred to as “45 ° polarized pumping light” in this specification. In order to prepare such pumping light, for example, a polarization plane controller is prepared in advance for the first input / output terminal 106-1 of the optical circulator 106, and the first input / output terminal 101-1 of the polarization splitting / combining module 101 is prepared. What is necessary is just to adjust the polarization state of the excitation light when it is input to the linearly polarized wave inclined by 45 ° from the s-polarization direction in the polarization beam splitting / combining module 101.

  The first and second half-wave plates 104 and 105 respectively operate as so-called half-wave plates in which the optical phase difference between the orthogonal optical axes is π with respect to the light of wavelength λp. It is. In this specification, unless otherwise specified, the wavelength (λ) when referred to as “1 / n wavelength plate” (n = 2, 3, 4,...) Refers to the excitation light wavelength λp. .

  The optical axis direction of the first half-wave plate 104 is prepared as shown in FIG. That is, in the polarization separation / combination module 101, the optical axis direction is rotated by 45 ° with respect to the polarization direction (s-polarization) of the input pump light output from the third input / output terminal 101-3. That is, the input excitation light output from the third input / output terminal 101-3 passes through the first half-wave plate 104, and then the polarization direction rotates by 90 ° to coincide with the p-polarization direction. It will be a thing. As will be described in detail later, this p polarization direction is a specific optical axis direction of the first and second second-order nonlinear optical media 102 and 103 (in the example described in the first embodiment, z light Each optical component is arranged so as to coincide with the axial direction.

  Each of the first and second second-order nonlinear optical media 102 and 103 is a nonlinear optical medium having a second-order nonlinear optical effect such as PPLN. Each of the second-order nonlinear optical media generates optical second harmonics (second harmonic generation light, hereinafter abbreviated as SHG light) of half the wavelength (λp / 2) when input excitation light of wavelength λp is input. To do. In this specification, this SHG light may be referred to as intermediate SHG light for convenience later.

  Furthermore, these two second-order nonlinear optical media 102 and 103 each receive the intermediate SHG light generated from one of the second-order nonlinear optical media, and perform the wavelength λs by an SPDC process using the SHG light as seed light. Signal light and idler light of wavelength λi are generated.

In the following description of the operation of the first embodiment, for the sake of simplicity, the input pump light and the intermediate SHG light generated by the input and output to and from the two second-order nonlinear optical media 102 and 103 are the same. Let it be a linearly polarized wave in the wave direction. Similarly, it is assumed that the SPDC correlated photon pair of the intermediate SHG light and the signal light and idler light generated by the intermediate SHG light are thereby linearly polarized in the same polarization direction. For example, when PPLN is used as the second-order nonlinear optical medium, the d ₃₃ component of the second-order nonlinear optical constant of PPLN is used for SHG generation and SPDC generation. Therefore, the input excitation light input to the PPLN crystal and the intermediate SHG light input to the PPLN crystal for SPDC correlation photon pair generation are linearly polarized in the z-axis direction, and the desired output from the PPLN crystal The signal light and idler light to be realized can also be realized by linearly polarized light polarized in the z-axis direction.

  According to the above-described example, the first second-order nonlinear optical medium 102, the second second-order nonlinear optical medium 103, and the second ½ wavelength arranged therebetween, which constitute the light conversion generation unit 100. The optical axis direction of the plate 105 is prepared as shown in FIG. That is, in the polarization separation / combination module 101, the first second-order nonlinear optical medium 102 and the second nonlinear optical medium 102 and the second polarization optical polarization direction (p-polarization) of the input pumping light output from the second input / output terminal 101-2. The z-axis direction of the second-order nonlinear optical medium 103 is the same. On the other hand, the optical axis direction of the second half-wave plate 105 is rotated by 45 ° from now. Therefore, when the input excitation light, the transient signal light that is not necessary in this case, which is described later, and idler light are output from one of the second-order nonlinear optical media with the polarization in the z-axis direction, these lights are The second half-wave plate 105 is converted to a linearly polarized wave that is orthogonal to the polarized wave (ie, s-polarized wave) and input to the other second-order nonlinear optical medium. .

  The optical low-pass filter 107 is a wavelength component (λp) of the intermediate SHG light generated in the first and second second-order nonlinear optical media 102 and 103 out of the output light from the third input / output terminal 106-3 of the optical circulator 106. / 2) is removed. For the reason described in detail later, the wavelength component of the light passing through the optical low-pass filter 107 is ideally only the signal light wavelength component (λs) and the idler light wavelength component (λi) in the first embodiment. The excitation light wavelength component (λp) is not included.

  The WDM filter 108 divides at least the signal light wavelength component (λs) and the idler light wavelength component (λi) out of the light passing through the optical low-pass filter 107 and outputs the separated light paths. As such a WDM filter 108, a so-called AWG type WDM filter having at least a signal light wavelength (λs) and an idler light wavelength (λi) as transmission wavelength components can be used.

  The signal light wavelength component and the idler light wavelength component that have passed through the WDM filter 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 desired quantum information communication technology by performing simultaneous measurement and the like.

(A-2) The following operation of the first embodiment, by applying the PPLN crystal as the first and second second-order nonlinear optical medium 102 and 103, SHG generating a _{d 33} component of its second-order nonlinear optical constant The operation of the polarization entangled photon pair generation device 10 of the first embodiment will be described as being used for SPDC generation.

  From the second input / output terminal 101-2 and the third input / output terminal 101-3 of the polarization beam splitting / combining module 101, pumping light (wavelength λp) having the same intensity is output for p-polarized light and s-polarized light, respectively. The PPLN crystal that is the first second-order nonlinear optical medium 102 is arranged so that the p-polarization direction and the z-axis direction of the PPLN crystal coincide. Similarly, the PPLN crystal that is the second-order nonlinear optical medium 103 is arranged so that the p-polarization direction and the z-axis direction of the PPLN crystal coincide.

  First, consider a process caused by pumping light propagating clockwise in the optical loop LP (pumping light component output as p-polarized light from the second input / output terminal 101-2 of the polarization separation / combination module 101).

  When this excitation light is input, intermediate SHG light is generated in the second secondary nonlinear optical medium 103. In addition, when the SPDC process using the intermediate SHG light as a seed light, a correlated photon pair of unwanted transient signal light and idler light is generated in the same PPLN crystal (second second-order nonlinear optical medium 103). There is also.

  The light output from the second second-order nonlinear optical medium 103 is input excitation light, intermediate SHG light, undesired transient signal light, and idler light. All of these lights are p-polarized light that matches the z-axis optical axis direction of the PPLN crystal.

  The output light from the second second-order nonlinear optical medium 103 then passes through the second half-wave plate 105. At this time, the excitation light, the transient signal light, and the idler light having a wavelength in the vicinity of λp are rotated by 90 ° to become s-polarized light and input to the first second-order nonlinear optical medium 102. The

  On the other hand, for the intermediate SHG light having a wavelength of λp / 2, the second half-wave plate 105 simply gives a phase difference of one wavelength, and thus no polarization rotation occurs. Therefore, the intermediate SHG light is input to the first second-order nonlinear optical medium 102 as p-polarized light.

  Next, in the first second-order nonlinear optical medium 102, since the input excitation light, transient signal light, and idler light are s-polarized light, no new SHG light is generated by them.

As a result, the second and third transient signal light and idler light photon pairs due to the SPDC process are not generated.

  On the other hand, since the intermediate SHG light is input to the first second-order nonlinear optical medium 102 with p polarization, new signal light and idler light are generated by the SPDC process. This photon pair of signal light and idler light becomes a desired correlated photon pair.

  As a result, the light output from the first second-order nonlinear optical medium 102 can be considered in two sets in the polarization state. That is, one set is input excitation light, transient signal light generated by the second second-order nonlinear optical medium 103, and idler light, and these are s-polarized light. The other set is intermediate SHG light, desired signal light generated by the first second-order nonlinear optical medium 102, and idler light, which are p-polarized light.

  Next, these lights pass through the first half-wave plate 104. As a result, the light other than the intermediate SHG light is rotated by 90 ° and then input to the third input / output terminal 101-3 of the polarization beam splitting / combining module 101.

  Since the input excitation light, the transient signal light generated by the second second-order nonlinear optical medium 103, and the idler light are input as p-polarized light, in this first embodiment of the polarization separation / combination module 101, The data is output to the fourth input / output terminal 101-4 which is not practically used. On the other hand, since the desired signal light and idler light generated in the first second-order nonlinear optical medium 102 are input as s-polarized light, they are output to the first input / output terminal 101-1 of the polarization beam splitting / combining module 101. . In other words, if the wavelength dependence of the polarization separation / combination module 101 is negligible, the intermediate SHG light is input as p-polarization, so that it is practical in the first embodiment of the polarization separation / combination module 101. Is output to the fourth input / output terminal 101-4 which is not used.

  That is, by the pumping light propagating clockwise in the optical loop LP, a desired correlated photon pair of s-polarized signal light and idler light is transmitted from the optical loop LP to the first input / output of the polarization separation / combination module 101. On the other hand, the input pumping light, the transient signal light generated by the second second-order nonlinear optical medium 103, the idler light, and the intermediate SHG light are output from the fourth input / output of the polarization splitting / combining module 101. It is output to the end 101-4.

  Next, consider a process caused by pump light propagating counterclockwise in the optical loop LP (pump light component output as s-polarized light from the third input / output terminal 101-3 of the polarization separation / combination module 101).

  First, when the excitation light passes through the first half-wave plate 104, polarization rotation occurs by 90 ° and becomes p-polarized light.

  When this excitation light is input, in the first second-order nonlinear optical medium 102, as in the case of the clockwise excitation light described above, intermediate SHG light, undesired transient signal light and idler light due to the SPDC process are used. Correlated photon pairs are generated.

  The input pump light, intermediate SHG light, unwanted transient signal light, and idler light output from the first second-order nonlinear optical medium 102 are also p-polarized light.

  The output light from the first second-order nonlinear optical medium 102 then passes through the second half-wave plate 105 and, as in the clockwise direction, the excitation light, the transient signal light, the idler The light is s-polarized light and the intermediate SHG light is p-polarized light and is input to the second second-order nonlinear optical medium 103.

  In the second second-order nonlinear optical medium 103, as described above, pump light, transient signal light, new SHG light by idler light, and new transient signal light by idler light, idler light. Does not occur.

  On the other hand, a photon pair of new signal light and idler light by the SPDC process is generated by the intermediate SHG light. This photon pair of signal light and idler light is a desired correlated photon pair.

  As a result, from the second second-order nonlinear optical medium 103, s-polarized input pump light, transient signal light generated in the first second-order nonlinear optical medium 102, idler light, and p-polarized light are generated. The intermediate SHG light, which is a wave, and the desired signal light and idler light generated in the second second-order nonlinear optical medium 103 are output.

  These lights are input to the second input / output terminal 101-2 of the polarization beam splitting / combining module 101.

  Since the input excitation light, the transient signal light generated by the first second-order nonlinear optical medium 102, and the idler light are input as s-polarized light, it is practically used in the first embodiment of the polarization separation / combination module 101. Is output to the fourth input / output terminal 101-4 which is not used. On the other hand, since the desired signal light and idler light generated in the second second-order nonlinear optical medium 103 are input as p-polarized light, they are output to the first input / output terminal 101-1 of the polarization beam splitting / combining module 101. . Speaking here, if the wavelength dependence of the polarization separation / combination module 101 can be ignored, the intermediate SHG light is also input as p-polarization, and therefore the first input / output terminal 101-of the polarization separation / combination module 101. 1 is output.

  That is, the excitation light propagating counterclockwise in the optical loop LP causes the signal light that is p-polarized light, the desired correlated photon pair of idler light, and the first second-order nonlinear optical medium. The intermediate SHG light generated at 102 is output from the first input / output terminal 101-1 of the polarization splitting / combining module 101, while the input pump light and the transient signal light generated at the first second-order nonlinear optical medium 102. The idler light is output to the fourth input / output terminal 101-4 of the polarization beam splitting / combining module 101.

  As in Non-Patent Document 1, when the pumping light intensity is sufficiently weak, the correlation photon pair of signal light and idler light output from the first input / output terminal 101-1 of the polarization separation / combination module 101 is an optical loop. An s-polarized correlated photon pair generated by pumping light propagating clockwise through LP, and a p-polarized correlated photon pair generated by pumping light propagating counterclockwise through the optical loop LP. That is, the state of the correlated photon pair generated from the device is a superposition state of the correlated photon pair propagating clockwise in the optical loop LP and the counter-clockwise correlated photon pair propagating counterclockwise. It becomes. That is, a polarization entangled photon pair is generated from this device.

  FIG. 4 is an explanatory diagram schematically showing the input / output relationship of each light and the input / output ends of the polarization splitting / combining module 101 described above. FIG. 4A is a schematic diagram when a polarization beam splitter of the type using the thin film 101R is used as the polarization separation / combination module 101, and FIG. It is a schematic diagram at the time of using what is called a polarizing prism using a birefringent crystal. In the first embodiment, either a polarizing beam splitter or a polarizing prism may be used.

  As shown in FIG. 4, a desired polarization entangled photon pair component (signal light, idler light) and an optical loop propagated counterclockwise from the first input / output terminal 101-1 of the polarization separation / combination module 101. An intermediate SHG light component generated by the excitation light is output. On the other hand, a strong input excitation light component, a clockwise intermediate SHG light component, a transiently generated signal light component, and an idler light component are theoretically generated by the fourth input / output terminal 101-of the polarization beam splitting / combining module 101. 4 is output.

  That is, from the input / output terminal (first input / output terminal 101-1 of the polarization separation / combination module 101) from which the desired polarization entangled photon pair is output, the light intensity is strong and the wavelength of the entangled photon pair is close. Ideally, the input excitation light component and unnecessary transient signal light / idler light are not output.

  The output light from the first input / output terminal 101-1 of the polarization splitting / combining module 101 is input to the second input / output terminal 106-2 of the optical circulator 106 and output to the third input / output terminal 106-3. However, here, the wavelength dependence of the optical circulator 106 is ignored for the time being.

  The output light from the third input / output terminal 106-3 of the optical circulator 106 first passes through the optical low-pass filter 107, and is generated by the SHG light component having the wavelength λp / 2 (excitation light propagating counterclockwise). Intermediate SHG light component) is removed.

  Next, the WDM filter 108 divides the signal light wavelength component (λs) and the idler light wavelength component (λi) into separate optical paths and outputs them. As such a WDM filter 108, a so-called AWG type WDM filter having at least a signal light wavelength (λs) and an idler light wavelength (λi) as transmission wavelength components can be used. The signal light wavelength component and the idler light wavelength component that have passed through the WDM filter 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 desired quantum information communication technology by performing simultaneous measurement and the like.

  As described above, in the first embodiment, the (intermediate) SHG light having the wavelength λp / 2, which is the seed light in the SPDC process, is supplied from the outside of the optical loop LP formed by the polarization splitting / combining module 101. Without being generated, it is autonomously generated in the second-order nonlinear optical medium 102 or 103 arranged in the optical loop LP. The intermediate SHG light is an unnecessary light component other than the optical path portion formed by the two second-order nonlinear optical media 102 and 103 and the second half-wave plate 105. That is, it is not necessary to propagate it without loss or to optically connect it to an optical component other than the optical path described above. As a result, the polarization splitting / combining module 101, the optical circulator 106, and the like may perform desired operations in the vicinity of the wavelength λp, and do not need to operate at the wavelength λp / 2. Need not be used. This point can also be said for a coupling optical system such as a lens system for inputting excitation light to the second-order nonlinear optical medium, that is, it is not necessary to be a coupling system that operates at both wavelengths λp / 2 and λp. It can be a coupled system that only works with. As a result, the manufacturing cost of the device is reduced, and excessive light loss and the like can be reduced, and a higher-purity quantum entangled state can be realized.

  Furthermore, since the intermediate SHG light of wavelength λp / 2 is an unnecessary light component other than in the two second-order nonlinear optical media, it receives a great amount of light loss in the polarization separation / combination module 101 and the coupling lens system. There is no problem at all, and in that case, it is desirable that the load on the filter transmission characteristic for removing the SHG light component required in the optical low-pass filter 107 in the subsequent stage is reduced.

  Furthermore, in the first embodiment, a desired quantum entangled photon pair is output from the optical output terminal (the first input / output terminal 101-1 of the polarization splitting / combining module 101) from which the desired quantum entangled photon pair is output outside the optical loop LP. In principle, light components such as the input excitation light component, transient signal light, and idler light that are close to the wavelength of the entangled photon pair and have the same or higher light intensity as the desired entangled photon pair Is never output.

  Therefore, when the desired entangled photon pair wavelength is sufficiently close to the input excitation light wavelength because the leaked light component of the strong excitation light input excitation component is not mixed at the output end of the desired entangled photon pair. But you can make it work. In principle, they can be the same (ie, λs = λi = λp).

  This is practically useful in that the entire wavelength band of the generated entangled photon pair is more effectively used. In particular, when the wavelength is the same correlated photon pair (ie, λs = λi), photon indistinguishability also occurs with respect to wavelength, which is also desirable for advanced quantum information communication technology. In addition, a high-efficiency quantum entangled photon pair with high generation efficiency of entangled photon pairs per unit wavelength is obtained by using a second-order nonlinear optical medium with a structural device that narrows the entire wavelength band of the entangled photon pair. It is also useful for occurrence.

As the first and second second-order nonlinear optical media 102 and 103, depending on the wavelength of the desired entangled photon pair, bulk crystals such as LiNbO ₃ crystal and PPLN crystal, and PPLN in which an optical waveguide structure is built in it. Various second-order nonlinear optical media such as waveguides can be used. When considering a entangled photon pair generator with a wavelength of 1.5 microns, the LiNbO ₃ crystal used in Patent Document 2 and Non-Patent Document 1 is a typical example.

  On the other hand, when executing the first embodiment, it is particularly important for industrial application to execute SHG generation and SPDC generation in the second-order nonlinear optical media 102 and 103 with high efficiency. For this purpose, the phase matching condition between the input excitation light and the SHG light and the phase matching condition between the SHG light, the signal light, and the idler light in each of the SHG generation process and the SPDC generation process are important.

  When a bulk crystal is used, it is common to use phase matching using angle matching. In this case, as shown in Patent Document 1 or the like, by spatial separation of signal light and idler light, The problem is that the purity of the entangled state deteriorates.

  On the other hand, using a structure in which the second-order nonlinear optical coefficient is spatially modulated, such as a periodic polarization inversion structure of a ferroelectric material, has an advantage that a pseudo phase matching condition can be realized regardless of angle matching. .

  Furthermore, in the second-order nonlinear optical medium incorporating the optical waveguide structure, it is possible to improve the above-described spatial separation problem, and to increase the effective second-order nonlinear optical coefficient due to strong optical confinement, thereby generating SHG and SPDC. There is an advantage that the probability of occurrence can be increased.

  For the above-described reason, the PPLN waveguide is optimal as the second-order nonlinear optical mediums 102 and 103 constituting the entangled photon pair generating device with a wavelength of 1.5 micron band intended by the first embodiment. It is an example.

The energy conservation law and the momentum conservation law (that is, the phase matching condition) in the SHG generation process, the SPDC generation process in the PPLN waveguide are as shown in Expressions (2) to (5). The wave number and the wavelength have a relationship of the expression (6) via the effective refractive index n in the PPLN waveguide.

  Equations (2) and (4) represent the wavelength relationship of each light component based on the energy conservation law. That is, the SHG light has a half wavelength of the input excitation light, and the sum of the optical frequencies of the signal light and idler light (= c / λ, where c is the speed of light in vacuum) generated using the SHG light as seed light. Means equal to the optical frequency of the SHG light and simultaneously equal to half the optical frequency of the input pumping light.

Equations (3) and (5) relate to phase matching. If the effective refractive index n _p and n _SHG are determined from the excitation light wavelength λp and the shape of the PPLN waveguide, the polarization inversion period Λ is determined based on the equations (3) and (6).

  When the wavelengths of the signal light and idler light are different (λs ≠ λp, λi ≠ λp), generally, the equations (3) and (5) are not compatible. That is, the phase matching conditions of the SHG process and the SPDC process are generally incompatible.

In the case of phase mismatch, the probability of occurrence decreases in proportion to {sin ² (δL / 2)} / (δL / 2) ² from the solution of the coupled mode equation describing the well-known nonlinear optical effect. it is conceivable that. Here, δ is a phase mismatch amount in the SPDC process expressed by the equation (7), and L is a PPLN waveguide length.

Assuming that the decrease in occurrence probability is 50% of the maximum value (value under phase matching) as an allowable mismatch amount, {sin ² (δL / 2)} / (δL / 2) ² = 0.5 ≒ 2.78. This limits the wavelength band of correlated photon pairs that occur in the SPDC process. That is, the wavelength range of signal light and idler light generated as a correlated photon pair is determined.

Further, when the wavelength change of the effective refractive index is substantially linear in the vicinity of the excitation light wavelength, that is, in the range where the effective refractive index in the vicinity of the excitation light wavelength is expressed by the expression (8), the expressions (4) and (6) Expression (9) is obtained using Expression (8) and Expression (8), that is, Expression (3) and Expression (5) of the phase matching condition are compatible within this range.

  From the above examination, in order to realize the quantum entangled photon pair generation device 10 of the first embodiment, first, the polarization inversion of the PPLN waveguide is satisfied so as to satisfy the expression (3) which is the phase matching condition for SHG generation. It can be seen that the period Λ may be determined. As a result, the correlated photon pair of the wavelength combination (λs, λi) that satisfies the phase matching in the SPDC generation process expressed by the equation (5) or at least within the phase mismatch amount within the allowable range is based on the SPDC process. As a result, it can be expected that a desired entangled photon pair can be generated.

(A-3) Effects of First Embodiment According to the quantum entangled photon pair generation device 10 of the first embodiment, the following effects can be expected.

  That is, unlike the prior art, the SHG light having the wavelength λp / 2, which is the seed light in the SPDC process, is not supplied from the outside of the optical path LP formed by the polarization beam splitting / combining module 101, but in the optical path of the optical loop LP Are generated by the two second-order nonlinear optical media 102 and 103 themselves. The SHG light generated here is an unnecessary component in the optical components other than the second-order nonlinear optical media 102 and 103. That is, in other device components, it is propagated without loss, or is transmitted to the optical components. There is no need for optical connection. As a result, it is not necessary to use special optical components that operate at both wavelengths λp and λp / 2 in the optical components that constitute the apparatus, such as the polarization separation / synthesis module and the coupling lens. In addition, at the output end of the desired entangled photon pair, an unnecessary optical component such as input excitation light whose wavelength is close to that of the entangled photon pair and whose light intensity is the same or higher than that of the entangled photon pair. However, in principle, they are not mixedly output. As a result, the device manufacturing cost can be reduced and excessive light loss can be reduced. Also, it is possible to provide a entangled photon pair generation device that can cope with more flexible device design such as widening or narrowing the wavelength band of entangled photon pairs to be used and can generate a high-purity quantum entangled state. it can.

(A-4) Modified Embodiments of the First Embodiment Various modifications can be made to the first embodiment in which the input / output configuration of the excitation light and the entangled photon pair is modified.

  FIG. 5 shows a modified embodiment in which the optical circulator is not used, and the same and corresponding parts as those in FIG. 1 are indicated by the same reference numerals.

  In the entangled photon pair generation device 10A shown in FIG. 5, an AWG is used as the WDM filter. As the AWG 108, an AWG having at least three wavelengths of a signal light wavelength (λs), an idler light wavelength (λi), and an excitation light wavelength (λp) as transmission wavelength components is used.

  The AWG 108 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 and output to the transmitted light input / output ports corresponding to the respective transmission wavelengths.

  Using the above-described characteristics of the AWG 108, the following configuration is adopted. That is, the pumping light is input to the transmitted light input / output port corresponding to the pumping 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 splitting / combining module 101. In the same manner as in the first embodiment shown in FIG.

  The output light from the optical loop LP output from the first input / output terminal 101-1 of the polarization separation / combination module 101 is input to the common input / output port of the AWG 108, and the transmitted light corresponding to the signal light and idler light, respectively. Output to I / O port. Furthermore, in order to remove the intermediate SHG wavelength component, if necessary, the optical low pass filters 107-s and 107-i are connected to the transmitted light input / output ports of the signal light and idler light to remove the SHG light wavelength component. .

  FIG. 6 shows another modified embodiment in which the optical circulator is not used, and the same and corresponding parts as those in FIG. 1 are indicated by the same reference numerals.

  In the entangled photon pair generation device 10B shown in FIG. 6, a narrow band optical bandpass filter 110 that selectively transmits (or reflects) only the wavelength component of the wavelength λp is used. Here, it is assumed that the wavelengths of the signal light and the idler light are sufficiently separated from the excitation light wavelength λp so as not to enter the transmission (or reflection) band of the optical bandpass filter 110.

  In FIG. 6, a reflective optical bandpass filter 110 is shown. Utilizing the reflection characteristics of the optical bandpass filter 110, the excitation light having the wavelength λp is input to the first input / output terminal 101-1 of the polarization beam splitting / combining module 101. Of the output light from the optical loop LP that is output from the first input / output terminal 101-1 of the polarization splitting / combining module 101, even if the pumping light wavelength component has a residual component, the reflective optical bandpass filter 110. And is not output to the transmission port. At this stage, the excitation light wavelength component is removed. Further, in order to remove the intermediate SHG wavelength component, an optical low-pass filter 107 is connected to remove the intermediate SHG light wavelength. Thereafter, the signal light and idler light are separated and output by being connected to the WDM filter 108.

  FIG. 7 is an explanatory diagram illustrating a configuration example other than the first embodiment of the light conversion generation unit. In the first embodiment shown in FIG. 1, the light conversion generation unit 100 includes a first second-order nonlinear optical medium 102, a second half-wave plate 105, and a second second-order nonlinear optical medium that are individual components. 103 was arranged in series. The light conversion generation unit 100C shown in FIG. 7 is obtained by forming each component at once on the same crystal substrate. This collective formation structure is substantially similar to the element structure of the integrated polarization-independent QPM-DFG wavelength conversion device shown in FIG. 7 on page 300 of the following reference.

Reference: Supervised by Shintaro Miyazawa and Nao Sasamura, "Basics and Applications of Polarization Inversion Devices", June 8, 2005, First Edition, First Edition, Publisher Optronics Inc. Photo-conversion generator shown in Figure 7 In realizing the structure of 100C, first, a polarization inversion structure (see FIG. 7B) and a waveguide structure (see FIG. 7A) are formed on a single LiNbO ₃ substrate, and a PPLN waveguide is formed. Form. Thereafter, the second half-wave plate 105 is embedded in the central portion of the waveguide. One PPLN waveguide portion sandwiching the second half-wave plate 105 is a first second-order nonlinear optical medium portion 102C that exhibits the same function as the first second-order nonlinear optical medium 102. The other PPLN waveguide portion serves as a second second-order nonlinear optical medium portion 103 </ b> C that exhibits a function similar to that of the second second-order nonlinear optical medium 103.

  As described in the reference literature, a Zn diffusion waveguide that guides both TE and TM polarized waves may be used as the waveguide structure. However, in this modified embodiment, a proton exchange waveguide is used. However, it functions as a waveguide only for one polarized wave (in this case, TM polarized wave) and does not function as a waveguide for a polarized wave orthogonal to it (TE polarized wave). It is rather effective to use a waveguide that is generated.

  That is, in this case, the p-polarization direction and the TM polarization direction of this modified embodiment match, but at this time, the input excitation light output from the first-order second-order nonlinear optical medium and the transient signal light This is because idler light is subjected to a large propagation loss by propagating through the second-stage second-order nonlinear optical medium as TE polarized light. That is, these lights are subjected to a large propagation loss in the second-order nonlinear optical medium in the second stage, and are polarized and separated from the desired entangled photon pair in the polarization separation / combination module 101. At the output terminal (first input / output terminal 101-1 of the polarization splitting / combining module 101) that outputs the entangled photon pair, it is possible to obtain a larger input pumping light, and an effect of reducing transient signal light and idler light. Can do.

  In the above description, the polarization directions of the input excitation light, the intermediate SHG light generated by the input excitation light, and the SPDC correlation photon pair generated by the intermediate SHG are the same.

On the other hand, even when the polarization directions of the input pump light, the intermediate SHG light, and the SPDC correlation photon pair are not the same, the same effect as that of the first embodiment can be obtained. For example, when the d ₃₁ component of the second-order nonlinear optical constant of PPLN is used, the arrangement location of the first half-wave plate 104 is closer to the second input / output end 101-2 of the polarization beam splitting / combining module 101. By doing so, the same effect as the first embodiment can be obtained.

(B) Second Embodiment Next, a second embodiment of the entangled photon pair generating apparatus according to the present invention will be described with reference to the drawings.

(B-1) Configuration of Second Embodiment FIG. 8 is an arrangement diagram of components showing the configuration of the polarization entangled photon pair generation device of the second embodiment, and is a diagram according to the first embodiment. The same and corresponding parts as those in FIG.

  The polarization quantum entangled photon pair generation device 10D of the second embodiment is based on the operation described in the first embodiment, and the input excitation light and the desired quantum entangled photon pair are different from each other in the polarization separation / synthesis module. Input / output is performed via the input / output terminals.

  In FIG. 8, the polarization quantum entangled photon pair generation device 10D of the second embodiment replaces the first ½ wavelength plate 104 used in the first embodiment with a Faraday rotator and a ½ wavelength. Two sets of non-reciprocal polarization plane conversion units (pairs of optical components) 211 and 212 composed of plates are used. Each of the nonreciprocal polarization plane conversion units 211 and 212 includes a first second-order nonlinear optical medium 102, a second half-wave plate 105, and a second second-order nonlinear optical medium 103 on the optical path of the optical loop LP. The light conversion generation unit 100 configured by Note that the optical circulator 106 used in the first embodiment is not used in the second embodiment.

  The constituent elements denoted by reference numerals 207 and 209 are first and second Faraday rotators that rotate the polarization by 45 ° with respect to the excitation light having the wavelength λp, respectively, and the constituent elements denoted by reference numerals 208 and 210 respectively Similarly to the first half-wave plate 104 used in the first embodiment, third and fourth half-wave plates that operate as a so-called half-wave plate for light of wavelength λp. It is.

  A pair of the first Faraday rotator 207 and the third half-wave plate 208 constitutes the first nonreciprocal polarization plane converter 211, and similarly, the second Faraday rotator 209 and the fourth 1 / A second non-reciprocal polarization plane conversion unit 212 is configured by a pair of two-wavelength plates 2. The first and second non-reciprocal polarization plane conversion units 211 and 212 are similar.

  The relationship in the optical axis direction of the constituent elements in each of the nonreciprocal polarization plane conversion units 211 and 212 is adjusted as shown in FIG. FIG. 9 shows the relationship in the optical axis direction of the components in the first non-reciprocal polarization plane conversion unit 211, but the same applies to the second non-reciprocal polarization plane conversion unit 212.

  In the first nonreciprocal polarization plane converter 211, linearly polarized excitation light in a specific polarization direction is input from the first Faraday rotator 207 side or the third half-wave plate 208 side. As will be described in detail in the operation section, this specific polarization direction coincides with either the p-polarization direction or the s-polarization direction in the polarization separation / combination module 101. The optical axis of the third half-wave plate 208 is adjusted to make an angle of 22.5 ° with this specific polarization direction (shown as the p-polarization direction in FIG. 9).

  The polarization separation / combination module 101, the first second-order nonlinear optical medium 102, the second half-wave plate 105, the second second-order nonlinear optical medium 103, the optical low-pass filter 107, and the WDM filter 108 are The function is the same as that of the embodiment, and the explanation of the function is omitted.

(B-2) Operation of the Second Embodiment Next, the operation of the polarization entangled photon pair generation device 10D of the second embodiment having the above configuration will be described.

  As in the case of the first embodiment, the first input / output terminal of the polarization splitting / combining module 101 is excited by light having a wavelength λp of linear polarization whose polarization direction is inclined by 45 ° with respect to the p polarization direction. When the signal is input to 101-1, it is separated into a p-polarization component and an s-polarization component having the same intensity, and the second input / output terminal 101-2 and the third input / output terminal 101-3 of the polarization beam splitting / combining module 101, respectively. Is output.

  In the following, an operation on pumping light that is output to the third input / output terminal 101-3 of the polarization splitting / combining module 101 and circulates the optical loop LP counterclockwise will be described.

  The s-polarized pump light output to the third input / output terminal 101-3 of the polarization splitting / combining module 101 passes through the first non-reciprocal polarization plane conversion unit 211. A change in the polarization state in the first nonreciprocal polarization plane converter 211 at this time will be described with reference to FIG.

  In FIG. 9A, the s-polarized pump light output to the third input / output terminal 101-3 of the polarization beam splitting / combining module 101 is represented by a right-pointing arrow. This s-polarized pump light passes through the first Faraday rotator 207. Passing the first Faraday rotator 207 causes a 45 ° polarization rotation counterclockwise in the figure. As a result, the polarization direction of the output light from the first Faraday rotator 207 is represented by an arrow directed obliquely upward 45 ° to the right. The output light from the first Faraday rotator 207 having such a polarization direction passes through the third half-wave plate 208. The polarization direction (optical axis) of the third half-wave plate 208 forms an angle of 22.5 ° with the specific polarization direction (p polarization direction). Since the polarization direction of the light input to the third half-wave plate 208 has an inclination of 45 ° with respect to the p-polarization direction, the optical axis of the third half-wave plate 208 is As a result, the polarization direction of the output light from the third half-wave plate 208 can be represented by the direction of the upward arrow.

  This means that the polarization direction of the pump light output through the third half-wave plate 208 is the p-polarization direction. That is, 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 third half-wave plate 208 has a polarization direction counterclockwise of 90. Rotated only by °. Note that the rotation in the polarization direction is the same in the second nonreciprocal polarization plane conversion unit 212. Such rotation occurs when the optical axis of the half-wave plate is adjusted to make an angle of 22.5 ° with the p-polarization direction or the s-polarization direction.

  The excitation light output from the first non-reciprocal polarization plane conversion unit 211 is input to the first second-order nonlinear optical medium 102, and as a result, the second second-order nonlinear optical as in the first embodiment. From the medium 103, s-polarized excitation light, transient signal light, idler light, p-polarized intermediate SHG light, and a desired SPDC correlation photon pair are generated.

  These lights are then input to the second nonreciprocal polarization plane converter 212. At this time, the excitation light and the transient signal light, idler light, and SPDC correlation photon pair having a wavelength close to that of the pump light are the same as when the second nonreciprocal polarization conversion unit 211 passes through the second By passing through the nonreciprocal polarization plane converter 212, the polarization state is rotated by 90 °. That is, excitation light, transient signal light, and idler light are converted to p-polarized light, and a desired SPDC correlation photon pair is converted to s-polarized light.

  Thereafter, the excitation light, the transient signal light, the idler light, and the SPDC correlation photon pair that have propagated counterclockwise in the optical loop LP as described above, 2 Input to the input / output terminal 101-2. At this time, the SPDC correlation photon pair whose polarization state is s-polarization is output to the fourth input / output terminal 101-4. That is, an SPDC correlation photon pair is output from a fourth input / output terminal different from the first input / output terminal to which excitation light is first input.

  On the other hand, pumping light whose polarization state is p-polarized light, transient signal light, and idler light are output to the first input / output terminal 101-1 that is the same as the input / output terminal to which pumping light is first input. Is done.

  Next, an operation for the pumping light that is output to the second input / output terminal 101-2 of the polarization splitting / combining module 101 and circulates in the optical loop LP clockwise will be described.

  The p-polarized excitation light output to the second input / output terminal 101-2 of the polarization splitting / combining module 101 passes through the second non-reciprocal polarization plane conversion unit 212, the third half-wave plate 210, the first The second Faraday element 209 passes in the order. The change in the polarization state at this time will be described with reference to FIG. FIG. 9 shows the first non-reciprocal polarization plane conversion unit 211, but as described above, the same applies to the second non-reciprocal polarization plane conversion unit 212, and reference numeral “207” in FIG. If “209” and “208” are replaced with “210”, FIG. 9 can be regarded as a drawing showing the second non-reciprocal polarization plane conversion unit 212.

  In FIG. 9B, since the excitation light output to the second input / output terminal 101-2 of the polarization beam splitting / combining module 101 is p-polarized light, it is represented by an upward arrow in the figure. This p-polarized excitation light passes through the fourth half-wave plate 210. Since the polarization direction of the excitation light to the fourth half-wave plate 210 forms an angle of 22.5 ° with the optical axis of the fourth half-wave plate 210, as a result, The polarization direction of the output light of the half-wave plate 210 can be represented by the direction of the arrow pointing 45 ° diagonally right upward. Next, when passing through the second Faraday rotator 209, it rotates by 45 ° counterclockwise, and as a result, the polarization direction of the output light of the second Faraday rotator 209 is represented by the direction of the upward arrow. be able to. That is, even if it passes through the second nonreciprocal polarization plane converter 212, it remains p-polarized.

  As described above, the polarization direction of the excitation light that has passed through the second non-reciprocal polarization plane converter 212 in the order of the fourth half-wave plate 210 and the second Faraday rotator 209 is not rotated. . Such non-rotation is the same in the first non-reciprocal polarization plane converter 211. This non-rotation occurs when the optical axis of the half-wave plate is adjusted to make an angle of 22.5 ° with the p-polarization direction or the s-polarization direction.

  As is clear from the above, the pumping light output from the second input / output terminal 101-2 of the polarization splitting / combining module 101 and having passed through the second nonreciprocal polarization plane converter 212 is used as p-polarized pumping light. , And input to the second second-order nonlinear optical medium 103. As a result, as in the first embodiment, s-polarized pump light, transient signal light, idler light, and p-polarized intermediate SHG light are desired from the first second-order nonlinear optical medium 102. An SPDC correlated photon pair is generated.

  These lights then pass through the first nonreciprocal polarization plane converter 211. Here, too, the excitation light and the transient signal light, idler light, and SPDC correlation photon pair having a wavelength close to that of the excitation light pass through the polarization state as they are.

  Thereafter, the excitation light, the transient signal light, the idler light, and the SPDC correlation photon pair that have propagated clockwise through the optical loop LP as described above are the third of the polarization separation / combination module 101. Input to the input / output terminal 101-3. At this time, the SPDC correlation photon pair whose polarization state is p-polarization is output to the fourth input / output terminal 101-4. That is, an SPDC correlation photon pair is output from a fourth input / output terminal different from the first input / output terminal to which excitation light is first input.

  The fourth input / output terminal to which the SPDC correlated photon pair propagating clockwise is input is the same as the input / output terminal to which the SPDC correlated photon pair propagating counterclockwise is output. However, the polarization direction of the SPDC correlation photon pair propagating clockwise is orthogonal to the polarization direction of the SPDC correlation photon pair propagating counterclockwise.

  On the other hand, the pumping light whose polarization state is s-polarized light, the transient signal light, and idler light are the same as the first input / output terminal 101-1, which is the same as the input / output terminal to which the pumping light is first input. Is output.

  That is, in the case of the second embodiment, a polarization quantum entangled photon pair is generated from the fourth input / output terminal 101-4 of the polarization beam splitting / combining module 101.

  Next, the intermediate SHG light having a half wavelength (λp / 2) generated from the respective second-order nonlinear optical media 102 and 103 is converted into the first nonreciprocal polarization plane converter 211 and the second nonreciprocal polarization. Consider the polarization state when passing through the wavefront converter 212. First, since the half-wave plate is a one-wave plate for SHG light, no polarization rotation occurs here. On the other hand, since the wavelength of the Faraday rotator is half that of the excitation light, the polarization rotation angle is 90 °.

  In other words, the nonreciprocal polarization plane conversion units (211 and 212) rotate the SHG light by 90 ° regardless of the order of passing through the Faraday rotator and the half-wave plate (see FIG. 9 (C), (D)).

  FIG. 10 is an explanatory diagram showing the input / output terminals of the polarization beam splitting / combining module 101 that inputs / outputs pumping light, transient signal light, idler light, intermediate SHG light, and correlated photon pairs (signal light, idler light). . As shown in FIG. 10, the desired entangled photon pair has a port (here, the fourth input / output terminal 101-4) different from the input / output terminal (here, the first input / output terminal 101-1) for inputting the excitation light. ) To be output together with the intermediate SHG light component generated by the excitation light propagating counterclockwise through the optical loop LP.

  On the other hand, as in the case of the first embodiment, the input excitation light component, the transient signal light, the idler light component, and the optical loop LP are transmitted from this port (here, the fourth input / output terminal 101-4). The intermediate SHG light component generated by the excitation light propagating clockwise is not output. All of these light components are output to the first input / output terminal 101-1 of the polarization beam splitting / combining module 101.

  Thereafter, in the same manner as in the first embodiment, the optical low-pass filter 107 and the WDM filter 108 remove the excess intermediate SHG light component and, in some cases, the residual excitation light wavelength component, and then the signal light and idler light. Branch out the component.

  When the configuration of the second embodiment is compared with the configuration of the first embodiment shown in FIG. 1 and the modified embodiment with respect to the first embodiment shown in FIGS. 5 and 6, there are the following differences. That is, in the second embodiment, in the polarization beam splitting / combining module 101, the pumping light input port (first input / output terminal 101-1) and the port from which the desired quantum entangled photon pair is output (fourth input). Unlike the output end 101-4), the excitation light does not return to the original excitation light input end and return. As a result, by installing an optical isolator at each input / output terminal, it is possible to eliminate the influence of the operational instability of the device due to the reflected return light, and to provide a device that operates more stably.

(B-3) Effects of Second Embodiment According to the entangled photon pair generation device 10D of the second embodiment, the following effects can be expected in addition to the effects of the first embodiment. That is, it is possible to realize an apparatus configuration in which the pumping light input port is different from the port from which a desired quantum entangled photon pair is output. As a result, by installing an optical isolator at each input / output terminal, it is possible to eliminate the influence of the operational instability of the device due to the reflected return light, and to provide a device that operates more stably.

(C) Third Embodiment Next, a third embodiment of the entangled photon pair generating apparatus according to the present invention will be described with reference to the drawings.

(C-1) Configuration of Third Embodiment FIG. 11 is an arrangement diagram of components showing the configuration of the polarization entangled photon pair generation device of the third embodiment, and is a diagram according to the first embodiment. The same and corresponding parts as those in FIG.

  In FIG. 11, the polarization entangled photon pair generation device 10E of the third embodiment optically connects the optical conversion generation unit 100 and the polarization separation / synthesis module 101 in addition to the same configuration as that of the first embodiment. Of these two optical paths, the polarization plane conversion unit 314 is provided in the optical path on the side where the first half-wave plate 104 is not inserted. The polarization plane conversion unit 314 is formed by cascading a fifth half-wave plate 311, a quarter-wave plate 312, and a sixth half-wave plate 313.

  The optical axis directions of the fifth half-wave plate 311, the quarter-wave plate 312, and the sixth half-wave plate 313 are selected as shown in FIG.

  The polarization plane converter 314 receives linearly polarized excitation light in a specific polarization direction. As will be described later, this specific polarization direction coincides with either the s-polarization direction or the p-polarization direction in the polarization separation / combination module 101. The optical axis of the fifth half-wave plate 311 and the sixth half-wave plate 313 is an angle of 22.5 ° with this specific polarization direction (shown as the p-polarization direction in the drawing). It is selected to make. On the other hand, the optical axis of the quarter wave plate 312 is adjusted to form an angle of 45 ° with the specific polarization direction.

  The optical conversion generator 100, the polarization separation / combination module 101, the first half-wave plate 104, the optical circulator 106, and the WDM filter 108 function in the same manner as in the first embodiment, and their functions I will omit the explanation.

(C-2) Operation of the Third Embodiment Next, the operation of the polarization entangled photon pair generating device 10E of the third embodiment having the above configuration will be described.

  As in the case of the first embodiment, the first input / output terminal of the polarization splitting / combining module 101 is excited by light having a wavelength λp of linear polarization whose polarization direction is inclined by 45 ° with respect to the p polarization direction. When the signal is input to 101-1, it is separated into a p-polarization component and an s-polarization component having the same intensity, and the second input / output terminal 101-2 and the third input / output terminal 101-3 of the polarization beam splitting / combining module 101, respectively. Is output.

  A characteristic effect described later of the third embodiment is brought about by the wavelength dependency of the polarization rotation in the polarization plane conversion unit 314. Hereinafter, this will be described with reference to FIG.

  FIG. 12A shows the change in the polarization state when the excitation light (wavelength λp) output to the second input / output terminal 101-2 of the polarization beam splitting / combining module 101 passes through the polarization plane converter 314. Show.

  The p-polarized excitation light (wavelength λp) output to the second input / output terminal 101-2 of the polarization beam splitting / combining module 101 is indicated by an upward arrow in the figure. This excitation light passes through the fifth half-wave plate 311. The optical axis direction of the fifth half-wave plate 311 is selected so as to form an angle of 22.5 ° with the p-polarization direction. Therefore, the polarization direction of the excitation light output from the fifth half-wave plate 311 is represented by an arrow directed obliquely upward 45 ° to the right.

  Next, the excitation light passes through the quarter wavelength plate 312. The optical axis direction of the quarter-wave plate 312 is arranged to form an angle of 45 ° with the p-polarization direction. Therefore, the polarization direction of the excitation light input to the quarter wavelength plate 312 matches one optical axis direction of the quarter wavelength plate 312, and as a result, passes through the quarter wavelength plate 312. However, no polarization rotation occurs.

  Next, the excitation light passes through the sixth half-wave plate 313. The optical axis direction of the sixth half-wave plate 313 is arranged to form an angle of 22.5 ° with the p-polarization direction. Therefore, the polarization direction of the excitation light output from the sixth half-wave plate 313 is rotated by 45 ° counterclockwise, that is, the light in the polarization direction represented by the upward arrow direction in the figure. Is output.

  That is, when the excitation light having the wavelength λp passes through the polarization plane converter 314, it is output while maintaining the original polarization direction (here, the p polarization direction). This is the same even when excitation light is input to the polarization plane converter 314 from the opposite side and output, as shown in FIG. Such a polarization state is maintained when the optical axis directions of the half-wave plates 311 and 313 and the quarter-wave plate 312 constituting the inside are the p-polarization direction or the s-polarization direction as described above. Occurs when selected to form 22.5 ° and 45 ° angles.

  Next, referring to FIGS. 12C and 12D, the state of the polarization direction conversion when the intermediate SHG light having the half wavelength (λp / 2) of the excitation light passes through the polarization plane conversion unit 314 is referred to. While explaining.

  The half-wave plates 311, 313 and the quarter-wave plate 312 operate as a one-wave plate and a half-wave plate for intermediate SHG light having a wavelength λp / 2, respectively.

  Accordingly, the p-polarized (upward arrow) intermediate SHG light input to the fifth half-wave plate 311 is output as p-polarized light.

  Next, when passing through the quarter-wave plate 312, the quarter-wave plate 312 operates as a half-wave plate for intermediate SHG light, and its optical axis direction is 45 ° with respect to the input polarization direction. As a result, the polarization direction of the output light is rotated by 90 ° (right arrow).

  Next, when passing through the sixth half-wave plate 313, no rotation in the polarization direction occurs.

  That is, when the intermediate SHG light having the wavelength λp / 2 passes through the polarization plane converter 314, it is rotated by 90 ° from the original polarization direction (here, the p polarization direction) (converted to s polarization). ) This is the same even when the intermediate SHG light is input and output from the reverse side as shown in FIG. This occurs when the optical axis direction of the quarter wavelength plate 312 constituting the inside is set to form an angle of 45 ° with the s-polarization direction or the p-polarization direction as described above. .

  The above results are summarized as follows. That is, when passing through the polarization plane conversion unit 314, the excitation light having the wavelength λp and the signal light and idler light having a wavelength close thereto are not polarized and rotated, whereas the intermediate SHG light having the wavelength λp / 2 is polarized by 90 °. Rotate. For such conversion of the polarization direction (polarization plane), the optical axis directions of the two half-wave plates 311 and 313 and one quarter-wave plate 312 constituting the inside are respectively p-polarized waves. The angle is selected to make an angle of 22.5 ° and 45 ° with respect to the direction or the s polarization direction.

  Based on the conversion operation of the polarization plane conversion unit 314 described above, the overall operation of the third embodiment will be described.

  The pumping light (p-polarized light) output from the second input / output terminal 101-2 of the polarization splitting / combining module 101 and propagating clockwise through the optical loop LP passes through the polarization plane converter 314 and is subjected to polarization conversion. Without being changed (p-polarized light), it is input to the second secondary nonlinear optical medium 103. As a result, as in the first embodiment, the s-polarized input excitation light, the transient signal light, the idler light, and the p-polarized intermediate SHG light, as desired, from the first second-order nonlinear optical medium 102. SPDC correlated photon pairs are generated.

  These lights are then input to the first half-wave plate 104, and the SDPC correlated photon pair is rotated s-polarized by 90 °, whereas the input pump light, transient Signal light, idler light, and intermediate SHG light are p-polarized light. Thereafter, these are input to the third input / output terminal 101-3 of the polarization separation / combination module 101, and the SPDC correlation photon pair whose polarization state is s-polarization is output to the first input / output terminal 101-1. On the other hand, the input excitation light, the transient signal light, the idler light, and the intermediate SHG light whose polarization state is p polarization are output to the fourth input / output terminal 101-4.

  On the other hand, the pumping light (s-polarized light) output from the third input / output terminal 101-3 of the polarization splitting / combining module 101 and propagating counterclockwise through the optical loop LP is transmitted through the first half-wave plate 104. The polarization is rotated by 90 °, converted to p-polarized light, and input to the first second-order nonlinear optical medium 102. As a result, as in the first embodiment, s-polarized input pump light, transient signal light, idler light, and p-polarized intermediate SHG light, desired from the second second-order nonlinear optical medium 103 SPDC correlated photon pairs are generated.

  These lights are then input to the polarization plane conversion unit 314, and the SDPC correlation photon pair is output as p-polarized light without rotating, whereas input pump light, transient signal light, idler Light and intermediate SHG light are output as s-polarized light. Thereafter, these are input to the second input / output terminal 101-2 of the polarization separation / combination module 101, and the SPDC correlation photon pair whose polarization state is p-polarization is output to the first input / output terminal 101-1. On the other hand, the input excitation light, the transient signal light, the idler light, and the intermediate SHG light whose polarization state is s-polarization are output to the fourth input / output terminal 101-4.

  Here, the SPDC correlation photon pair output from the first input / output terminal 101-1 of the polarization demultiplexing / combining module 101 is generated by the pumping light propagating clockwise in the optical loop LP is the s-polarization. On the other hand, since the p-polarized light is generated by the pumping light propagating counterclockwise in the optical loop LP, as in the first embodiment, the first input / output terminal 101-of the polarization splitting / combining module 101 is used. From 1 a polarization entangled photon pair is generated.

  FIG. 13 is an explanatory diagram showing the input / output ends of the polarization splitting / combining module 101 that inputs / outputs pumping light, transient signal light, idler light, intermediate SHG light, and correlated photon pairs (signal light and idler light). . As shown in FIG. 13, the desired entangled photon pair is output from the same input / output terminal (here, the first input / output terminal 101-1) as the input / output terminal to which the excitation light is input, whereas the input excitation Light, transient signal light, idler light, and intermediate SHG light components are output to the fourth input / output terminal 101-4 regardless of the propagation direction of the optical loop LP.

  That is, the desired quantum entangled photon pair and the other light components are output to different input / output terminals of the polarization splitting / combining module 101 and are not output to the same input / output terminal. As a result, unlike the first and second embodiments, the optical low-pass filter 107 for removing the intermediate SHG light is not necessary in principle.

  Although not shown, the input / output configuration of the excitation light and the entangled photon pair is the same as that of the modified embodiment (FIGS. 5 and 6) with respect to the first embodiment of the AWG filter used as the WDM filter 108. Modified embodiments such as a configuration using bidirectional input / output characteristics (FIG. 5) and a configuration using optical bandpass filter 110 (FIG. 6) can be cited. The same effect as that of the third embodiment can be obtained.

(C-3) Effects of Third Embodiment According to the entangled photon pair generation device 10E of the third embodiment, the following effects can be expected in addition to the effects of the first and second embodiments. That is, in principle, the optical low-pass filter 107 for removing SHG light is not necessary. In reality, even if the polarization extinction ratio with respect to the intermediate SHG light in the polarization demultiplexing / combining module 101 is insufficient (in this case, the polarization demultiplexing / combining module 101 uses the long-wavelength excitation light and the SPDC correlation photon pair. Since an optimized circuit is assumed, the polarization extinction ratio for the short-wavelength intermediate SHG light is not necessarily guaranteed), and at least the residual intermediate SHG light component is lower than that in the first and second embodiments. Therefore, at least the effect of reducing the load of removing the intermediate SHG light component of the optical low-pass filter 107 can be expected. As a result, a reduction in device cost can be expected.

(D) Fourth Embodiment Next, a fourth embodiment of the entangled photon pair generating apparatus according to the present invention will be described with reference to the drawings.

(D-1) Configuration of the Fourth Embodiment FIG. 14 is an arrangement diagram of the components showing the configuration of the polarization entangled photon pair generation device of the fourth embodiment, and FIG. The same and corresponding parts as those in FIG. 11 are indicated by the same and corresponding reference numerals.

  The polarization entangled photon pair generation device 10F of the fourth embodiment introduces both the characteristic configuration of the second embodiment and the characteristic configuration of the third embodiment, and the operational effects of the second embodiment In addition, the operational effects of the third embodiment can be obtained together. That is, an optical low-pass filter for removing the intermediate SHG light has a configuration for inputting / outputting the input pumping light and the desired quantum entangled photon pair via different input / output terminals of the polarization splitting / combining module. In principle, it becomes unnecessary.

  The polarization entangled photon pair generation device 10F of the fourth embodiment shown in FIG. 14 has the polarization plane conversion unit 314 described in the third embodiment in addition to the configuration of the second embodiment shown in FIG. Is inserted between the second second-order nonlinear optical medium 103 and the second Faraday rotator 209.

(D-2) Operation of the Fourth Embodiment Next, the operation of the polarization entangled photon pair generation device 10F of the fourth embodiment having the above configuration will be described.

  As described in the operation section of the third embodiment, even when the polarization plane conversion unit 314 is inserted, polarization rotation of excitation light, signal light, and idler light having wavelengths close to each other does not occur.

  Therefore, considering the light component propagating clockwise in the optical loop LP, the p-polarized pumping light output from the second input / output terminal 101-2 of the polarization splitting / combining module 101 is the second non-reciprocal polarization. Since the wavefront conversion unit 212 and the polarization plane conversion unit 314 do not rotate the plane of polarization, they are input to the second second-order nonlinear optical medium 103 as p-polarized waves. As a result, the first second-order nonlinear optical medium 102 receives s Polarized input excitation light, transient signal light, idler light, p-polarized intermediate SHG light, and a desired SPDC correlation photon pair are generated.

  Since the excitation light, the transient signal light, the idler light, and the SPDC correlation photon pair do not undergo polarization rotation even in the first nonreciprocal polarization plane conversion unit 211, the input excitation light, the transient signal light, and the idler light are , S-polarized light is output from the first input / output terminal 101-1 of the polarization beam splitting / combining module 101. On the other hand, the SPDC correlation photon pair is output from the fourth input / output terminal 101-4 of the polarization beam splitting / combining module 101 with p polarization. Further, the intermediate SHG light is converted into s-polarized light and output to the first input / output terminal 101-1 of the polarization beam splitting / combining module 101.

  Next, considering the light component propagating counterclockwise in the optical loop LP, the s-polarized pumping light output from the third input / output terminal 101-3 of the polarization splitting / combining module 101 is the first non-polarized light. It passes through the reciprocal polarization plane converter 211 in the order of the first Faraday rotator 207 and the third half-wave plate 208 and is converted into p-polarized light. Then, it is input to the first second-order nonlinear optical medium 102, and as a result, from the second second-order nonlinear optical medium 103, s-polarized input excitation light, transient signal light, idler light, and p-polarized light Intermediate SHG light, the desired SPDC correlated photon pair is generated. Then, it passes through the polarization plane conversion unit 314. By the operation described in the third embodiment, the excitation light, the transient signal light, the idler light, the intermediate SHG light are s-polarized, and the SPDC correlation photon pair is Output from the polarization plane converter 314 as p-polarized light.

  Next, when passing through the second non-reciprocal polarization plane conversion unit 212, the excitation light, the transient signal light, the idler light, the intermediate SHG light, and the SPDC correlation photon pair are all rotated by 90 ° in the polarization direction. Therefore, it is output as p polarized wave, p polarized wave, p polarized wave, and s polarized wave. As a result, the SPDC correlation photon pair is output from the fourth input / output terminal 101-4 of the polarization beam splitting / combining module 101 with s polarization, while the excitation light, the transient signal light, the idler light, and the intermediate SHG light are The signal is output to the first input / output terminal 101-1 of the polarization beam splitting / combining module 101 as p-polarized light.

  FIG. 15 is an explanatory diagram showing the input / output terminals of the polarization splitting / combining module 101 that inputs / outputs pumping light, transient signal light, idler light, intermediate SHG light, and correlated photon pairs (signal light and idler light). . As shown in FIG. 15, the desired entangled photon pair is output to the fourth input / output terminal 101-4 of the polarization splitting / combining module 101, while other light components (excitation light, transient signal light, All of the idler light and intermediate SHG light) are output to the first input / output terminal 101-1.

  That is, in the fourth embodiment, the input pumping light and the desired quantum entangled photon pair, which are the operational effects of the second embodiment, are respectively input / output via different input / output terminals of the polarization splitting / combining module. The desired quantum entangled photon pair and the other optical components, which are the operational effects of the third embodiment, are output to different input / output terminals of the polarization splitting / combining module, and the same input / output ends. The effect of not being output to the output end can be realized at the same time.

(D-3) Effects of the fourth embodiment According to the fourth embodiment, both of the effects of the second and third embodiments can be obtained.

(E) Other Embodiments In each of the above embodiments, the case where a PPLN crystal is used as the second-order nonlinear optical medium has been described. However, as described in the text, the effect of each of the above embodiments is other than the PPLN crystal. This can occur even when a second-order nonlinear optical medium is used. As the second-order nonlinear optical medium, not only a waveguide type device such as a bulk crystal or a PPLN waveguide in which an optical waveguide structure is formed, but also various other forms of second-order nonlinear optical media can be used. .

Further, it is a design matter that the first half-wave plate 104 used in the first embodiment or the third embodiment is disposed in the optical path of the optical loop LP. That is, for example, when the effect of each embodiment is obtained using the d ₁₁ component of the second-order nonlinear optical medium, the first input / output of the polarization splitting / combining module 101 is connected to the first half-wave plate 104. You may arrange | position to the optical path which connects the end 101-2 and the 1st second order nonlinear optical medium 102. That is, the arrangement location can be set flexibly as appropriate based on the optical axis direction of the second-order nonlinear optical medium and the polarization directions of the excitation light and intermediate SHG light input thereto. Similarly, with respect to the polarization plane conversion unit 314 described in the second to fourth embodiments and the first and second non-reciprocal polarization plane conversion units 211 and 212, the optical axis direction of the second-order nonlinear optical medium Based on the polarization directions of the excitation light and intermediate SHG light input thereto, it can be set appropriately and flexibly.

  In each of the above embodiments, the optical loop LP is shown as a wired loop using a polarization-maintaining optical fiber. However, for example, a partial section may be a spatial section.

  DESCRIPTION OF SYMBOLS 10, 10A-10F ... Quantum entangled photon pair generation apparatus, 100 ... Light conversion production | generation part, 101 ... Polarization demultiplexing synthesis module, 102 ... 1st second order nonlinear optical medium, 103 ... 2nd second order nonlinear optical medium, 104, 105, 208, 210, 311, 313 ... 1/2 wavelength plate, 106 ... optical circulator, 107, 107-s, 107-i ... optical low pass filter, 108 ... WDM filter, 207,209 ... Faraday rotator, 211, 212 ... non-reciprocal polarization plane conversion unit, 312 ... quarter wave plate, 314 ... polarization plane conversion unit, LP ... optical loop.

Claims (14)

A polarization-maintaining loop optical path;
The input excitation light from the first optical path outside the loop is branched into first and second demultiplexed excitation lights, which are polarization components orthogonal to each other, and the first demultiplexed excitation light is Excitation light demultiplexing input means for inputting the second demultiplexed excitation light counterclockwise to the loop optical path, and
The first nonlinear optical medium, the second half-wave plate and the second nonlinear optical medium are connected in series in this order counterclockwise, and the plane of polarization is orthogonal. Light conversion generation means for generating first natural parametric down-converted light propagating in the direction and the second natural parametric down-converted light propagating counterclockwise in the loop optical path;
The first natural parametric down-converted light propagating clockwise in the loop optical path and the second natural parametric down-converted light propagating counterclockwise in the loop optical path are combined and combined. Converted light combining output means for outputting the wave light to the second optical path outside the loop;
Polarization of at least one of the first demultiplexed pump light, the second demultiplexed pump light, the first natural parametric down-converted light, and the second natural parametric down-converted light Polarization plane operating means for operating the direction,
The first nonlinear optical medium generates a first optical second harmonic with respect to the second demultiplexed excitation light propagating counterclockwise in the loop optical path, and the second nonlinear optical medium is Generating the first natural parametric down-converted light propagating clockwise by the conversion to the second optical second harmonic propagating clockwise in the generated loop optical path;
The second nonlinear optical medium generates the second optical second harmonic with respect to the first demultiplexed excitation light propagating clockwise in the loop optical path, and the first nonlinear optical medium is Generating the second natural parametric down-converted light propagating counterclockwise by conversion to the first optical second harmonic propagating counterclockwise through the generated loop optical path;
The second half-wave plate is output from one of the first or second nonlinear optical medium and input to the other of the first or second nonlinear optical medium. Rotate the polarization plane of the demultiplexed excitation light by 90 ° and output it.
The first and second optical second harmonics and the first and second natural parametric down-converted lights are generated in the first or second nonlinear optical medium disposed in the loop optical path. A entangled photon pair generator characterized by: Loop optical path,
The input excitation light from the first optical path outside the loop is branched into first and second demultiplexed excitation lights, which are polarization components orthogonal to each other, and the first demultiplexed excitation light is Excitation light demultiplexing input means for inputting the second demultiplexed excitation light counterclockwise to the loop optical path, and
The first nonlinear optical medium, the second half-wave plate and the second nonlinear optical medium are connected in series in this order counterclockwise, and the plane of polarization is orthogonal. Light conversion generation means for generating first natural parametric down-converted light propagating in the direction and the second natural parametric down-converted light propagating counterclockwise in the loop optical path;
Combining the first natural parametric downconverted light propagating clockwise in the loop optical path and the second natural parametric downconverted light propagating counterclockwise in the loop optical path; Converted light combining output means for outputting the combined light to the second optical path outside the loop;
Aligning the plane of polarization of the first demultiplexed pump light with the plane of polarization of the second demultiplexed pump light when input to the nonlinear optical medium, or the second in the converted light multiplexing output means The first natural parametric down-converted light and the second natural parametric down-converted light to the out-of-loop optical path at least execute a combined output in which polarization planes are orthogonal to each other. The polarization direction of at least one of the first demultiplexed pump light, the second demultiplexed pump light, the first natural parametric down-converted light, and the second natural parametric down-converted light. Polarization plane operating means
The first nonlinear optical medium generates a first optical second harmonic with respect to the second demultiplexed excitation light propagating counterclockwise in the loop optical path, and the second nonlinear optical medium is Generating the first natural parametric down-converted light propagating clockwise by the conversion to the second optical second harmonic propagating clockwise in the generated loop optical path;
The second nonlinear optical medium generates the second optical second harmonic with respect to the first demultiplexed excitation light propagating clockwise in the loop optical path, and the first nonlinear optical medium is Generating the second natural parametric down-converted light propagating counterclockwise by conversion to the first optical second harmonic propagating counterclockwise through the generated loop optical path;
The second half-wave plate is output from one of the first or second nonlinear optical medium and input to the other of the first or second nonlinear optical medium. Rotate the polarization plane of the demultiplexed excitation light by 90 ° and output it.
The first and second optical second harmonics and the first and second natural parametric down-converted lights are generated in the first or second nonlinear optical medium disposed in the loop optical path. A entangled photon pair generator characterized by:   3. The entangled photon pair generation device according to claim 1, wherein the first nonlinear optical medium and the second nonlinear optical medium are second-order nonlinear optical media.   The quantum according to any one of claims 1 to 3, wherein the conversion with respect to the first optical second harmonic and the conversion with respect to the second optical second harmonic are natural parametric down conversions. Tangle photon pair generator.   The quantum entanglement according to any one of claims 1 to 4, wherein the pumping light demultiplexing input means and the converted light multiplexing output means are composed of the same polarization splitting / combining module having a plurality of input / output terminals. Photon pair generator.   The polarization plane manipulating means is a first half-wave plate, and the first half-wave plate is a polarization plane of the first natural parametric down-converted light that propagates clockwise in the loop optical path. 6. The quantum entangled photon pair generating apparatus according to claim 5, wherein the quantum entangled photon pair generating apparatus outputs the light after rotating by 90 °. The polarization plane operating means includes first and second non-reciprocal polarization plane conversion units,
In the loop optical path, the first non-reciprocal polarization plane conversion unit, the light conversion generation unit, and the second non-reciprocal polarization plane conversion unit are arranged in this order counterclockwise,
The first non-reciprocal polarization plane converter is configured by arranging a first 45 ° Faraday rotator and a third half-wave plate in this order counterclockwise,
The second non-reciprocal polarization conversion unit is configured by arranging a second 45 ° Faraday rotator and a fourth half-wave plate in this order counterclockwise. The entangled photon pair generating device according to claim 5. The polarization plane operating means is composed of a first half-wave plate and a polarization plane converter.
In the loop optical path, the polarization plane conversion unit, the light conversion generation means, and the first half-wave plate are arranged in this order in the clockwise direction,
The polarization plane conversion unit is configured by arranging a fifth half-wave plate, a quarter-wave plate, and a sixth half-wave plate in this order in a clockwise direction,
The first half-wave plate rotates the polarization plane of the first natural parametric down-converted light propagating clockwise in the loop optical path by 90 ° and outputs the rotated light. The entangled photon pair generator described. The polarization plane operating means includes first and second non-reciprocal polarization plane conversion units, and a polarization plane conversion unit,
In the loop optical path, the first non-reciprocal polarization plane conversion unit, the light conversion generation unit, the polarization plane conversion unit, and the second non-reciprocal polarization plane conversion unit are counterclockwise. Arranged in order,
The first non-reciprocal polarization plane converter is configured by arranging a first 45 ° Faraday rotator and a third half-wave plate in this order counterclockwise,
The second non-reciprocal polarization conversion unit is configured by arranging a second 45 ° Faraday rotator and a fourth half-wave plate in this order counterclockwise,
The polarization plane conversion unit is configured by arranging a fifth half-wave plate, a quarter-wave plate, and a sixth half-wave plate in this order in a clockwise direction. Item 6. The entangled photon pair generator according to Item 5.   A wavelength component of the input excitation light is removed in the second optical path outside the second loop through which the first and second natural parametric down-converted lights that have been combined and output pass, and the first and second The first wavelength selective filter that outputs the signal light wavelength component and the idler light wavelength component of the natural parametric down-converted light to different light output ends, respectively, according to claim 1, Quantum entangled photon pair generator.   The entangled photon pair generation device according to claim 10, wherein the first wavelength selective filter is an AWG filter.   An optical low pass that removes at least one of the first and second optical second harmonic components in the second optical path outside the second loop through which the first and second natural parametric down-converted lights that have been combined and output pass. The quantum entangled photon pair generation device according to claim 1, further comprising a filter.   13. The optical path outside the first loop and the optical path outside the second loop have a common part, and an optical circulator is provided at an end of the common part. The entangled photon pair generator according to any one of the above.   The first optical path outside the loop and the second optical path outside the loop have a common part, and the wavelength component of the input excitation light transmitted through the optical path outside the second loop is transmitted to the end of the common part or 7. A second wavelength selective filter that reflects and reflects or transmits a signal light wavelength component and an idler wavelength component of the first and second natural parametric down-converted lights is provided. , 8, 10-12. The entangled photon pair generating device according to any one of the above.

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