JP2017167489A - Quantum entanglement generation device, and method for improving quantum entanglement fidelity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maximize the fidelity of quantum entanglement in accordance with the characteristic of a photon count detector used in measurement.SOLUTION: A quantum entanglement generation device of the present invention comprises: a first light source unit and a second light source unit for outputting photons in Raman scattering by irradiation of a pump light; a first detector and a second detector for detecting the photons outputted from the first light source unit and second light source unit through a beam splitter; a single photon source for measuring the characteristic of at least one of the first detector and the second detector; a first optical path from the single photon source to the first detector and/or the second detector; an optical system for switching a second optical path from the first light source unit and second light source unit to the first detector and second detector through the beam splitter; and a control unit for specifying the characteristic value of at least one of the first detector and the second detector in the first optical path, and generating, on the basis of outputs of the first detector and second detector, quantum entanglement in the second optical path with the intensity of pump light that corresponds to the characteristic value.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a quantum entanglement generating device and a method of improving quantum entanglement fidelity.

  In quantum cryptography communication and quantum computation, quantum entanglement is often used as a protocol resource. In particular, quantum communication that is not wiretapped is realized by generating quantum entanglement in two matter systems that are separated from each other. Two particles in a quantum entanglement relationship overlap each other, and measuring the state of one particle instantly determines the state of the other particle that is spatially separated.

  The method of generating entanglement involves first interacting two matter systems with photons, then transmitting those photons as needed, and finally performing two-photon interferometry, resulting in two matter systems. Generate entanglement between systems.

  A method using Raman scattering of an asymmetric three-level (lambda) system and Fock basis of Stokes photons has been proposed for the generation of entanglement (see, for example, Patent Document 1 and Non-Patent Document 1). As the lambda system, non-patent document 1 uses an atomic group, and patent document 1 uses a group of coupled quantum dots.

  FIG. 1 shows a known device configuration used to generate entanglement. Asymmetric three-level (lambda) systems α and β are prepared, and Stokes photons due to Raman scattering when pump light is incident are transmitted through optical paths 107 and 108 such as optical fibers, respectively. The pump light is blocked by the filters 109 and 111, and Stokes photons are simultaneously incident on the beam splitter 125.

  Each Stokes photon is transmitted or reflected by the beam splitter 125 with a 50 to 50 probability. Quantum interference occurs because the stochastic photons incident with a probability of 1 or less from two lambda systems interfere with each other. The output (interference result) of the beam splitter 125 is detected by two photon number detectors 127 and 128 (also indicated as “PND1” and “PND2” in the figure). Observation of the state caused by quantum interference is called Bell measurement.

  The Bell measurement is successful when only one photon is detected by either one of the photon number detectors 127 or 128 from the output of the beam splitter 125. At this time, one of the four states of 2 qubits is specified, and quantum entanglement is generated.

Japanese Patent Application Laid-Open No. 2006-12877

L-M. Duan et al., “Long-distance quantum communication with atomicensembles and linear optics”, NATURE 414, 413-418 (2001)

  In general, the higher the fidelity of the generated entanglement, the better the protocol delivery distance, processing speed, noise immunity, and eavesdropping safety using the entanglement. However, the performance of a photon number detector (PND) is not necessarily constant or complete. Each photon number detector has a limited quantum efficiency and a non-zero dark count rate, and its characteristics vary depending on the photon number detector.

  In the conventional method, the optimum condition for improving the fidelity according to the characteristics of the photon number detector is not considered. That is, regardless of the performance of the photon number detector used, in order to eliminate the two-photon state at a certain level or more in the first stage, the material system and the photon are entangled with a sufficiently low pump light intensity. Therefore, if the performance of the photon number detector is low, only fidelity lower than the possible limit can be obtained. Conversely, when the performance of the photon number detector is high, the opportunity to obtain the entanglement generation rate commensurate with it is lost.

  An object of the present invention is to provide a quantum entanglement generation device and a fidelity improvement method that maximize the fidelity according to the characteristics of the photon number detector.

In one aspect, the quantum entanglement generating device comprises:
A first light source unit and a second light source unit for outputting photons by Raman scattering by irradiation of pump light;
A first detector and a second detector for detecting photons output from the first light source unit and the second light source unit through a beam splitter;
A single photon source for measuring characteristics of at least one of the first detector and the second detector;
A first optical path from the single photon source to the first detector and / or the second detector, the first light source unit and the second light source unit to the first detector and the second light source through the beam splitter; An optical system that switches between a second optical path leading to two detectors;
The characteristic value of at least one of the first detector and the second detector is specified in the first optical path, and the first detector and the intensity of pump light according to the characteristic value are determined in the second optical path. A controller that generates quantum entanglement based on the output of the second detector;
Have

  As one aspect, the entanglement fidelity can be maximized according to the characteristics of the photon number detector used for measurement.

It is the schematic of the conventional quantum entanglement production | generation apparatus. It is the schematic of the quantum entanglement production | generation apparatus of embodiment. It is a principle diagram of Bell measurement and entanglement generation by a photon number detector. It is a principle diagram of entanglement generation of a lambda system and Stokes photon by Raman scattering. It is the figure which plotted the amplitude dependence of the Stokes light of the entanglement fidelity using the quantum efficiency η and the dark count rate ν as parameters. It is a flowchart of maximization of the entanglement fidelity of embodiment. It is a figure which shows the optical system at the time of the characteristic measurement of the photon number detector of S11 of FIG. It is a figure which shows the optical system at the time of the measurement of the light source intensity and Raman amplitude of S12 of FIG. It is a figure which shows the optical system at the time of the quantum entanglement production | generation of S15 of FIG. It is a figure which shows the example of arrangement | positioning of the optical system in each process. It is a figure which shows the example of rotation of a movable mirror.

  Before describing the embodiment of the present invention, the above-described technical problems will be described in a little more detail.

When the lambda system α in the ground state | G> α is irradiated with non-resonant pump light with respect to the energy difference from the excited state | E> α, the probability amplitude f ₁ ^α proportional to the square of the pump light intensity is immediately obtained. One Stokes photon having a wavelength longer than that of the pump light is generated, and transitions to a one-particle excited metastable state | S ₁ > α. By adjusting the intensity of the pump light, the probability amplitude f ₁ ^α at which the pump light is Raman scattered in the lambda system is adjusted, and an entangled state between the lambda system and the Stokes photons is generated. Accordingly, the wave function after scattering is approximately represented by the equation (1).

ここで、|1>aは準安定状態への遷移による光路ａの１ストークス光子状態である。なお、式（１）では規格化定数を省略している。

Here, | 1> a is one Stokes photon state of the optical path a due to the transition to the metastable state. Note that the normalization constant is omitted in Equation (1).

  Similarly, the wave function after scattering in another lambda system β is approximately expressed by Expression (2).

ここで、|1>bは準安定状態への遷移による光路ｂの１ストークス光子状態である。式（２）では規格化定数を省略している。

Here, | 1> b is one Stokes photon state of the optical path b due to the transition to the metastable state. In equation (2), the normalization constant is omitted.

  The Stokes photons are guided to the Bell measuring device by the optical path a (for example, optical path 107 in FIG. 1) and the optical path b (for example, optical path 108 in FIG. 1). The Bell measuring instrument includes a beam splitter 125 and two photon number detectors (PND) 127 and 128. By performing the stochastic Bell measurement by matching the timing of the two Stokes light pulses into the beam splitter 125, the quantum entanglement of the equation (3) is established between the original two lambda systems with a certain success probability. It is formed.

確率的Bell測定とは、ビームスプリッタ１２５を二つのパルスが同時に通過することにより起こる量子干渉の結果を２つの光子数検出器１２７，１２８で検出するものである。片方の光子数検出器に一つだけ光子が検出された時に、Bell測定が成功したことになる。

The stochastic Bell measurement is a method in which two photon number detectors 127 and 128 detect the result of quantum interference caused by two pulses simultaneously passing through the beam splitter 125. The Bell measurement was successful when only one photon was detected in one photon number detector.

  However, the state of the lambda system and the Stokes photon after Raman scattering also includes a term proportional to the fourth power of the pump light intensity. Therefore, the expression (1) is more accurately expressed by the expression (4).

ここで、|S2>αは２粒子励起準安定状態、|2>aは光路ａにおける２ストークス光子状態を表す。

Here, | S2> α represents a two-particle excited metastable state, and | 2> a represents a two-Stokes photon state in the optical path a.

  Adjusting the pump light intensity without reflecting the term proportional to the fourth power of the pump light intensity to adjust the Raman scattering probability amplitude f, combined with the variation in the characteristics of the photon number detector, increases the fidelity of the entanglement. It can be a factor that cannot be improved sufficiently.

Therefore, the embodiment provides a configuration and method for maximizing the fidelity of the generated quantum entanglement regardless of the characteristics of the photon number detector.
<Device configuration>
FIG. 2 is a schematic diagram of the quantum entanglement generating device 1 of the embodiment. The quantum entanglement generating apparatus 1 includes pump light sources 11 and 12, intensity modulators 13 and 14 that adjust the intensity of pump light, material systems 15 and 16 that generate photons, a beam splitter 25, and a photon number detector 27. 28, single photon sources 31, 32, an optical system 50 for switching the optical path to the photon number detectors 27, 28, and the photon number detectors 27, 28 based on the outputs of the photon number detectors 27, 28. A control unit 30 that performs characteristic analysis and control of the pump light source is included. Single photon sources 31 and 32 are used to measure the performance of the photon number detectors 27 and 28.

  The pump light sources 11 and 12 are laser light sources that emit pump pulses, for example. The intensity of the pump pulse is adjusted by the intensity modulators 13 and 14 based on a control signal from the control unit 30. Specifically, the intensity of the pump pulse is adjusted so that the fidelity of the generated quantum entanglement is maximized. By irradiating the corresponding substance systems 15 and 16 with the pump pulses adjusted in intensity, one Stokes photon can be generated with high probability while suppressing the two-photon state. The mechanism of photon generation will be described later.

  The pump light source 11, the intensity modulator 13, and the lambda system 15 form the first Stokes light source unit 2. The pump light source 12, the intensity modulator 14, and the lambda system 16 form the second Stokes light source unit 3.

  The beam splitter 25 causes quantum interference between photons generated in the two material systems 15 and 16. The photon number detectors 27 and 28 measure the result of quantum interference. The beam splitter 25 and the photon number detectors 27 and 28 form a Bell measuring device 5.

  The quantum entanglement generating apparatus 1 also includes optical transmission paths 17 and 18 that guide photons output from the material systems 15 and 16 to the optical system 50, and filters 19 and 21. The filters 19 and 21 block the pump light and allow only photons generated by Raman scattering to pass.

As one of the features of the embodiment, in the optical system 50,
(A) the stage of measuring the performance of the photon number detectors 27, 28;
(B) the stage of acquiring the relationship between the light source intensity and the probability amplitude of Raman scattering (abbreviated as “Raman amplitude” as appropriate) f; and (c) the stage of quantum entanglement generation (this operation)
At each stage, the optical path is switched to achieve the maximum fidelity of quantum entanglement.

  The optical system 50 includes mirrors 51a, 51b, 52a, 52b, 53a, 53b, 55 and 56, and at least some of these mirrors are movable mirrors. The movable mirror of the optical system 50 is operated according to each of the steps (a), (b), and (c) described above to form a predetermined optical path. The movable mirror may be a MEMS (Micro Electro Mechanical Systems) mirror or a piezoelectric drive type mirror.

  The control unit 30 includes a processor 301 and a memory 302. In the step (a) of the performance measurement of the photon number detectors 27 and 28, the control unit 30 performs these based on the output results of the photon number detectors 27 and 28 measured using the single photon sources 31 and 32. Identify the characteristics and performance of the detector.

  As examples of parameters representing the characteristics, the quantum efficiency η and the dark count rate ν of the photon number detectors 27 and 28 are estimated. The dark count rate ν is the probability of being detected even though no photons are incident within a unit time. The quantum efficiency η and the dark count rate ν of the photon number detectors 27 and 28 can be measured by directly irradiating the photon number detectors 27 and 28 with the single photon sources 31 and 32. In order to accurately measure the characteristics of the photon number detectors 27 and 28, it is desirable to use single photon sources 31 and 32 having the highest possible accuracy. For example, a single photon source that irradiates a uniform quantum dot with an ultrashort pulse with a narrow pulse width and generates a single photon with high purity by minimizing the simultaneous generation of a plurality of photons is used. The identified characteristic parameter value is recorded in the memory 302.

  The control unit 30 also calculates the Raman scattering probability amplitude f that maximizes the entanglement fidelity at the specified quantum efficiency η and dark count rate ν. This process will be described later.

  In the step (b) of acquiring the relationship between the light source intensity and the Raman amplitude f, the control unit 30 acquires the output results of the photon number detectors 27 and 28 while changing the intensity of the pump pulse to obtain the intensity of the pump pulse. And the correspondence between Raman amplitudes. This correspondence relationship is recorded in the memory 302. Based on the acquired correspondence, the control unit 30 determines the intensity of the pump light corresponding to the probability amplitude f of Raman scattering that maximizes the fidelity of the entanglement.

  In the step (c) of this operation, the control unit 30 controls the intensity modulators 13 and 14 so as to obtain the determined pump light intensity, observes the result of quantum interference in the beam splitter 25, and entangles the quantum. Is generated. When one photon is detected by either one of the photon number detectors 27 and 28, this photon is output assuming that quantum entanglement is formed. Assuming that the photon number detectors 27 and 28 are the photon number detectors c and d, respectively, the state is “| 0> c, | 1> d” or “| 1> c, | 0> d” in qubits. , One state among the four Bell states represented by Expression (3) is specified.

FIG. 3 is a diagram for explaining the principle of Bell measurement and quantum entanglement generation by the photon number detectors 27 and 28. For example, when only one photon is detected by the photon number detector 27, quantum entanglement | Ψ ⁺ > α, β is formed. When only one photon is detected by the photon number detector 28, the quantum entanglement | ψ ⁻ > α, β is formed. The control unit 30 outputs the formed | Ψ ⁺ > α, β or | Ψ ⁻ > α, β. In the configuration of FIG. 2, since the intensity of the pump pulse is adjusted to an intensity that optimizes the Raman amplitude, the fidelity of the entanglement generated by the control unit 30 is maximized.
<Principle explanation>
FIG. 4 is a schematic diagram of a substance system irradiated by the output pulse of the intensity modulator 13 or 14. The pump pulse whose intensity is adjusted is incident on the corresponding material system 15 or 16. There are no particular limitations on the type of material system, and any material system can be used, such as a system that utilizes the spin state and vibrational level of the nucleus. 2 to 4, a three-level (lambda) system using three energy levels is used. Specifically, as in Patent Document 1, a state transition of a coupled quantum dot by two adjacent quantum dots having different sizes may be used.

  The wavelength of the pump pulse whose intensity is adjusted is set to a non-resonant wavelength smaller than the energy difference between the ground state | G> α and the excited state | E> α of the lambda system.

By the irradiation of the pump pulse, the substance of the lambda system α in the ground state | G> α is excited to the excited state | E> α, and has a certain probability amplitude (Raman amplitude) f ₁ ^α and a wavelength longer than the pump pulse wavelength. One Stokes photon is generated (Raman scattering) and transitions to a one-particle excited metastable state | S1> α. At another probability amplitude (Raman amplitude) f ₂ ^α , _two Stokes photons are generated and a transition is made to the two-particle excited metastable state | S2> α.

  The state of the Stokes photon in the optical path a after Raman scattering is that no Stokes photon is output | 0> a, one Stokes photon is output | 1> a, and two Stokes photons are output | 2>. Includes a. The state (wave function Ψ) of the lambda system α and the Stokes photon after Raman scattering is described by Expression (5).

| Ψ> α, a = | G> α | 0> a + f ₁ ^α | S1> α | 1> a + f ₂ ^α | S2> α | 2> a (5)
The third term on the right side is a term proportional to the fourth power of the intensity of the pump pulse, and is a term resulting from two-photon excitation. If there is a two-Stokes photon state, the determination by the photon number detectors 27 and 28 is likely to contain an error. Therefore, we want to reduce the influence of the third term as much as possible.

However, if the intensity of the pump light is lowered by the intensity modulator 13 (or 14) in order to reduce f ₂ ^α , the probability amplitude f ₁ ^α of one-photon excitation is also reduced, and the quantum efficiency is deteriorated. Since f ₁ ^α and f ₂ ^α are in a trade-off relationship, it is desired to control the pump light intensity to an optimal level for the generation of quantum entanglement.

  The inventor finds that the optimum condition of the pump light intensity depends on the performance of the photon number detectors 27 and 28, and adjusts the pump light intensity based on this knowledge to maximize the fidelity of quantum entanglement. That is, the optimum operating point is determined by evaluating the characteristics or performance of the photon number detectors 27 and 28. On the other hand, the statistics of Stokes photons generated while changing the intensity of the pump pulse are evaluated to obtain the correspondence between the intensity and the probability amplitude of Raman scattering, and the fidelity according to the characteristics of the photon number detectors 27 and 28. Calculate the optimal probability amplitude that maximizes. Fidelity is maximized by generating quantum entanglement by irradiating the material system with a pump pulse whose intensity is adjusted to the calculated probability amplitude.

The graph (a) in FIG. 5 is a diagram in which the relationship between the probability amplitude (Raman amplitude) f and the fidelity F of the entanglement is plotted with the dark count rate ν as a parameter, and the graph (b) is the probability amplitude f and the fidelity. It is the figure which plotted the relationship of F by using quantum efficiency (eta) as a parameter. In graph (a), the quantum efficiency η is fixed at 0.9, and the dark count rate ν is changed. In the graph (b), the dark count rate ν is fixed to 10 ⁻³ and the quantum efficiency η is changed. Both the quantum efficiency η and the dark count rate ν depend on the performance or characteristics of the photon number detectors 27 and 28.

  The probability amplitude f giving the maximum point of the graph, that is, the maximum fidelity F is defined as fmax (η, ν). If the quantum efficiency η and the dark count rate ν are known by measuring the performance of the photon number detectors 27 and 28, the fmax (η, ν) at that value can be obtained.

  Next, calculation of fmax (η, ν) will be described. fmax (η, ν) is calculated by the following equation.

ただし、

However,

ここで、Ｆは忠実度、ｆは１ストークス光子発生の確率振幅、C_n ^mは２項係数である。Ｎは測定回数、ｍは実際に光子が検出された回数を示す。式（Ｂ）の量子効率ηと暗計数率νは、単一光子源３１、３２を用いた光子の検出で特定されるので、その値を代入すればよい。理論式（Ａ）の忠実度Ｆを最大にするｆがｆmax(η,ν)である。

Here, F is fidelity, f is the probability amplitude of 1 Stokes photon generation, is C _n ^m is a binomial coefficient. N represents the number of measurements, and m represents the number of times that a photon was actually detected. Since the quantum efficiency η and the dark count rate ν in the formula (B) are specified by the detection of photons using the single photon sources 31 and 32, the values may be substituted. F that maximizes the fidelity F of the theoretical formula (A) is fmax (η, ν).

  More specifically, the states of lambda systems α and β after Raman scattering are expressed by the following equations.

ここで、|S2>αは２次のディッケ状態であり、式５．２で表される。

Here, | S2> α is a second-order Dicke state, and is represented by Expression 5.2.

２つの光子数検出器ｃとｄの量子効率ηと暗計数率νを

Quantum efficiency η and dark count rate ν of two photon number detectors c and d

とすると、光子数検出器ｃが１個の光子を検出し、光子数検出器ｄが光子を検出しないときのＰＯＶＭ（positive-Operator Valued Measure）要素は、以下のようになる。

Then, the POVM (positive-Operator Valued Measure) element when the photon number detector c detects one photon and the photon number detector d does not detect the photon is as follows.

ここで、

here,

であり、式（Ｂ）の根拠である。簡略化のために、

And is the basis of the formula (B). For simplicity,

とおくと、Bell測定により選択された状態の忠実度Ｆは、

The fidelity F of the state selected by the Bell measurement is

となる。これが式（Ａ）の根拠である。

It becomes. This is the basis of the formula (A).

  As is apparent from the graph (a) in FIG. 5, the fidelity is sensitive to the dark count rate ν. If the quantum efficiency η is 1 (η = 1), the fidelity is

と近似される。この場合、確率振幅（ラマン振幅）ｆを増大させてｆ>>√νとすることで忠実度Ｆは１に近づく。

Is approximated by In this case, the fidelity F approaches 1 by increasing the probability amplitude (Raman amplitude) f so that f >> √ν.

  If the ideal state ν = 0 is obtained, the fidelity F is

と近似される。この場合、ｆ<<１とすることで、忠実度は１に近づく。この数学的検証は図５によって裏付けられている。

Is approximated by In this case, the fidelity approaches 1 by setting f << 1. This mathematical verification is supported by FIG.

In the graph (a) of FIG. 5, assuming that the quantum efficiency η = 0.9, when the dark count rate ν is increased from 10 ⁻³ to 10 ⁻¹ (when the performance of the photon number detector is deteriorated), Raman The probability amplitude f of scattering must be improved from 0.1 to 0.6. On the other hand, when the dark count rate ν is very small, it is desirable to maintain the Raman scattering probability amplitude f near 0.1 in order to maximize the fidelity.

Thus, it is demonstrated that the entanglement generation rate is greatly influenced according to the characteristics of the photon number detector.
<Maximization of entanglement fidelity>
FIG. 6 is a flowchart of maximizing entanglement fidelity. First, the characteristics of the photon number detectors 27 and 28 used for quantum entanglement generation (Bell measurement) are specified (S11). In the embodiment, the quantum efficiency η and the dark count rate ν are measured as the characteristics of the photon number detector, but other parameters such as response speed and jitter may be measured. As described above, the high-performance single photon sources 31 and 32 are used to measure the characteristics of the photon number detectors 27 and 28.

  FIG. 7 shows the state of the optical system 50 of the quantum entanglement generating device 1 when step S11 of FIG. 6 is performed. The Stokes light source units 2 and 3 are in the OFF state, and the mirrors 53a and 53b are removed from the optical path. Photons output from the single photon source 31 are reflected by the mirror 51 a and directly enter the photon number detector 27. Photons output from the single photon source 32 are reflected by the mirror 51 b and directly enter the photon number detector 28. Since the Stokes light source units 2 and 3 are OFF, the other mirrors 52a, 52b, 55, 56 do not affect the measurement of single photons. The beam splitter 25 does not affect the measurement.

  The control unit 30 specifies the quantum efficiency η and the dark count rate ν based on the count values in the photon number detectors 27 and 28, and records the specification result in the memory 302.

  Returning to FIG. 6, the relationship between the intensity of the pump light sources 11 and 12 and the probability amplitude f of Raman scattering is measured in advance (S12). In this case, the single photon sources 31 and 32 are turned OFF, and the output light from the pump light sources 11 and 12 is directly detected by the photon number detectors 27 and 28 without passing through the beam splitter 25.

  FIG. 8 shows the state of the optical system 50 of the quantum entanglement generating device 1 when step S12 of FIG. 6 is performed. The outputs of the single photon sources 31 and 32 are turned off, and the mirrors 53a and 53b that have been removed from the optical path are returned to their initial positions.

  The number of generated Stokes photons is counted by the photon number detectors 27 and 28 while controlling the intensity modulators 13 and 14 and gradually changing the intensity of the pump pulses output from the pump light sources 11 and 12. Based on the count value, the control unit 30 calculates the probability amplitude f at which one Stokes photon and / or two Stokes photons are output to the optical path by Raman scattering, and associates them with the intensity level of the pump light source. Record in 302. Note that the order of step S11 and step S12 may be reversed.

  Returning to FIG. 6, using the theoretical formulas (A) and (B), the Raman scattering probability amplitude fmax that maximizes the fidelity F with the specified quantum efficiency η and dark count rate ν is calculated (S13). . In this process, the processor 301 of the control unit 30 reads the theoretical expressions (A) and (B), the measured quantum efficiency η, and the dark count rate ν from the memory 302, and calculates fmax.

  Next, the control unit 30 refers to the relationship between the pump light intensity recorded in the memory and the probability amplitude of Raman scattering, specifies the intensity of the pump light from which the maximum probability amplitude fmax is obtained, and the intensity modulator 13, 14 is controlled to adjust the intensity of the pump light incident on the lambda systems 15 and 16 (S14).

  Finally, the lambda systems 15 and 16 are irradiated with the pump light whose intensity is optimized, and Bell measurement is performed to generate quantum entanglement (S15).

  FIG. 9 shows the state of the optical system 50 of the quantum entanglement generating device 1 at S15 (this operation) in FIG. The mirrors 51a, 51b, 52a, 52b are removed from the optical path. The single photon sources 31 and 32 are turned off, and the mirrors 53a and 53b may be inserted because they do not affect the optical path, but may be excluded. Thereby, the lambda systems 15 and 16 are irradiated with the pump light intensity optimized according to the characteristics of the photon number detectors 27 and 28 to be used, and the fidelity of the entanglement is maximized.

  FIG. 10 is a diagram illustrating an arrangement state of the optical elements of the optical system 50 according to the operation stage. In FIG. 10A, only the mirrors 51a and 51b are inserted and excluded after the characteristic measurement of the photon number detectors 27 and 28 (S11). Since the beam splitter 25 does not affect the optical path of the single photon sources 31, 32, it may be inserted as shown in FIG. Alternatively, as shown in FIG. 7, the mirrors 52a, 52b, 53a, 53b may be inserted by turning off the Stokes light source units 2 and 3.

  In FIG. 10A, when measuring the relationship between the pump light intensity and the Raman amplitude (S12), the mirrors 51a, 51b, 52a, 52b, 53a, 53b may be excluded. In this case, the incident surfaces of the photon number detectors 27 and 28 are directed to the outgoing optical paths of the optical transmission paths 17 and 18. Alternatively, as shown in FIG. 10B, all optical elements may be inserted. This is consistent with the configuration of FIG.

  In this operation, only the beam splitter 25 is inserted and the mirrors 51a, 51b, 52a, 52b, 53a, 53b are excluded in FIGS. 10 (a) and 10 (b). Of course, the mirrors 53a and 53b may be inserted by turning off the single photon sources 31 and 32 as shown in FIG.

  FIG. 11 is a diagram illustrating the rotation of the mirror. As shown in FIG. 11A, the mirror M may be raised in the Z direction when in use, and tilted in the XY plane when not in use. Alternatively, as shown in FIG. 11B, it may be configured to rotate in the XY plane and inserted into the optical path when used.

  By using the quantum entanglement generating device 1 having such a configuration, the fidelity of quantum entanglement is set to the maximum theoretically possible value according to the characteristics of the photon number detectors 27 and 28 actually used. be able to. As a result, the quantum information processing using the obtained quantum entanglement and the processing speed, processing efficiency, and security safety of the quantum cryptography communication device can be optimized. As a result, device resources are made more efficient, leading to reductions in initial costs and running costs.

  The characteristics of the photon number detector gradually deteriorate or change during use. By performing the procedure for optimizing the operating point at any time in the quantum entanglement generating apparatus 1, the performance of the apparatus can always be maintained optimally.

  In the embodiment, the Stokes Raman scattering generates a Stokes photon having a wavelength longer than the wavelength of the irradiation light. However, the anti-Stokes Raman scattering generates an anti-Stokes photon having a wavelength shorter than the wavelength of the irradiation light. A light source unit may be used. In this case, the initial state of the material system is a metastable state, and transitions to the ground state by the emission of anti-Stokes photons. Also with this configuration, since irradiation is performed with a pump pulse having an intensity optimized in accordance with the performance of the photon number detector, one anti-Stokes photon is output with a probability amplitude fmax that maximizes fidelity.

  In the embodiment, the Stokes photon statistics are evaluated using both the 1-Stokes photon generation and the 2-Stokes photon generation based on the expression (5). However, the probability amplitude of the 1-Stokes photon generation and the probability of the 2-Stokes photon generation are described. The correspondence with the pump light intensity may be acquired using either one of the amplitudes. Also in this case, it is possible to determine the pump light intensity that gives fmax that maximizes the fidelity from the correspondence.

  Further, fmax may be obtained using the quantum efficiency η and the dark count rate ν of any one of the photon number detectors 27 and 28, or the average of the characteristic values of the two detectors may be used. Alternatively, the characteristic values of either one or both of the photon number detectors 27 and 28 may be obtained using one single photon source 31 or 32. Also in this case, fmax that maximizes the fidelity can be calculated using the specified characteristic value.

ただし、

を用いて、前記最適な確率振幅を算出することを特徴とする付記６に記載の量子もつれ合い生成装置。
（付記８）
前記第１光源ユニットは、第１ポンプ光源と、前記第１ポンプ光源の強度を調整する第１強度変調器と、強度変調された前記ポンプ光の入射により光子を出力する３準位の第１物質系を有し、
前記第２光源ユニットは、第２ポンプ光源と、前記第２ポンプ光源の強度を調整する第２強度変調器と、強度変調された前記ポンプ光の入射により光子を出力する３準位の第２物質系を有し、
前記制御部は、前記第１強度変調器と前記第２強度変調器を制御して、前記ポンプ光の強度を調整することを特徴とする付記６に記載の量子もつれ合い生成装置。
（付記９）
ラマン散乱で生成される光子の量子干渉を測定することにより量子もつれ合いを生成する量子もつれ合い生成装置において、単一光子源を用いて前記光子を検出する検出器の特性値を特定し、
前記量子もつれ合い生成装置のポンプ光強度とラマン散乱の確率振幅との対応関係をあらかじめ取得し、
前記特性値を用いて、量子もつれ合いの忠実度を最大にする最適な確率振幅を算出し、
前記対応関係に基づいて、前記最適な確率振幅を与えるポンプ光強度に調整し、
前記調整されたポンプ光強度で前記量子もつれ合いを生成する、
ことを特徴とする量子もつれ合い忠実度向上方法。
（付記１０）
前記検出器の特性の特定と、前記対応関係の取得と、前記量子もつれ合いの生成で、前記検出器に至る光路を切り替えることを特徴とする付記９に記載の量子もつれ合い忠実度向上方法。 The following notes are presented for the above explanation.
(Appendix 1)
A first light source unit and a second light source unit for outputting photons by Raman scattering by irradiation of pump light;
A first detector and a second detector for detecting photons output from the first light source unit and the second light source unit through a beam splitter;
A single photon source for measuring characteristics of at least one of the first detector and the second detector;
A first optical path from the single photon source to the first detector and / or the second detector, the first light source unit and the second light source unit to the first detector and the second light source through the beam splitter; An optical system that switches between a second optical path leading to two detectors;
The characteristic value of at least one of the first detector and the second detector is specified in the first optical path, and the first detector and the intensity of pump light according to the characteristic value are determined in the second optical path. A controller that generates quantum entanglement based on the output of the second detector;
A quantum entanglement generating device characterized by comprising:
(Appendix 2)
The control unit includes the first detector and the second detection based on an output of at least one of the first detector and the second detector irradiated by the single photon source in the first optical path. The quantum entanglement generating device according to appendix 1, wherein the quantum efficiency (η) and dark count rate (ν) of the device are specified.
(Appendix 3)
The optical system is switchable to a third optical path from the first light source unit and the second light source unit to the first detector and the second detector without passing through the beam splitter,
The supplementary note 1 or 2, wherein the control unit obtains a correspondence relationship between the intensity of pump light of the first light source unit and the second light source unit and the probability amplitude of Raman scattering in the third optical path. The quantum entanglement generator described.
(Appendix 4)
The control unit acquires the correspondence relationship by evaluating statistics of photons detected by the first detector and the second detector while changing the intensity of the pump light. 3. The quantum entanglement generating device according to 3.
(Appendix 5)
The quantum entanglement generating device according to appendix 4, wherein the control unit acquires a correspondence relationship between at least one of the probability amplitude of one-photon generation and the probability amplitude of two-photon generation and the intensity of the pump light.
(Appendix 6)
The control unit calculates an optimal probability amplitude that maximizes the fidelity of the generated quantum entanglement using the characteristic value, specifies an optimal intensity that gives the optimal probability amplitude from the correspondence relationship, and 4. The quantum entanglement generating device according to appendix 3, wherein the intensity of pump light is adjusted to be the optimum intensity.
(Appendix 7)
The control unit is a theoretical formula representing fidelity of the entanglement.

However,

The quantum entanglement generating device according to appendix 6, wherein the optimal probability amplitude is calculated using
(Appendix 8)
The first light source unit includes a first pump light source, a first intensity modulator that adjusts the intensity of the first pump light source, and a three-level first that outputs photons upon incidence of the intensity-modulated pump light. Have a material system,
The second light source unit includes a second pump light source, a second intensity modulator that adjusts the intensity of the second pump light source, and a second three-level output that outputs a photon upon incidence of the intensity-modulated pump light. Have a material system,
The quantum entanglement generating apparatus according to appendix 6, wherein the control unit controls the first intensity modulator and the second intensity modulator to adjust the intensity of the pump light.
(Appendix 9)
In a quantum entanglement generator that generates quantum entanglement by measuring quantum interference of photons generated by Raman scattering, a characteristic value of a detector that detects the photon using a single photon source is specified,
Obtaining in advance the correspondence between the pump light intensity of the quantum entanglement generator and the probability amplitude of Raman scattering,
Using the characteristic value, the optimal probability amplitude that maximizes the fidelity of the entanglement is calculated,
Based on the correspondence, adjust the pump light intensity to give the optimal probability amplitude,
Generating the quantum entanglement with the adjusted pump light intensity;
Quantum entanglement fidelity improvement method characterized by this.
(Appendix 10)
The quantum entanglement fidelity improvement method according to appendix 9, wherein the optical path to the detector is switched by specifying the characteristics of the detector, acquiring the correspondence, and generating the quantum entanglement.

1 Quantum entanglement generator 2, 3 Stokes light source unit 5 Bell measuring device (quantum interference measuring device)
11, 12 Pump light source 13, 14 Intensity modulator 15, 16 Material system (3 level system)
17, 18 Optical transmission line 19, 21 Filter 25 Beam splitter 27, 28 Photon number detector (detector)
30 Control unit 31, 32 Single photon source 50 Optical system 51a, 51b, 52a, 52b, 53a, 53b, 55, 56 Mirror 301 Processor 302 Memory

Claims (6)

A first light source unit and a second light source unit for outputting photons by Raman scattering by irradiation of pump light;
A first detector and a second detector for detecting photons output from the first light source unit and the second light source unit through a beam splitter;
A single photon source for measuring characteristics of at least one of the first detector and the second detector;
A first optical path from the single photon source to the first detector and / or the second detector, the first light source unit and the second light source unit to the first detector and the second light source through the beam splitter; An optical system that switches between a second optical path leading to two detectors;
The characteristic value of at least one of the first detector and the second detector is specified in the first optical path, and the first detector and the intensity of pump light according to the characteristic value are determined in the second optical path. A controller that generates quantum entanglement based on the output of the second detector;
A quantum entanglement generating device characterized by comprising:   The control unit includes the first detector and the second detector based on an output of at least one of the first detector and the second detector irradiated by the single photon source in the first optical path. The quantum entanglement generation device according to claim 1, wherein the quantum efficiency and the dark count rate of the quantum entanglement are specified. The optical system is switchable to a third optical path from the first light source unit and the second light source unit to the first detector and the second detector without passing through the beam splitter,
The said control part acquires the correspondence of the intensity | strength of the pump light of a said 1st light source unit and a said 2nd light source unit, and the probability amplitude of a Raman scattering by a said 3rd optical path, The said 1 or 2 characterized by the above-mentioned. Quantum entanglement generator described in 1.   The control unit calculates an optimal probability amplitude that maximizes the fidelity of the generated quantum entanglement using the characteristic value, specifies an optimal intensity that gives the optimal probability amplitude from the correspondence relationship, and 4. The quantum entanglement generating apparatus according to claim 3, wherein the intensity of pump light is adjusted to be the optimum intensity. In a quantum entanglement generator that generates quantum entanglement by measuring quantum interference of photons generated by Raman scattering, a characteristic value of a detector that detects the photon using a single photon source is specified,
Obtaining in advance the correspondence between the pump light intensity of the quantum entanglement generator and the probability amplitude of Raman scattering,
Using the characteristic value, the optimal probability amplitude that maximizes the fidelity of the entanglement is calculated,
Based on the correspondence, adjust the pump light intensity to give the optimal probability amplitude,
Generating the quantum entanglement with the adjusted pump light intensity;
Quantum entanglement fidelity improvement method characterized by this. 6. The method of improving quantum entanglement fidelity according to claim 5, wherein the optical path to the detector is switched by specifying the characteristics of the detector, acquiring the correspondence relationship, and generating the quantum entanglement.

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