Quantum entangled photon pair generation device

Abstract

To provide a quantum entangled photon pair generation device that facilitates use of various quantum memories and an excitation laser.SOLUTION: A quantum entangled photon pair generation device (1) comprises: a first laser beam source (21) that emits a laser beam with a first wavelength (λ1); a second laser beam source (22) that emits a laser beam with a second wavelength (λ2); a wavelength conversion element (23) that generates wavelength conversion light from the laser beams emitted from the first and second laser beam sources through sum frequency generation; a nonlinear optical element (31) that generates a quantum entangled photon pair from the wavelength conversion light; and a resonator (32) that causes the quantum entangled photon pair to make a circuit, and emits the quantum entangled photon pair making a circuit.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a quantum entangled photon pair generating device that generates quantum entangled photon pairs, and more particularly to a quantum entangled photon pair generating device that is used for long-distance quantum communication using a quantum repeater. [Background technology]

[0002] Quantum communication is a communication technology that uses quantum entangled photon pairs. Quantum communication is expected to be applied to encrypted communications, which require a high level of information security, in communication over the Internet or proprietary networks. For example, quantum communication can be used to generate and share cryptographic keys, which are random bit strings, in encrypted communications, and in cloud quantum computing. The encryption technology currently used in communications is secure in the sense that it is virtually impossible to decrypt because it requires a massive amount of calculation to decrypt (this is called computational security). On the other hand, encryption technology using quantum communication can guarantee even more complete information security.

[0003] In quantum communication, the transmission loss and decoherence in the optical fiber connecting the nodes that make up the network cause the quantum entangled photons propagating through the optical fiber to attenuate exponentially, limiting the communication distance. For this reason, quantum repeater technology that relays information between nodes in quantum communication is being developed (for example, Non-Patent Document 1). Non-Patent Document 1 discloses quantum repeater technology that uses quantum memory.

[0004] Patent Document 1 discloses a technology that uses optical second harmonic generation (SHG) to generate SHG light having an optical frequency twice that of an excitation laser, thereby generating quantum entangled photon pairs (signal photon and idler photon).When the technology described in Patent Document 1 is applied to the quantum communication system described in Non-Patent Document 1, the optical frequency of at least the photon (idler photon) propagating through the optical fiber corresponds to the communication wavelength band. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-035892 [Non-patent literature]

[0006] [Non-Patent Document 1] LM. Duan, MD Lukin, JICirac and P. Zoller, "Long-distance quantum communication with atomic ensembles and linear optics", NATURE, Macmillan Magazines Ltd, November 22, 2001, Vol. 414, p. 413-418. Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the technology of Patent Document 1, the sum of the optical frequencies of the signal photon and the idler photon is limited to twice the optical frequency of the pump laser, which results in restrictions on quantum memory, communication wavelength bands, and the wavelength of the pump laser.

[0008] One aspect of the present invention has been made in view of the above-mentioned problems, and aims to provide a quantum entangled photon pair generating device that can easily use a variety of quantum memories and pump lasers. [Means for solving the problem]

[0009] In order to solve the above problem, a quantum entangled photon pair generating device according to one aspect of the present invention includes: a first laser light source that emits a first laser light having a first wavelength; a second laser light source that emits a second laser light having a second wavelength different from the first wavelength; a wavelength conversion element that generates wavelength-converted light having a third wavelength different from both the first and second wavelengths by optical sum frequency generation from the first and second laser lights emitted from the first and second laser light sources, respectively; a nonlinear optical element that generates quantum entangled photon pairs from the wavelength-converted light generated by the wavelength conversion element; and a resonator that circulates the quantum entangled photon pairs generated by the nonlinear optical element and emits the circulated quantum entangled photon pairs. [Effects of the Invention]

[0010] According to one aspect of the present invention, it is possible to provide a quantum entangled photon pair generating device that can easily use a variety of quantum memories and pump lasers. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a block diagram illustrating a quantum communication system according to an embodiment of the present invention. [Figure 2] 1 is a diagram showing a quantum entangled photon pair generating device according to a first embodiment of the present invention. [Figure 3] FIG. 10 is a diagram showing a quantum entangled photon pair generating device according to a second embodiment of the present invention. [Figure 4] FIG. 10 is a diagram showing a quantum entangled photon pair generating device according to a third embodiment of the present invention. [Figure 5] FIG. 10 is a diagram showing a part of a quantum entangled photon pair generating device according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0012] (Quantum Communication Systems) DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Fig. 1 is a diagram showing a quantum communication system S1 according to an embodiment of the present invention. The quantum communication system S1 has nodes A1 to A3.

[0013] Nodes A1 and A2 perform quantum communication via intermediate station B1. Nodes A1 and A3 perform quantum communication via intermediate station B2. As a result, communication between nodes A2 and A3 becomes possible as shown below. Note that intermediate stations B1 and B2 may also be referred to as Bell measurement stations or optical Bell measurement stations.

[0014] Node A1 and intermediate station B1 are connected by optical fiber F11, intermediate station B1 and node A2 are connected by optical fiber F22, node A1 and intermediate station B2 are connected by optical fiber F12, and intermediate station B2 and node A3 are connected by optical fiber F31.

[0015] Node A1 has quantum entanglement light sources A11 and A12, quantum memories A13 and A14, and Bell measure A15. Node A2 has quantum entanglement light sources A21 and A22, quantum memories A23 and A24, and Bell measure A25. Node A3 has quantum entanglement light sources A31 and A32, quantum memories A33 and A34, and Bell measure A35.

[0016] The quantum entangled light source A11, quantum memory A13, intermediate station B1, quantum entangled light source A22, and quantum memory A24 constitute an elementary link E1 as one communication unit. The quantum entangled light source A12, quantum memory A14, intermediate station B2, quantum entangled light source A31, and quantum memory A33 also constitute an elementary link E2 as one communication unit. By connecting multiple elementary links Ei (i: an integer greater than or equal to 1) in series, communication between multiple nodes Ai becomes possible, and the communication distance can be extended.

[0017] At node A1, quantum entanglement light source A11 generates a quantum entangled photon pair consisting of light LT1-1 and light LT1-2. Also, quantum entanglement light source A12 generates a quantum entangled photon pair consisting of light LT1-3 and light LT1-4. Similarly, at node Ai (i: an integer greater than or equal to 2), quantum entanglement light source Ai1 generates a quantum entangled photon pair consisting of light LTi-1 and light LTi-2. Also, quantum entanglement light source Ai2 generates a quantum entangled photon pair consisting of light LTi-3 and light LTi-4.

[0018] The photon states of light LT1-1, light LT1-3, light LT2-1, light LT2-3, light LT3-1, and light LT3-3 (signal photons, hereinafter referred to as "light LT1-1, etc.") are stored in quantum memory A13, quantum memory A14, quantum memory A23, quantum memory A24, quantum memory A33, and quantum memory A34 (hereinafter referred to as "quantum memory A13, etc."), respectively. The quantum memories A13, etc. have a memory material (as an example, a Pr-doped YSO crystal) that stores the photon states of light LT1-1, etc. When the memory material is a Pr-doped YSO crystal, the wavelength (optical frequency) of the absorption wavelength band is 605.977 [nm] (optical frequency: 494.726 [THz]).

[0019] Light LT1-2, light LT1-4, light LT2-2, light LT2-4, light LT3-2, and light LT3-4 (idler photons, hereinafter referred to as "light LT1-2, etc.") are used for communication between nodes Ai. The wavelengths of light LT1-2, etc. are in the communication wavelength band, and are, for example, 1.5 μm.

[0020] Light LT1-2 and light LT2-4 are transmitted to intermediate station B1 via optical fiber F11 and optical fiber F22, respectively, while light LT1-4 and light LT3-2 are transmitted to intermediate station B2 via optical fiber F12 and optical fiber F31, respectively.

[0021] The intermediate station B1 has a Bell measurement device and performs Bell measurements with light LT1-2 and light LT2-4. As a result, the quantum memories A13 and A24 of the elementary link E1 share quantum entanglement, and their states become correlated (elementary link). In other words, the nodes A1 and A2 can securely share common bit information with each other.

[0022] The intermediate station B2 also has a Bell measurement device and performs Bell measurements using light LT1-4 and light LT3-2. As a result, the quantum memories A14 and A33 of the elementary link E2 share quantum entanglement, and their states are correlated (elementary link). In other words, the nodes A1 and A3 can securely share common bit information with each other.

[0023] At node A1, Bell counter A15 reproduces the quantum states stored in quantum memories A13 and A14 and performs Bell measurements. As a result, quantum memories A13 and A14, and in turn quantum memories A24 and A33, share quantum entanglement and their states become correlated. This can be considered as communication between nodes A2 and A3 via node A1.

[0024] Nodes A2 and A3 also form elementary links with their respective neighboring nodes, and perform Bell measurements between the quantum memories in each node, enabling quantum entanglement sharing between distant nodes Ai. This means that the communication distance of the quantum communication system S1 can be extended. This allows, for example, common bit information to be securely shared between them.

[0025] (Photon pair generator) The following describes a quantum entangled photon pair generating device 1 according to the present invention. This quantum entangled photon pair generating device 1 can be used as the quantum entangled light source A11 or the like in the quantum communication system described above.

[0026] (Embodiment 1) An embodiment of the present invention will be described in detail below. Fig. 2 is a diagram showing an example of a quantum entangled photon pair generating device 1 according to embodiment 1 of the present invention. The quantum entangled photon pair generating device 1 includes a wavelength converting unit 2, a resonator unit 3, and an optical input unit 4.

[0027] (wavelength conversion part) The wavelength conversion unit 2 includes a first excitation laser 21, a second excitation laser 22, beam splitters SP1 to SP3, a wavelength conversion element 23, a first frequency stabilizing gas cell 24, and a second frequency stabilizing gas cell 25, and outputs wavelength-converted light LT100.

[0028] The first pump laser 21 functions as a first laser light source and emits a first laser light (light LT11) having a first wavelength λ1. The first wavelength λ1 is, for example, 894.593 nm (optical frequency: 335.116 THz) corresponding to the D1 line of cesium. The first pump laser 21 is, for example, an external cavity type semiconductor laser.

[0029] The second pump laser 22 functions as a second laser light source and emits a second laser light (light LT12) having a second wavelength λ2 different from the first wavelength. The second wavelength λ2 is, for example, 852.347 nm (optical frequency: 351.726 THz) corresponding to the D2 line of cesium. The second pump laser 22 is, for example, an external cavity type semiconductor laser.

[0030] The beam splitter SP1 receives the light LT11, transmits a portion of it and outputs it as light LT11-1, and reflects a portion of it and outputs it as light LT11-2.

[0031] The beam splitter SP2 receives the light LT12, transmits a portion of it and outputs it as light LT12-1, and reflects a portion of it and outputs it as light LT12-2.

[0032] The beam splitter SP3 functions as a combining element, combining the first and second laser lights (light LT11-1, light LT12-1) emitted respectively from the first and second laser light sources (first excitation laser 21, second excitation laser 22) and emitting the combined light.

[0033] A wavelength filter may be used instead of the beam splitter SP3. This wavelength filter preferably transmits one of the light beams having the first wavelength λ1 and the second wavelength λ2 and reflects the other. For example, assume that the wavelength filter transmits the light beam having the first wavelength λ1 and reflects the light beam having the second wavelength λ2. A first laser beam (light LT11-1) is irradiated onto one of the front and back surfaces of this wavelength filter (referred to as the "first surface"), and a second laser beam (light LT12-1) is irradiated onto the surface opposite the first surface (referred to as the "second surface"). In this case, the light beam having the first wavelength λ1 is transmitted and the light beam having the second wavelength λ2 is reflected on the second surface. As a result, the transmitted light beam having the first wavelength λ1 and the reflected light beam having the second wavelength λ2 can be combined on the second surface to generate combined light. Note that when a wavelength filter is used, the splitting loss that would normally occur with a beam splitter is eliminated.

[0034] The wavelength conversion element 23 generates wavelength-converted light LT100 having a third wavelength λ3 different from both the first and second wavelengths λ1 and λ2 by optical sum frequency generation from the first and second laser light (light LT11-1, light LT12-1) emitted from the first and second laser light sources (first pump laser 21, second pump laser 22), respectively. Here, the wavelength conversion element 23 generates and outputs the wavelength-converted light LT100 of the third wavelength λ3 by optical sum frequency generation (SFG) of the combined light LT11-1 and light LT12-1 output from the beam splitter SP3. As an example of a material constituting the wavelength conversion element 23, periodically poled lithium niobate (PPLN) is used.

[0035] When the first and second wavelengths λ1 and λ2 are 894.593 [nm] and 852.347 [nm], respectively, the wavelength of the wavelength-converted light LT100 (SFG light) is 436.480 [nm] (optical frequency: 686.842 [THz]).

[0036] The first frequency stabilization gas cell 24 is filled with a gas G1 having an absorption wavelength band including the first wavelength λ1, and functions as a first stabilization gas cell that stabilizes the frequency of the first laser light emitted from the first laser light source. When the first wavelength λ1 is 894.593 [nm] (optical frequency: 335.116 [THz]), cesium can be used as the gas G1, and the D1 line can be used as the absorption line.

[0037] The second frequency stabilization gas cell 25 is filled with a gas G2 having an absorption wavelength band including the second wavelength λ2, and functions as a second stabilization gas cell that stabilizes the frequency of the first laser light emitted from the second laser light source. When the second wavelength λ2 is 852.347 [nm] (optical frequency: 351.726 [THz]), cesium can be used as the gas G2, and the D2 line can be used as the absorption line.

[0038] The first frequency stabilization gas cell 24 and the second frequency stabilization gas cell 25 can respectively stabilize the wavelength linewidth of the light LT11 output by the first pump laser 21 and the light LT11 output by the second pump laser 22 at or below a predetermined value. The predetermined value is the absorption linewidth of the quantum memory used in the quantum communication system, and is, for example, several MHz in terms of frequency.

[0039] Light LT11-2 is incident on first frequency-stabilized gas cell 24, and a portion of it is absorbed by gas G1. The portion of light LT11-2 that is not absorbed by gas G1 is emitted as light LT11-3 from first frequency-stabilized gas cell 24. At this time, when the wavelength of LT11-2 coincides with the center of an absorption line of gas G1 (for example, the D1 line of cesium), the absorption of light LT11-2 reaches a maximum, and as the wavelength of LT11-2 deviates from the center of the absorption line of gas G1, the absorption of light LT11-2 decreases.

[0040] The first photodetector 26 generates a signal SG1 based on the rate at which the light LT11-2 is absorbed by the gas G1. For example, the voltage of the signal SG1 is zero at the center of the absorption line spectrum of the gas G1, and takes on a positive or negative value depending on the rate at which the light LT11-2 is shifted from the center of the absorption line spectrum.

[0041] The signal SG1 is input to a driver for the first pump laser 21. The driver monitors changes in the ratio of the signal SG1 and adjusts the wavelength of the light LT11 output by the first pump laser 21, for example, so that the voltage value of the signal SG1 becomes zero. If the first pump laser 21 is an external cavity semiconductor laser, the wavelength can be adjusted by finely adjusting the drive current. As a result, the wavelength (first wavelength λ1) of the light LT11 emitted from the first pump laser 21 is stabilized. For example, if the D1 line of a cesium atom is used, it is possible to stabilize the optical frequency to approximately 1 / 1000 of the transition linewidth (approximately 5 MHz). The wavelength of the light LT11-1 transmitted through the first beam splitter SP1 is also stabilized to a similar width.

[0042] Light LT12-2 is incident on second frequency stabilization gas cell 25, and a portion of it is absorbed by gas G2. The portion of light LT12-2 that is not absorbed by gas G2 is emitted as light LT12-3 from second frequency stabilization gas cell 25. At this time, when the wavelength of LT12-2 coincides with the center of an absorption line of gas G2 (for example, the D2 line of cesium), the absorption of light LT12-2 reaches a maximum, and as the wavelength of LT12-2 deviates from the center of the absorption line of gas G2, the absorption of light LT12-2 decreases.

[0043] The second photodetector 27 generates a signal SG2 based on the rate at which the light LT12-2 is absorbed by the gas G2. For example, the voltage of the signal SG2 is zero at the center of the absorption line spectrum of the gas G2, and takes on a positive or negative value depending on the rate at which the light LT12-2 is shifted from the center of the absorption line spectrum.

[0044] As with the first photodetector 26, the wavelength of the light LT12 output by the second pump laser 22 is adjusted so that the voltage value of the signal SG2 becomes zero. As a result, the wavelength (first wavelength λ1) of the light LT12 from the second pump laser 22 is stabilized. For example, when the D2 line of a cesium atom is used, it is possible to stabilize the optical frequency to about 1 / 1000 of the transition linewidth (about 5 MHz). The wavelength of the light LT12-1 transmitted through the second beam splitter SP2 and output is also stabilized to a similar width.

[0045] (Optical input section) The light input unit 4 inputs the wavelength-converted light LT100 generated by the wavelength conversion element 23 into the resonator unit 3. The light input unit 4 shapes the wavelength-converted light LT100 and inputs it as light LT20 into the resonator unit 3. Shaping the wavelength-converted light LT100 into a suitable beam shape enables efficient generation of quantum entangled photon pairs in the resonator unit 3. The light input unit 4 is, for example, a focusing lens 34, and preferably has a focal length that enables the wavelength-converted light LT100 to be focused on the nonlinear optical element 31 via the mirror M1.

[0046] (Resonator section) The resonator section 3 includes a nonlinear optical element 31 , a resonator 32 , and a band-pass filter 33 .

[0047] The nonlinear optical element 31 is, for example, a nonlinear optical crystal, and generates a quantum entangled photon pair EP1 from the wavelength-converted light LT100 (light LT21) generated by the wavelength conversion element 23. More specifically, the light LT100 having a third wavelength λ3 is condensed via the light input unit 4 and a mirror M1 (described later) and input to the nonlinear optical element 31, where the quantum entangled photon pair EP1 is generated. The nonlinear optical element 31 generates the quantum entangled photon pair EP1 through a PDC (Parametric Down Conversion) process using the light LT100 (light LT21) as excitation light. As an example, a periodically poled lithium niobate crystal can be used as the nonlinear optical element 31, as in the case of the wavelength conversion element 23. Note that the nonlinear optical element 31 may be amorphous as long as it has desired nonlinear optical properties (e.g., second-order nonlinear optical properties).

[0048] The wavelength of light LT1-1 is, for example, the absorption band wavelength of the quantum memory. When a Pr-doped YSO crystal is used as the quantum memory, the wavelength of light LT1-1 is 605.977 [nm]. The wavelength of light LT1-2 is in the communications wavelength band (1.5 [μm], for example). When the first and second wavelengths λ1 and λ2 are 894.593 [nm] and 852.347 [nm], respectively, the wavelength of wavelength-converted light LT100 (SFG light) is 436.480 [nm] (optical frequency: 686.842 [THz]), and the wavelength of light LT1-2 is 1560.476 [nm].

[0049] The resonator 32 circulates the quantum entangled photon pairs generated by the nonlinear optical element 31 and emits the circulated quantum entangled photon pairs.

[0050] The resonator 32 includes a mirror M1, a mirror M2, a mirror M3, and a mirror M4. The shapes of the mirrors M1 to M4 can be set appropriately. As an example, the mirrors M1 and M2 may be spherical mirrors, and the mirrors M3 and M4 may be flat mirrors.

[0051] The entangled photon pair EP1 is reflected in turn by mirror M2, mirror M3, mirror M4, and mirror M1 as entangled photon pair EP2, entangled photon pair EP3, entangled photon pair EP4, and entangled photon pair EP5.

[0052] A part of the quantum entangled photon pair EP3 incident on mirror M4 passes through mirror M4 and is output as quantum entangled photon pair EP6. This quantum entangled photon pair EP6 is output from quantum entangled photon pair generating device 1. The quantum entangled photon pair EP5 is incident on nonlinear optical element 31, and a part of it is transmitted and superimposed on the quantum entangled photon pair EP1.

[0053] Here, the resonator 32 is a so-called bowtie resonator. As shown in FIG. 2, the bowtie resonator has optical paths where the paths along which quantum entangled photon pairs circulate intersect. That is, unlike a Fabry-Perot resonator or the like, in the bowtie resonator, the photon pairs EP1 to EP5 circulating within the resonator 32 are incident on the nonlinear optical element 31 from one direction. As a result, it is possible to prevent the quantum entangled photon pairs from returning from the resonator 32 to the first and second pump lasers 21 and 22 and interfering with frequency stabilization. Furthermore, in the bowtie resonator, the light LT21 is focused on the nonlinear optical element 31 by the focusing lens 34 and the spherical mirror M1, facilitating the generation of quantum entangled photon pairs EP1 with high efficiency. However, the resonator 32 may have a resonator structure other than the bowtie structure.

[0054] By setting the reflectivity of the mirrors M1 to M4 to a high reflectivity for quantum entangled photon pairs, the resonator unit 3 can operate as a resonator for desired quantum entangled photon pairs. The reflectivity of each mirror can be determined appropriately depending on the required performance. As an example, the reflectivity of the mirrors M1, M2, and M3 for quantum entangled photon pairs is set to 99.9% and the reflectivity of the mirror M4 used to output the quantum entangled photon pairs to the outside is set to 95%.

[0055] The resonance condition is satisfied for both the signal photon (light LT1-1) and the idler photon (light LT1-2) that make up the quantum entangled photon pair EP1, thereby increasing the intensity of the signal photon and the idler photon in the resonator 32 and narrowing the linewidth. In this case, this is called the double resonance condition. Achieving the double resonance condition is preferable for improving the efficiency of generating quantum entangled photon pairs in the quantum entangled photon pair generating device 1. The double resonance condition can be achieved by appropriately designing and adjusting the circuit length.

[0056] Here, it is possible to increase the transmittance of the mirror M1 at the third wavelength λ3 of the wavelength-converted light LT20, etc. In this case, the wavelength-converted light LT100 (PDC pump light) from the wavelength converter 2 can be efficiently incident on the nonlinear optical element 31. In addition, simple and stable operation of the quantum entangled photon pair generating device 1 is facilitated. That is, if the transmittance of the mirror M1 at the wavelength λ3 is increased, the wavelength-converted light LT22 is significantly attenuated by the mirror M1 when circulating within the resonator 32, and the resonator 32 no longer operates as a resonator at the wavelength λ3. As a result, it is not necessary to control the resonance conditions in consideration of the influence of multiple resonances including the PDC pump light (light LT100), and simple and stable operation is facilitated. As a condition for this, it is preferable that the transmittance of the mirror M1 is in the range of 0.001 to 0.1% (for example, 0.01%) for quantum entangled photon pairs, and in the range of 20 to 99% (for example, 90%) for the third wavelength λ3.

[0057] The entangled photon pair EP6 that has passed through mirror M4 is incident on bandpass filter 33. Bandpass filter 33 transmits light of the wavelength of the desired quantum entangled photon pair and attenuates other light, for example, light of a third wavelength λ3 such as wavelength-converted light LT24. Bandpass filter 33 transmits the incident entangled photon pair EP6 as light LT1-1 and light LT1-2. As an example, light LT1-1 and light LT1-2 are quantum entangled photon pairs generated at node A1 of the quantum communication system, with light LT1-1 stored in quantum memory A13 and light LT1-2 transmitted to repeater B via optical fiber F11.

[0058] The circuit length and average circuit number of the resonator 32 are determined based on the linewidth based on the absorption wavelength band of the quantum memory that absorbs the entangled photon pairs (light LT1-1 and light LT1-2) emitted by the quantum entangled photon pair generating device 1. Here, the narrow linewidth of the absorption wavelength band of the quantum memory that absorbs the quantum entangled photon pairs (light LT1-1 and light LT1-2) is several [MHz] or less.

[0059] In general, the linewidth of light emitted from the resonator 32 is inversely proportional to the lifespan of the light circulating within the resonator 32. Furthermore, the lifespan of the light circulating within the resonator 32 is proportional to the circumferential length of the resonator 32. Therefore, the linewidth of the emitted light is inversely proportional to the circumferential length of the resonator 32. This means that increasing the circumferential length of the resonator 32 is preferable in terms of narrowing the linewidth of the emitted light.

[0060] However, if the circuit length of the resonator 32 is made too long, the amount of light emitted from the resonator 32 decreases due to optical loss caused by diffraction and optical loss caused by passing through the nonlinear optical element 31. For this reason, it is preferable to set the circuit length so as to balance both the narrowness of the line width of the emitted light and the large amount of light.

[0061] The circuit length of the resonator 32 is preferably, for example, 1 cm to 300 cm (for example, 60 cm). The average number of turns of the resonator 32 is preferably, for example, 5 to 200 (for example, 50).

[0062] In a bowtie resonator, by setting the circuit length of the photon pair to, for example, 60 cm, it is possible to emit quantum entangled light with a narrow linewidth based on the absorption wavelength band of the quantum memory. Although a bowtie resonator is large for a resonator, it occupies a smaller area than a ring resonator with the same circuit length.

[0063] As described above, the quantum entangled photon pair generating device 1 according to this embodiment is a quantum entangled light source used for quantum communication, and includes a first laser light source (first pump laser 21), a second laser light source (second pump laser 22), a wavelength conversion element 23, a nonlinear optical element 31, and a resonator 32. The first laser light source emits a first laser light having a first wavelength λ1. The second laser light source emits a second laser light having a second wavelength λ2 different from the first wavelength λ1. The wavelength conversion element 23 generates wavelength-converted light LT20 having a third wavelength λ3 different from both the first and second wavelengths λ1 and λ2 from the first and second laser light sources, respectively. The nonlinear optical element 31 generates quantum entangled photon pairs from the wavelength-converted light LT20 generated by the wavelength conversion element 23. The resonator 32 circulates the quantum entangled photon pairs generated by the wavelength conversion element 23 and emits the circulated quantum entangled photon pairs (light LT1-1, LT1-2).

[0064] The quantum entangled photon pairs are generated by the PDC in the nonlinear optical element 31. The pump light for exciting the PDC is generated by SFG of two pump lasers whose frequencies are stabilized by a frequency-stabilizing gas cell.

[0065] In the SFG and PDC processes, the one-photon and two-photon states respectively satisfy the law of energy conservation, i.e., the following equation (1) is satisfied: f p1 +f p2 =f SFG =f s +f i ... Equation (1) f p1 : Optical frequency of the first pump laser beam f p2 : Optical frequency of the second pump laser beam f SFG : Optical frequency of SFG light f s : The optical frequency of the PDC signal photon (the photon stored in the quantum memory) f i: Optical frequency of idler photons of PDC (photons in the communication wavelength band sent to intermediate stations)

[0066] On the other hand, in the SHG and PDC processes, the following equation (2) is satisfied: 2×f p =f SHG =f s +f i ... Equation (2) f p : Optical frequency of the pump laser light f SHG : Optical frequency of SHG light

[0067] As can be seen by comparing equations (1) and (2), the optical frequency f of the pump light for SHG p are specified, whereas the first and second optical frequencies f p1 , f p2 There are many combinations of these, and the wavelength of the generated quantum entangled light can be more flexibly adapted depending on the absorption wavelength band of the quantum memory to be used.

[0068] In this embodiment, the optical frequencies of the first and second excitation lasers are stabilized by using two frequency stabilizing gas cells in which gases G1 and G2 are sealed, respectively, to obtain an optical frequency f SFG The optical frequency f SFG The pump light is used to generate a narrow linewidth and stabilized optical frequency f s , f i It is possible to generate quantum entangled photon pairs.

[0069] That is, in this embodiment, two gases G1 and G2 can be selected from a large number of gases that can be used and controlled, and whose sum of absorption wavelengths (more precisely, optical frequencies) corresponds to the optical frequency of the desired SFG light.

[0070] optical frequency f s , f iIf one or the other of these is changed, for example, due to the required specifications of the system, a corresponding gas may not always exist in the SHG / PDC system. On the other hand, in the present SFG / PDC system, there are many combinations of optical frequencies of the two pump lasers for SFG, so an available gas combination can be selected from these.

[0071] (Embodiment 2) A second embodiment of the present invention will be described below. For ease of explanation, components having the same functions as those described in the above embodiments will be denoted by the same reference numerals, and their description will not be repeated. FIG. 3 is a diagram showing an example of a quantum entangled photon pair generating device 1 according to a second embodiment of the present invention. The quantum entangled photon pair generating device 1 includes a wavelength conversion unit 2, a resonator unit 3, and an optical input unit 4.

[0072] (wavelength conversion part) The wavelength conversion section 2 in the second embodiment includes a first excitation laser 21, a second excitation laser 22, a wavelength conversion element 23, and a third frequency stabilizing gas cell 41, and outputs converted wavelength light LT100.

[0073] The third frequency stabilization gas cell 41 is filled with a gas G3 having an absorption wavelength band including both the first wavelength λ1 and the second wavelength λ2, and stabilizes the wavelengths λ1 and λ2 of the first laser light emitted from the first laser light source and the second laser light emitted from the second laser light source. Cesium can be used as the gas G3. The D1 line (wavelength 894.593 nm (optical frequency: 335.116 THz)) and D2 line (wavelength 852.347 nm (optical frequency: 351.726 THz)) of cesium atoms can be used to stabilize the frequencies of the first pump laser 21 and the second pump laser 22.

[0074] Light LT11-2 and light LT12-2 are both incident on the third frequency stabilization gas cell 41. Part of the light LT11-2 is absorbed by gas G3 (the D1 line of cesium atoms), and the part that is not absorbed by gas G3 exits the third frequency stabilization gas cell 41 as light LT11-3. Part of the light LT12-2 is absorbed by gas G3 (the D2 line of cesium atoms), and the part that is not absorbed by gas G3 exits the third frequency stabilization gas cell 41 as light LT12-3.

[0075] At this time, it is preferable that the beams of light LT11-2 and light LT11-3 and the beams of light LT12-2 and light LT12-3 are spatially separated within the third frequency stabilizing gas cell 41. For example, it is preferable that the distance D0 between the first and second laser beams (lights LT11-2 and LT12-2) passing through the third frequency stabilizing gas cell 41 is 2.0 times or more the beam radii r1 and r2 of the first and second laser beams (lights LT11-2 and LT12-2). This means, for example, that "D0>2×r1" or "D0>2×r2" holds. Alternatively, it may be "D0>r1+r2".

[0076] In this case, the optical window of the third frequency-stabilized gas cell 41 may be made relatively large relative to the beam diameter. This optical window is for inputting and outputting light LT11-2, light LT12-2, light LT11-3, and light LT12-3. The diameter of this optical window is, for example, 10 mm, and the beam diameter of light LT11-2 and light LT12-2 is, for example, 3 mm. The light-receiving aperture (diameter) of photodetectors 26 and 27 is, for example, about 3 mm. In this way, signals SG1 and SG2 can be acquired without light LT11-2 and light LT12-2 interfering with each other.

[0077] Here, in the wavelength conversion unit 2, a third frequency stabilization gas cell 41 filled with gas G3 having an absorption wavelength band that includes both the first wavelength λ1 and the second wavelength λ2 is used to stabilize the frequencies λ1 and λ2 of both the first excitation laser 21 and the second excitation laser 22. The roles and configurations of the resonator unit 3 and the optical input unit 4 are the same as those in the first embodiment, so detailed explanations thereof will be omitted.

[0078] In the second embodiment, similarly to the first embodiment, the optical frequency of the first pump laser 21 can be stabilized by controlling the first pump laser 21 based on the output of the light LT11-3. Furthermore, the second pump laser 22 can be stabilized by controlling the second pump laser 22 based on the output of the light LT12-3. As a result, the wavelength (optical frequency) of the wavelength-converted light LT100 is stabilized, and further, the quantum entangled photon pair light LT1-1 and light LT1-2 have a narrow linewidth.

[0079] In the second embodiment, in addition to the effects of the first embodiment, the size, cost, and power consumption of the device configuration can be reduced. The optical frequencies of the first pump laser 21 and the second pump laser 22 can be stabilized by the third frequency stabilization gas cell 41.

[0080] (Embodiment 3) A third embodiment of the present invention will be described below. For ease of explanation, components having the same functions as those described in the above embodiments will be denoted by the same reference numerals, and their description will not be repeated. FIG. 4 is a diagram showing an example of a quantum entangled photon pair generating device 1 according to a third embodiment of the present invention. The quantum entangled photon pair generating device 1 includes a wavelength conversion unit 2, a resonator unit 3, an optical input unit 4, and a wavelength filter 42.

[0081] Light LT12-2 is multiplexed with light LT11-2 by the first beam splitter SP1, output from the same output port, and then incident on the third frequency stabilization gas cell 41. Light LT11-3 and light LT12-3 emitted from the third frequency stabilization gas cell 41 are emitted as light LT40 that is not spatially separated within the third frequency stabilization gas cell 41.

[0082] The wavelength filter 42 passes the first and second laser beams (light LT11-3, LT12-3) emitted from the third frequency-stabilized gas cell 41. The wavelength filter 42 has a first region R1 that passes light of the first wavelength λ1 and blocks light of the second wavelength λ2, and a second region R2 that passes light of the second wavelength λ2 and blocks light of the first wavelength λ1. Spatially separated light LT11-3 of the first wavelength λ1 and light LT12-3 of the second wavelength λ2 are emitted from the first region R1 and the second region R2, respectively. That is, spatially separated light LT11-3 and light LT12-3 can be formed using the wavelength filter 42 having the first region R1 and the second region R2. Separation of light by such a wavelength filter is easy when the wavelength difference between the first wavelength λ1 and the second wavelength λ2 is large.

[0083] Here, a third region R3 and a fourth region R4 are arranged in layers between the first and second regions R1 and R2. The third region R3 is arranged on the incident side of the light LT40, and the fourth region R4 is arranged on the opposite side. The third region R3 passes both the first and second wavelengths λ1 and λ2, increasing the intensity of the light LT11-3 and the light LT12-3. The fourth region R4 is an area that does not transmit either the first or second wavelengths λ1 or λ2. By arranging the fourth region R4, which does not transmit either the light of the first wavelength λ1 or the light of the second wavelength λ2, between the first region R1, which passes only the light of the first wavelength λ1, and the second region R2, which passes only the light of the second wavelength λ2, the spatial separation of the light LT11-3 and the light LT12-3 is further ensured.

[0084] Light LT40 is incident on the third region R3. The light of the first wavelength λ1 in the light LT40 passes through the first region R1 and is emitted from the wavelength filter 42 as light LT11-3. The light of the second wavelength λ2 in the light LT40 passes through the second region R2 and is emitted from the wavelength filter 42 as light LT12-3. At this time, the fourth region R4 does not pass any of the light of the first or second wavelength λ1, λ2, so no light exists between the light LT11-3 and the light LT12-3, and the light LT11-3 and the light LT12-3 are effectively separated.

[0085] In the above, it is assumed that light LT40 is extracted as a single light beam from third frequency stabilised gas cell 41. Light LT11-3 and light LT12-3 may be generated by extracting two light beams from third frequency stabilised gas cell 41 and making them incident on a first filter that passes light of a first wavelength λ1 and a second filter that passes light of a second wavelength λ2, respectively.

[0086] In the above, the wavelength filter 42 transmits both the light of the first wavelength and the light of the second wavelength in the light LT40. Alternatively, the wavelength filter 42 may transmit either the light of the first wavelength or the light of the second wavelength and reflect the other. For example, suppose the wavelength filter 42 transmits the light of the first wavelength λ1 and reflects the light of the second wavelength λ2. In this case, the wavelength filter 42 receives the light LT40, transmits the light LT11-3, and reflects the light LT12-3. By arranging the first photodetector 26 and the second photodetector 27 so that the light LT11-3 (transmitted light) and the light LT12-3 (reflected light) are incident on them, the light LT40 can be separated into the light LT11-3 and the light LT12-3 and can be incident on the first photodetector 26 and the second photodetector 27, respectively. In this case, the filter 42 does not need to have multiple regions R1 to R4.

[0087] The light LT11-3 and light LT12-3 spatially separated by the wavelength filter 42 are received by the first photodetector 26 and the second photodetector 27, respectively, and the signals SG1 and SG2 can be obtained without interfering with each other.

[0088] In the third embodiment, in addition to the effects of the first embodiment, the size, cost, and power consumption of the device configuration can be reduced. The optical frequencies of the first pump laser 21 and the second pump laser 22 can be stabilized by the third frequency stabilization gas cell 41.

[0089] (Variation) Modulation transfer spectroscopy (MTS) may be applied as a frequency stabilization technique for the first pump laser 21 and the second pump laser 22 in the first to third embodiments. Fig. 5 is a diagram showing a part of a quantum entangled photon pair generating device according to a modified example. Here, an example in which MTS is applied to the first pump laser 21 is shown. As in the first to third embodiments, the oscillation frequency of the first pump laser 21 is stabilized by a signal SG1.

[0090] As in the first to third embodiments, the light LT11 emitted from the first excitation laser 21 is split into light LT11-1 and light LT11-2 by the beam splitter SP1.

[0091] Here, the signal SG1 is generated using a beam splitter BS, a high-frequency oscillator RF, an electro-optic modulator EOM, a beam splitter BS1, a photodetector PD, and a double-balanced mixer DBM in addition to the first stabilizing gas cell 24. The first stabilizing gas cell 24, the beam splitter BS, the electro-optic modulator EOM, the multiplexer GT, and the photodetector PD constitute the MTS.

[0092] Beam splitter BS splits light LT11-2 into light LT11-4 and light LT11-5. Light LT11-4 and light LT11-5 are signal light and reference light, respectively. Light LT11-5 is phase-modulated by electro-optical modulator EOM, which is controlled by modulation signal SG3-1 from high-frequency oscillator RF, and emitted as light LT11-6. A portion of light LT11-6 is emitted as light LT11-7 by beam splitter BS1. Light LT11-7 is incident on first stabilizing gas cell 24 so as to face light LT11-4. As a result, light LT11-4 is modulated by light LT11-7 in first stabilizing gas cell 24 through a nonlinear optical process such as four-wave mixing, and is emitted as light LT11-8. A part of the modulated light LT11-8 is emitted as light LT11-9 via a beam splitter BS1. A light receiving element PD converts the light LT11-9 into an electrical signal SG4.

[0093] The double-balanced mixer DBM generates a signal SG1 based on an electrical signal SG4 from the photodetector PD and a signal SG3-2 from the high-frequency oscillator RF. By appropriately selecting the relative phase of SG3-1 and SG3-2, the signal SG1 outputs a positive voltage when the frequency is lower than the resonant frequency of the atoms in the stabilizing gas cell, and a negative voltage when the frequency is higher than the resonant frequency. In other words, the sign of the signal is reversed at the resonant frequency. This signal can be used as an error signal to stabilize the frequency.

[0094] (summary) The above first to third embodiments can be summarized as follows.

[0095] (1) A quantum entangled photon pair generating device (1) according to a first aspect includes: a first laser light source (first excitation laser 21) that emits a first laser light (light LT11-1) of a first wavelength (λ1); a second laser light source that emits a second laser light (light LT12-1) of a second wavelength (λ2) different from the first wavelength; a wavelength conversion element (23) that generates wavelength-converted light (LT20) having a third wavelength (λ3) different from both the first and second wavelengths by optical sum frequency generation from the first and second laser light sources, respectively; a nonlinear optical element (31) that generates quantum entangled photon pairs (light LT1-, LT-2) from the wavelength-converted light generated by the wavelength conversion element; and a resonator (32) that circulates the quantum entangled photon pairs generated by the nonlinear optical element and emits the circulated quantum entangled photon pairs. By generating wavelength-converted light (LT20) having a third wavelength (λ3) from the first and second laser beams and further generating quantum entangled photon pairs, it becomes easy to generate quantum entangled photon pairs of various optical frequencies, which makes it easy to apply various quantum memories, etc.

[0096] (2) A quantum entangled photon pair generating device according to a second aspect is the first aspect, and includes a first stabilizing gas cell (first frequency stabilizing gas cell 24) filled with a gas (G1) having an absorption wavelength band including the first wavelength and stabilizing the frequency of the first laser light emitted from the first laser light source, and a second stabilizing gas cell (second frequency stabilizing gas cell 25) filled with a gas (G2) having an absorption wavelength band including the second wavelength and stabilizing the wavelength of the second laser light emitted from the second laser light source. The first wavelength (λ1), the second wavelength (λ2), and ultimately the third wavelength of the wavelength-converted light (LT20) are stabilized, facilitating stable quantum communication.

[0097] (3) A quantum entangled photon pair generating device according to a third aspect is the first aspect, and further includes a third frequency stabilizing gas cell (third frequency stabilizing gas cell 41) that is filled with a gas (G3) having an absorption wavelength band that includes both the first wavelength and the second wavelength, and that stabilizes the wavelengths of both the first laser light emitted from the first laser light source and the second laser light emitted from the second laser light source. The third frequency stabilizing gas cell stabilizes the first and second frequencies, making it easy to miniaturize the quantum entangled photon pair generating device.

[0098] (4) A quantum entangled photon pair generating device according to a fourth aspect is the third aspect, wherein the interval (D0) between the first and second laser beams passing through the third frequency-stabilized gas cell is 2.0 times or more the beam radii (r1, r2) of the first and second laser beams. The first and second laser beams can be spatially separated within the third frequency-stabilized gas cell.

[0099] (5) A quantum entangled photon pair generating device according to a fifth aspect is the third aspect, further comprising an optical filter (wavelength filter 42) that passes the first and second laser beams emitted from the third frequency-stabilized gas cell, the optical filter having a first region (R1) that passes the light of the first wavelength and blocks the light of the second wavelength, and a second region (R2) that passes the light of the second wavelength and blocks the light of the first wavelength. The optical filter having the first region (R1) and the second region (R2) can spatially separate the first and second laser beams emitted from the third frequency-stabilized gas cell.

[0100] (6) A quantum entangled photon pair generating device according to a sixth aspect is any of the first to fifth aspects, further comprising a beam splitter SP3 that combines first and second laser beams emitted from the first and second laser light sources, respectively, to emit combined light, and the wavelength conversion element generates the wavelength-converted light from the combined light emitted from the beam splitter SP3. By combining the first and second laser beams, it becomes easier to reliably generate the wavelength-converted light in the wavelength conversion element.

[0101] (7) A quantum entangled photon pair generating device according to a seventh aspect is the quantum entangled photon pair generating device of the sixth aspect, wherein the nonlinear optical element is disposed within the resonator, and the quantum entangled photon pair generating device includes an optical input unit (4) that inputs the multiplexed light output from the multiplexing element to the nonlinear optical element via the resonator, and the nonlinear optical element generates the quantum entangled photon pairs from the wavelength-converted light input by the optical input unit. The optical input unit ensures that the multiplexed light output from the multiplexing element is input to the nonlinear optical element disposed within the resonator.

[0102] (8) A quantum entangled photon pair generating device according to an eighth aspect is any of the second, sixth, and seventh aspects, wherein the gas sealed in the first and second frequency-stabilized gas cells is cesium, and the first and second wavelengths are 894.593 [nm] (optical frequency: 335.116 terahertz) and 852.347 [nm] (optical frequency: 351.726 terahertz), respectively.

[0103] (9) A quantum entangled photon pair generating device according to a ninth aspect is any of the third to seventh aspects, wherein the gas sealed in the third frequency-stabilized gas cell is cesium, and the first and second wavelengths are 894.593 [nm] (optical frequency: 335.116 terahertz) and 852.347 [nm] (optical frequency: 351.726 terahertz), respectively.

[0104] (10) A quantum entangled photon pair generating device according to a tenth aspect is any of the first to ninth aspects, wherein the wavelength of one photon of the quantum entangled photon pair is 605.977 [nm] (optical frequency: 494.726 terahertz) and the wavelength of the other photon of the quantum entangled photon pair is 1560.476 [nm] (optical frequency: 192.116 terahertz).

[0105] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0106] For example, the gas enclosed in the frequency-stabilized gas cell can be obtained for combinations of atomic beams of gases other than cesium.

[0107] Furthermore, the quantum memory may use materials other than Pr-doped YSO crystals.

[0108] The wavelength band of the light LT1-2 may be other than the 1.5 μm band, for example, the 1.3 μm band, in which the group velocity dispersion of communication optical fibers is generally minimized.

[0109] The wavelength conversion element 23 and the nonlinear optical element 31 may be made of a nonlinear optical crystal other than PPLN, or a medium having optical nonlinearity different from that of PPLN.

[0110] The resonator configuration of the resonator section 3 may be a resonator structure other than the bowtie type.

[0111] The application of the quantum entangled photon pair generating device 1 is not limited to quantum communication systems based on quantum repeaters. The quantum entangled photon pair generating device 1 may also be used in quantum communication that does not require quantum repeaters. The quantum entangled photon pair generating device 1 may also be used in communication using optical fibers to convert light with a wavelength in the communication wavelength band into an optical frequency with a narrow linewidth of several MHz or less. [Explanation of symbols]

[0112] S1 Quantum Communication System 1. Quantum entangled photon pair generator 2 Wavelength conversion section 3 Resonator section 4 Optical input section A13, A14, A23, A24, A33, A34 quantum memory 21, 22 Pump laser 23 Wavelength conversion element 24, 25, 41 Frequency-stabilized gas cell 26, 27 Photodetector 31 Nonlinear optical elements 32 resonator 33 Bandpass Filter 34 Condenser Lens 42 Wavelength Filter A1~A3 nodes A11, A12, A21, A22, A31, A32 Quantum Entanglement Light Source A15, A25, A35 Bell Measuring Instruments B1, B2 intermediate stations E1, E2, Ei Elementary Link EP1~EP6 photon pairs F11, F12, F21, F22, F31, F32 optical fiber G1, G2, G3 Gas LT1-1, LT1-2 photon pairs LT100, LT20, LT21, LT22, LT23, LT24, LT25 Wavelength conversion light M1~M4 mirrors SP1, SP2, SP3 Beam Splitters

Claims

1. a first laser light source that emits a first laser beam having a first wavelength; a second laser light source that emits a second laser light having a second wavelength different from the first wavelength; a wavelength conversion element that generates wavelength-converted light having a third wavelength different from both the first and second wavelengths by optical sum frequency generation from the first and second laser beams emitted from the first and second laser beam sources, respectively; a nonlinear optical element that generates quantum entangled photon pairs from the wavelength-converted light generated by the wavelength conversion element; a resonator that circulates the quantum entangled photon pairs generated by the nonlinear optical element and emits the circulated quantum entangled photon pairs; A quantum entangled photon pair generating device comprising:

2. a first stabilizing gas cell in which a gas having an absorption wavelength band including the first wavelength is sealed, and which stabilizes the frequency of the first laser light emitted from the first laser light source; a second stabilizing gas cell in which a gas having an absorption wavelength band including the second wavelength is sealed, and which stabilizes the wavelength of the second laser light emitted from the second laser light source; The quantum entangled photon pair generating device according to claim 1 , comprising:

3. 2. The quantum entangled photon pair generating device of claim 1, further comprising: a third frequency stabilizing gas cell in which a gas having an absorption wavelength band including both the first wavelength and the second wavelength is sealed, and which stabilizes the wavelengths of both the first laser light emitted from the first laser light source and the second laser light emitted from the second laser light source.

4. 4. The quantum entangled photon pair generating device according to claim 3, wherein the spacing between the first and second laser beams passing through the third frequency-stabilized gas cell is 2.0 times or more the beam radius of the first and second laser beams.

5. an optical filter that passes the first and second laser beams emitted from the third frequency-stabilized gas cell; 4. The quantum entangled photon pair generating device of claim 3, wherein the optical filter has a first region that passes light of the first wavelength and blocks light of the second wavelength, and a second region that passes light of the second wavelength and blocks light of the first wavelength.

6. a multiplexing element that multiplexes the first and second laser beams emitted from the first and second laser light sources, respectively, and outputs multiplexed light, The quantum entangled photon pair generating device according to claim 1 , wherein the wavelength conversion element generates the wavelength-converted light from multiplexed light output from the multiplexing element.

7. the nonlinear optical element is disposed within the resonator; the quantum entangled photon pair generating device includes an optical input unit that inputs the combined light output from the combining element to the nonlinear optical element via the resonator; The quantum entangled photon pair generating device according to claim 6 , wherein the nonlinear optical element generates the quantum entangled photon pairs from wavelength-converted light input by the optical input section.

8. the gas sealed in the first and second frequency stabilizing gas cells is cesium; 3. The quantum entangled photon pair generating device according to claim 2, wherein the first and second wavelengths are 894.593 [nm] (optical frequency: 335.116 terahertz) and 852.347 [nm] (optical frequency: 351.726 terahertz), respectively.

9. the gas enclosed in the third frequency stabilizing gas cell is cesium; 4. The quantum entangled photon pair generating device according to claim 3, wherein the first and second wavelengths are 894.593 [nm] (optical frequency: 335.116 terahertz) and 852.347 [nm] (optical frequency: 351.726 terahertz), respectively.

10. The wavelength of one photon of the quantum entangled photon pair is 605.977 [nm] (optical frequency: 494.726 terahertz), 10. The quantum entangled photon pair generating device according to claim 1, wherein the wavelength of the other photon of the quantum entangled photon pair is 1560.476 nm (optical frequency: 192.116 terahertz).

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