A system that shares quantum entanglement

Abstract

We provide a system for efficiently sharing quantum entanglement. [Solution] In a quantum entanglement sharing system (100), the system comprises a first device (10) and a plurality of second devices (20), a classical communication channel (2) connecting the first device (10) and each of the plurality of second devices (20), a multiplexing device (30) connected to the second devices (20), and a quantum communication channel (1) connecting the first device (10) and the multiplexing device (30), wherein the first device (10) has a single quantum light source that outputs photons that have undergone multiplexing processing based on combinations of time, frequency, and spatial degrees of freedom to the quantum communication channel.

Description

[Technical Field]

[0001] This invention relates to a system for sharing quantum entanglement. [Background technology]

[0002] Quantum communication systems that use quantum mechanics to send and receive signals are known. Patent Document 1 discloses a multiplexed quantum communication system that sends and receives photons by multiplexing them with optical wavelengths (paragraph

[0018] , Figure 1, claim 9). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-157405 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] However, the aforementioned multiplexed quantum communication system is limited to multiplexing at optical wavelengths, and therefore cannot necessarily be said to have good communication efficiency.

[0005] One aspect of the present invention aims to realize a system for efficiently sharing quantum entanglement. [Means for solving the problem]

[0006] To solve the above problems, a system according to one aspect of the present invention is a system for sharing quantum entanglement, comprising: a first device; a plurality of second devices; a classical communication channel connecting the first device and each of the plurality of second devices; and a multiplexing device connected to the second devices. The system comprises a quantum communication channel connecting the first device and the multiplexing / decoupling device, the first device having a single quantum light source that outputs photons that have undergone multiplexing processing using combinations of time, frequency, and spatial degrees of freedom to the quantum communication channel, the single quantum light source comprising an optical crystal and a resonator, and performing wavelength multiplexing processing using a resonant wavelength mode, time multiplexing processing utilizing pulse train formation by mode-locking of the resonator, and spatial multiplexing processing by forming higher-order Hermitian or Laguerre-Gaussian spatial modes using the transverse modes of the resonator, and the multiplexing / decoupling device performing demultiplexing processing by combining time, frequency, and spatial degrees of freedom. [Effects of the Invention]

[0007] According to one aspect of the present invention, efficient sharing of quantum entanglement can be achieved in a system that shares quantum entanglement. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic diagram showing a multiplex quantum communication system according to an embodiment of the present invention. [Figure 2] This is a schematic diagram illustrating a more concrete example of a multiplex quantum communication system according to an embodiment of the present invention. [Figure 3] This is a schematic diagram illustrating another example that more specifically describes the multiple quantum communication system according to the embodiment of the present invention. [Figure 4] This is a schematic diagram illustrating the state of frequency multiplexing. [Figure 5] This is a schematic diagram illustrating the state of time multiplexing. [Figure 6] This is a schematic diagram representing a spatially multiplexed state. [Figure 7] This is a schematic diagram representing a state that combines frequency multiplexing, time multiplexing, and spatial multiplexing. [Figure 8] This is a flowchart illustrating the procedure for generating an encryption key. [Modes for carrying out the invention]

[0009] Hereinafter, an embodiment of the present invention will be described in detail. FIG. 1 is a diagram showing the device configuration of a multi - quantum communication system 100 according to the embodiment.

[0010] The multi - quantum communication system 100 includes a quantum communication channel 1, classical communication channels 2(1), 2(2), a transmission device 10, receiving devices 20(1), 20(2), and a multiplexing / demultiplexing device 30, and shares quantum entanglement between the transmission device 10 and the receiving devices 20(1), 20(2).

[0011] Hereinafter, when the classical communication channels 2(1), 2(2) are described together, they are represented as the classical communication channel 2, and when the receiving devices 20(1), 20(2) are described together, they are represented as the receiving device 20.

[0012] The quantum communication channel 1 is an optical fiber that transmits and receives single photons, and connects the transmission device 10 (the first device) and the multiplexing / demultiplexing device 30. Note that a plurality of quantum communication channels 1 may be arranged in parallel. As will be described later, spatial multiplexing using a plurality of quantum communication channels 1 becomes possible. Here, the plurality of quantum communication channels 1 may be composed not only of a plurality of optical fibers but also of a single optical fiber. If a single optical fiber has a plurality of propagation modes, it can function as a plurality of quantum communication channels 1.

[0013] The transmission device 10 (the first device) sequentially generates a pair of photons corresponding to information, multiplexes one of the pair of photons, and transmits it to the multiplexing / demultiplexing device 30, and thus to the receiving device 20 via the quantum communication channel 1.

[0014] The pair of photons is in a state of quantum entanglement, and when the state of one photon is determined, the state of the other photon is also determined accordingly. The transmission device 10 transmits one of the pair of photons and holds the other photon.

[0015] The transmission device 10 transmits photons (one of the pair of photons) multiplexed by a combination of each degree of freedom of time, frequency, and space. That is, one of a plurality of pairs of photons (eventually, a plurality of photons) is multiplexed by a combination of each degree of freedom of time, frequency, and space.

[0016] Figures 4 to 6 are schematic diagrams respectively representing frequency multiplexing, time multiplexing, and spatial multiplexing of photons.

[0017] In frequency multiplexing, one of a plurality of pairs of photons (hereinafter referred to as "a plurality of photons") is multiplexed and transmitted so that the frequencies (wavelengths) are different. The range of frequencies to be multiplexed may be broadband, for example, larger than 100 [GHz].

[0018] In time multiplexing, a plurality of photons are arranged at narrow time intervals. For example, photons may be arranged at time intervals smaller than 10 [ns], and a time multiplexing band with a bandwidth larger than 100 [ns] may be formed.

[0019] In FIG. 6, spatial multiplexing is represented as states held at different positions such as quantum memories. When performing communication in spatial multiplexing, it is conceivable to use a plurality of quantum communication paths 1 (for example, a plurality of optical fibers arranged in parallel).

[0020] FIG. 7 is a schematic diagram representing a state in which frequency multiplexing, time multiplexing, and spatial multiplexing are combined. By combining each degree of freedom of time, frequency, and space, more efficient transmission and reception of information becomes possible. Here, it represents the case where the phases of all the multiplexed photons are equal (for example, e 0 = 1). The multiplicity (number of modes) at this time is represented as the number of grids inside the rectangular parallelepiped in FIG. 7. In addition to frequency multiplexing, time multiplexing, and spatial multiplexing, when performing phase multiplexing, the number of modes increases dramatically.

[0021] Here, frequency multiplexing, time multiplexing, and spatial multiplexing can be represented by the following equations (1), (2), and (3) respectively.

Number

Number

Number

[0022] Furthermore, combinations of frequency multiplexing, time multiplexing, and spatial multiplexing can be expressed by the following equation (4).

number

[0023] In other words, in this embodiment, the multiplexed quantum state obtained by combining the degrees of freedom of time, frequency, and space is expressed as |φ>, which is the product of the right-hand side of equation (4). Note that multiplexing is not required in all three dimensions: time, frequency, and space. It is also acceptable for no one of the three dimensions to be used for multiplexing.

[0024] The multiplexing / demultiplexing device 30 receives the multiplexed photons from the transmitting device 10, performs demultiplexing by combining the degrees of freedom of time, frequency, and space, and sends the demultiplexed photons to the receiving devices 20(1) and 20(2).

[0025] The receiving devices 20(1) and 20(2) (multiple second devices) receive the demultiplexed photons and reconstruct the information transmitted from the transmitting device 10.

[0026] Here, the transmitting device 10 encrypts the information using an encryption key (shared key) and transmits it as encrypted information, and the receiving devices 20(1) and (2) decrypt the received encrypted information using the encryption key (shared key). For this reason, processing to generate an encryption key is performed between the transmitting device 10 and the receiving devices 20(1) and (2).

[0027] Classical communication channels 2(1) and 2(2) transmit and receive control information for generating cryptographic keys. The control information is, for example, a synchronization signal for synchronizing between the transmitting device 10 and the receiving device 20. Classical communication channel 2 may be wired, such as an optical fiber, wireless, or a combination of wired and wireless.

[0028] In this embodiment, for the sake of explanation, the device that transmits photons is referred to as the transmitting device 10, but the transmitting device 10 may also have the function of receiving photons. Similarly, the receiving device 20 may have the function of transmitting photons.

[0029] [Example of Functional Configuration] Figure 2 shows an example of the functional configuration of the transmitter 10 and receiver 20 of the embodiment. The transmitter 10 comprises a generation unit 11 and a control unit 12. The receiver 20(1) comprises a detection unit 21(1) and a control unit 22(1). The receiver 20(2) comprises a detection unit 21(2) and a control unit 22(2).

[0030] The generation unit 11 is a quantum light source that generates photons (photon pairs) in wavelength multiplexing states. The quantum light source is composed of, for example, an optical crystal and a resonator. As shown below, by combining the optical crystal and the resonator, photons of multiple wavelength modes (photons in wavelength multiplexing states) can be generated.

[0031] Optical crystals possess high nonlinear optical constants (or high effective nonlinear optical constants) and form photons. Here, optical crystals can generate quantum-entangled photon pairs across a wider wavelength band than the wavelength used for communication through spontaneous parametric downconversion that propagates forward. A resonator restricts (subdivides) this wide wavelength band into multiple wavelength modes (repeating structures) by causing light to travel back and forth (resonate) within it. As a result, by combining an optical crystal and a resonator, photons with multiple wavelength modes (wavelength multiplexing states) can be generated. Note that the photons do not necessarily have to travel back and forth (circulate) within the resonator a predetermined number of times. To improve fidelity to the Bell state of the quantum state, the optical crystal may include not only a crystal that induces spontaneous parametric downconversion but also a birefringent phase-compensating crystal.

[0032] Here, the optical crystal may include a dispersion-compensated crystal or a birefringent phase-compensated crystal. Using a dispersion-compensated crystal allows for a wider wavelength bandwidth for the existence of each wavelength mode (peak). Using a birefringent phase-compensated crystal makes it possible to increase the fidelity to the ideal quantum entanglement state of the generated state, thereby reducing the number of entanglement purification steps and improving the communication rate.

[0033] The generation unit 11 is capable of not only wavelength multiplexing (i.e., frequency multiplexing), but also time multiplexing and spatial multiplexing combinations. In addition to frequency multiplexing, time multiplexing can be used in combination, for example, by utilizing pulse train generation through mode synchronization of the resonators constituting the generation unit 11. Furthermore, in addition to wavelength multiplexing, spatial multiplexing is possible by using the transverse modes of the resonators to form higher-order Hermitian or Laguerre Gaussian spatial modes.

[0034] The control unit 12 controls the operation of the generation unit 11. The control unit 12 also performs processing to generate an encryption key.

[0035] The detection unit 21 detects photons output from the multiplexing device 30, and, for example, decodes the received bits from a single photon based on polarization to reconstruct the encrypted information.

[0036] The detection unit 21 may receive (detect) photons using a photon detection element such as an avalanche photodiode (APD). The APD is a photodetector made of, for example, indium gallium arsenide, silicon, germanium, or gallium nitride.

[0037] The APD operates in an operating mode called Geiger mode. In Geiger mode, the reverse voltage of the APD is set to be above the breakdown voltage, and a large pulse is generated in response to the incoming received photon due to the avalanche effect. As a result, single-photon detection becomes possible. The detection unit 21 continuously performs single-photon detection by supplying a square wave or sinusoidal voltage consisting of a voltage above the breakdown voltage and a voltage below the breakdown voltage.

[0038] The control unit 22 controls the operation of the detection unit 21 to output encrypted information and uses the encryption key to reconstruct the information before encryption. The control unit 22 also performs processing to generate the encryption key.

[0039] Figure 3 shows another example of the functional configuration of the transmitter 10 and receiver 20 of the embodiment. The transmitter 10 comprises a generation unit 11(1), 11(2), a control unit 12, and a multiplexing unit 13. The receiver 20(1) comprises a detection unit 21(1) and a control unit 22(1). The receiver 20(2) comprises a detection unit 21(2) and a control unit 22(2).

[0040] The generation units 11(1) and 11(2) are a plurality of spatially arranged quantum light sources. As previously described, the quantum light source may be composed of, for example, an optical crystal and a resonator. Here, for the sake of clarity, two generation units 11 are shown, but there can be three or more generation units 11. Also, the number of generation units 11 and the number of receiving devices 20 are shown to be the same, but these numbers may be different.

[0041] Here, the phase relationship is crucial for quantum (photon) multiplexing. That is, if there is variation in the phase of the multiplexed quanta, this variation becomes noise and hinders demultiplexing. By using a quantum light source with a pulsed correlation function, the phase relationship at locations where the probability of finding photons is non-zero can be aligned. Similarly, there is a Fourier transform relationship in which pulsation occurs at the timing when the phases of the frequencies align, and this can be realized by including a resonator in the quantum light source.

[0042] For spatial modes, it is possible to align their phases by acquiring interference data using a separate laser.

[0043] The multiplexing unit 13 performs multiplexing processing on light from the generation units 11(1) and 11(2) (multiple quantum light sources) by combining the degrees of freedom of time, frequency, and space. As previously described, the generation unit 11 is capable of not only wavelength multiplexing (i.e., frequency multiplexing) but also combinations of time multiplexing and spatial multiplexing. The degree of multiplexing can be increased by adding the multiplexing unit 13. In this case, a single generation unit 11 may not have multiplexing, and multiplexing may be ensured only by the multiplexing unit 13.

[0044] The multiplexing unit 13 can be configured by a combination of a frequency shifter, a delay unit, and a distributor.

[0045] A frequency shifter shifts the frequencies of photons from the generating units 11(1) and 11(2) so that they differ from each other. Examples of frequency shifters include acousto-optic modulators and electro-optic modulators. Frequency multiplexing of light can be achieved by changing the modulation frequency applied to the acousto-optic modulator. Furthermore, by applying a sawtooth wave to the electro-optic modulator, the frequency (wavelength) of light can be shifted at high speed. When light passes through an acousto-optic modulator, it undergoes diffraction (generation of diffraction angle) that depends on the modulation frequency, but the effect of diffraction can be reduced by having the light travel back and forth within the acousto-optic modulator.

[0046] The delay device delays the photons from the generators 11(1) and 11(2) so that they pass through the quantum communication channel 1 at different times. The delay device may be constructed by combining a beam splitter with multiple optical fibers of different lengths. For example, the delay device can be constructed from a beam splitter that decomposes light into two 50:50 beams and two optical fibers of different lengths. Here, a polarizing beam splitter may be used as the beam splitter to decompose light into two polarizations.

[0047] The delay element may be included in the multiplexing unit 13 as part of a Mach-Zehnder interferometer or a Michelson interferometer. In this case, the Mach-Zehnder interferometer may be a multi-arm Mach-Zehnder interferometer using multiple optical fibers of different lengths.

[0048] The distributor distributes photons from the generators 11(1) and 11(2) to different quantum communication channels 1. The distributor is, for example, a switching mechanism that switches between the quantum communication channels 1 to which the generators 11(1) and 11(2) are connected. Examples of distributors include optical switches and acousto-optic modulators. High-speed switching of quantum communication channels 1 is possible using optical switches. With an acousto-optic modulator, the quantum communication channels 1 can be switched by changing the diffraction angle of light depending on the modulation frequency. Alternatively, the direction of light may be changed by slightly rotating a lens as an optical switch. Furthermore, a distributor may be configured by combining a frequency shifter (e.g., an acousto-optic modulator and an electro-optic modulator) and a spectrometer (e.g., a diffraction grating).

[0049] Furthermore, the distributor (e.g., an optical switch) may be time-synchronized with the pump light pulses from the generation unit 11 (quantum light source). The pulses can be distributed using feedforward control.

[0050] Here, by connecting the generation unit 11(1) to the quantum communication channel 1(1) and the generation unit 11(2) to the quantum communication channel 1(2), it is possible to configure a distributor (fixed connection without switching the connection destination).

[0051] The multiplexing unit 13 may have a selection of frequency shifters, delayers, and distributors, or it may have all of them. For example, the multiplexing unit 13 can be configured by combining the following frequency shifter (1), delayer (2), and distributor (3).

[0052] (1) Multiple frequency modes are generated and frequency multiplexed using multiple electro-optic modulators as a frequency shifter. That is, multiple frequency modes are generated by applying sinusoidal waves with different frequencies and voltages greater than half a wavelength voltage to each of the multiple electro-optic modulators.

[0053] (2) A multi-arm Mach-Zehnder interferometer is used as a delay element for time multiplexing. In this way, a time mode can be equivalently assigned to each frequency mode that has been frequency multiplexed, without being affected by frequency. In other words, interference between frequency multiplexing and time multiplexing, which would result in decoherence, can be prevented.

[0054] (3) Spatial multiplexing is performed using a star coupler or a tree coupler as the distributor. In this way, a spatial mode can be equivalently assigned to each frequency mode and each time mode that have been frequency and time multiplexed, without being affected by frequency and time.

[0055] As described above, by combining (1) to (3), frequency, time, and space multiplexing can be performed, interference between frequency multiplexing, time multiplexing, and space multiplexing can be prevented, and a multiplexing unit 13 that is optical fiber based and easy to assemble can be constructed. Note that two of (1) to (3) may be combined as appropriate, or the multiplexing unit 13 may be constructed using only one of (1) to (3).

[0056] Furthermore, the multiplexing unit 13 may have a diffraction grating or a fiber Bragg grating, enabling wavelength resolution or projection calculations onto the time axis.

[0057] [Quantum key generation process] An example of the secret key generation process in the embodiment will be described. Figure 8 is a diagram showing an example of the secret key generation process in the embodiment. The secret key generation process will be described below based on Figure 8 and Figure 1.

[0058] (1) Quantum communication (steps S1a, S1b) The transmitting device 10 transmits one of the entangled photon pairs to the receiving device 20 via the quantum communication channel 1 and the multiplexing / decoupling device 30. N(1+2ζ) photons are multiplexed and transmitted, and the multiplexing / decoupling device 30 resolves the multiplexing before the receiving device 20 receives them. As a result, the receiving device 20 retains one of the N(1+2ζ) photon pairs (N(1+2ζ) photons), and the transmitting device 10 retains the other of the N(1+2ζ) photon pairs (N(1+2ζ) photons).

[0059] (2) Quantum selection and measurement (Step S2a) The transmitting device 10 randomly selects Nζ photons from N(1+2ζ) photons, measures them in either the HV basis or DA basis, and transmits the results to the receiving device 20 via the classical communication channel 2. Here, the transmitting device 10 will perform the measurement in the HV basis.

[0060] Here, the HV and DA grounds are grounds corresponding to the polarization states of photons. The H (Horizontal), V (Vertical), D (Diagonal), and A (Anti-diagonal) states represent linear polarization states in the horizontal, vertical, +45°, and -45° diagonal directions, respectively. The H and V states are orthogonal to each other, and the D and A states are orthogonal to each other.

[0061] (3) Quantum selection and measurement (Step S2b) The receiving device 20 selects Nζ photons from the N(1+2ζ) photons received that correspond to the photons measured by the transmitting device 10, and measures them using the HV basis. The receiving device 20 calculates the error rate by comparing this measurement result with the measurement result of the transmitting device 10. If the error rate exceeds a predetermined value, the generation of the encryption key is stopped.

[0062] (4) Quantum selection and measurement (Step S3a) The transmitting device 10 randomly selects Nζ photons from the remaining N(1+ζ) photons, measures them in the DA basis, and transmits the measurement results to the receiving device 20 via the classical communication channel 2.

[0063] (5) Quantum selection and measurement (Step S3b) The receiver 20 measures Nζ corresponding photons using a DA basis. The receiver 20 calculates the error rate by comparing this measurement result with the measurement result of the transmitter 10. If the error rate exceeds a predetermined value, the generation of the encryption key is stopped.

[0064] As a result, the transmitting device 10 and the receiving device 20 each hold N unmeasured photons.

[0065] (5) Quantum transformation (steps S4a, S4b) The transmitting device 10 determines an N×N holomorphic 2-step sequence C1 and a mapping h1, and transmits them to the receiving device 20 via the classical communication channel 2. The transmitting device 10 applies a unitary transformation Ua(C1) to the N photons on the transmitting device 10 side. On the other hand, the receiving device 20 applies a unitary transformation Ub(C1) to the N photons on the receiving device 20 side.

[0066] (6) Quantum measurement (Step S5a) The transmitter 10 measures the Kz photons from the (N-Kz+1)th to the Nth photon using the HV basis and obtains a sequence of Kz bits z A This is transmitted to the receiving device 20 via classical communication channel 2.

[0067] (7) Quantum measurement, s1 calculation, and inversion (step S5b) The receiver 20 measures the Kz photons from the (N-Kz+1)th to the Nth photon using the HV basis and obtains a sequence of Kz bits z B And the result z of the transmitting device 10 A Therefore, s1=z B -z A The following is calculated. The receiver 20 performs HV inversion using s1 on the (N-Kz) photons from the 1st to the (N-Kz)th photons.

[0068] (8) Quantum transformation (steps S6a, S6b) The transmitting device 10 selects a regular binary matrix C2 of (N-Kz)×(N-Kz) and a mapping h2, and transmits them to the receiving device 20 via the classical communication channel 2. The transmitting device 10 performs a unitary transformation UA(C2) on (N-Kz) photons. On the other hand, the receiving device 20 performs a unitary transformation UB(C2) on the (N-Kz) photons from the first to the (N-Kz)th.

[0069] (9) Quantum measurement (step S7a) The transmitting device 10 measures the Kx photons from the (N-Kz-Kx+1)th to the (N-Kz)th in the DA basis, and the obtained Kx-bit sequence x A is transmitted to the receiving device 20 via the classical communication channel 2.

[0070] (10) Quantum measurement · s2 calculation · inversion (step S7b) The receiving device 20 measures the Kx photons from the (N-Kz-Kx+1)th to the (N-Kz)th according to the DA rule, and the obtained Kx-bit sequence x B and the result x A from the transmitting device 10, calculate s2 = x B -x A The receiving device 20 performs a DA inversion on the (N-Kz-Kx) photons from the first to the (N-Kz-Kx)th using s2.

[0071] (11) Quantum measurement (steps S8a, S8b) The transmitting device 10 measures the remaining photons in the HV basis, and uses the obtained bit sequence as the encryption key. The receiving device 20 measures the remaining photons in the HV basis, and uses the obtained bit sequence as the encryption key.

[0072] Summarizing the above, it is as follows. That is, the transmitting device 10 generates a plurality of pairs of photons in an entangled state, and transmits one of the pairs of photons to the receiving device 20 via the multiplexing separation device 30. The receiving device 20 measures a part of the received plurality of photons, and the transmitting device 10 measures a part of the plurality of photons and transmits the measurement results. If the measurement results correspond, an encryption key sequence is generated based on the plurality of photons for which the measurement results have not been transmitted.

[0073] The above example uses a polarization basis, but a time bin basis may be used instead. In this case, time bin basis 1-0, 1-1, 2-0, and 2-1 are randomly selected and used for measurement. When the value on the left matches between the transmitter 10 and the receiver 20, the value on the right is used as the key sequence.

[0074] Here, the time bin base 1-0 etc. represent the following: Time bin basis 1-0: Selection of "early" from two basis {early, late}. Time bin base 1-1: Selection of "late" from two bases {early, late} Time bin basis 2-0: Selection of early+late from two basis {early+late, early-late} Time bin basis 2-1: Selection of early-late from two basis {early+late, early-late}

[0075] The present invention is not limited to the embodiments described above, 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.

[0076] (summary) (1) A system according to embodiment 1 of the present invention is a quantum entanglement sharing system comprising a first device (transmitter 10), a plurality of second devices (receivers 20(1), 20(2)), classical communication channels (2(1), 2(2)) connecting the first device and each of the plurality of second devices, a multiplexing / deselection device (30) connected to the second devices, and a quantum communication channel (1) connecting the first device and the multiplexing / deselection device, wherein the first device outputs photons multiplexed by combinations of time, frequency, and spatial degrees of freedom to the quantum communication channel, and the multiplexing / deselection device performs demultiplexing processing by combining time, frequency, and spatial degrees of freedom.

[0077] In one aspect of the present invention, quantum entanglement can be efficiently shared by multiplexing, which involves combining the degrees of freedom of time, frequency, and space.

[0078] (2) In the system according to embodiment 2 of the present invention, in embodiment 1, the first device is a transmitting device and the second device is a receiving device.

[0079] In aspect 2 of the present invention, efficient communication using quantum entanglement becomes possible by multiplexing, which combines the degrees of freedom of time, frequency, and space.

[0080] (3) In the system according to embodiment 3 of the present invention, in embodiment 1 or 2, the first apparatus comprises a quantum light source (generation unit 11) that generates photons in wavelength multiplexing state.

[0081] In embodiment 3 of the present invention, multiplexing becomes possible using a quantum light source that generates photons in wavelength multiplexing states.

[0082] (4) In the system according to embodiment 4 of the present invention, in embodiment 1 or 2, the first apparatus has a plurality of spatially arranged quantum light sources (generation units 11(1), 11(2)).

[0083] In aspect 4 of the present invention, photons from multiple spatially arranged quantum light sources can be multiplexed.

[0084] (5) In the system according to aspect 5 of the present invention, in aspect 4, the first apparatus has a multiplexing unit (13) that performs multiplexing processing by combining the degrees of freedom of time, frequency, and space of photons from the plurality of quantum light sources.

[0085] In aspect 5 of the present invention, the multiplexing unit can multiplex photons from multiple quantum light sources.

[0086] (6) In the system according to embodiment 6 of the present invention, in embodiment 5, the multiplexing unit has a frequency shifter that makes the frequencies of photons from the plurality of quantum light sources different from each other.

[0087] In embodiment 6 of the present invention, a frequency shifter can be used to frequency multiplex photons from multiple quantum light sources.

[0088] (7) In the system according to embodiment 7 of the present invention, in embodiment 5 or 6, the multiplexing unit has a delay unit that causes the times at which photons from the plurality of quantum light sources pass through the quantum communication channel to be different from each other.

[0089] In aspect 7 of the present invention, a delay device can be used to time-multiplex photons from multiple quantum light sources.

[0090] (8) A system according to aspect 8 of the present invention, in any of aspects 5 to 6, comprises a plurality of quantum communication channels connecting the first device and the multiplexing device, wherein the multiplexing unit has a distributor that distributes photons from the plurality of quantum light sources to different quantum communication channels.

[0091] In aspect 8 of the present invention, photons from multiple quantum light sources can be spatially multiplexed using multiple quantum communication channels (e.g., multiple optical fibers) and a distributor. [Explanation of Symbols]

[0092] 1. Quantum communication channel 2 Classical Communication Channels 10 Transmitter 11 Generation part 12, 22 Control Unit 13 Multiplexer 20 Receiving device 21 Detection unit 30 Demultiplexer 100 Multiple Quantum Communication Systems

Claims

1. In a system that shares quantum entanglement, The first device and Multiple second devices, A classical communication channel connecting the first device and each of the plurality of second devices, A multiplexing device connected to the second device, A quantum communication channel connecting the first device and the multiplexing device, Equipped with, The first apparatus has a single quantum light source that outputs photons, which have been multiplexed using combinations of time, frequency, and spatial degrees of freedom, to the quantum communication channel. The single quantum light source is It is composed of an optical crystal and a resonator, Wavelength multiplexing processing is performed using the resonant wavelength mode. At least one of the following is performed: a time-multiplexing process utilizing pulse train generation by mode-locking of the resonator, and a spatial-multiplexing process by forming higher-order Hermitian or Laguerre-Gaussian spatial modes using the transverse modes of the resonator. The aforementioned multiplexing device is a system that performs demultiplexing processing by combining the degrees of freedom of time, frequency, and space.

2. The first device is a transmitting device, The system according to claim 1, wherein the second device is a receiving device.

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