Pulse length optimization device, pulse length optimization method, and program

Abstract

To assist in determining a pulse length optimized for bell measurement in quantum repeating using a quantum memory.SOLUTION: For quantum repeating using a quantum memory, a pulse length optimization device comprises a measurement information acquisition part which acquires measurement information representing a measurement result of photons sent from a plurality of repeaters, an entanglement generation rate calculation part which calculates a generation rate of entanglement shared by the plurality of repeaters based upon the measurement information, and an optimum pulse length determination part which determines an optimum pulse length based upon the generation rate of entanglement.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a pulse length optimization device, a pulse length optimization method, and a program. [Background technology]

[0002] Quantum repeaters for long-distance quantum communication and quantum memories used in quantum repeaters are being researched worldwide. Quantum repeaters enable entanglement to be shared more efficiently than direct transmission by storing the state in a quantum memory.

[0003] When using quantum repeaters, it is necessary to first communicate and perform Bell measurements between the repeaters to share entanglement between them. Bell measurements are measurements of two photons sent from each repeater. There is a known method of using memoryless measurements for Bell measurements, but there is a problem that this method is not successful unless photons arrive from both repeaters without loss.

[0004] Therefore, Non-Patent Document 1 discloses a method for improving efficiency by providing a quantum memory at the measurement point and storing photons that arrive from one side without loss, thereby enabling measurements across two photons to be performed even if the photons are not successfully communicated without loss at the same time.

[0005] In order to use quantum memory as a component of a quantum repeater that shares entanglement at multiple distant locations, multiple quantum memories must be connected. Therefore, Non-Patent Documents 2 and 3 disclose the use of a resonator QED system in which atoms are trapped in an ultra-low-loss nano-optical fiber resonator with low optical loss at the connection location as a quantum memory. Writing to this quantum memory is performed by inputting a photon pulse. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Bhaskar, MK, Riedinger, R., Machielse, B. et al. Experimental demonstration of memory-enhanced quantum communication. Nature 580, 60-64 (2020). https: / / doi.org / 10.1038 / s41586-020-2103-5 [Non-Patent Document 2] Kato S, Aoki T. Strong Coupling between a Trapped Single Atom and an All-Fiber Cavity. Physical Review Letters. 2015 8 26;115(9). 093603. https: / / doi.org / 10.1103 / PhysRevLett.115.093603 [Non-Patent Document 3] Kato, S., Nemet, N., Senga, K. et al. Observation of dressed states of distant atoms with delocalized photons in coupled-cavities quantum electrodynamics. Nat Commun 10, 1160 (2019). https: / / doi.org / 10.1038 / s41467-019-08975-8 Summary of the Invention [Problem to be solved by the invention]

[0007] In order to maximize the efficiency of Bell measurements in quantum repeaters using quantum memories in cavity QED systems, there is a need to determine the optimized pulse length. However, in conventional techniques, determining the optimized pulse length involves multiple factors and is computationally difficult.

[0008] The disclosed technique aims to assist in determining pulse lengths optimized for Bell measurements in quantum repeaters using quantum memories. [Means for solving the problem]

[0009] The disclosed technology is a pulse length optimization device in a quantum repeater using a quantum memory, the device comprising: a measurement information acquisition unit that acquires measurement information indicating measurement results for photons sent from multiple repeaters; an entanglement generation rate calculation unit that calculates a generation rate of entanglement shared by the multiple repeaters based on the measurement information; and an optimal pulse length determination unit that determines an optimal pulse length based on the entanglement generation rate. Effect of the Invention

[0010] The disclosed technique can assist in determining a pulse length optimized for Bell measurements in quantum repeaters using quantum memories. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram illustrating an example of the configuration of a quantum memory. [Diagram 2] FIG. 1 is a diagram for explaining Bell measurement in quantum repeaters. [Diagram 3] FIG. 2 illustrates an example of a functional configuration of a pulse length optimization device. [Figure 4] 13 is a flowchart showing an example of the flow of a pulse length optimization process. [Diagram 5] FIG. 11 is a diagram for explaining a method for calculating an error probability. [Figure 6] FIG. 13 is a diagram illustrating an example of a calculation result of an error probability. [Figure 7] FIG. 13 is a diagram showing an example of the relationship between the entanglement generation rate and the pulse length. [Figure 8] FIG. 2 illustrates an example of a hardware configuration of a computer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention (the present embodiment) will be described with reference to the drawings. The embodiment described below is merely an example, and the embodiment to which the present invention is applied is not limited to the following embodiment.

[0013] The reference numbers and names of reference documents related to the reference techniques of the present embodiment are listed at the end of the present embodiment. In the following description, the reference numbers of related reference documents are indicated as "[1]" etc.

[0014] (Outline of this embodiment) The pulse length optimization device according to the present embodiment calculates an entanglement generation rate based on a measurement result by Bell measurement in a quantum repeater using a quantum memory, and outputs information indicating an optimal pulse length, thereby assisting in the determination of a pulse length optimized for Bell measurement in a quantum repeater using a quantum memory.

[0015] (Quantum memory configuration) 1 is a diagram showing an example of the configuration of a quantum memory. A quantum memory 10 according to this embodiment includes a resonator QED system in which atoms are trapped in an ultra-low-loss nano-optical fiber resonator with low optical loss at connection points.

[0016] Specifically, the quantum memory 10 is a physical system that has a reflectance of 0 in state |u〉 and a reflectance of 1 in state |g〉, and is made of an ultra-low-loss nano-optical fiber resonator and atoms.

[0017] (Overview of Bell measurements in quantum repeaters) 2 is a diagram for explaining Bell measurement in quantum repeaters. A measurement system 1 for performing Bell measurement in quantum repeaters includes a measurement device 2, a first repeater 3, and a second repeater 4.

[0018] The measuring device 2 is a device that performs Bell measurement in the quantum repeater between the first repeater 3 and the second repeater 4. The measuring device 2 includes a quantum memory 10-2. In this way, by providing a quantum memory at the measurement point and storing photons that arrive from one side without loss, it is possible to perform measurements across two photons even if the photons are not successfully communicated at the same time without loss.

[0019] The first repeater 3 includes a quantum memory 10-3. The second repeater 4 includes a quantum memory 10-4. The quantum memories 10-2, 10-3, and 10-4 are physical systems that have a reflectance of 0 in state |u〉 and a reflectance of 1 in state |g〉, similar to the quantum memory 10 shown in Fig. 1, and are made of ultra-low-loss nano-optical fiber resonators and atoms.

[0020] Next, a method to aid in optimizing the pulse length in the Bell measurement shown in FIG. 2 will be described.

[0021] (Functional configuration of the pulse length optimization device) 3 is a diagram showing an example of the functional configuration of a pulse length optimization apparatus 20. The pulse length optimization apparatus 20 includes a measurement information acquisition section 21, an entanglement creation rate calculation section 22, an optimal pulse length determination section 23, and an output section 24.

[0022] The measurement information acquiring unit 21 acquires the measurement information 11. The measurement information 11 is information indicating the result of measurement by the measuring device 2 shown in FIG. 2. The measurement information 11 includes, for example, m1, m2, m3, etc. Here, m1 is a value obtained by measuring a photon of the first repeater 3. m2 is a value obtained by measuring a photon of the second repeater 4. m3 is a value read out from the quantum memory 10-2 of the measuring device 2.

[0023] The entanglement creation rate calculation unit 22 calculates the entanglement creation rate based on the measurement information 11. Specifically, the entanglement creation rate calculation unit 22 calculates the entanglement creation rate by Equation 1 for key rate calculation shown in [1].

[0024] G = R × [1-h(e x )-h(e y )]...(Formula 1)

[0025] Here, G is the key rate, i.e., the rate at which entanglement is generated after distillation. R is the success rate. e x and e y are the error rates when a photon is transmitted in the X basis and Y basis, respectively. h(x) represents the binary entropy.

[0026] Furthermore, the entanglement creation rate calculation unit 22 may use the key distribution protocol using time bin qubits shown in [2] as the protocol for calculating the key rate.

[0027] The entanglement generation rate calculation unit 22 considers the herald probability of successful memory writing and the operation rate to the memory as the value to which the pulse length contributes among the success rate R. Here, the herald probability of successful memory writing is the average value of the reflectance when the atomic state is |g〉 and the reflectance when the atomic state is |u〉.

[0028] In addition, in the measurement of the time bin qubit assumed in this embodiment, a total of five pulses are required for the write and read operations. Therefore, since the next communication cannot be performed during the five pulses, the operation rate to the memory may be the reciprocal of the length of the five pulses.

[0029] Furthermore, the entanglement generation rate calculation unit 22 may determine the error probability by taking into consideration an error caused by the inability to obtain the intended reflectance in the quantum memory of the measurer due to the short pulse length of the photon, and an error caused by the phase relaxation of atoms during the memory time. The consideration of these errors will be described later.

[0030] The optimum pulse length determination unit 23 determines the pulse length at which the entanglement generation rate is maximized as the optimum value based on the relationship between the entanglement generation rate and the pulse length. An example of the relationship between the entanglement generation rate and the pulse length will be described later.

[0031] The output unit 24 outputs optimal pulse length information 12 indicating the determined optimal value of the pulse length. The measuring device 2 can perform Bell measurement for quantum repeaters using a pulse length based on the output optimal pulse length information 12.

[0032] (Overview of pulse length optimization device operation) Next, the operation of the pulse length optimization device will be described with reference to the drawings.

[0033] 4 is a flowchart showing an example of the flow of a pulse length optimization process. The measurement information acquisition unit 21 acquires the measurement information 11 (step S11). The measurement information 11 is information indicating the results of measurement by the measuring device 2 shown in FIG.

[0034] Next, the entanglement generation rate calculation unit 22 calculates the entanglement generation rate based on the measurement information 11 (step S12). The optimum pulse length determination unit 23 determines the optimum pulse length based on the calculated entanglement generation rate (step S13). The output unit 24 outputs optimum pulse length information indicating the determined optimum pulse length (step S14).

[0035] Next, the error probability taken into consideration for the calculation of the entanglement creation rate by the entanglement creation rate calculation unit 22 in step S12 of FIG. 4 will be described.

[0036] FIG. 5 is a diagram for explaining a method of calculating the error probability. FIG. 5 shows, as an example, a flow when the first repeater 3 and the second repeater 4 send information in the Y basis. Here, α is the probability of a phase inversion error. s is the probability that m1m2m3=1. m1 is a value obtained by measuring a photon of the first repeater 3. m2 is a value obtained by measuring a photon of the second repeater 4. m3 is a value read from the quantum memory 10-2 of the measuring device 2. The entanglement generation rate calculation unit 22 calculates α and s based on the measurement information 11 including m1, m2, and m3, as shown in FIG. 5.

[0037] Fig. 6 is a diagram showing an example of the calculation result of the error probability. For example, assume that the first repeater 3 and the second repeater 4 both send bit 0 in the Y basis, that is, the phase of the time bin quantum bit is π / 2. In this case, the calculation result of the error probability is as shown in Fig. 6.

[0038] That is, the entanglement generation rate calculation unit 22 calculates the error rate when transmitted in the Y basis by Equation 2.

[0039] e y =((1-α)s+α(1-s)+α) / 2...(Equation 2)

[0040] Similarly, the entanglement creation rate calculation unit 22 calculates the error rate when transmitted in the X basis by Equation 3.

[0041] e x =α...(Equation 3)

[0042] The entanglement creation rate calculation unit 22 calculates the entanglement creation rate from these values ​​using the above-mentioned equation 1.

[0043] Fig. 7 is a diagram showing an example of the relationship between the entanglement generation rate and the pulse length. In step S13 shown in Fig. 4, the optimum pulse length determination unit 23 determines the pulse length (about 0.51 μs in the case of Fig. 7) that maximizes the entanglement generation rate based on the relationship between the entanglement generation rate and the pulse length calculated as in Fig. 7.

[0044] (Example of hardware configuration of pulse length optimization device) The pulse length optimization device 20 can be realized, for example, by making a computer execute a program in which the processing contents described in this embodiment are described. Note that this "computer" may be a physical machine or a virtual machine on the cloud. When a virtual machine is used, the "hardware" described here is virtual hardware.

[0045] The above program can be recorded on a computer-readable recording medium (such as a portable memory) and can be stored or distributed. The above program can also be provided via a network such as the Internet or e-mail.

[0046] Fig. 8 is a diagram showing an example of the hardware configuration of the computer. The computer in Fig. 8 has a drive device 1000, an auxiliary storage device 1002, a memory device 1003, a CPU 1004, an interface device 1005, a display device 1006, an input device 1007, an output device 1008, etc., which are all connected to each other via a bus B.

[0047] The program for realizing the processing on the computer is provided by a recording medium 1001 such as a CD-ROM or a memory card. When the recording medium 1001 storing the program is set in the drive device 1000, the program is installed from the recording medium 1001 to the auxiliary storage device 1002 via the drive device 1000. However, the program does not necessarily have to be installed from the recording medium 1001, but may be downloaded from another computer via a network. The auxiliary storage device 1002 stores the installed program as well as necessary files, data, etc.

[0048] When a program start instruction is received, the memory device 1003 reads out and stores the program from the auxiliary storage device 1002. The CPU 1004 realizes the functions related to the device according to the program stored in the memory device 1003. The interface device 1005 is used as an interface for connecting to a network. The display device 1006 displays a GUI (Graphical User Interface) or the like according to a program. The input device 1007 is composed of a keyboard, a mouse, a button, a touch panel, or the like, and is used to input various operation instructions. The output device 1008 outputs the calculation result. Note that the above computer may be equipped with a GPU (Graphics Processing Unit) or a TPU (Tensor processing unit) instead of the CPU 1004, or may be equipped with a GPU or a TPU in addition to the CPU 1004. In that case, the processes may be shared and executed, for example, the GPU or the TPU executes processes that require special calculations, and the CPU 1004 executes other processes.

[0049] (Effects of this embodiment) According to the pulse length optimization device 20 of the present embodiment, the entanglement generation rate is calculated based on the measurement result by Bell measurement in the quantum repeater using the quantum memory, and information indicating the optimal pulse length is output. This makes it possible to assist in the determination of the pulse length optimized for Bell measurement in the quantum repeater using the quantum memory.

[0050] [References] [1] van Loock, P., Alt, W., Becher, C., Benson, O., Boche, H., Deppe, C., Eschner, J., Hofling, S., Meschede, D., Michler, P., Schmidt, F. and Weinfurter, H. (2020), Extending Quantum Links: Modules for Fiber‐ and Memory‐Based Quantum Repeaters. Adv. Quantum Technol., 3: 1900141. https: / / doi.org / 10.1002 / qute.201900141 [2] Bhaskar, MK, Riedinger, R., Machielse, B. et al. Experimental demonstration of memory-enhanced quantum communication. Nature 580, 60-64 (2020). https: / / doi.org / 10.1038 / s41586-020-2103-5

[0051] (Summary of the embodiment) This specification describes at least the pulse length optimization apparatus, pulse length optimization method, and program described in the following sections. (Section 1) In a quantum repeater using a quantum memory, a measurement information acquisition unit that acquires measurement information indicating measurement results for photons sent from a plurality of repeaters; an entanglement generation rate calculation unit that calculates a generation rate of entanglement shared by the plurality of repeaters based on the measurement information; and an optimal pulse length determination unit that determines an optimal pulse length based on the entanglement generation rate. Pulse length optimization device. (Section 2) The entanglement generation rate calculation unit calculates the entanglement generation rate by using a key distribution protocol using time bin quantum bits. 2. A pulse length optimization apparatus as described in claim 1. (Section 3) the entanglement generation rate calculation unit calculates a success rate, an error rate when a photon is transmitted in the X basis, and an error rate when a photon is transmitted in the Y basis, and calculates the entanglement generation rate based on the calculated success rate, the error rate when a photon is transmitted in the X basis, and the error rate when a photon is transmitted in the Y basis; 3. A pulse length optimization apparatus according to claim 1 or 2. (Section 4) the optimum pulse length determination unit determines, based on a relationship between the entanglement generation rate and the pulse length, a pulse length at which the entanglement generation rate is maximized as the optimum pulse length; 4. A pulse length optimization device according to any one of claims 1 to 3. (Section 5) 1. A computer implemented method for optimizing pulse length, comprising: In a quantum repeater using a quantum memory, a step of acquiring measurement information indicating measurement results for photons sent from a plurality of repeaters; calculating a generation rate of entanglement shared among the repeaters based on the measurement information; determining an optimal pulse length based on the entanglement generation rate. Pulse length optimization method. (Section 6) A program for causing a computer to function as each unit in the pulse length optimization device according to any one of claims 1 to 4.

[0052] Any of the above configurations provides a technique that can assist in determining an optimized pulse length for Bell measurements in quantum repeaters using quantum memories. According to the second term, the entanglement generation rate can be calculated using a key distribution protocol using time bin qubits. According to the third term, the entanglement generation rate can be calculated taking into account the error rate. According to the fourth term, the optimal pulse length can be determined based on the relationship between the entanglement generation rate and the pulse length.

[0053] Although the present embodiment has been described above, the present invention is not limited to such a specific embodiment, and various modifications and variations are possible within the scope of the gist of the present invention described in the claims. [Explanation of symbols]

[0054] 1. Measurement system 2 Measuring instrument 3 First Repeater 4 Second repeater 10 Quantum Memory 11 Measurement Information 12 Optimal pulse length information 20 Pulse length optimization device 21 Measurement information acquisition section 22 Entanglement generation rate calculation unit 23 Optimum pulse length determination section 24 Output section 1000 Drive Device 1001 Recording media 1002 Auxiliary storage 1003 Memory device 1004 CPU 1005 Interface device 1006 Display device 1007 Input Device 1008 Output device

Claims

1. In a quantum repeater using a quantum memory, a measurement information acquisition unit that acquires measurement information indicating measurement results for photons sent from a plurality of repeaters; an entanglement generation rate calculation unit that calculates a generation rate of entanglement shared by the plurality of repeaters based on the measurement information; and an optimal pulse length determination unit that determines an optimal pulse length based on the entanglement generation rate. Pulse length optimization device.

2. The entanglement generation rate calculation unit calculates the entanglement generation rate by using a key distribution protocol using time bin quantum bits.

2. The pulse length optimization apparatus of claim 1.

3. the entanglement generation rate calculation unit calculates a success rate, an error rate when a photon is transmitted in the X basis, and an error rate when a photon is transmitted in the Y basis, and calculates the entanglement generation rate based on the calculated success rate, the error rate when a photon is transmitted in the X basis, and the error rate when a photon is transmitted in the Y basis; 3. The pulse length optimization apparatus according to claim 1 or 2.

4. the optimum pulse length determination unit determines, based on a relationship between the entanglement generation rate and the pulse length, a pulse length at which the entanglement generation rate is maximized as the optimum pulse length; 4. A pulse length optimization apparatus according to claim 1.

5. 1. A computer implemented method for optimizing pulse length, comprising: In a quantum repeater using a quantum memory, a step of acquiring measurement information indicating measurement results for photons sent from a plurality of repeaters; calculating a generation rate of entanglement shared among the repeaters based on the measurement information; determining an optimal pulse length based on the entanglement generation rate. Pulse length optimization method.

6. A program for causing a computer to function as each unit in the pulse length optimization device according to any one of claims 1 to 4.

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