Bell state measurement device and quantum key distribution system

Abstract

The present application relates to the field of optical communication and quantum key distribution, and discloses a Bell state measurement device and a quantum key distribution system, the Bell state measurement device comprising two polarization mode dispersion cancellation modules, an unequal arm polarization interferometer and two single photon detectors, the unequal arm polarization interferometer is capable of polarization beam splitting on the phase encoding quantum state after polarization randomization, delaying one polarization component after the beam splitting, rotating the two polarization components by 45 degrees respectively, and finally performing secondary polarization beam splitting on the rotated polarization components. Compared with the prior art, the present application completely eliminates the influence of polarization state fluctuation, fiber birefringence, polarization mode dispersion and other factors on Bell state measurement, does not require additional polarization tracking and compensation modules, significantly reduces the system complexity and cost, and improves the environmental adaptability of the system; the accuracy of HOM interference can be improved, and the bit error rate of the quantum key distribution system can be reduced.

Description

Technical Field

[0001] This invention relates to the fields of optical communication and quantum key distribution technology, and particularly to a Bell state measurement device and a quantum key distribution system. Background Technology

[0002] Quantum key distribution can provide theoretical security for both communicating parties; however, due to the imperfections of actual devices, security vulnerabilities exist in the system, with the most common vulnerabilities found in the measurement device. The introduction of measurement-device-independent quantum key distribution (MDIQKD) removes the requirement for trust at the measurement end, making it immune to all attacks targeting the measurement end and greatly improving the actual security of the system.

[0003] Since the core of MDIQKD is Bell state measurement, the key is how to achieve stable (Hong-Ou-Mandel, HOM) interference. This requires that the two photons involved in the interference be indistinguishable, that is, the polarization state, arrival time, and frequency of the two photons must be kept consistent. For polarization state consistency, conventional MDIQKD uses real-time polarization compensation at the measurement end, which consumes resources and reduces the stability of the system. For time phase encoding, in order to solve the polarization problem, the literature Wang C, Yin ZQ, Wang S, et al. Measurement-device-independent quantum key distribution robust against environmental disturbances[J]. Optica, 2017, 4(9): 1016-1023. proposes a scheme that uses two transmitters to actively perturb the polarization, and after polarization beam splitting at the measurement end, the two orthogonal polarization components are subjected to independent HOM interference. This can realize polarization-independent Bell state measurement with time phase encoding. However, this scheme requires active polarization perturbation and two beam splitters and four single-photon detectors (SPDs), which increases the complexity and cost of the system. Chinese invention patent CN202310442366.3 proposes a method to improve the stability of Bell state measurement by adding a depolarization module at the transmitting end, performing polarization beam splitting on the two input quantum states at the measuring end, and then interfering with the orthogonal polarization components using a bidirectional multiplexing beam splitter before polarization beam combining. This method makes the Bell state measurement results immune to the influence of channel polarization disturbances, improves system stability, and reduces the number of single-photon detectors. This scheme requires two polarization beam splitters and one beam splitter, with HOM interference performed on the beam splitter. The paper Wang C, et al. Realistic device imperfections affect the performance of HOM interference with weak coherent states[J]. Journ. Of Ligh. Tech. 2017, 35:4996. shows that an unsatisfactory beam splitting ratio of the beam splitter has a significant impact on the HOM interference results. The beam splitting ratio error of a conventional 50:50 beam splitter is difficult to achieve above 23dB, while the polarization extinction ratio of a polarization beam splitter can reach above 30dB. Using a polarization beam splitter for HOM interference can improve the interference effect. However, there is currently a lack of schemes for Bell state measurements using polarization beam splitters for time-phase encoding. Summary of the Invention

[0004] To address the aforementioned shortcomings of existing technologies, this invention proposes a Bell state measurement device and a quantum key distribution system.

[0005] The technical solution of this invention is implemented as follows: A Bell state measurement device for performing Bell state joint measurements of phase-encoded quantum states, comprising: The first depolarization module DEP1 and the second depolarization module DEP2 are used to perform polarization randomization processing on the phase-encoded quantum states input to the two input ports of the unequal-arm polarization interferometer, so that the polarization degrees of freedom of the quantum states are completely decoherent and the influence of polarization state fluctuations on the measurement results is eliminated. The unequal-arm polarization interferometer matches the time difference between the early pulse E and the late pulse L of the two time modes of the phase-encoded quantum state. It can perform polarization beam splitting on the phase-encoded quantum state after polarization randomization, delay one polarization component after beam splitting, rotate the two polarization components by 45° respectively, and finally perform secondary polarization beam splitting on the rotated polarization components to produce two mutually orthogonal interference results. The first single-photon detector SPD1 and the second single-photon detector SPD2 are connected to the two output terminals of the unequal-arm polarization interferometer, respectively, to detect the two interference results and realize Bell state measurement based on the triggering condition of the detection signal.

[0006] Preferably, the unequal-arm polarization interferometer is an unequal-arm Mach-Zehnder interferometer composed of a first polarization beamsplitter PBS1 and a second polarization beamsplitter PBS2. The first polarization beamsplitter PBS1 serves as an input beamsplitter, used to perform the first polarization beam splitting on the phase-encoded quantum state after polarization randomization. The second polarization beamsplitter PBS2 serves as an output beam combiner, used to perform a second polarization beam splitting and beam combining interference on the polarization components after delay and polarization rotation. In the short-arm and long-arm optical paths of the unequal-arm Mach-Zehnder interferometer, a first polarization rotation module PR1 and a second polarization rotation module PR2 are respectively provided. Both polarization rotation modules are used to rotate the polarization components passing through them by 45° to realize the Hadamard basis transformation.

[0007] Preferably, the unequal-arm polarization interferometer is a Faraday-Michelson interferometer, which includes a third polarization beam splitter PBS3, a first Faraday rotator FRM1, and a second Faraday rotator FRM2. Two Faraday rotating mirrors are respectively set on the two output arms of the third polarization beam splitter PBS3, forming an unequal arm structure. The difference in arm length is matched with the time difference between the two time modes of the phase-encoded quantum state. The magneto-optical crystal in the Faraday rotating mirror has an optical rotation angle of 22.5°. When a photon passes through the Faraday rotating mirror, its polarization state is cumulatively rotated by 45°, thus achieving a 45° polarization rotation function. At the two input ports of the third polarization beam splitter PBS3, a first circulator CIR1 and a second circulator CIR2 are respectively set. Both circulators are used to transmit the incident phase-encoded quantum state to the third polarization beam splitter PBS3, and to transmit the two interference results after interference by the Faraday Michelson interferometer in the opposite direction to the two single-photon detectors.

[0008] Preferably, the function of each polarization beam splitter is as follows: when a horizontally polarized light signal or a vertically polarized light signal is incident from its first input port, it will be emitted from its first output port or its second output port, both of which are horizontally polarized; when a horizontally polarized light signal or a vertically polarized light signal is incident from its second input port, it will be emitted from its first output port or its second output port, both of which are vertically polarized.

[0009] Preferably, both polarization rotation modules are polarization-maintaining fiber 45° fusion splicing structures. By splicing the fast axis or slow axis of the two polarization-maintaining fiber segments at a 45° angle, the polarization component passing through the fusion splicing structure undergoes a 45° polarization rotation.

[0010] Preferably, the phase-coded quantum state is a time-dual-mode phase-coded quantum state, with the encoding basis being the X basis and Y basis in the BB84 protocol, wherein the X basis corresponds to a phase difference of 0 or π between the two time modes, and the quantum states are respectively... , The phase difference between the two time modes corresponding to the Y basis is π / 2 or 3π / 2, and the quantum states are respectively , ; The arm length difference ΔL of the unequal-arm polarization interferometer satisfies , where c is the speed of light in the medium, and Δt is the time interval between the two time modes E and L, so that the two time modes can effectively interfere after being transmitted through an unequal arm optical path.

[0011] Preferably, the inherent phase difference between the short-arm and long-arm optical paths of the unequal-arm Mach-Zehnder interferometer Where λ is the wavelength of the phase-encoded quantum state, and ΔL is the difference in arm length between the short and long arms; the unequal-arm Mach-Zehnder interferometer also includes a phase feedback module for real-time adjustment of the inherent phase difference. ,make To ensure that the interference is at the constructive operating point and the interference contrast is ≥98%.

[0012] Preferably, the magneto-optical crystals of the two Faraday rotators are made of terbium gallium garnet (TGG) material, with a Verdet constant ≥ 15 rad / (T·m). At the working wavelength, the optical rotation angle stability is ≤ 0.1° / ℃, ensuring that the polarization rotation angle of the photon after round trip is precisely maintained at 45°.

[0013] Preferably, both depolarization modules are passive polarization scramblers, which enable rapid randomization of polarization states, decoherence of the polarization degrees of freedom of quantum states, and polarization randomization uniformity ≥98%.

[0014] The present invention also discloses a quantum key distribution system, including a first sender, a second sender, and a measuring party, wherein the first sender and the second sender are connected to the measuring party through a first optical fiber channel and a second optical fiber channel, respectively; Both the first and second senders contain a quantum state preparation module; The quantum state preparation module is used to generate phase-encoded quantum states; The measuring method includes the Bell state measuring device as described in any one of claims 1-9.

[0015] Preferably, the two depolarization modules are respectively located on the first transmitter and the second transmitter, or both are located on the measuring side, or one is located on one transmitter and the other is located on the measuring side.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention proposes a Bell state measurement device and a quantum key distribution system. By using a polarization depolarization module (DEP) to randomize the polarization of the input quantum state, combined with a 45° polarization rotation design of an unequal-arm polarization interferometer, the influence of polarization state fluctuations, fiber birefringence, and polarization mode dispersion on Bell state measurement is completely eliminated. No additional polarization tracking or compensation modules are required, significantly reducing system complexity and cost, and improving system environmental adaptability. Furthermore, the use of a polarization beamsplitter with a high polarization extinction ratio instead of a beamsplitter improves the accuracy of HOM interferometry and reduces the bit error rate of the quantum key distribution system. Attached Figure Description

[0017] Figure 1 This is a block diagram illustrating the principle of the Bell state measurement device of the present invention. Figure 2 This is a schematic diagram of a first embodiment of the Bell state measuring device of the present invention; Figure 3 This is a schematic diagram of the second embodiment of the Bell state measuring device of the present invention; Figure 4 This is a schematic diagram of an embodiment of the quantum key distribution system based on a Bell state measurement device according to the present invention. Detailed Implementation

[0018] The present invention will now be clearly and completely described with reference to the accompanying drawings in the embodiments of the present invention.

[0019] like Figure 1As shown, a Bell state measurement device is used to perform Bell state joint measurement on phase-encoded quantum states, including a first depolarization module DEP1, a second depolarization module DEP2, an unequal-arm polarization interferometer, and a first single-photon detector SPD1 and a second single-photon detector SPD2. The first depolarization module DEP1 and the second depolarization module DEP2 are respectively used to perform polarization randomization processing on the phase-encoded quantum states input to the two input ports of the unequal arm polarization interferometer, so that the polarization degrees of freedom of the quantum states are completely decoherent and the influence of polarization state fluctuations on the measurement results is eliminated. The arm length difference of the unequal arm polarization interferometer matches the time difference between the two time modes (early pulse E and late pulse L) of the phase-encoded quantum state. It can perform polarization beam splitting on the phase-encoded quantum state after polarization randomization, delay one polarization component after beam splitting, rotate the two polarization components by 45° respectively, and finally perform secondary polarization beam splitting on the rotated polarization components, ultimately producing two mutually orthogonal interference results. SPD1 and SPD2 are connected to the two outputs of the unequal-arm polarization interferometer, respectively, to detect the two interference results and realize Bell state measurement based on the triggering of the detection signal.

[0020] The specific work process is as follows: The four commonly used phase-encoded quantum states are: ,in This indicates that the photon in the quantum state is in the early time mode E. This indicates a late-time mode L. Two phase-encoded quantum states, Q1 and Q2, enter the two input ports of the Bell state measurement device, respectively. They are first perturbed by DEP1 and DEP2 to achieve polarization randomization, and then enter the two input ports of the unequal-arm polarization interferometry module. The specific transmission process and formula derivation are as follows: The two input quantum states can be written as follows: , Both are subjected to polarization randomization processes via DEP1 and DEP2 respectively. After depolarization, the polarization degrees of freedom of the quantum state are completely decoherent, retaining only the phase information of the time dual-mode. Let the quantum state before depolarization be... The density matrix of the quantum states after debiasing is: , in Let be the identity matrix of the polarization space, and Tr denotes the trace operation, which is used to eliminate polarization degrees of freedom. After depolarization, the quantum state can be simplified to a pure state of time dual mode.

[0021] The depolarized two-photon state is incident on the PBS, whose role is to separate the horizontal polarization component. Transmitted into the short-arm optical path, the vertical polarization component... The Jones matrices of the reflected light onto the long arm optical path are as follows: , Since the polarization degrees of freedom are decoherent after depolarization, the beam splitting process of the PBS can be simplified to a time-mode beam splitting after tracing the polarization space. Two quantum states, after being simultaneously split by the PBS, enter photon polarization states that are orthogonal to each other in the short or long arm (H and V). Therefore, the quantum states can be written as follows: , The quantum states of both the long and short arms undergo a 45° polarization rotation via the polarization rotation module, which is essentially a Hadamard transformation, and the corresponding Jones matrix is: , This transformation will Mapped to ,Will Mapped to This decouples the polarization space from the time mode space, ensuring that the time mode information of the quantum state is not affected after polarization rotation.

[0022] Because the longer optical path of the long arm is longer than that of the short arm, the longer propagation time of the long-arm quantum state is significantly longer than that of the short-arm quantum state. This delay is exactly equal to the time interval between the two time modes (E and L), ensuring that the E and L pulses can effectively interfere at the polarization beam splitter. The phase difference corresponding to the delay is... Combined with phase feedback module adjustment, the inherent phase difference is reduced. At this point, the transmission phases of the short-arm and long-arm quantum states are respectively , The quantum state after transmission is: , The quantum states after the short and long arms are rotated re-enter the polarization beam splitter for secondary beam splitting and beam combining interference. The interference results are then fed into SPD1 and SPD2, respectively. Because... and , and , and Patterns cannot overlap in time, therefore they cannot interfere; only and Patterns overlapping in time can cause interference; gating the SPD can then be used to target only the affected areas. and The detection is performed within the time window corresponding to the mode, and the corresponding quantum state is... , in, and These represent the 1-photon state and the 2-photon state, respectively.

[0023] Simultaneous responses from SPD1 and SPD2 are considered successful responses to Bell state measurements; responses from only one single-photon detector are discarded. It can be seen that when... When the values ​​are the same, SPD1 and SPD2 respond simultaneously, resulting in a successful response. However, when... When the values ​​of SPD1 and SPD2 differ by π, that is, when they have the same basis but different phases, SPD1 and SPD2 will only have one response, meaning the probability of obtaining a successful response is 0.

[0024] like Figure 2 As shown, this is Embodiment 1 of the present invention: The unequal-arm polarization interferometer is an unequal-arm Mach-Zehnder interferometer composed of two polarization beamsplitters (PBS). The first polarization beamsplitter, PBS1, serves as the input beamsplitter and is used to perform the first polarization beam splitting on the phase-encoded quantum state after polarization randomization. The second polarization beamsplitter, PBS2, serves as the output beam combiner and is used to perform a second polarization beam splitting and beam combining interference on the polarization components after delay and polarization rotation. In the short-arm and long-arm optical paths of the unequal-arm MZ interferometer, a polarization rotation module (PR1 and PR2) is respectively set. The polarization rotation module is used to rotate the polarization components passing through it by 45° to realize the Hadamard basis transformation.

[0025] The specific work process is as follows: The two input quantum states can be written as follows: , Both are subjected to polarization randomization processes via DEP1 and DEP2 respectively. After depolarization, the polarization degrees of freedom of the quantum state are completely decoherent, retaining only the phase information of the time dual-mode. Let the quantum state before depolarization be... The density matrix of the quantum states after debiasing is: , in Let be the identity matrix of the polarization space, and Tr denotes the trace operation, which is used to eliminate polarization degrees of freedom. After depolarization, the quantum state can be simplified to a pure state of time dual mode.

[0026] After depolarization, the two-photon state is incident on PBS1. Since the polarization degrees of freedom are decoherent after depolarization, the beam splitting process of PBS1 can be simplified to time-mode beam splitting after tracing the polarization space. The photon polarization states of the two quantum states entering the short arm or long arm after passing through the beam split of PBS1 are orthogonal (H and V). Therefore, the quantum state can be written as follows: , The quantum states of the long and short arms undergo a 45° polarization rotation via polarization rotation modules PR1 and PR2, respectively. This is essentially a Hadamard transformation, and the corresponding Jones matrix is: , This transformation will Mapped to ,Will Mapped to This decouples the polarization space from the time mode space, ensuring that the time mode information of the quantum state is not affected after polarization rotation.

[0027] Because the longer optical path of the long arm is longer than that of the short arm, the longer propagation time of the long-arm quantum state is significantly longer than that of the short-arm quantum state. This delay is exactly equal to the time interval between the two time modes (E and L), ensuring that the E and L pulses can effectively interfere at the polarization beam splitter. The phase difference corresponding to the delay is... Combined with phase feedback module adjustment, the inherent phase difference is reduced. At this point, the transmission phases of the short-arm and long-arm quantum states are respectively , The quantum state after transmission is: , The quantum states after the short and long arms are rotated enter PBS2 for secondary beam splitting and beam combining interference, and the interference results enter SPD1 and SPD2 respectively. Because... and , and , and Patterns cannot overlap in time, therefore they cannot interfere; only and Patterns overlapping in time can cause interference; gating the SPD can then be used to target only the affected areas. and The detection is performed within the time window corresponding to the mode, and the corresponding quantum state is... , in, and These represent the 1-photon state and the 2-photon state, respectively.

[0028] Simultaneous responses from SPD1 and SPD2 are considered successful responses to Bell state measurements; responses from only one single-photon detector are discarded. It can be seen that when... When the values ​​are the same, SPD1 and SPD2 respond simultaneously, resulting in a successful response. However, when... When the values ​​of SPD1 and SPD2 differ by π, that is, when they have the same basis but different phases, SPD1 and SPD2 will only have one response, meaning the probability of obtaining a successful response is 0.

[0029] like Figure 3 As shown, this is embodiment two of the present invention: The unequal-arm polarization interferometer is a Faraday-Michelson interferometer, which includes a third polarization beam splitter PBS3 and two Faraday rotating mirrors (FRM1 and FRM2). The two Faraday rotating mirrors (FRM1 and FRM2) are respectively set on the two output arms of PBS3, forming an unequal arm structure. The difference in arm length matches the time difference between the two time modes of the phase-encoded quantum state. The magneto-optical crystal in the Faraday rotating mirror has an optical rotation angle of 22.5°. When a photon passes through the Faraday rotating mirror, its polarization state is cumulatively rotated by 45°, thus achieving a 45° polarization rotation function. At each of the two input ports of the PBS3, a circulator (CIR1 and CIR2) is set. The circulator is used to transmit the incident phase-encoded quantum state to the PBS3 and to transmit the two interference results after interference by the Faraday Michelson interferometer in the reverse direction to the two single-photon detectors.

[0030] The magneto-optical crystals of FRM1 and FRM2 are made of terbium gallium garnet (TGG) material, with a Verdet constant ≥ 15 rad / (T·m). At the working wavelength, the optical rotation angle stability is ≤ 0.1° / ℃, ensuring that the polarization rotation angle of the photon is accurately maintained at 45° after round trip.

[0031] The specific work process is as follows: The two input quantum states can be written as follows: , Both are subjected to polarization randomization processes via DEP1 and DEP2 respectively. After depolarization, the polarization degrees of freedom of the quantum state are completely decoherent, retaining only the phase information of the time dual-mode. Let the quantum state before depolarization be... The density matrix of the quantum states after debiasing is: , in Let be the identity matrix of the polarization space, and Tr denotes the trace operation, which is used to eliminate polarization degrees of freedom. After depolarization, the quantum state can be simplified to a pure state of time dual mode.

[0032] After depolarization, the two-photon states are incident on the two input ports of PBS3 via CIR1 and CIR2, respectively. Since the polarization degrees of freedom are decoherent after depolarization, the beam splitting process of PBS3 can be simplified to time-mode beam splitting after tracing the polarization space. The photon polarization states of the two quantum states entering the short arm or long arm after being simultaneously split by PBS3 are orthogonal (H and V). Therefore, the quantum states can be written as follows: , The photon travels back and forth within the arms containing FRM1 and FRM2. Upon passing through the FRM, its polarization state rotates due to the magneto-optical effect of the FRM. According to Faraday's rotation principle, the optical rotation angle of the photon in a single pass through the FRM is... After the round trip, the cumulative optical rotation angle is 45°, achieving a 45° polarization rotation. The formula for optical rotation is: (in It is the optical rotation angle. It is the Verdet constant. The magnetic field strength, (where is the length of the magneto-optical crystal). After round-trip transmission, the Jones matrix corresponding to the polarization rotation is: , This transformation will Mapped to ,Will Mapped to This decouples the polarization space from the time mode space, ensuring that the time mode information of the quantum state is not affected after polarization rotation.

[0033] Because the longer optical path of the long arm is longer than that of the short arm, the longer propagation time of the long-arm quantum state is significantly longer than that of the short-arm quantum state. This delay is exactly equal to the time interval between the two time modes (E and L), ensuring that the E and L pulses can effectively interfere at the polarization beam splitter. The phase difference corresponding to the delay is... Combined with phase feedback module adjustment, the inherent phase difference is reduced. At this point, the transmission phases of the short-arm and long-arm quantum states are respectively , The quantum state after transmission is: , The quantum states after rotational reflection of the short and long arms return to the two output ports of PBS3 for secondary beam splitting and beam combining interference. The interference results are then fed into SPD1 and SPD2, respectively. Because... and , and , and Patterns cannot overlap in time, therefore they cannot interfere; only and Patterns overlapping in time can cause interference; gating the SPD can then be used to target only the affected areas. and The detection is performed within the time window corresponding to the mode, and the corresponding quantum state is... , in, and These represent the 1-photon state and the 2-photon state, respectively.

[0034] Simultaneous responses from SPD1 and SPD2 are considered successful responses to Bell state measurements; responses from only one single-photon detector are discarded. It can be seen that when... When the values ​​are the same, SPD1 and SPD2 respond simultaneously, resulting in a successful response. However, when... When the values ​​of SPD1 and SPD2 differ by π, that is, when they have the same basis but different phases, SPD1 and SPD2 will only have one response, meaning the probability of obtaining a successful response is 0.

[0035] like Figure 4 As shown, a quantum key distribution system based on the Bell state measurement device includes a first sender, a second sender, and a measurer; the first sender and the second sender are connected to the measurer through a first optical fiber channel and a second optical fiber channel, respectively. Both the first and second senders contain a quantum state preparation module; The quantum state preparation module is used to generate phase-encoded quantum states; The measuring method includes the Bell state measuring device.

[0036] The work process is as follows: The optical pulse signal generated by the laser LD of the first transmitter is randomly modulated into a signal state or a decoy state by the intensity modulator IM, and then enters the phase encoding module, where four phase-encoded states are randomly prepared. These states are then attenuated to the single-photon level by the adjustable attenuator VOA, becoming the time-phase-encoded quantum state Q1, and finally enters the first optical fiber channel. Similarly, the second transmitter undergoes the same process to randomly prepare the time-phase-encoded quantum state Q2, which then enters the second optical fiber channel.

[0037] The two quantum states entering the measurement side can be written as follows: , Both are subjected to polarization randomization processes via DEP1 and DEP2 respectively. After depolarization, the polarization degrees of freedom of the quantum state are completely decoherent, retaining only the phase information of the time dual-mode. Let the quantum state before depolarization be... The density matrix of the quantum states after debiasing is: in Let be the identity matrix of the polarization space, and Tr denotes the trace operation, which is used to eliminate polarization degrees of freedom. After depolarization, the quantum state can be simplified to a pure state of time dual mode.

[0038] After depolarization, the two-photon state is incident on PBS1. Since the polarization degrees of freedom are decoherent after depolarization, the beam splitting process of PBS can be simplified to time-mode beam splitting after tracing the polarization space. The photon polarization states of the two quantum states entering the short arm or long arm after being simultaneously split by PBS1 are orthogonal (H and V). Therefore, the quantum state can be written as follows: , The quantum states of the long and short arms undergo a 45° polarization rotation via polarization rotation modules PR1 and PR2, respectively. This is essentially a Hadamard transformation, and the corresponding Jones matrix is: This transformation will Mapped to ,Will Mapped to This decouples the polarization space from the time mode space, ensuring that the time mode information of the quantum state is not affected after polarization rotation.

[0039] Because the longer optical path of the long arm is longer than that of the short arm, the longer propagation time of the long-arm quantum state is significantly longer than that of the short-arm quantum state. This delay is exactly equal to the time interval between the two time modes (E and L), ensuring that the E and L pulses can effectively interfere at the polarization beam splitter. The phase difference corresponding to the delay is... Combined with phase feedback module adjustment, the inherent phase difference is reduced. At this point, the transmission phases of the short-arm and long-arm quantum states are respectively , The quantum state after transmission is: , The quantum states after the short and long arms are rotated enter PBS2 for secondary beam splitting and beam combining interference, and the interference results enter SPD1 and SPD2 respectively. Because... and , and , and Patterns cannot overlap in time, therefore they cannot interfere; only and Patterns overlapping in time can cause interference; gating the SPD can then be used to target only the affected areas. and The detection is performed within the time window corresponding to the mode, and the corresponding quantum state is... , in, and These represent the 1-photon state and the 2-photon state, respectively.

[0040] Simultaneous responses from SPD1 and SPD2 are considered successful responses to Bell state measurements; responses from only one single-photon detector are discarded. It can be seen that when... When the values ​​are the same, SPD1 and SPD2 respond simultaneously, resulting in a successful response. However, when... When the values ​​of SPD1 and SPD2 differ by π, that is, when they have the same basis but different phases, SPD1 and SPD2 will only have one response, meaning the probability of obtaining a successful response is 0.

[0041] The measuring party publishes the response results of the first and second single-photon detectors to the first and second senders, and then performs processes such as basis setting and post-processing according to the measurement device-independent quantum key distribution protocol to finally generate a secure key.

[0042] In addition, the two depolarization modules are respectively set on the first transmitter and the second transmitter, or both are set on the measurement side, or one is set on one transmitter and the other is set on the measurement side.

Claims

1. A Bell state measurement device for performing Bell state joint measurements of phase-encoded quantum states, characterized in that, include: The first depolarization module DEP1 and the second depolarization module DEP2 are used to perform polarization randomization processing on the phase-encoded quantum states input to the two input ports of the unequal-arm polarization interferometer, so that the polarization degrees of freedom of the quantum states are completely decoherent. The unequal-arm polarization interferometer matches the time difference between the early pulse E and the late pulse L of the two time modes of the phase-encoded quantum state. It can perform polarization beam splitting on the phase-encoded quantum state after polarization randomization, delay one polarization component after beam splitting, rotate the two polarization components by 45° respectively, and finally perform secondary polarization beam splitting on the rotated polarization components to produce two mutually orthogonal interference results. The first single-photon detector SPD1 and the second single-photon detector SPD2 are connected to the two output terminals of the unequal-arm polarization interferometer, respectively, to detect the two interference results and realize Bell state measurement based on the triggering condition of the detection signal.

2. The Bell state measuring device according to claim 1, characterized in that, The unequal-arm polarization interferometer is an unequal-arm Mach-Zehnder interferometer composed of a first polarization beamsplitter PBS1 and a second polarization beamsplitter PBS2. The first polarization beamsplitter PBS1 serves as the input beamsplitter, used to perform the first polarization beam splitting on the phase-encoded quantum state after polarization randomization. The second polarization beamsplitter PBS2 serves as the output beam combiner, used to perform a second polarization beam splitting and beam combining interference on the polarization components after delay and polarization rotation. In the short-arm and long-arm optical paths of the unequal-arm Mach-Zehnder interferometer, a first polarization rotation module PR1 and a second polarization rotation module PR2 are respectively set. Both polarization rotation modules are used to rotate the polarization components passing through them by 45° to realize the Hadamard basis transformation.

3. The Bell state measuring device according to claim 1, characterized in that, The unequal-arm polarization interferometer is a Faraday-Michelson interferometer, which includes a third polarization beam splitter PBS3, a first Faraday rotator FRM1, and a second Faraday rotator FRM2. Two Faraday rotating mirrors are respectively set on the two output arms of the third polarization beam splitter PBS3, forming an unequal arm structure. The difference in arm length is matched with the time difference between the two time modes of the phase-encoded quantum state. The optical rotation angle of the magneto-optical crystal in the Faraday rotating mirror is 22.5°. When a photon passes through the Faraday rotating mirror back and forth, its polarization state is cumulatively rotated by 45°, thus achieving a 45° polarization rotation function. At the two input ports of the third polarization beam splitter PBS3, a first circulator CIR1 and a second circulator CIR2 are respectively set. Both circulators are used to transmit the incident phase-encoded quantum state to the third polarization beam splitter PBS3, and to transmit the two interference results after interference by the Faraday Michelson interferometer in the opposite direction to the two single-photon detectors.

4. The Bell state measuring device according to claim 2 or 3, characterized in that, The functions of each polarization beam splitter are as follows: when a horizontally polarized light signal or a vertically polarized light signal is incident from its first input port, it will be emitted from its first output port or its second output port, both of which are horizontally polarized; when a horizontally polarized light signal or a vertically polarized light signal is incident from its second input port, it will be emitted from its first output port or its second output port, both of which are vertically polarized.

5. The Bell state measuring device according to claim 2, characterized in that, Both polarization rotation modules are polarization-maintaining fiber 45° fusion splicing structures. By splicing the fast or slow axes of two polarization-maintaining fiber segments at a 45° angle, the polarization components passing through the fusion splicing structure undergo a 45° polarization rotation.

6. The Bell state measuring device according to claim 1, characterized in that, The phase-coded quantum state is a time-dual-mode phase-coded quantum state, with the encoding basis being the X basis and Y basis in the BB84 protocol. The X basis corresponds to a phase difference of 0 or π between the two time modes, and the quantum states are respectively... , The phase difference between the two time modes corresponding to the Y basis is π / 2 or 3π / 2, and the quantum states are respectively , ; The arm length difference ΔL of the unequal-arm polarization interferometer satisfies , where c is the speed of light in the medium, and Δt is the time interval between the two time modes E and L, so that the two time modes can effectively interfere after being transmitted through an unequal arm optical path.

7. The Bell state measuring device according to claim 2, characterized in that, The inherent phase difference between the short-arm and long-arm optical paths of the unequal-arm Mach-Zehnder interferometer Where λ is the wavelength of the phase-encoded quantum state, and ΔL is the difference in arm length between the short and long arms; the unequal-arm Mach-Zehnder interferometer also includes a phase feedback module for real-time adjustment of the inherent phase difference. ,make Ensure that the interference is at the constructive operating point and the interference contrast is ≥98%.

8. The Bell state measuring device according to claim 3, characterized in that, The magneto-optical crystals of the two Faraday rotators are made of terbium gallium garnet (TGG) material, with a Verdet constant ≥ 15 rad / (T·m). At the working wavelength, the optical rotation angle stability is ≤ 0.1° / ℃, ensuring that the polarization rotation angle of the photon after round trip is precisely maintained at 45°.

9. The Bell state measuring device according to claim 1, characterized in that, Both depolarization modules are passive polarization scramblers, which enable rapid randomization of polarization states, decoherence of the polarization degrees of freedom of quantum states, and polarization randomization uniformity ≥98%.

10. A quantum key distribution system, characterized in that, It includes a first transmitter, a second transmitter, and a measuring party, wherein the first transmitter and the second transmitter are connected to the measuring party through a first optical fiber channel and a second optical fiber channel, respectively; Both the first and second senders contain a quantum state preparation module; The quantum state preparation module is used to generate phase-encoded quantum states; The measuring method includes the Bell state measuring device as described in any one of claims 1-9.

11. The quantum key distribution system according to claim 10, characterized in that, The two depolarization modules are set on the first transmitter and the second transmitter respectively, or both are set on the measurement side, or one is set on one transmitter and the other is set on the measurement side.

Images