Quantum key distribution system based on PZT phase modulation and PZT phase correction method

Abstract

The invention discloses a quantum key distribution system based on PZT phase modulation and a PZT phase correction method. The method specifically comprises the following steps: determining expected working voltages of a first PZT and a second PZT; dynamically adjusting the working voltage of the first PZT based on the current detection error rate of the first zero-beat single-photon detector SPD or the second zero-beat single-photon detector SPD and the deviation distance between the current working voltage of the first PZT and the expected working voltage; and dynamically adjusting the working voltage of the second PZT based on the detection error rate of the third zero-beat single-photon detector SPD or the fourth zero-beat single-photon detector SPD and the deviation distance between the current working voltage and the expected working voltage of the second PZT. The PZT is used as the phase modulator, phase modulation is performed on light quantum, passive decoding is performed according to the response of the detector, the phase modulator does not need to participate in modulation, high-speed quantum random number resources of 2bit / photon are reduced, and the practical application cost and difficulty of the high-speed quantum key distribution system are greatly reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of quantum key distribution. More specifically, the present invention relates to a quantum key distribution system based on PZT phase modulation and a PZT phase correction method. Background Art

[0002] With the rapid development of modern communication technologies, the communication environment has become increasingly complex, and communication security issues have become increasingly serious. All walks of life attach more and more importance to communication security. At present, the most commonly used RSA encryption algorithm has difficulty in ensuring security under the impact of quantum computing. Quantum secure communication is based on the basic laws of quantum mechanics, the quantum no-cloning property and the Heisenberg uncertainty principle. Encrypting information in the "one-time pad" manner ensures the unconditional security of quantum cryptography. The application of quantum key distribution technology has become increasingly widespread.

[0003] The commonly used coding methods in quantum key distribution technology mainly include polarization coding and phase coding. In the quantum key distribution of the polarization coding optical fiber transmission scheme, due to the inherent birefringence effect of the optical fiber in the channel, the polarization state of photons will randomly change during transmission, and it is greatly affected by the external environment. As a result, the polarization state of photons reaching the receiving end cannot be predicted. If the measurement is performed according to the agreed polarization direction, incorrect detection results may occur, leading to problems such as short transmission distance and high bit error rate. For the quantum key distribution system using the phase coding scheme, the influence of polarization perturbation in the optical fiber channel on the system can be eliminated, and it has stronger environmental robustness. Therefore, the commonly used quantum key distribution scheme is mainly phase coding, and the key information is transmitted by encoding the phase difference of photons.

[0004] However, in the quantum key distribution system, for each 1-bit optical quantum modulated by the sending end, 4 bits of random number resources are required: whether the laser is turned on (1 / 0, 1 means there is a photon, 0 means the vacuum state), whether the intensity modulator modulates the signal state or the decoy state, whether the basis bit is modulated to the 0 phase or the π / 2 phase, and whether the key bit is modulated to the 0 phase or the π phase. At the same time, the receiving end also requires 2 bits: whether the basis bit is modulated to the 0 phase or the π / 2 phase, and whether the key bit is modulated to the 0 phase or the π phase. The decoding of the high-speed phase modulator at the receiving end is consistent with the encoding frequency of the high-speed encoder at the sending end. When the optical quantum sending frequency doubles, the random number requirement at the receiving end also doubles, resulting in insufficient random number resources. Moreover, most of the photons output by the sending end are lost in the channel and cannot reach the receiving end. According to the estimation of the 50 km channel attenuation and optical loss, the probability of photons reaching the receiving end is only on the order of 1‰. If the receiving end also uses the same high-speed phase modulation frequency, it will cause a great waste of random number resources. Summary of the Invention

[0005] The present invention provides a quantum key distribution system based on PZT phase modulation, aiming to improve at least one of the above problems.

[0006] The present invention is implemented as follows. A quantum key distribution system based on PZT phase modulation, the system includes: a sending end and a receiving end. Quantum communication is carried out between the sending end and the receiving end through a quantum state transmission channel, where;

[0007] The sending end generates a quantum optical pulse of single photons, randomly modulates the quantum optical pulse into a signal state optical pulse or a decoy state optical pulse with different phases, and sends it to the receiving end through the quantum state transmission channel;

[0008] The receiving end includes: a beam splitter that divides the optical quantum arriving at the receiving end into two identical optical signals, and inputs them into the first PZT and the second PZT respectively. The first PZT maintains a 0-phase modulation on the input optical quantum by controlling its operating voltage, and the second PZT maintains a π / 2-phase modulation on the input optical quantum by controlling its operating voltage. The first homodyne single photon detector SPD and the second homodyne single photon detector SPD connected to the first PZT, and the third homodyne single photon detector SPD and the fourth homodyne single photon detector SPD connected to the second PZT. The first homodyne single photon detector SPD applies a 0-phase difference after the first PZT through a circulator, the second homodyne single photon detector SPD applies a π-phase difference after the first PZT through a circulator, the third homodyne single photon detector SPD applies a 0-phase difference after the second PZT through a circulator, and the fourth homodyne single photon detector SPD applies a π-phase difference after the second PZT through a circulator.

[0009] The present invention is implemented as follows. A PZT phase correction method for a quantum key distribution system based on PZT phase modulation, the method is specifically as follows:

[0010] (1) Determine the expected operating voltages of the first PZT and the second PZT;

[0011] (2) Dynamically adjust the operating voltage of the first PZT based on the current detection error rate of the first homodyne single photon detector SPD or the second homodyne single photon detector SPD and the deviation distance between the current operating voltage and the expected operating voltage of the first PZT;

[0012] (3) Dynamically adjust the operating voltage of the second PZT based on the detection error rate of the third homodyne single photon detector SPD or the fourth homodyne single photon detector SPD and the deviation distance between the current operating voltage and the expected operating voltage of the second PZT.

[0013] Furthermore, the method for determining the expected operating voltage of the first PZT is specifically as follows:

[0014] The control base-bit phase modulator sends the 0101 base-bit clock code, and the key-bit phase modulator sends the 0101 key-bit clock code. Define the first bit of the base-bit clock encoding and the key-bit clock encoding as the 0 serial number. Change the voltage value of the first PZT according to the set step size. Each time the voltage value is changed, the transmitting end sends a certain amount of optical pulses. Respectively count the responses of the first homodyne single-photon detector SPD to the odd-count optical pulses and even-count optical pulses sent by the transmitting end. Take the voltage value with the highest response to the even-count optical pulses as the expected operating voltage of the first PZT.

[0015] Further, the method for determining the expected operating voltage of the second PZT is as follows:

[0016] The control base-bit phase modulator sends the 0101 base-bit clock code, and the key-bit phase modulator sends the 0101 key-bit clock code. Define the first bit of the base-bit clock encoding and the key-bit clock encoding as the 0 serial number. Change the voltage value of the second PZT according to the set step size. Each time the voltage value is changed, the transmitting end sends a certain amount of optical pulses. Respectively count the responses of the fourth homodyne single-photon detector SPD to the odd-count optical pulses and even-count optical pulses sent by the transmitting end. Take the voltage value with the highest response to the odd-count optical pulses as the expected operating voltage of the second PZT.

[0017] Further, the dynamic adjustment process of the operating voltage of the PZT is as follows:

[0018] (21) At the end of this round, determine the average bit error rate err_currt of the corresponding homodyne single-photon detector SPD. Detect whether the average bit error rate err_currt of this round is greater than the bit error rate threshold. If the detection result is yes, execute step (22);

[0019] (22) Determine whether the average bit error rate err_currt of this round has decreased compared to the average bit error rate err_old of the previous round. If the detection result is yes, take the voltage adjustment direction of the PZT in this round as the voltage adjustment direction of the next round. If the detection result is no, take the reverse of the voltage adjustment direction of the PZT in this round as the voltage adjustment direction of the next round;

[0020] (23) Calculate the adjustment step size volt_step of the PZT operating voltage, volt_step = |err_currt - err_old|·100·ρ, where ρ is the proportionality coefficient, which reflects the ratio of the bit error rate to the deviation distance between the PZT operating voltage and the expected operating voltage;

[0021] (24) On the voltage adjustment direction of the next round, adjust the operating voltage of this round by volt_step to form the operating voltage of the PZT in the next round.

[0022] Further, before step (21), it further includes:

[0023] Detect whether the average bit error rate err_currt is greater than the upper limit err_thld_up of the bit error rate threshold. If the detection result is yes, return to step (1). If the detection result is no, execute step (21).

[0024] Further, the PZT is the first PZT or the second PZT. When the PZT is the first PZT, the average bit error rate err_currt is the average of the detection bit error rates of the first homodyne single-photon detector SPD and the second homodyne single-photon detector SPD. When the PZT is the second PZT, the average bit error rate err_currt is the average of the detection bit error rates of the third homodyne single-photon detector SPD and the fourth homodyne single-photon detector SPD.

[0025] Further, the calibration method of the proportionality coefficient ρ is specifically as follows:

[0026] After determining the expected working voltage of the PZT, change the working voltage in a certain voltage step step = 2 near the expected working voltage. Each time the working voltage is changed, send a certain number N of photons, count the detection bit error rate of the corresponding homodyne single-photon detector SPD, and record the current detection bit error rate and the difference between the working voltage of the PZT and the expected working voltage. Finally, fit the proportionality coefficient ρ between the detection bit error rate and the difference between the working voltage of the PZT and the expected working voltage.

[0027] Further, the method for determining the detection bit error rate of the homodyne single-photon detector SPD is specifically as follows:

[0028] The sending end randomly modulates the basis bit, key bit, and intensity modulator modulation signal of the optical quantum, and records the basis bit, key bit, and intensity modulator modulation signal of the current randomly modulated optical quantum in Alice rawkey;

[0029] After the optical pulse reaches the receiving end, if the first homodyne single-photon detector SPD responds, Bob raw key records the basis bit as 0 and the key bit as 0. If the second homodyne single-photon detector SPD responds, Bob raw key records the basis bit as 0 and the key bit as 1. If the third homodyne single-photon detector SPD responds, Bob raw key records the basis bit as 1 and the key bit as 0. If the fourth homodyne single-photon detector SPD responds, Bob raw key records the basis bit as 1 and the key bit as 1.

[0030] Send the Bob raw key to the receiver. Based on the Alice raw key, determine the signal-state photons in the Bob raw key, and retain the key bits where the basis bits of the signal-state photons in the Bob raw key and the Alice raw key are the same. Randomly announce some key bits to determine the current bit error rate of the corresponding homodyne single-photon detector SPD.

[0031] In the present invention, a PZT is used as a phase modulator to perform phase modulation on optical quanta, and passive decoding is performed according to the response of the detector, without the participation of a phase modulator in modulation, and the high-speed quantum random number resource of 2 bit / photon is reduced, greatly reducing the cost and difficulty of the actual application of the high-speed quantum key distribution system, thereby improving the secure key generation rate of the quantum key distribution system and expanding the scope of its application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic structural diagram of a quantum key distribution system based on PZT phase modulation provided by an embodiment of the present invention;

[0033] Figure 2 It is a schematic diagram of the expected working voltage of the first PZT provided by an embodiment of the present invention;

[0034] Figure 3 It is a schematic diagram of the expected working voltage of the second PZT provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] The following will further describe in detail the specific embodiments of the present invention with reference to the accompanying drawings and through the description of the embodiments, so as to help those skilled in the art have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention.

[0036] Figure 1 It is a schematic structural diagram of a quantum key distribution system based on PZT phase modulation provided by an embodiment of the present invention. For the convenience of description, only the parts related to the embodiments of the present invention are shown. The system includes:

[0037] A sending end and a receiving end. Quantum communication is carried out between the sending end and the receiving end through a quantum state transmission channel. Among them, the sending end includes:

[0038] A semiconductor laser (LD) generates quantum optical pulses of single photons by high-frequency triggering; a high-speed encoder (Enc) randomly selects a 1-bit basis (1 or 0) and a 1-bit key (1 or 0), phase-modulates the optical quantum, then randomly selects a 1-bit intensity modulator to modulate it into a signal state or a decoy state optical pulse (1 or 0), and sends it to the receiving end through a quantum state transmission channel; the 1-bit basis is also called a basis bit, which is used to determine which group of bases to select, and the 1-bit key is also called a key bit, which is used to determine which quantum state of the selected basis to use.

[0039] The receiving end includes: a beam splitter (BS) that divides the optical quantum reaching the receiving end into two identical optical signals, and inputs them into the first PZT and the second PZT respectively. The first PZT maintains a 0-phase modulation on the input optical quantum by controlling its working voltage, and the second PZT maintains a π / 2-phase modulation on the input optical quantum by controlling its working voltage. The first homodyne single-photon detector SPD and the second homodyne single-photon detector SPD connected to the first PZT, and the third homodyne single-photon detector SPD and the fourth homodyne single-photon detector SPD connected to the second PZT. The first homodyne single-photon detector SPD applies a 0-phase difference after the first PZT through a circulator, the second homodyne single-photon detector SPD applies a π-phase difference after the first PZT through a circulator, the third homodyne single-photon detector SPD applies a 0-phase difference after the second PZT through a circulator, and the fourth homodyne single-photon detector SPD applies a π-phase difference after the second PZT through a circulator. The above four homodyne single-photon detectors SPD are all used for the detection of optical signals.

[0040] Since the amplitude value of the interference pulse depends on the phase difference of the phase encoding at both ends, without considering the detection efficiency, dark count and afterpulse influence of the detector, according to the interference formula:

[0041] I = 1 / 2(1 + cos(φ Alice - φ Bob )) (1)

[0042] where I represents the interference probability, and φ Alice , φ Bob represent the phase of the optical quantum modulated at the sending end and the phase of the optical quantum detected at the receiving end respectively.

[0043] When the sending end (Alice) randomly modulates the basis bit 0 and the key bit 0 of the optical quantum, the basis bit is recorded as 0 and the key bit is recorded as 0 in Alice raw key, and φ Alice= 0 × π / 2 + 0 × π = 0, where π / 2 is the phase of the bit encoding and π is the phase of the key bit encoding. If this optical pulse reaches the receiving end and passes through the beam splitter to the channel where the first PZT is located, the probability that the first homodyne single-photon detector SPD can detect is I = 100%, where φ Bob = 0 × π / 2 + 0 × π = 0. If the first homodyne single-photon detector SPD responds, the Bob raw key records the basis bit as 0 and the key bit as 0; the probability that the second homodyne single-photon detector can detect is I = 0%, where, φ Bob = 0 × π / 2 + 1 × π = π. If the second homodyne single-photon detector SPD responds, the Bob raw key records the basis bit as 0 and the key bit as 1; when it passes through the beam splitter and is split to the channel where the second PZT is located, the probability that the third homodyne single-photon detector SPD can detect is I = 50%, where, φ Bob = 1 × π / 2 + 0 × π = 0. If the third homodyne single-photon detector SPD responds, then the Bob raw key records the basis bit as 1 and the key bit as 0; the probability that the fourth homodyne single-photon detector SPD can detect is I = 50%, where φ Bob = 1 × π / 2 + 1 × π = 3π / 2. If the fourth homodyne single-photon detector SPD responds, then the Bob raw key records the basis bit as 1 and the key bit as 1; after obtaining the raw key and announcing the corresponding basis bits of both parties, it can be found that if the first homodyne single-photon detector responds, the basis bits are the same and the key bits are also the same; if the second homodyne single-photon detector responds, the basis bits are the same but the key bits are opposite; if the third homodyne single-photon detector or the fourth homodyne single-photon detector responds, the basis bits are different. If the detection probability is 100%, it means that both the basis bit and the key bit are the same. If the detection probability is 0%, it indicates an extremely low probability. If the homodyne single-photon detector SPD responds, it means that the basis bits are the same but the key bits are opposite, which is an error code. The detection probability of 50% corresponds to the situation where the homodyne single-photon detector SPD may or may not respond. If it responds, by comparing the basis bits, it is found that the basis bits are different and will be discarded, which does not constitute an error code. According to the basis rule, only the key bits with the same basis bits need to be retained. When the basis bits are the same, the key bits are very likely (100%) to be the same and extremely unlikely (0%) to be different.

[0044] When the sender (Alice) randomly modulates the optical quantum basis bit 1 and the key bit 0, the Alice raw key records the basis bit as 1 and the key bit as 0, φ Alice = 1 × π / 2 + 0 × π = π / 2. If this optical pulse reaches the receiving end and passes through the beam splitter to the channel where the first PZT is located, the probability that the first homodyne single-photon detector SPD can detect is I = 50%, where φ Bob= 0×π / 2 + 0×π = 0. If the first homodyne single-photon detector SPD responds, the base bit recorded in Bob's raw key is 0, and the key bit is 0; the probability that the second homodyne single-photon detector SPD can detect is I = 50%, where φ Bob = 0×π / 2 + 1×π = π. If the second homodyne single-photon detector SPD responds, the base bit recorded in Bob's raw key is 0, and the key bit is 1; when it is split by the beam splitter and enters the channel where the second PZT is located, the probability that the third homodyne single-photon detector SPD can detect is I = 100%, where, φ Bob = 1×π / 2 + 0×π = π / 2. If the third homodyne single-photon detector SPD responds, the base bit recorded in Bob's raw key is 1, and the key bit is 0; the probability that the fourth homodyne single-photon detector SPD can detect is I = 0%, where φ Bob = 1×π / 2 + 1×π = 3π / 2. If the fourth homodyne single-photon detector SPD responds, the base bit recorded in Bob's raw key is 1, and the key bit is 1; after obtaining the raw key, when the corresponding base bits of both parties are announced, it can be found that if the third homodyne single-photon detector SPD responds, the base bits are the same and the key bits are also the same; if the fourth homodyne single-photon detector SPD responds, the base bits are the same but the key bits are opposite; if the first homodyne single-photon detector SPD or the second homodyne single-photon detector SPD responds, the base bits are different. According to the base rule, only the key bits with the same base bits need to be retained. When the base bits are the same, the key bits are very likely (100%) to be the same and very unlikely (0%) to be different.

[0045] When the sender (Alice) randomly modulates the other two phases: optical quantum base bit 0 and key bit 1, optical quantum base bit 1 and key bit 1, the processing method is the same as above. Only the key bits with the same base bits in the signal-state photons are retained in Bob's raw key. By sampling some key bits, it is used to determine the current bit error rate of the system. When the detected key bits in Bob's raw key are inconsistent with the key bits sent in Alice's raw key, it is determined that there is a bit error.

[0046] Before the system starts working, determine the working parameters of the system first, including the laser trigger delay, intensity modulator trigger delay, base bit phase modulator delay, key bit phase modulator delay, base bit phase modulator π / 2 phase voltage (0 voltage is the 0 phase voltage), key bit phase modulator π phase voltage (0 voltage is the 0 phase voltage), and the expected working voltage of the first PZT (expected 0 phase modulation voltage) and the expected working voltage of the second PZT (expected π / 2 phase modulation voltage). After determining other parameters except the expected working voltages of the first PZT and the second PZT, according to the requirements of the BB84 protocol, the sending end triggers the laser at a high speed frequency to generate a single-photon quantum optical pulse, and randomly modulates a certain optical intensity (such as three states: signal state, decoy state, and vacuum state; or two states: signal state and decoy state) and phase (jointly determined by the base bit and the key bit) of the optical quantum. After transmission through the quantum state transmission channel, (if possible) it reaches the receiving end, undergoes beam splitter and PZT voltage phase modulation, records the response result of a certain detector, and the sending end and the receiving end respectively record the random numbers used for modulation of this optical quantum to generate their respective raw key information. The sending end raw key includes the laser random number bit, intensity modulator bit, base bit, and key bit. The receiving end raw key includes the base bit and the key bit (it can be determined only by checking which detector responds, and the four detectors correspond to four combinations of base bits and key bits). During the data processing process, only the key bits with signal state photon detection and consistent base bits need to be retained, and the counts of decoy state and vacuum state are discarded. Through algorithms such as error correction and confidentiality enhancement, the security and randomness of the final key can be guaranteed.

[0047] The PZT phase correction method provided by the embodiment of the present invention is specifically as follows:

[0048] (1) First, determine the expected working voltages of the first PZT and the second PZT;

[0049] (2) Dynamically adjust the working voltage of the first PZT based on the current detection error rate of the first homodyne single-photon detector SPD or the second homodyne single-photon detector SPD and the deviation distance between the current working voltage and the expected working voltage of the first PZT;

[0050] (3) Dynamically adjust the working voltage of the second PZT based on the detection error rate of the third homodyne single-photon detector SPD or the fourth homodyne single-photon detector SPD and the deviation distance between the current working voltage and the expected working voltage of the second PZT.

[0051] (11) The expected working voltage of the first PZT is determined;

[0052] After the phase modulation delay and phase modulation voltage of the high-speed encoder at the transmitting end are determined, the high-speed encoder includes: a base bit phase modulator and a key bit phase modulator. When in the usage stage, the key bit phase modulator randomly generates key bits, and the base bit phase modulator randomly generates base bits. When in the test stage, the base bit phase modulator is controlled to send a 0101 base bit clock code, and the key bit phase modulator sends a 0101 key bit clock code. Define the first bit of the base bit clock encoding and the key bit clock encoding as the 0 sequence number, that is, the optical pulse with an odd sequence number (i.e., odd counting) has a modulation phase of 3π / 2, and the optical pulse with an even sequence number has a modulation phase of 0 phase; according to the working voltage range (such as 0 to 4095, the digital signal corresponds to 0 to 10v, etc.) change the voltage value of the first PZT in accordance with the set step size. Each time the voltage value is changed, the transmitting end sends a certain amount of optical pulses. Respectively count the responses of the first homodyne single-photon detector SPD to the odd-counting optical pulses (optical pulses corresponding to the modulation phase of 3π / 2) and even-counting optical pulses (optical pulses corresponding to the modulation phase of 0 phase) emitted by the transmitting end. Take the voltage value with the highest response to the even-counting optical pulses as the expected working voltage of the first PZT, which is also the expected 0-phase modulation voltage (corresponding to the odd counting, this should be the 1 / 2 counting position at this time). Since the voltage range selected in this system (such as 0 to 4095) may include 2 - 3 working cycles of the first PZT (the voltage range of one cycle is about 1500), it is better to select the voltage value of the working point of the first PZT in the middle part of the voltage range, about 1500 - 3000, with better linearity, which is convenient for voltage tracking and correction during operation, such as Figure 2 as shown

[0053] (12) Determination of the expected working voltage of the second PZT;

[0054] After the phase modulation delay and phase modulation voltage of the high-speed encoder at the transmitting end are determined, the high-speed encoder includes: a base bit phase modulator and a key bit phase modulator. During the usage phase, the key bit phase modulator randomly generates key bits, and the base bit phase modulator randomly generates base bits. During the test phase, the base bit phase modulator is controlled to send a 0101 base bit clock code, and the key bit phase modulator sends a 0101 key bit clock code. The first bit of the base bit clock encoding and the key bit clock encoding is defined as the 0 sequence number, that is, the optical pulse with an odd sequence number has a modulation phase of 3π / 2, and the optical pulse with an even sequence number has a modulation phase of 0 phase; according to the working voltage range (such as 0 to 4095, the digital signal corresponds to 0 to 10v, etc.), the voltage value of the second PZT is changed in accordance with the set step size. Each time the voltage value is changed, the transmitting end sends a certain amount of optical pulses. The responses of the fourth zero-beat single-photon detector SPD to the odd-count optical pulses (corresponding to the optical pulses with a modulation phase of 3π / 2) and even-count optical pulses (corresponding to the optical pulses with a modulation phase of 0 phase) emitted by the transmitting end are respectively counted. The voltage value with the highest response to the odd-count optical pulses is used as the expected working voltage of the second PZT, which is also the expected π / 2 phase modulation voltage (corresponding to the even count, which should be at the 1 / 2 count position at this time). Since the selected voltage range (such as 0 to 4095) of this system may cover 2 - 3 working cycles of the second PZT (one cycle is about 1500 or so), it is better to select the voltage value of the working point of the second PZT in the middle part of the voltage range, about 1500 to 3000, with better linearity, which is convenient for voltage tracking and correction during operation, such as Figure 3 as shown

[0055] When the bit error rate is extremely low (the overall bit error rate of the system is less than 5%, and the drift of the PZT working voltage value is within 100), the change in the bit error rate is proportional to the distance between the actual working voltage of the PZT and the expected working voltage. The proportionality coefficient is ρ, and the proportionality coefficient ρ can be estimated through a calibration experiment and does not need to be very accurate. In practical applications, through dynamic adjustment, the bit error rate of the system can also be stabilized within a relatively ideal range. If it exceeds this range, the method for determining the working point voltage described above needs to be used to re-determine the expected working point voltage of the PZT once

[0056] After determining the relationship between the bit error rate fluctuation and the working voltage change, the two PZTs respectively dynamically correct the current working voltage of the PZT according to the detection bit error rate of the corresponding detector. The dynamic correction processes of the two are carried out separately. To reduce the error fluctuation caused by counting fluctuations, the system is run n (such as n = 5) times as a statistical period for the average bit error rate of the detection count. The dynamic correction or adjustment process of the current working voltage of the PZT is specifically as follows

[0057] (21) At the end of this round, determine the average bit error rate err_currt of the system in this round, and detect whether this average bit error rate err_currt is greater than the upper limit err_thld_up of the bit error rate threshold. The upper limit err_thld_up of the bit error rate threshold is less than 5%. If the detection result is yes, return to step (1). If the detection result is no, execute step (22);

[0058] (22) Detect whether the average bit error rate err_currt of this round is greater than the bit error rate threshold. If the detection result is no, use the working voltage of PZT in this round as the working voltage of PZT in the next round. If the detection result is yes, execute step (23);

[0059] (23) Determine whether the average bit error rate err_currt of this round has decreased compared to the average bit error rate err_old of the previous round. If the detection result is yes, use the voltage adjustment direction of PZT in this round as the voltage adjustment direction of the next round. If the detection result is no, use the reverse of the voltage adjustment direction of PZT in this round as the voltage adjustment direction of the next round;

[0060] The voltage adjustment direction includes an increasing direction (positive direction) and a decreasing direction (negative direction). The reverse of the positive direction is the negative direction, and the reverse of the negative direction is the positive direction. If the average bit error rate err_currt of this round has decreased compared to the average bit error rate err_old of the previous round, and the voltage adjustment direction of PZT in the current round is the positive direction, then the voltage adjustment direction of PZT in the next round is still the positive direction. If the average bit error rate err_currt of this round has increased compared to the average bit error rate err_old of the previous round, and the voltage adjustment direction of PZT in the current round is the positive direction, then the voltage adjustment direction of PZT in the next round is still the negative direction, that is, adjust in the direction of decreasing the working voltage of PZT.

[0061] (24) Calculate the adjustment step size volt_step of the PZT working voltage, volt_step = |err_currt - err_old|·100·ρ, where ρ is a proportionality coefficient representing the ratio of the bit error rate to the deviation distance between the PZT working voltage and the expected working voltage;

[0062] (25) Adjust the working voltage of this round by volt_step in the working voltage adjustment direction of the next round to form the working voltage of PZT in the next round.

[0063] The above PZT is the first PZT or the second PZT. When the PZT is the first PZT, the average bit error rate err_currt is the average of the detection bit error rates of the first homodyne single-photon detector SPD and the second homodyne single-photon detector SPD. When the PZT is the second PZT, the average bit error rate err_currt is the average of the detection bit error rates of the third homodyne single-photon detector SPD and the fourth homodyne single-photon detector SPD.

[0064] If there is a slight drift in the phase voltage of the transmitting end, it can also be equivalently considered that the phase voltage of the transmitting end has not changed, but the phase modulation voltage of the receiving end has changed. Thus, the above tracking method is used to adjust the actual working voltage of the PZT.

[0065] After determining the expected working voltage of the PZT, within a small range (±50) around the expected working voltage, the working voltage is changed according to a certain voltage step size step = 2, and a certain number N of photons are sent (to avoid the counting fluctuation error of the detector, N is generally above the order of 1M). The system runs normally, and the bit error rates of the detection counts of the corresponding detectors of the system (the first zero-beat single-photon detector SPD and the second zero-beat single-photon detector SPD are used for the first PZT, and the third zero-beat single-photon detector SPD and the fourth zero-beat single-photon detector SPD are used for the second PZT) are statistically analyzed. The current detection bit error rate and the difference between the working voltage of the PZT and the expected working voltage are recorded. Finally, the proportionality coefficient ρ between the detection bit error rate and the difference between the working voltage of the PZT and the expected working voltage is fitted. The proportionality coefficients ρ of the first PZT and the second PZT are calibrated separately, and the above method is used to calibrate the proportionality coefficient ρ.

[0066] The present invention uses a PZT as a phase modulator to perform phase modulation on optical quanta, passively decodes according to the response of the detector, does not require the phase modulator to participate in the modulation, and reduces the high-speed quantum random number resources of 2 bit / photon, greatly reducing the cost and difficulty of the actual application of the high-speed quantum key distribution system, thereby improving the secure key generation rate of the quantum key distribution system and expanding the scope of its application scenarios.

[0067] The present invention has been described by way of example. Obviously, the specific implementation of the present invention is not limited by the above methods. As long as various non-substantive improvements are made by adopting the method concept and technical solution of the present invention, or the concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. A quantum key distribution system based on PZT phase modulation, characterized in that: The system comprises: a transmitting end and a receiving end, wherein the transmitting end performs quantum communication with the receiving end through a quantum state transmission channel, wherein; The transmitter generates a single-photon quantum light pulse, randomly modulates the quantum light pulse into a signal state light pulse or a decoy state light pulse with different phases, and sends it to the receiver through a quantum state transmission channel; The receiving end includes: a beam splitter, which divides the photon arriving at the receiving end into two identical optical signals, which are input into a first PZT and a second PZT respectively, wherein the first PZT keeps loading a 0 phase modulation on the input photon by controlling its working voltage, and the second PZT keeps loading a π / 2 phase modulation on the input photon by controlling its working voltage, a first zero-beat single-photon detector SPD and a second zero-beat single-photon detector SPD connected to the first PZT, and a third zero-beat single-photon detector SPD and a fourth zero-beat single-photon detector SPD connected to the second PZT, wherein the first zero-beat single-photon detector SPD applies a 0 phase difference through a circulator after the first PZT, the second zero-beat single-photon detector SPD applies a π phase difference through a circulator after the first PZT, the third zero-beat single-photon detector SPD applies a 0 phase difference through a circulator after the second PZT, and the fourth zero-beat single-photon detector SPD applies a π phase difference through a circulator after the second PZT.

2. The PZT phase correction method of the quantum key distribution system based on PZT phase modulation according to claim 1, characterized in that: The method is specifically as follows: (1) Determine the desired operating voltage of the first PZT and the second PZT; (2) dynamically adjusting the operating voltage of the first PZT based on the current detection bit error rate of the first zero-beat single-photon detector SPD or the second zero-beat single-photon detector SPD and the deviation distance between the current operating voltage of the first PZT and the expected operating voltage; (3) Dynamically adjust the operating voltage of the second PZT based on the detection bit error rate of the third zero-beat single-photon detector SPD or the fourth zero-beat single-photon detector SPD and the deviation distance between the current operating voltage of the second PZT and the expected operating voltage.

3. The PZT phase correction method according to claim 2, characterized in that: The method for determining the expected operating voltage of the first PZT is as follows: The base bit phase modulator is controlled to send a 0101 base bit clock code, and the key bit phase modulator is controlled to send a 0101 key bit clock code. The first bit of the base bit clock code and the key bit clock code is defined as a 0 sequence number. The voltage value of the first PZT is changed according to the set step size. Each time the voltage value is changed, the transmitting end sends a certain amount of light pulses, and the responses of the first zero-beat single photon detector SPD to the odd-count light pulses and the even-count light pulses sent by the transmitting end are counted respectively. The voltage value with the highest response to the even-count light pulse is used as the expected working voltage of the first PZT.

4. The PZT phase correction method according to claim 2, characterized in that: The method for determining the expected operating voltage of the second PZT is as follows: The base bit phase modulator is controlled to send a 0101 base bit clock code, and the key bit phase modulator is controlled to send a 0101 key bit clock code. The first bit of the base bit clock code and the key bit clock code is defined as a 0 sequence number. The voltage value of the second PZT is changed according to the set step size. Each time the voltage value is changed, the transmitting end sends a certain amount of light pulses. The responses of the fourth zero-beat single photon detector SPD to the odd-count light pulses and the even-count light pulses sent by the transmitting end are counted respectively, and the voltage value with the highest response to the odd-count light pulse is used as the expected working voltage of the second PZT.

5. The PZT phase correction method according to claim 2, characterized in that: The dynamic adjustment process of the working voltage of the PZT is as follows: (21) At the end of this round, determine the average bit error rate err_currt of the corresponding zero-beat single photon detector SPD, and detect whether the average bit error rate err_currt of this round is greater than the bit error rate threshold. If the detection result is yes, execute step (22); (22) Determine whether the average bit error rate err_currt of this round is lower than the average bit error rate err_old of the previous round. If the detection result is yes, the voltage adjustment direction of the PZT in this round is used as the voltage adjustment direction of the next round. If the detection result is no, the reverse direction of the voltage adjustment direction of the PZT in this round is used as the voltage adjustment direction of the next round. (23) Calculate the adjustment step length volt_step of the PZT working voltage, volt_step = |err_currt-err_old|·100·ρ, ρ is a proportional coefficient, reflecting the ratio of the bit error rate to the deviation distance between the PZT working voltage and the expected working voltage; (24) The working voltage of the current round is adjusted by volt_step in the direction of the working voltage adjustment of the next round to form the working voltage of the PZT in the next round.

6. The PZT phase correction method according to claim 5, characterized in that: Before step (21), the method further includes: Check whether the average bit error rate err_currt is greater than the bit error rate upper threshold err_thld_up. If the detection result is yes, return to step (1); if the detection result is no, execute step (21).

7. The PZT phase correction method according to claim 5, characterized in that: The PZT is the first PZT or the second PZT. When the PZT is the first PZT, the average bit error rate err_currt is the average of the detection bit error rate of the first zero-beat single-photon detector SPD and the detection bit error rate of the second zero-beat single-photon detector SPD. When the PZT is the second PZT, the average bit error rate err_currt is the average of the detection bit error rate of the third zero-beat single-photon detector SPD and the detection bit error rate of the fourth zero-beat single-photon detector SPD.

8. The PZT phase correction method according to claim 5, characterized in that: The calibration method of the proportional coefficient ρ is as follows: After determining the expected working voltage of the PZT, the working voltage is changed near the expected working voltage according to a certain voltage step size step = 2. Each time the working voltage is changed, a certain number N of photons are sent, and the detection bit error rate of the corresponding zero-beat single-photon detector SPD is counted. The current detection bit error rate and the difference between the working voltage of the PZT and the expected working voltage are recorded. Finally, the proportional coefficient ρ between the detection bit error rate and the difference between the working voltage of the PZT and the expected working voltage is fitted.

9. The PZT phase correction method according to claim 2, characterized in that: The method for determining the detection bit error rate of the zero-beat single photon detector SPD is as follows: The sending end randomly modulates the basis bit, key bit and intensity modulator modulation signal of the light quantum, and records the basis bit, key bit and intensity modulator modulation signal of the current random modulated light quantum in Alice raw key; After the optical pulse reaches the receiving end, if the first zero-beat single-photon detector SPD responds, the Bob raw key record base bit is 0 and the key bit is 0; if the second zero-beat single-photon detector SPD responds, the Bob raw key record base bit is 0 and the key bit is 1; if the third zero-beat single-photon detector SPD responds, the Bob raw key record base bit is 1 and the key bit is 0; if the fourth zero-beat single-photon detector SPD responds, the Bob raw key record base bit is 1 and the key bit is 1. The Bob raw key is sent to the receiving end, and the signal state photons in the Bob raw key are determined based on the Alice raw key. The key bits of the signal state photons in the Bob raw key and Alice raw key are retained, and some key bits are randomly published to determine the current bit error rate of the corresponding zero-beat single photon detector SPD.

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