CN114465717B - Quantum key distribution system capable of controlling pulse delay and method thereof - Google Patents
Quantum key distribution system capable of controlling pulse delay and method thereof Download PDFInfo
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Abstract
The invention discloses a quantum key distribution system with controllable pulse delay and a method thereof, wherein the quantum key distribution system comprises n sender ALICEs and n receiver BOBs, the sender ALICEs are connected with a first DWDM, the receiver BOBs are connected with a second DWDM, and quantum channels and classical channels in each sender ALICE and receiver BOB are coupled to one optical fiber for transmission through the DWDM; in the sender, a pulse laser, a phase modulator, an intensity modulator and a receiver are sequentially connected through optical fibers and are used for transmitting quantum signals; in the receiver, the circulator, the 2x2 coupler and the pulse delay unit are sequentially connected through optical fibers, so that accurate pulse delay is realized. The system adopts a network structure of the Sagnac ring, has better stability, can compensate phase jitter caused by an optical path, ensures that two pulses pass through the same optical path and simultaneously reach the effect that the coupler successfully and stably interferes, greatly reduces the requirement of the whole system on the number of signals, and improves the code rate of the system.
Description
Technical Field
The invention belongs to the technical field of quantum information and optical communication, and particularly relates to a quantum key distribution system and a quantum key distribution method capable of controlling pulse delay.
Background
Since the proposal of quantum key distribution (Quantum Key Distribution, QKD) technology in 1984, the technology has the characteristic of generating a theoretically unconditional secure key, and becomes the basis of quantum secret communication, and is widely applied to the technical field of quantum communication. The quantum communication technology is an important means for guaranteeing information safety in the future, and is an industry which is mainly supported and developed by the country. The fourteen-fifth construction period provides that the innovative application of the key digital technology is enhanced, the front edge technology of layout quantum communication, quantum calculation and the like is quickened, a complete world-to-world integrated wide area quantum communication network technology system is constructed in China, and the quantum communication technology is promoted to be widely applied to the fields of finance, government affairs, energy and the like.
In the technical field of optical communication, it is often necessary to delay each optical pulse in a pulse sequence by different periods T, so as to ensure that two different optical pulses arrive at the coupler to interfere. Particularly in a conventional mach-zehnder interferometer, when a pulse is split into two pulses by a first beam splitter and then reaches a second beam splitter through different optical paths, it is difficult to ensure that the two optical pulses reach the second beam splitter through different optical paths to achieve stable interference. At this time, a pulse delay unit can be connected to the Sagnac loop, and the two light pulses respectively pass through the same light path clockwise and anticlockwise, so that one light pulse is delayed in a proper period, and the two light pulses reach the coupler at the same time to realize stable interference.
Prior art patent: (CN 101866091B) proposes a one-way periodic programmable optical pulse retarder and an electrical pulse retarder. By designing the optical pulse delayers with different optical fiber lengths, the time delay of different periods is realized. The phase of the pulse is changed by adjusting the voltage value, so that the period delay of integer times is realized. However, the delay device can only realize unidirectional transmission of the optical pulse, and is not considered to be bidirectional transmission of the optical pulse, so that the delay device is limited in practical application and cannot be applied to a large-scale network.
Prior art patent: (CN 108123803 a) provides a quantum key distribution system and method. The method can reduce the requirement of the whole system on the number of signals, improve the code rate of the system, and improve the transmission distance of the quantum key and the key generation rate. However, when different pulses in the same pulse sequence of the BOB end in the system reach the second beam splitter through different light paths to interfere, the time that the two pulses actually reach the second beam splitter is not necessarily the same due to factors such as different channel noise, link loss and the like, so that the two pulses cannot interfere, pulse resources are wasted, and the final result is that the code rate is reduced.
Disclosure of Invention
The invention aims to provide a quantum key distribution system and a method capable of controlling pulse delay, which apply a new pulse delay technology, wherein a pulse delay unit adopts a network structure of a Sagnac ring, so that light pulses needing to be delayed and light pulses not needing to be delayed are ensured to be output to a coupler through the same light path to interfere; in addition, in quantum key distribution, the required original key can be generated by using a small amount of pulse quantity, so that the requirement of the whole system on the signal quantity is greatly reduced, and the code rate of the system is improved.
The calculation scheme adopted for realizing the purpose of the invention is as follows: a quantum key distribution system capable of controlling pulse delay comprises n sending ends and n receiving ends, wherein n is an integer greater than 2;
the n sending ends are respectively a first sending end ALICE1, a second sending end ALICE2, … … and an nth sending end ALICEn; the n receiving ends are respectively a first receiving end BOB1, a second receiving end BOB2, a … … and an nth receiving end BOBn;
the first transmitting end ALICE1, the second transmitting end ALICE2, … … and the nth transmitting end ALICEn are respectively connected with the first DWDM; the first receiving end BOB1, the second receiving end BOB2, … … and the nth receiving end BOBn are respectively connected with a second DWDM;
The first DWDM and the second DWDM are connected with each other through optical fibers and used for transmitting optical pulse signals;
a quantum channel for transmitting quantum signals and a classical channel for transmitting classical signals are arranged between the n sending ends and the n receiving ends;
the first transmitting end ALICE1 includes: the device comprises a pulse laser, a first phase modulator, an intensity modulator, a first quantum random number generator, a second quantum random number generator, a first quantum key manager and a first classical signal receiving and transmitting end; the pulse laser is connected with a first input end of the first phase modulator, and an output end of the first phase modulator is connected with a first input end of the intensity modulator; the first quantum random number generator is respectively connected with the second input end of the first phase modulator and the first input end of the first quantum key manager, the second quantum random number generator is respectively connected with the second input end of the intensity modulator, the second input end of the first quantum key manager and the first input end of the first classical signal receiving and transmitting end, and the second output end of the first classical signal receiving and transmitting end is mutually connected with the third input end of the first quantum key manager;
the first receiving terminal BOB1 includes: the device comprises a circulator, a first 2x2 coupler, a pulse delay unit, a third quantum random number generator, a first detector, a second quantum key manager and a second classical signal receiving and transmitting end; the circulator comprises a first port, a second port and a third port, wherein the first port is connected with the intensity modulator through quantum channels by wavelength division multiplexing, the second port is connected with the first 2x2 coupler, and the third port is connected with the second detector; the pulse delay unit is respectively connected with two output ends of the first 2x2 coupler and the third quantum random number generator to form a Sagnac loop, and the first 2x2 coupler is also connected with the first detector; the output end of the first detector and the output end of the second detector are respectively connected with a second quantum key manager, and the second quantum key manager is connected with a second classical signal receiving and transmitting end;
The output end of the intensity modulator is connected with the first port of the circulator through the quantum channel by wavelength division multiplexing;
the first output end of the first classical signal receiving and transmitting end is connected with the second classical signal receiving and transmitting end through classical channels by wavelength division multiplexing;
the structures and the connection of the second transmitting end ALICE2 and … … and the nth transmitting end ALICEn are consistent with those of the first transmitting end ALICE 1; the structures and the connection of the second receiving ends BOB2, … … and the nth receiving end BOB are consistent with those of the first receiving end BOB 1.
Preferably, in the Sagnac loop, two output ends of the four ports of the first 2x2 coupler are respectively connected to two ends of the pulse delay unit to form a loop, the third quantum random number generator is connected to the third end of the pulse delay unit, and the delay period of the pulse delay unit is controlled by the third quantum random number generator.
Preferably, the pulse delay unit performs cycle delay control on the arrived light pulse; the pulse delay unit comprises a second phase modulator, a second 2x2 coupler, a 2x1 coupler and an optical switch; the four ports of the second 2x2 coupler are respectively connected to two ends of the second phase modulator to form a loop, and the other two ports are respectively connected with one input end of the optical switch and one input end of the 2x1 coupler; the other input end of the 2x1 coupler is connected with one output end of the first 2x2 coupler; the optical switch is a 2-selection-1 optical switch, and two selection ports of the 2-selection-1 optical switch are respectively connected with the output end of the 2x1 coupler and the other output end of the first 2x2 coupler; the port of the second 2x2 coupler connected with the optical switch can be selectively connected with the output end of the 2x1 coupler or the other output end of the first 2x2 coupler through the 2 selection 1 operation of the optical switch, and the input and output of the optical pulse can be controlled through the switching selection of the two selection ports of the optical switch.
Preferably, the third quantum random number generator is connected to the second phase modulator, and the random binary number generated by the third quantum random number generator adjusts the input of the voltage of the second phase modulator, and changes the phase of the optical pulse sequence passing through the second phase modulator so as to control the delay of the optical pulse.
Preferably, the first 2x2 coupler includes a first input end, a second input end, a first output end and a second output end, and the first input end of the first 2x2 coupler is connected with the second port of the circulator and is used for receiving the optical pulse signal; the first 2x2 coupler is shown at 50:50, dividing an input optical pulse signal into two paths, and respectively inputting the optical pulse signal into a clockwise optical fiber link and a counterclockwise optical fiber link through a first output end and a second output end; the optical pulse signals in the clockwise or anticlockwise optical fiber links are respectively connected to the pulse delay units and then return to the first 2x2 coupler again, and interference occurs at the first 2x2 coupler; the second input of the first 2x2 coupler is connected to the first detector, which measures interference occurring at the first 2x2 coupler via the second input of the first 2x2 coupler.
Preferably, the first detector and the second detector are single photon detectors, and the two detectors are set to be 1 when one of the two detectors detects a photon and 0 when the other detector detects a photon.
Preferably, the n transmitting ends remotely transmit quantum signals and classical signals to the receiving ends by multiplexing the signals onto one optical fiber through a first DWDM, and the n receiving ends receive the signals through a second DWDM and demultiplex the signals to the corresponding receiving ends.
Another scheme adopted for achieving the purpose of the invention is as follows: a quantum key distribution method according to the above-described quantum key distribution system, comprising the steps of:
step A, preparing M quantum states consisting of L pulses by a pulse laser at a transmitting end, preparing a random binary number for each pulse of each quantum state by a first quantum random number generator, and transmitting the random binary number to a first phase modulator to perform phase modulation on the pulse, wherein M and L are natural numbers;
step B, the intensity modulator adjusts the signal state and the decoy state of each quantum state after the phase modulation according to the random binary number generated by the second quantum random number generator;
step C1, the first quantum random number generator and the second quantum random number generator send the random binary numbers generated by the first quantum random number generator and the second quantum random number generator to the first quantum key manager for recording and storing;
Step C2, the second quantum random number generator also sends the generated random binary number to the first classical signal receiving and transmitting end, and sends the random binary number to the second classical signal receiving and transmitting end of the receiving end and the second quantum key manager through the classical channel by the first classical signal receiving and transmitting end;
step D1, a transmitting end transmits the encoded signal state and the encoded decoy state to a receiving end through a quantum channel;
step D2, after receiving the signal state and the decoy state, the receiving end sends the signal state and the decoy state into a Sagnac loop through a circulator, and a third quantum random number generator generates random binary numbers and sends the random binary numbers to a second phase modulator to carry out period delay on a pulse sequence entering a pulse delay unit clockwise or anticlockwise;
step D3, returning the pulse sequence transmitted clockwise and anticlockwise in the Sagnac loop to the first 2x2 coupler again to generate interference, and measuring interference results of the first detector and the second detector respectively through the first 2x2 coupler and the circulator;
step D4, the second quantum key manager receives and records the measurement result, and the position (x, y) of the pulse in the pulse sequence when the first detector and the second detector respond, and sends the pulse to the second classical signal receiving and sending end, and then the second classical signal receiving and sending end sends the pulse to the sending end through a classical channel;
Step D5, a first classical signal transceiver of the transmitting end receives the measurement result and the position (x, y) information and transmits the measurement result and the position (x, y) information to the first quantum key manager;
and E, communication is carried out in a double mode, a first quantum key manager of the transmitting end and a second quantum key manager of the receiving end compare and evaluate channel parameters and original keys according to decoy state information, and if the error rate of the channel parameters is smaller than a threshold value, the first quantum key manager and the second quantum key manager carry out confidentiality enhancement and error correction on the obtained original quantum keys to obtain final quantum keys.
Preferably, the optical pulse sequence input from the circulator enters the first 2x2 coupler through the output port of the circulator, and the optical pulse sequence is split into two beams from the first 2x2 coupler, namely a first optical pulse sequence component and a second optical pulse sequence component; when the first optical pulse sequence component reaches the second phase modulator, if the random binary number generated by the third quantum random number generator is 1, the pulse of the first optical pulse sequence component is delayed, and if the random binary number generated by the third quantum random number generator is 0, the pulse of the first optical pulse sequence component is not delayed; when the second optical pulse sequence component reaches the phase modulator, if the random binary number generated by the third quantum random number generator is 0, the second optical pulse sequence component is not delayed and does not perform any operation, and if the random binary number generated by the third quantum random number generator is 1, the pulse of the second optical pulse sequence component is delayed; when the second optical pulse sequence component is not delayed, the second optical pulse sequence component directly passes through the phase modulator and returns to the first 2x2 coupler; the two light pulse components interfere after reaching the first 2x2 coupler again.
Preferably, in step E, if the error rate of the channel parameter is greater than the threshold, the process returns to step a to restart.
Compared with the prior art, the invention has the beneficial effects that:
(1) The invention adopts a network structure based on the Sagnac ring, has better stability, can compensate phase jitter brought by an optical path, and ensures that two pulses pass through the same optical path and reach the coupler to generate stable interference. In a quantum key distribution protocol in the prior art, two pulses simultaneously reach a coupler to interfere through a Sagnac ring by carrying out accurate period delay on the pulses needing to interfere; the invention can generate the needed original key by using a small amount of pulse quantity, thereby greatly reducing the requirement of the whole system on the pulse quantity and further improving the code rate of the system.
(2) The pulse delay unit is a reversible unit device, allows pulses to pass through bidirectionally, and can be applied to a Sagnac loop. And is widely used in large-scale networks in practical applications.
(3) The method satisfies the wavelength division multiplexing, can access a plurality of sending terminals and a plurality of receiving terminals, and maximally supports the requirement of multiple users.
Drawings
FIG. 1 is a block diagram of the overall architecture of a quantum key distribution system of the present invention;
FIG. 2 is a block diagram of the sender architecture of the quantum key distribution system of the present invention;
FIG. 3 is a block diagram of a receiver end of the quantum key distribution system of the present invention;
FIG. 4 is a block diagram of a particular architecture of a quantum key distribution system of the present invention;
FIG. 5 is a diagram of a pulse delay unit of the quantum key distribution system of the present invention;
FIG. 6 is a flow chart of a quantum key distribution method of the present invention;
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and specifically described below with reference to the drawings in the embodiments of the present invention. It will be apparent that the described embodiments are only some, but not all, embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without any inventive effort, are intended to be within the scope of the invention.
As shown in fig. 1, the quantum key distribution system with controllable pulse delay in this embodiment includes n transmitting ends and n receiving ends, where n is an integer greater than 2; the n sending ends are respectively a first sending end ALICE1, a second sending end ALICE2, a third sending end ALICE3 … … and an nth sending end ALICEn; the n receiving ends are respectively a first receiving end BOB1, a second receiving end BOB2, a third receiving end BOB3, … … and an nth receiving end BOBn.
The first transmitting end ALICE1, the second transmitting end ALICE2, … … and the nth transmitting end ALICEn are respectively connected with the first DWDM; the first receiving end BOB1, the second receiving end BOB2, … … and the nth receiving end BOBn are respectively connected with a second DWDM; the first DWDM and the second DWDM are connected with each other through optical fibers and used for transmitting optical pulse signals; a quantum channel 03 for transmitting quantum signals and a classical channel 04 for transmitting classical signals are arranged between the n transmitting ends and the n receiving ends.
The structures and the connection of the second transmitting end ALICE2 and … … and the nth transmitting end ALICEn are consistent with those of the first transmitting end ALICE 1; the structures and the connection of the second receiving ends BOB2, … … and the nth receiving end BOB are consistent with those of the first receiving end BOB 1. The n transmitting ends remotely transmit quantum signals and classical signals to the receiving ends through multiplexing the signals to one optical fiber through a first DWDM, and the n receiving ends receive the signals through a second DWDM and de-multiplex the signals to the corresponding receiving ends.
As shown in fig. 2 and 4, taking the structure of the first transmitting end ALICE1 of the transmitting end 01 as an example:
the first transmitting end ALICE1 includes: a pulse laser 11, a first phase modulator 12, an intensity modulator 13, a first quantum random number generator 14, a second quantum random number generator 15, a first quantum key manager 16 and a first classical signal transceiver 17.
The pulse laser 11 is connected to a first input of a first phase modulator 12, and the output of the first phase modulator 12 is connected to a first input of an intensity modulator 13.
The first quantum random number generator 14 is connected to the second input terminal of the first phase modulator 12 and the first input terminal of the first quantum key manager 16, respectively, and the second quantum random number generator 15 is connected to the second input terminal of the intensity modulator 13, the second input terminal of the first quantum key manager 16, and the first input terminal of the first classical signal receiving and transmitting terminal 17, respectively, and the second output terminal of the first classical signal receiving and transmitting terminal 17 is connected to the third input terminal of the first quantum key manager 16, respectively.
As shown in fig. 3 and 4, taking the structure of the first receiving end BOB1 of the receiving end 02 as an example:
the first receiving terminal BOB1 includes: the device comprises a circulator 21, a first 2x2 coupler 22, a pulse delay unit 23, a third quantum random number generator 24, a first detector (D1) 25, a second detector (D2) 26, a second quantum key manager 27 and a second classical signal receiving and transmitting end 28.
The circulator 21 comprises a first port 1, a second port 2 and a third port 3, the first port 1 being connected to the intensity modulator 13 via a quantum channel via wavelength division multiplexing, the second port 2 being connected to a first 2x2 coupler 22, and the third port 3 being connected to a second detector 26. The output of the intensity modulator 13 is connected to the first port 1 of the circulator 21 via a quantum channel 03 via wavelength division multiplexing.
The pulse delay unit 23 is respectively connected with two output ends of the first 2x2 coupler 22 and the third quantum random number generator 24 to form a Sagnac loop.
The first 2x2 coupler 22 is further connected to a first detector 25, the output of the first detector 25 and the output of a second detector 26 are respectively connected to a second quantum key manager 27, and the second quantum key manager 27 is connected to a second classical signal receiving and transmitting terminal 28. The first output of the first classical signal transmission and reception terminal 17 is connected to a second classical signal transmission and reception terminal 28 via a classical channel 04 via wavelength division multiplexing.
As shown in fig. 3 and 5, in the Sagnac loop, four ports of the first 2x2 coupler 22 are respectively connected to two ends of the pulse delay unit 23, that is, one end in the clockwise CW direction < 11 >, and the other end in the counterclockwise CCW direction < 22 >, so as to form a loop, and the third quantum random number generator 24 is connected to the third end of the pulse delay unit 23, and the delay period of the pulse delay unit 23 is controlled by using the third quantum random number generator 24.
The pulse delay unit 23 shown in fig. 5 has the following structure: the pulse delay unit 23 includes a second Phase Modulator (PM) 44, a second 2x2 coupler 33, a 2x1 coupler, and an optical switch.
The four ports e, d, f, g of the second 2x2 coupler 33 are respectively connected to two ends of the second phase modulator 44 to form loops, namely a clockwise loop CW and a counterclockwise loop CCW, and the other two ports, wherein the port e is connected with the input terminal b of the 2x1 coupler, and the port d is connected with the port 122 of the 2-selected 1 optical switch.
The other input a of the 2x1 coupler is connected to one output of the first 2x2 coupler 22; the two selection ports 121, 132 of the 2-by-1 optical switch are respectively connected with the output terminal c of the 2x1 coupler and the other output terminal of the first 2x2 coupler 22. In use, through the 2-to-1 operation of the optical switch, the port 122 connected to the optical switch connected to the second 2x2 coupler can select the port 121 connected to the output terminal c of the 2x1 coupler to be conducted, or select the port 132 connected to the other output terminal of the first 2x2 coupler to be conducted, and through the switching selection of the two selected ports of the optical switch, the input and output of the optical pulse can be controlled.
The third quantum random number generator 24 is connected to the second phase modulator 44, and the random binary number generated by the third quantum random number generator 24 adjusts the input of the voltage of the second phase modulator 44, and changes the phase of the optical pulse sequence passing through the second phase modulator to control the delay of the optical pulse.
The first 2x2 coupler 22 includes a first input end, a second input end, a first output end and a second output end, and the first input end of the first 2x2 coupler 22 is connected with the second port 2 of the circulator and is used for receiving the optical pulse signal; the first 2x2 coupler 22 is shown at 50: the 50 ratio divides the incoming optical pulse signal into two paths into the clockwise CW optical fiber link and the counterclockwise CCW optical fiber link, respectively. After the optical pulse signals in the clockwise or anticlockwise optical fiber links are respectively connected to the pulse delay units 23, the optical pulse signals return to the first 2x2 coupler 22 again, and interference occurs at the first 2x2 coupler 22; the second input of the first 2x2 coupler 22 is connected to the first detector 25, and the first detector 25 measures the interference occurring at the first 2x2 coupler via the second input of the first 2x2 coupler. The second detector 26 then measures the interference occurring at the first 2x2 coupler via the circulator 21 and the first input of the first 2x2 coupler 22.
In the above system, the pulse laser 11 at the transmitting end generates M pulse sequences at time intervals T, and transmits the generated pulse sequences to the first phase modulator 12. Wherein a pulse train consists of L pulses, so that a pulse train can be regarded as a quantum state.
The values of M and L may be chosen appropriately according to the number of keys to be prepared. In this embodiment, when the receiving end interferes two pulses in the same input pulse sequence, by controlling the optical switch of the pulse delay unit and the input of the voltage, the two pulses input into the Sagnac loop can be interfered successfully, so that the number of input pulses is greatly saved. Because each pulse sequence is used as a quantum state, different M can be set according to the number of keys to be prepared in the actual preparation process, and the value of L can be set smaller.
In addition, the first quantum random number generator generates M random binary sequences. For example, the number of the random binary numbers is 0 or 1, and the generated random binary numbers are sent to the first phase modulator, the first quantum key manager and the first classical signal receiving and transmitting end, and the number of the random binary numbers generated by the first quantum random number generator is equal to the number M of pulse sequences generated by the pulse laser.
The first phase modulator modulates each pulse in a pulse train generated by the pulse laser according to a random binary sequence generated by the first quantum random number generator.
For example, if the first (m.ltoreq.M) th random binary number in the mth (i.ltoreq.L) th random binary sequence generated by the first quantum random number generator is 1, the phase of the first pulse in the mth pulse sequence generated by the pulse laser is modulated to be shifted by 0 with respect to the total phase of the pulse sequence (i.e., the phase shift of the first pulse is 0), and if the first (1.ltoreq.L) th random binary number in the mth (m.ltoreq.M) th random binary sequence generated by the first quantum random number generator is 0, the phase of the first pulse in the mth signal state or spoof state generated by the pulse laser is modulated to be shifted by pi with respect to the total phase of the pulse sequence (i.e., the phase shift of the first pulse is pi), or vice versa. Alternatively, other modulation schemes are also possible.
The first phase modulator sends the modulated pulse sequences to the intensity modulator, and the intensity modulator can modulate the intensity of each pulse sequence generated by the first phase modulator according to the random number generated by the second quantum random number generator, wherein one pulse sequence is in a signal state, and the other pulse sequences are in a decoy state. So that an attacker cannot distinguish between the signal state and the spoofing state, preventing PNS attacks. Wherein the total pulsed light intensity of the signal state is greater than the total pulsed light intensity of the decoy state.
The second quantum random number generator may generate M random numbers, and then the second quantum random number generator may transmit the generated random numbers to the intensity modulator and the first quantum key manager. Thus, the number of random numbers generated by the second quantum random number generator is equal to the number of pulse trains generated by the pulse laser.
By this intensity modulation, the intensity modulation for each light pulse train can be completed. And encoded into a signal state and a spoofing state. PNS attacks can be prevented. Here, the transmitting end ALICE and the receiving end BOB may approximate the encoding scheme in advance. For example, it is previously agreed that a pulse whose phase is shifted by 0 is regarded as being encoded as 0, and a pulse whose phase is shifted by pi is regarded as being encoded as 1; vice versa. Alternatively, other modulation schemes are also possible.
The signal state or decoy state modulated by the intensity modulator is sent to the circulator. In the circulator, light can only be transmitted in the direction of the designated arrow.
The first end interface of the circulator sends the received signal state or the decoy state to the first 2x2 coupler through the second port, the signal state or the decoy state passing through the coupler is respectively divided into two beams, the pulses respectively pass through the pulse delay unit in the clockwise or anticlockwise direction, then reach the coupler again simultaneously, interference occurs at the coupler, and finally measurement is carried out through the first detector D1 and the second detector D2. Wherein, the function of pulse delay unit does: the first 2x2 coupler splits and beam-combines the arriving pulses with a suitable period delay.
The random binary number generated by the third quantum random number generator modulates the input of the pulse delay unit PM voltage (for example, half-wave voltage), and modulates the phase of the optical pulse so that the pulse is delayed. For example, when the random binary number generated by the third quantum random number generator is 0, the voltage of the pulse delay unit PM is not modulated, and the pulse is not delayed. When the random binary number generated by the third quantum random number generator is 1, the voltage of the pulse delay unit PM is modulated, and the pulse is delayed, or vice versa, other modulation methods are also possible.
The specific delay transmission of the pulse sequence into the Sagnac loop via the circulator is as follows:
first: as shown in fig. four, the pulse train output through the first 2×2 coupler 22 in the first receiving terminal BOB1 is split into two beams, which are transmitted along clockwise and counterclockwise optical paths in the Sagnac loop, respectively. The pulse train is transmitted in a clockwise CW link in a Sagnac loop into a pulse delay unit.
At this time, as shown in fig. five, in the specific structure diagram of the pulse delay unit 23, the pulse sequence transmitted along the clockwise CW link enters the pulse delay unit from the < 11 > port, and is input through the a port and output through the c port of the 2x1 coupler. The optical switch is connected with the links 121 to 122, the pulse sequence is transmitted to the second 2x2 coupler through the d port by the link communicated by the optical switch, and the pulse sequence is divided into two identical optical pulse sequences, namely an optical pulse sequence 1 and an optical pulse sequence 2 by one pulse sequence of the second 2x2 coupler 33. Where the optical pulse train 1 is output by the f-port, the CW link in the clockwise direction is returned to the g-port of the second 2x2 coupler 33 by the phase modulator 44. The optical pulse train 2 is input by the g-port and the CCW link in the counter-clockwise direction is returned to the f-port of the second 2x2 coupler 33 by the phase modulator 44.
When the optical pulse sequence 1 reaches the second phase modulator, if the random binary number generated by the third quantum random number generator is 1, the optical pulse sequence 1 is delayed, and when the optical pulse sequence 2 reaches the phase modulator, if the random binary number generated by the third quantum random number generator is 0, no operation is performed on the optical pulse sequence component 2, and the optical pulse sequence component 2 directly passes through the phase modulator and returns to the f port of the second 2x2 coupler 33. Since the optical pulse train 1 and the optical pulse train 2 pass through the same path, there is no time difference between the optical pulse train 1 and the optical pulse train 2 in its own pulse laser transmission. By delaying the pulse sequence, the interference of different pulses in the same pulse sequence at the second 2x2 coupler can be realized. When the delayed optical pulse train 1, 1 and the undelayed optical pulse train 2 reach the second 2x2 coupler again, the phase difference between the pulse of the optical pulse train component 1 and the pulse of the optical pulse train component 2 after the interference of the optical pulse train component 1 and the pulse of the optical pulse train component 2 returns to the second 2x2 coupler is pi, and the pulse synthesized after the interference is output from the first port e. The delayed pulse reaches the b port of the 2x1 coupler, is output from the c port of the 2x1 coupler, and reaches the d port of the second 2x2 coupler again. Repeating the steps, and continuing to delay the pulse sequence when the third quantum random number generator generates the random binary number 1. When the third quantum random number generator generates a random binary number 0, the pulse train is no longer delayed. The third quantum random number generator does not modulate the input of the voltage of the phase modulator, the second phase modulator does not perform any operation on the clockwise and anticlockwise passing light pulses 1 and 2, the light pulse sequences 1 and 2 reach the second 2x2 coupler again, after interference beam combination, the phase difference is 0, and the light pulse sequences are output from the d port of the second 2x2 coupler. The optical switch switches on the 122-132 link and the optical pulses output via the second port d are output from the < 22 > port via the link to which the 122, 132 is connected.
Second,: as shown in fig. four, the pulse train output through the first 2x2 coupler 22 in the first receiving terminal BOB1 is split into two beams, which are transmitted along clockwise and counterclockwise optical paths in the Sagnac loop, respectively. The pulse train is transmitted in the CCW link in the counterclockwise direction in the Sagnac loop, entering the pulse delay unit.
At this time, as shown in fig. five, in the specific structure diagram of the pulse delay unit, the pulse sequence transmitted along the CCW link in the counterclockwise direction enters the pulse delay unit from the port < 22 > and the optical switch is connected to the links 132-122, the pulse sequence is transmitted to the second 2x2 coupler through the d port of the link connected by the optical switch, and is divided into two identical optical pulse sequences, namely, the optical pulse sequence 1 and the optical pulse sequence 2, through the second 2x2 coupler 33. Where the optical pulse train 1 is output by the f-port, the CW link in the clockwise direction is returned to the g-port of the second 2x2 coupler 33 by the phase modulator 44. The optical pulse train 2 is input by the g-port and the CCW link in the counter-clockwise direction is returned to the f-port of the second 2x2 coupler 33 through the second phase modulator 44.
When the optical pulse sequence 1 reaches the second phase modulator, if the random binary number generated by the third quantum random number generator is 1, the optical pulse sequence 1 is delayed, and when the optical pulse sequence 2 reaches the second phase modulator, if the random binary number generated by the third quantum random number generator is 0, no operation is performed on the optical pulse sequence 2, and the optical pulse sequence 2 directly passes through the second phase modulator and returns to the f port of the second 2x2 coupler 33. Since the optical pulse train 1 and the optical pulse train 2 pass through the same path, there is no time difference between the optical pulse train 1 and the optical pulse train 2 in its own pulse laser transmission. By delaying the pulse sequence, the interference of different pulses in the same pulse sequence at the 2x2 coupler can be realized. The phase difference between the pulse of the optical pulse train 1 and the pulse of the optical pulse train 2 after the interference of the second 2x2 coupler is pi, and the pulse train synthesized after the interference is output from the first port e. The delayed pulse sequence reaches the b port of the 2x1 coupler, is output from the c port of the 2x1 coupler, adjusts the optical switch, turns on the 121-122 link, and reaches the d port of the second 2x2 coupler again. Repeating the steps, and continuing to delay the pulse sequence when the third quantum random number generator generates the random binary number 1. When the third quantum random number generator generates a random binary number 0, the pulse train is no longer delayed. The third quantum random number generator does not modulate the input of the voltage of the phase modulator, the second phase modulator does not perform any operation on the light pulse sequence components passing clockwise and anticlockwise, the light pulse sequence components reach the second 2x2 coupler again respectively, after interference beam combination, the phase difference is 0, the light pulse sequence is output from the d port of the second 2x2 coupler, and the pulse sequence is output from the < 11 > port.
Therefore, the input of the voltage of the phase modulator is regulated through the random binary number generated by the third quantum random number generator, the phase of one light pulse sequence clockwise or anticlockwise or two light pulse sequences anticlockwise and clockwise is changed, the accurate pulse delay can be realized, the number of input pulses at the transmitting end is reduced, and the required original secret key can be generated only by using a small number of pulse numbers. The requirement of the whole system on the pulse number is reduced to a great extent, and the code rate of the system is further improved. The length of the optical fiber used by the pulse delay unit is reasonably selected according to requirements, and the delay of different periods t (the period of the pulse in the pulse sequence sent by the pulse laser is t) of the pulse sequence is realized through the random binary number generated by the third quantum random number generator. The pulse delay unit is arranged in the Sagnac ring, and can realize bidirectional pulse delay as required, and two ports of the pulse delay unit can be used as an input port and an output port. The optical switch is flexibly adjusted to realize the input and output of the pulse.
In this embodiment, it may be set in advance such that when the phase difference between pulses in which interference occurs in the coupler is 0, the first detector D1 responds, that is, detects a photon, and the code corresponding to the successful detection event is 0. When the phase difference between the two pulses interfering at the coupler is pi, the second detector D2 responds, i.e. detects a photon, and the code corresponding to the successful detection event is 1. Vice versa.
When either the first detector D1 or the second detector D2 detects a photon (i.e. a successful detection event), the measurement result and the position (x, y) of the two pulses in the pulse sequence are recorded and sent to the quantum key manager 2.
The second quantum key manager may send the positions (x, y) of the two pulses in the pulse sequence to the second classical signal transmission and reception end, which sends the received positions (x, y) to the first classical signal transmission and reception end via classical channel 04. The first classical signal receiving and transmitting end sends the received position (x, y) to the first quantum key manager, and the random binary sequence generated by the second quantum random number generator is sent to the first quantum key manager.
Data post-processing stage: the first classical signal receiving and transmitting end can also transmit the random binary number generated by the second quantum random number generator to the second classical signal receiving and transmitting end through the classical channel 04. And the second classical signal receiving and transmitting end transmits the received random binary number generated by the second quantum random number generator to the second quantum key manager.
At this time, the first quantum key manager and the second quantum key manager can both obtain the random number sequence generated by the second quantum random number generator, the positions (x, y) of the two pulses corresponding to the measurement result are located, and the original quantum key can be obtained according to the measurement result. Then, the first quantum key manager and the second quantum key manager perform security enhancement and error correction on the obtained original quantum key to obtain a final quantum key.
The structures and the connections of the second transmitting end ALICE2, the third transmitting end ALICE3, … … and the nth transmitting end ALICEn are consistent with those of the first transmitting end ALICE1, n transmitting ends are connected through a first DWDM, and n transmitting ends ALICE are multiplexed onto one optical fiber through the first DWDM, so that distance selection transmission is realized, and optical fiber resources are effectively saved. And the quantum signal is positioned in a lower wave band, and the classical signal is positioned in a higher wave band, so that the interference of the classical signal on the quantum signal is reduced as much as possible. The structures and the connections of the second receiving end BOB2, the third receiving end BOB3, … … and the nth receiving end BOBn are consistent with those of the first receiving end BOB 1. And n receiving ends are connected through a second DWDM and respectively demultiplex the signals to the corresponding receiving ends.
The quantum key distribution method according to the quantum key distribution system in this embodiment includes the following steps:
step A, preparing M quantum states consisting of L pulses by a pulse laser at a transmitting end, preparing a random binary number for each pulse of each quantum state by a first quantum random number generator, and transmitting the random binary number to a first phase modulator to perform phase modulation on the pulse, wherein M and L are natural numbers;
step B, the intensity modulator adjusts the signal state and the decoy state of each quantum state after the phase modulation according to the random binary number generated by the second quantum random number generator;
Step C1, the first quantum random number generator and the second quantum random number generator send the random binary numbers generated by the first quantum random number generator and the second quantum random number generator to the first quantum key manager for recording and storing;
step C2, the second quantum random number generator also sends the generated random binary number to the first classical signal receiving and transmitting end, and sends the random binary number to the second classical signal receiving and transmitting end of the receiving end and the second quantum key manager through the classical channel by the first classical signal receiving and transmitting end;
step D1, a transmitting end transmits the encoded signal state and the encoded decoy state to a receiving end through a quantum channel;
step D2, after receiving the signal state and the decoy state, the receiving end sends the signal state and the decoy state into a Sagnac loop through a circulator, and a third quantum random number generator generates random binary numbers and sends the random binary numbers to a second phase modulator to carry out period delay on a pulse sequence entering a pulse delay unit clockwise or anticlockwise;
step D3, returning the pulse sequence transmitted clockwise and anticlockwise in the Sagnac loop to the first 2x2 coupler again to generate interference, and measuring interference results of the first detector and the second detector respectively through the first 2x2 coupler and the circulator;
step D4, the second quantum key manager receives and records the measurement result, and the position (x, y) of the pulse in the pulse sequence when the first detector and the second detector respond, and sends the pulse to the second classical signal receiving and sending end, and then the second classical signal receiving and sending end sends the pulse to the sending end through a classical channel;
Step D5, a first classical signal transceiver of the transmitting end receives the measurement result and the position (x, y) information and transmits the measurement result and the position (x, y) information to the first quantum key manager;
and E, communication is carried out in a double mode, a first quantum key manager of the transmitting end and a second quantum key manager of the receiving end compare and evaluate channel parameters and original keys according to decoy state information, and if the error rate of the channel parameters is smaller than a threshold value, the first quantum key manager and the second quantum key manager carry out confidentiality enhancement and error correction on the obtained original quantum keys to obtain final quantum keys.
In step E, if the error rate of the channel parameter is greater than the threshold, the process returns to step a to restart.
The optical pulse sequence input from the circulator enters the first 2x2 coupler through the output port of the circulator, and the optical pulse sequence is split into two beams from the first 2x2 coupler and is respectively a first optical pulse sequence component and a second optical pulse sequence component; when the first optical pulse sequence component reaches the second phase modulator, if the random binary number generated by the third quantum random number generator is 1, the pulse of the first optical pulse sequence component is delayed, and if the random binary number generated by the third quantum random number generator is 0, the pulse of the first optical pulse sequence component is not delayed; when the second optical pulse sequence component reaches the phase modulator, if the random binary number generated by the third quantum random number generator is 0, the second optical pulse sequence component is not delayed and does not perform any operation, and if the random binary number generated by the third quantum random number generator is 1, the pulse of the second optical pulse sequence component is delayed; when the second optical pulse sequence component is not delayed, the second optical pulse sequence component directly passes through the phase modulator and returns to the first 2x2 coupler; the two light pulse components interfere after reaching the first 2x2 coupler again.
Compared with the prior art, the beneficial effects of the embodiment are as follows:
(1) The network structure based on the Sagnac ring for 2 times has better stability, can compensate phase jitter caused by an optical path, and ensures that two pulses pass through the same optical path and reach the coupler to generate stable interference. In a quantum key distribution protocol in the prior art, two pulses simultaneously reach a coupler to interfere through a Sagnac ring by carrying out accurate period delay on the pulses needing to interfere; the invention can generate the needed original key by using a small amount of pulse quantity, thereby greatly reducing the requirement of the whole system on the pulse quantity and further improving the code rate of the system.
(2) The pulse delay unit is a reversible unit device, allows pulses to pass through bidirectionally, and can be applied to a Sagnac loop. And is widely used in large-scale networks in practical applications.
(3) The method satisfies the wavelength division multiplexing, can access a plurality of sending terminals and a plurality of receiving terminals, and maximally supports the requirement of multiple users.
The above embodiments are only for illustrating the technical solution of the present invention, and are not limiting; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some of the technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims (10)
1. A quantum key distribution system capable of controlling pulse delay comprises n sending ends and n receiving ends, wherein n is an integer greater than 2;
the n sending ends are respectively a first sending end ALICE1, a second sending end ALICE2, … … and an nth sending end ALICEn; the n receiving ends are respectively a first receiving end BOB1, a second receiving end BOB2, a … … and an nth receiving end BOBn;
the first transmitting end ALICE1, the second transmitting end ALICE2, … … and the nth transmitting end ALICEn are respectively connected with the first DWDM; the first receiving end BOB1, the second receiving end BOB2, … … and the nth receiving end BOBn are respectively connected with a second DWDM;
the first DWDM and the second DWDM are connected with each other through optical fibers and used for transmitting optical pulse signals; the method is characterized in that:
a quantum channel for transmitting quantum signals and a classical channel for transmitting classical signals are arranged between the n sending ends and the n receiving ends;
the first transmitting end ALICE1 includes: the device comprises a pulse laser, a first phase modulator, an intensity modulator, a first quantum random number generator, a second quantum random number generator, a first quantum key manager and a first classical signal receiving and transmitting end; the pulse laser is connected with a first input end of the first phase modulator, and an output end of the first phase modulator is connected with a first input end of the intensity modulator; the first quantum random number generator is respectively connected with the second input end of the first phase modulator and the first input end of the first quantum key manager, the second quantum random number generator is respectively connected with the second input end of the intensity modulator, the second input end of the first quantum key manager and the first input end of the first classical signal receiving and transmitting end, and the second output end of the first classical signal receiving and transmitting end is mutually connected with the third input end of the first quantum key manager;
The first receiving terminal BOB1 includes: the device comprises a circulator, a first 2x2 coupler, a pulse delay unit, a third quantum random number generator, a first detector, a second quantum key manager and a second classical signal receiving and transmitting end; the circulator comprises a first port, a second port and a third port, wherein the first port is connected with the intensity modulator through quantum channels by wavelength division multiplexing, the second port is connected with the first 2x2 coupler, and the third port is connected with the second detector; the pulse delay unit is respectively connected with two output ends of the first 2x2 coupler and the third quantum random number generator to form a Sagnac loop, and the first 2x2 coupler is also connected with the first detector; the output end of the first detector and the output end of the second detector are respectively connected with a second quantum key manager, and the second quantum key manager is connected with a second classical signal receiving and transmitting end;
the output end of the intensity modulator is connected with the first port of the circulator through the quantum channel by wavelength division multiplexing;
the first output end of the first classical signal receiving and transmitting end is connected with the second classical signal receiving and transmitting end through classical channels by wavelength division multiplexing;
the structures and the connection of the second transmitting end ALICE2 and … … and the nth transmitting end ALICEn are consistent with those of the first transmitting end ALICE 1; the structures and the connection of the second receiving end BOB2, … … and the nth receiving end BOBn are consistent with those of the first receiving end BOB 1;
The pulse laser at the transmitting end prepares M quantum states composed of L pulses, the first quantum random number generator prepares a random binary number for each pulse of each quantum state, and the random binary number is transmitted to the first phase modulator to carry out phase modulation on the pulses, wherein M and L are natural numbers;
the intensity modulator adjusts the signal state and the decoy state of each quantum state after the phase modulation according to the random binary number generated by the second quantum random number generator;
the first quantum random number generator and the second quantum random number generator send the random binary numbers generated by the first quantum random number generator and the second quantum random number generator to the first quantum key manager for recording and storing;
the transmitting end transmits the encoded signal state and the decoy state to the receiving end through a quantum channel;
the receiving end receives the signal state and the decoy state, then sends the signal state and the decoy state into a Sagnac loop through a circulator, and the third quantum random number generator generates random binary numbers and sends the random binary numbers to the second phase modulator to carry out period delay on a pulse sequence entering the pulse delay unit clockwise or anticlockwise;
the pulse sequence transmitted clockwise and anticlockwise in the Sagnac loop returns to the first 2x2 coupler again to interfere, and the interference of the first detector and the second detector is measured by the first 2x2 coupler and the circulator respectively;
The first quantum key manager and the second quantum key manager can both obtain a random number sequence generated by the second quantum random number generator, the positions (x, y) of two pulses corresponding to the measurement result are located, and the original quantum key can be obtained according to the measurement result; the first quantum key manager and the second quantum key manager perform confidentiality enhancement and error correction on the obtained original quantum key to obtain a final quantum key.
2. The quantum key distribution system of claim 1, wherein: in the Sagnac loop, four ports of a first 2x2 coupler are respectively connected with two ends of a pulse delay unit to form a loop, a third quantum random number generator is connected with a third end of the pulse delay unit, and the delay period of the pulse delay unit is controlled by the third quantum random number generator.
3. The quantum key distribution system of claim 1 or 2, wherein: the pulse delay unit performs period delay control on the arrived light pulse; the pulse delay unit comprises a second phase modulator, a second 2x2 coupler, a 2x1 coupler and an optical switch; the four ports of the second 2x2 coupler are respectively connected to two ends of the second phase modulator to form a loop, and the other two ports are respectively connected with one input end of the optical switch and one input end of the 2x1 coupler; the other input end of the 2x1 coupler is connected with one output end of the first 2x2 coupler; the optical switch is a 2-selection-1 optical switch, and two selection ports of the 2-selection-1 optical switch are respectively connected with the output end of the 2x1 coupler and the other output end of the first 2x2 coupler; the port of the second 2x2 coupler connected with the optical switch can be selectively connected with the output end of the 2x1 coupler or the other output end of the first 2x2 coupler through the 2 selection 1 operation of the optical switch, and the input and output of the optical pulse can be controlled through the switching selection of the two selection ports of the optical switch.
4. A quantum key distribution system as claimed in claim 3, wherein: the third quantum random number generator is connected to the second phase modulator, and the random binary number generated by the third quantum random number generator is used for adjusting the input of the voltage of the second phase modulator, so that the phase of the optical pulse sequence passing through the second phase modulator is changed, and the time delay of the optical pulse is controlled.
5. The quantum key distribution system of claim 1, wherein: the first 2x2 coupler comprises a first input end, a second input end, a first output end and a second output end, wherein the first input end of the first 2x2 coupler is connected with the second port of the circulator and is used for receiving optical pulse signals; the first 2x2 coupler is shown at 50:50, dividing an input optical pulse signal into two paths, and respectively inputting the optical pulse signal into a clockwise optical fiber link and a counterclockwise optical fiber link through a first output end and a second output end; the optical pulse signals in the clockwise or anticlockwise optical fiber links are respectively connected to the pulse delay units and then return to the first 2x2 coupler again, and interference occurs at the first 2x2 coupler; the second input of the first 2x2 coupler is connected to the first detector, which measures interference occurring at the first 2x2 coupler via the second input of the first 2x2 coupler.
6. The quantum key distribution system of claim 1, wherein: the first detector and the second detector are single photon detectors, and the two detectors are set to be 1 when one of the two detectors detects photons and 0 when the other detector detects photons.
7. The quantum key distribution system of claim 1, wherein: the n sending ends remotely transmit quantum signals and classical signals to the receiving ends through multiplexing the signals to one optical fiber through a first DWDM, and the n receiving ends receive the signals through a second DWDM and demultiplex the signals to the corresponding receiving ends.
8. A quantum key distribution method of a quantum key distribution system according to any one of claims 1 to 7, comprising the steps of:
step A, preparing M quantum states consisting of L pulses by a pulse laser at a transmitting end, preparing a random binary number for each pulse of each quantum state by a first quantum random number generator, and transmitting the random binary number to a first phase modulator to perform phase modulation on the pulse, wherein M and L are natural numbers;
step B, the intensity modulator adjusts the signal state and the decoy state of each quantum state after the phase modulation according to the random binary number generated by the second quantum random number generator;
Step C1, the first quantum random number generator and the second quantum random number generator send the random binary numbers generated by the first quantum random number generator and the second quantum random number generator to the first quantum key manager for recording and storing;
step C2, the second quantum random number generator also sends the generated random binary number to the first classical signal receiving and transmitting end, and sends the random binary number to the second classical signal receiving and transmitting end of the receiving end and the second quantum key manager through the classical channel by the first classical signal receiving and transmitting end;
step D1, a transmitting end transmits the encoded signal state and the encoded decoy state to a receiving end through a quantum channel;
step D2, after receiving the signal state and the decoy state, the receiving end sends the signal state and the decoy state into a Sagnac loop through a circulator, and a third quantum random number generator generates random binary numbers and sends the random binary numbers to a second phase modulator to carry out period delay on a pulse sequence entering a pulse delay unit clockwise or anticlockwise;
step D3, returning the pulse sequence transmitted clockwise and anticlockwise in the Sagnac loop to the first 2x2 coupler again to generate interference, and measuring interference results of the first detector and the second detector respectively through the first 2x2 coupler and the circulator;
step D4, the second quantum key manager receives and records the measurement result, and the position (x, y) of the pulse in the pulse sequence when the first detector and the second detector respond, and sends the pulse to the second classical signal receiving and sending end, and then the second classical signal receiving and sending end sends the pulse to the sending end through a classical channel;
Step D5, a first classical signal transceiver of the transmitting end receives the measurement result and the position (x, y) information and transmits the measurement result and the position (x, y) information to the first quantum key manager;
and E, communication is carried out in a double mode, a first quantum key manager of the transmitting end and a second quantum key manager of the receiving end compare and evaluate channel parameters and original keys according to decoy state information, and if the error rate of the channel parameters is smaller than a threshold value, the first quantum key manager and the second quantum key manager carry out confidentiality enhancement and error correction on the obtained original quantum keys to obtain final quantum keys.
9. The quantum key distribution method of claim 8, wherein: the optical pulse sequence input from the circulator enters the first 2x2 coupler through the output port of the circulator, and the optical pulse sequence is split into two beams from the first 2x2 coupler and is respectively a first optical pulse sequence component and a second optical pulse sequence component; when the first optical pulse sequence component reaches the second phase modulator, if the random binary number generated by the third quantum random number generator is 1, the pulse of the first optical pulse sequence component is delayed, and if the random binary number generated by the third quantum random number generator is 0, the pulse of the first optical pulse sequence component is not delayed; when the second optical pulse sequence component reaches the phase modulator, if the random binary number generated by the third quantum random number generator is 0, the second optical pulse sequence component is not delayed and does not perform any operation, and if the random binary number generated by the third quantum random number generator is 1, the pulse of the second optical pulse sequence component is delayed; when the second optical pulse sequence component is not delayed, the second optical pulse sequence component directly passes through the phase modulator and returns to the first 2x2 coupler; the two light pulse components interfere after reaching the first 2x2 coupler again.
10. The quantum key distribution method of claim 8, wherein: in step E, if the error rate of the channel parameter is greater than the threshold, the process returns to step a to restart.
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