QKD system, signal receiving end, calibration method and calibration system

Abstract

The invention discloses a QKD system, a signal receiving end, a calibration method and a calibration system. The signal receiving end can detect synchronous light through a first detection unit to obtain a first detection signal, and detect signal light through a second detection unit to obtain a second detection signal; the gating signal output circuit can output a first gating signal and a second gating signal based on the first detection signal; the first output circuit can output a gate signal for controlling the working state of the second detection unit based on the first gate control signal; and the second output circuit can filter the initial avalanche signal through the coincidence gate based on the input second gating signal, and filter out an interference signal of an avalanche transition region to form an effective avalanche signal, thereby ensuring that the interference signal of the avalanche transition region does not enter a key extraction process, and preventing an external interference signal from influencing a communication result of the QKD system.

Description

Technical Field

[0001] The present invention relates to the technical field of Quantum Key Distribution (QKD), and more specifically, to a QKD system, a signal receiving end, a calibration method, and a calibration system. Background Art

[0002] The security of the QKD system has been strictly proven from the principles of quantum physics to the communication protocol. However, in actual engineering, due to the imperfections of the devices in the system, there are deviations or loopholes from the theoretical assumptions in the proof process. To improve the security of the QKD system, it is necessary to discover and adopt appropriate countermeasures for defense. Such as anti-bright light attack schemes, anti-dead time attack schemes, anti-avalanche transition region attack schemes, etc.

[0003] The QKD system consists of multiple components, and the single-photon detector is a very important component, and most of them work in the gated mode. For a gated single-photon detector, its working principle is as follows: when inside the gate, the single-photon detector is in the Geiger mode and can detect single photons; when outside the gate, the single-photon detector is in the linear mode and cannot detect single photons. Due to the imperfections of the device, both the rise and fall of the gate signal require a certain amount of time to complete the opening and closing actions. Therefore, during this period, the single-photon detector neither belongs to the Geiger mode that can detect single photons nor belongs to the linear mode that cannot detect single photons. This time region is called the avalanche transition region. In the avalanche transition region, the single-photon detector has superlinear characteristics, hardly responds to single-photon signals, but has a high response probability for signals exceeding a certain threshold, similar to the detection principle in the linear region, but the photon number threshold is much lower.

[0004] Based on the above description, it can be seen that the QKD system is vulnerable to external interference signals in the avalanche transition region of the single-photon detector. Therefore, in the avalanche transition region of the single-photon detector, how to avoid the influence of external signals on the communication result of the QKD system is an urgent problem to be solved in the QKD technical field. Summary of the Invention

[0005] In view of this, the present application provides a QKD system, a signal receiving end, a calibration method, and a calibration system, and the solutions are as follows:

[0006] A signal receiving end of a QKD system, the signal transmitting end of the QKD system is used to emit synchronous light and signal light, and the signal receiving end includes:

[0007] A first detection unit, configured to detect the synchronous light and obtain a first detection signal;

[0008] The second detection unit is used to detect the signal light and obtain a second detection signal; the second detection signal includes an initial avalanche signal obtained by the second detection unit in the avalanche transition region.

[0009] The gating signal output circuit is used to output a first gating signal and a second gating signal based on the first detection signal.

[0010] The first output circuit is used to output a gate signal for controlling the working state of the second detection unit based on the first gating signal.

[0011] The second output circuit is used to filter the initial avalanche signal through a coincidence gate based on the input second gating signal to form an effective avalanche signal.

[0012] Preferably, in the above signal receiving end, the gating signal output circuit includes:

[0013] A frequency multiplier for increasing the frequency of the first detection signal.

[0014] A filter amplification circuit for amplifying and filtering the output signal of the frequency multiplier.

[0015] The first adjustable delay circuit for delaying the output signal of the filter amplification circuit to calibrate the delay between the signal light and the synchronization light.

[0016] A controllable attenuation circuit for intensity modulation of the output signal of the first adjustable delay circuit to output the first gating signal and the second gating signal.

[0017] Preferably, in the above signal receiving end, the first output circuit is used to amplify and power-couple the first gating signal to output a stable gate signal.

[0018] Preferably, in the above signal receiving end, the first output circuit includes:

[0019] A gate signal amplification circuit for amplifying the first gating signal output by the gating signal output circuit.

[0020] A power coupling circuit for power-coupling the amplified first gating signal to output a gate signal.

[0021] Preferably, in the above signal receiving end, the second output circuit includes:

[0022] The second adjustable delay circuit for delaying based on the second gating signal to calibrate the delay of the initial avalanche signal and the coincidence gate signal.

[0023] An adjustable discrimination circuit for discriminating the output signal of the second adjustable delay circuit to output a coincidence gate signal.

[0024] A discriminator, configured to discriminate an amplified initial avalanche signal based on a set avalanche discrimination threshold, and output an avalanche discrimination signal;

[0025] A coincidence and pulse width shaping circuit, configured to perform coincidence on the avalanche discrimination signal and a coincidence gate signal, so as to filter out interference pulses in the avalanche transition region, and perform pulse width shaping, and output an effective avalanche signal.

[0026] The present application further provides a QKD system, including:

[0027] A signal transmitting end, configured to emit synchronous light and signal light;

[0028] The signal receiving end of any one of the above, configured to detect synchronous light and signal light.

[0029] The present application further provides a calibration method, configured to calibrate the gate width of a coincidence gate in any one of the above signal receiving ends, including:

[0030] Configure a signal source, where the signal source is configured to output a first signal, a second signal, and a third signal; wherein, the first signal is used to trigger a weak coherent pulse light source; the second signal is used as a gate signal for controlling the working state of a second detection unit; the third signal is used to provide a synchronous signal for a time-to-digital converter;

[0031] Based on the first signal, the second signal, and the third signal, plot a first curve and a second curve; wherein, the first curve and the second curve are respectively curves showing the relationship between multiple different delay positions and corresponding counts when the weak coherent pulse light source emits different pulse lights;

[0032] Based on the first curve and the second curve, plot a third curve, where the third curve is a curve showing the relationship between multiple different delay positions and corresponding count ratios;

[0033] Based on the third curve, determine the gate width of the coincidence gate, and store the gate width of the coincidence gate in the signal receiving end;

[0034] Wherein, the time-to-digital converter is used to record the counts of the second detection unit at different delay positions.

[0035] Preferably, in the above calibration method, the method for plotting the first curve includes:

[0036] Adjust and calibrate the weak coherent pulse light source to emit a first pulse light through an optical power meter and an optical attenuator;

[0037] Traverse the relative delay between the first signal and the second signal, and the delay range is the period of the gate signal;

[0038] Through the time-to-digital converter, record the counts respectively corresponding to the second detection unit at N different delay positions;

[0039] Draw a first curve based on N delay positions and their corresponding counts.

[0040] Preferably, in the above calibration method, the method for drawing the second curve includes:

[0041] Adjust and calibrate the second pulsed light emitted by the weak coherent pulse light source through an optical power meter and an optical attenuator; the second pulsed light has a different average number of photons per pulse from the first pulsed light;

[0042] Traverse the relative delay between the first signal and the second signal, and the delay range is the period of the gate signal;

[0043] Through a time-to-digital converter, record the counts respectively corresponding to the second detection unit at N delay positions;

[0044] Draw a second curve based on N delay positions and their corresponding counts.

[0045] Preferably, in the above calibration method, both the first curve and the second curve include the counts corresponding to the 1st delay position to the Nth delay position;

[0046] The method for drawing the third curve includes:

[0047] Determine the count ratio of the first curve and the second curve at the same delay position;

[0048] Based on the respective count ratios of the 1st delay position to the Nth delay position, draw a relationship curve between different delay positions and the corresponding count ratios.

[0049] Preferably, in the above calibration method, in the third curve, the duration of the delay position interval corresponding to the count ratio interval greater than the set protection threshold on the vertical axis on the horizontal axis is T';

[0050] Based on the third curve, determine the gate width of the coincidence gate, including:

[0051] Set the gate width of the coincidence gate as τ = T - 2T';

[0052] where T is the period of the gate signal.

[0053] This application also provides a calibration system for calibrating the gate width of the coincidence gate in any one of the above signal receivers, including:

[0054] Weak coherent pulse light source;

[0055] A signal source for outputting a first signal, a second signal, and a third signal; wherein, the first signal is used to trigger the weak coherent pulse light source; the second signal is used as a gate signal for controlling the working state of the second detection unit; the third signal is used to provide a synchronization signal for the time-to-digital converter;

[0056] An optical power meter and a variable optical attenuator, which are used to calibrate the pulse intensity of the optical pulses emitted by a weak coherent pulse light source to set the average number of photons per pulse;

[0057] A host, which is used to draw a first curve and a second curve based on a first signal, a second signal and a third signal, draw a third curve based on the first curve and the second curve, determine the gate width of a coincidence gate based on the third curve, and store the gate width of the coincidence gate into a signal receiving end;

[0058] Wherein, the first curve and the second curve are respectively relationship curves between multiple different delay positions and corresponding counts when the weak coherent pulse light source emits different pulsed lights, and the third curve is a relationship curve between multiple different delay positions and corresponding count ratios; The time-to-digital converter is used to record the counts of the second detection unit at different delay positions.

[0059] From the above description, it can be seen that in the QKD system, signal receiving end, calibration method and calibration system provided by the technical solution of the present application, the signal receiving end can detect the synchronization light through the first detection unit to obtain a first detection signal, and detect the signal light through the second detection unit to obtain a second detection signal; The gating signal output circuit can output a first gating signal and a second gating signal based on the first detection signal; The first output circuit can output a gate signal for controlling the working state of the second detection unit based on the first gating signal; The second output circuit can filter the initial avalanche signal through the coincidence gate based on the input second gating signal, filter out the interference signals in the avalanche transition region, and form an effective avalanche signal, so as to ensure that the interference signals in the avalanche transition region do not enter the key extraction process and avoid external interference signals from affecting the communication result of the QKD system. Description of the Drawings

[0060] In order to more clearly illustrate the technical solutions in the embodiments of the present application or related technologies, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained according to the provided drawings.

[0061] The structures, ratios, sizes, etc. shown in the drawings of this specification are only used to cooperate with the content disclosed in the specification for those familiar with this technology to understand and read, and are not used to limit the limited conditions under which the present application can be implemented. Therefore, they do not have technical substance. Any modification of the structure, change of the proportional relationship or adjustment of the size, without affecting the effects that the present application can produce and the purposes that can be achieved, should still fall within the scope covered by the technical content disclosed in the present application.

[0062] Figure 1 The signal receiving end of a QKD system provided by an embodiment of the present application;

[0063] Figure 2 Another signal receiving end of a QKD system provided by an embodiment of the present application;

[0064] Figure 3 For Figure 2 The signal timing diagram in the shown signal receiving end;

[0065] Figure 4 The structural schematic diagram of a QKD system provided by an embodiment of the present application;

[0066] Figure 5 The flow schematic diagram of a coincidence gate width calibration method provided by an embodiment of the present application;

[0067] Figure 6 The method flow chart of a method for drawing a first curve provided by an embodiment of the present application;

[0068] Figure 7 The method flow chart of a method for drawing a second curve provided by an embodiment of the present application;

[0069] Figure 8 The method flow chart of a method for drawing a third curve provided by an embodiment of the present application;

[0070] Figure 9 The structural schematic diagram of a calibration system provided by an embodiment of the present application. Detailed implementation manners

[0071] Next, the embodiments in the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0072] Without departing from the spirit or scope of the present application, various modifications and variations can be made in the present application, which are obvious to those skilled in the art. Therefore, the present application is intended to cover the modifications and variations of the present application that fall within the scope of the corresponding claims (the claimed technical solutions) and their equivalents. It should be noted that the implementation manners provided by the embodiments of the present application can be combined with each other without contradiction.

[0073] The QKD system includes a signal transmitting end and a signal receiving end. The signal transmitting end can transmit synchronous light and signal light, and the signal receiving end can detect the synchronous light and the signal light and obtain the communication information contained in the signal light based on the detection results.

[0074] In the BB84 protocol, a third party uses random basis vectors to measure the quantum states sent by the signal transmitter, and then the third party resends the measurement results. The measurement result is not a single photon but a known optical pulse called a trigger pulse. Then, if the third party uses a basis vector that matches the signal receiver to detect, the trigger pulse sent by the third party will always be detected by a detector with the same encoded value at the same basis vector end of the signal receiver, and will not be detected by a detector with a different encoded value. On the contrary, if the third party uses a basis vector that does not match the signal receiver to measure the quantum state of the signal light sent by the signal transmitter, the energy of its trigger pulse will be evenly distributed to two detectors with different encoded values at different basis vector ends of the signal receiver and will not be detected. In this case, the situation where the third party introduces bit errors will not occur, and the signal receiver will not monitor the bit errors introduced by the third party. At this time, the third party can obtain the key information without being discovered.

[0075] The avalanche transition region attack (ATR attack) is an attack method in which a third party uses the above method combined with the superlinear characteristics of the avalanche transition region to obtain the key of the QKD system without introducing bit errors.

[0076] A scheme for defending against the ATR attack is to resist the ATR attack by monitoring the bias current of the avalanche photodiode (APD). When the third party uses the avalanche transition region of the detector to implement the ATR attack, if the number of photons sent is only a few hundred photons, since the reverse bias voltage of the APD is smaller than the voltage value during normal single-photon detection, the avalanche current amplification will also be smaller than that during normal detection. Therefore, the current flowing through the APD during the implementation of the ATR attack is basically equivalent to or even smaller than the avalanche current during normal single-photon detection. So, the method of judging whether there is an ATR attack based on the magnitude of the APD bias current cannot resist the ATR attack under weak light.

[0077] Another scheme for defending against the ATR attack is to resist the ATR attack based on the characteristics of active gate signal switching. This method requires a clock fan-out, a multiplexer, a delay module, and a random source device, with high system complexity and great difficulty in engineering implementation.

[0078] Based on the above description, it can be seen that among the above two methods for defending against the ATR attack, this type of patchy defense measure based on monitoring the APD bias current parameter makes relatively small changes to the original system and is easy to implement, but this type of defense measure does not completely plug the loopholes. And the scheme for defending against the avalanche transition region attack by using the characteristics of active gate signal switching has a higher complexity compared to the original system.

[0079] To solve the above problems, the inventors have found through research that by using a time-to-digital converter (TDC) for measurement, it can be known that the distribution of the arrival times of normal detector pulses is different from that of the detection pulses when performing an ATR attack using the superlinear characteristic. There is a gap in the peak positions of the two types of pulses. Specifically, during the rise and fall of the gate signal, the detector efficiency of the detector in the signal receiving end is superlinear; at the same time, for the detection pulses of the signal light, when the system is operating normally and when under attack, due to the different numbers of photons used, the time distribution of the in-gate detection counts of the detector is different. Generally, the time distribution is wider when under attack. Therefore, the pulses in the avalanche transition region can be filtered out by a hardware coincidence gate, thereby effectively defending against avalanche transition region attacks. Based on this, the embodiments of the present application provide a defense solution for ATR attacks on single-photon detectors without active gate signal switching. This solution filters out ATR pulses through a hardware coincidence gate and only retains valid avalanche signals to ensure that ATR attack data does not enter the key extraction process. This solution is simple to implement and has good versatility.

[0080] To make the above objects, features, and advantages of the present application more obvious and understandable, the present application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0081] Refer to Figure 1 , Figure 1 A signal receiving end of a QKD system provided by an embodiment of the present application. The QKD system includes a signal transmitting end and a signal receiving end. The signal transmitting end is used to emit synchronous light and signal light, and the signal receiving end is used to detect the synchronous light and the signal light and obtain the communication information carried by the signal light based on the detection results.

[0082] The signal receiving end shown includes:

[0083] A first detection unit, which is used to detect the synchronous light and obtain a first detection signal;

[0084] A second detection unit, which is used to detect the signal light and obtain a second detection signal; the second detection signal includes an initial avalanche signal obtained by the second detection unit in the avalanche transition region;

[0085] A gating signal output circuit 10, which is used to output a first gating signal and a second gating signal based on the first detection signal;

[0086] A first output circuit 11, which is used to output a gating signal for controlling the working state of the second detection unit based on the first gating signal;

[0087] A second output circuit 12, which is used to filter the initial avalanche signal through a coincidence gate based on the input second gating signal to form a valid avalanche signal.

[0088] The first detection unit and the second detection unit may be located in the same single-photon detector in the signal receiving end. Optionally, the first detection unit is a PIN-type photodiode, and the second detection unit is an APD. Figure 1 The first detection unit and the second detection unit are not shown.

[0089] Those skilled in the art can know that the first detection unit is the detection element of the synchronous optical detector in the signal receiving end, and the second detection unit is the detection element of the single-photon detector in the signal receiving end. The first detection unit can also be arranged in the single-photon detector of the signal receiving end to multiplex control resources. This application does not limit this.

[0090] In the embodiment of the present application, the gating signal output circuit 10 of the signal receiving end can output a first gating signal and a second gating signal based on the first detection signal. The first output circuit 11 can output a gate signal for controlling the working state of the second detection unit based on the first gating signal. The second output circuit 12 can filter the initial avalanche signal through a coincidence gate based on the input second gating signal, filter out the interference signals in the avalanche transition region, and form an effective avalanche signal, so as to ensure that the interference signals in the avalanche transition region do not enter the key extraction process and avoid external interference signals from affecting the communication result of the QKD system.

[0091] Reference Figure 2 as shown Figure 2 is the signal receiving end of another QKD system provided by the embodiment of the present application. On the basis of the Figure 1 way shown Figure 2 In the signal receiving end shown, the gating signal output circuit 10 includes:

[0092] A frequency multiplier (PLL) 101, and the frequency multiplier 101 is used to perform frequency increase processing on the first detection signal;

[0093] A filter amplification circuit 102, and the filter amplification circuit 102 is used to perform amplification processing and filtering processing on the output signal of the frequency multiplier 101;

[0094] A first adjustable delay circuit 103, and the first adjustable delay circuit 103 is used to perform delay processing based on the output signal of the filter amplification circuit 102 to calibrate the delay of the signal light and the synchronous light;

[0095] A controllable attenuation circuit 104, and the controllable attenuation circuit 104 is used to perform intensity modulation on the output signal of the first adjustable delay circuit 103 to output a first gating signal and a second gating signal.

[0096] In the QKD system, a synchronization light of 100 KHz can be adopted. The first detection unit can convert the received 100 KHz synchronization light into an electrical signal of 100 KHz, and the frequency multiplier 101 converts this electrical signal into a signal of 1.25 GHz for output.

[0097] Based on Figure 2 the circuit structure shown, the first detection signal can be converted into the first gating signal and the second gating signal for output after passing through frequency increase, amplification processing, filtering processing, delay processing, and intensity modulation in the gating signal output circuit 10 in sequence. The controllable attenuation circuit 104 performs intensity modulation on the output signal of the first adjustable delay circuit 103 based on the feedback information of the first output circuit 11 to output stable first and second gating signals.

[0098] In the embodiment of the present application, the first output circuit 11 is used to perform amplification processing and power coupling processing on the first gating signal and output a stable gating signal.

[0099] As Figure 2 shown, the first output circuit 11 includes:

[0100] a gating signal amplification circuit 111, which is used to perform amplification processing on the first gating signal output by the gating signal output circuit 10;

[0101] a power coupling circuit 112, which is used to perform power coupling processing on the amplified first gating signal to output a gating signal.

[0102] Among them, the gating signal amplification circuit 111 can provide feedback information for the controllable attenuation circuit 104, so that the controllable attenuation circuit 104 performs intensity modulation on the output signal of the first adjustable delay circuit 103 to output stable first and second gating signals. The power coupling circuit 112 can perform power modulation on the input signal to output a stable gating signal.

[0103] Based on the above description, it can be known that the first output circuit 11 can perform amplification processing and power coupling processing on the first gating signal and output a stable gating signal.

[0104] As Figure 2 shown, the second output circuit 12 includes:

[0105] a second adjustable delay circuit 121, which is used to perform delay processing based on the second gating signal to perform delay calibration on the initial avalanche signal and the coincidence gating signal;

[0106] an adjustable discrimination circuit 122, which is used to discriminate the output signal of the second adjustable delay circuit 121 to output a coincidence gating signal;

[0107] Discriminator 123 is used to discriminate the amplified initial avalanche signal based on a set avalanche discrimination threshold and output an avalanche discrimination signal.

[0108] Coincidence and pulse width shaping circuit 124 is used to perform coincidence on the avalanche discrimination signal and the coincidence gate signal to filter out interference pulses in the avalanche transition region and perform pulse width shaping, and output an effective avalanche signal.

[0109] In Figure 2 In the second output circuit 12 shown, the second adjustable delay circuit 121 can perform delay calibration on the avalanche signal and the coincidence gate signal at the back end of the detector. After combining with the adjustable discrimination circuit 122 to discriminate the output signal of the second adjustable delay circuit 121, the required coincidence gate signal can be output through the adjustable discrimination circuit 122. The initial avalanche signal and the gate signal are superimposed together, and after filtering and amplification, the amplified initial avalanche signal is obtained. After being discriminated by the discriminator 123, the avalanche discrimination signal is output, and by performing coincidence on the avalanche discrimination signal with the coincidence gate signal, an effective avalanche signal can be generated.

[0110] Based on the above description, it can be known that in Figure 2 In the signal receiving end of the QKD system shown, the PIN photodiode is the first detection unit, and the APD is the second detection unit. The PIN photodiode converts the received 100KHz synchronous optical signal into a 100KHz electrical signal, and this electrical signal can be used as the clock reference for generating the subsequent gate signal. After frequency increase processing, a 1.25GHz signal is generated, and then through amplification processing, filtering processing, delay processing, and intensity modulation, two gate control signals can be output. One gate control signal can output the required stable gate signal after passing through the gate signal amplification circuit 111 and the power coupling circuit 112 in the first output circuit 11. The other gate control signal, in the second output circuit 12, outputs a coincidence gate signal after passing through the second adjustable delay circuit 121 and the adjustable discrimination circuit 122. The amplified initial avalanche signal outputs an avalanche discrimination signal after being discriminated by the discriminator 123. In the coincidence and pulse width shaping circuit 124, by performing coincidence on the avalanche discrimination signal with the coincidence gate signal, the interference pulses in the avalanche transition region can be filtered out, and then through pulse width shaping, an effective avalanche signal is output. The waveform timings of each signal are as Figure 3 shown.

[0111] Refer to Figure 3 shown, Figure 3 is Figure 2 the signal timing diagram in the signal receiving end shown, Figure 3Among them, S1 is the gate signal, S2 is the superposition of the initial avalanche signal and the gate signal, S3 is the amplified initial avalanche signal, S4 is the avalanche discrimination signal, S5 is the coincidence gate signal, and S6 is the effective avalanche signal.

[0112] In the embodiment of the present application, the effective avalanche signal output by the coincidence and pulse width shaping circuit 124 can be further converted into a digital pulse signal for subsequent data processing.

[0113] It can be seen from the above description that in the QKD signal receiver provided by the embodiment of the present application, the interference pulses in the avalanche transition region can be filtered out by the hardware coincidence gate, and the effective avalanche signal is retained, so as to ensure that the attack data in the avalanche transition region does not enter the key extraction process.

[0114] Based on the description of the above embodiment, another embodiment of the present application further provides a QKD system, as Figure 4 shown.

[0115] Refer to Figure 4 shown, Figure 4 which is a schematic structural diagram of a QKD system provided by the embodiment of the present application. The QKD system shown includes:

[0116] A signal transmitting end 21, which is used to emit synchronous light and signal light;

[0117] A signal receiving end 22, which is the signal receiving end described in any one of the above embodiments, and the signal receiving end 22 is used to detect synchronous light and signal light.

[0118] In the QKD system provided by the embodiment of the present application, by adopting the signal receiving end provided by the above embodiment, the interference pulses in the avalanche transition region can be filtered out by the hardware coincidence gate, and the effective avalanche signal is retained, so as to ensure that the attack data in the avalanche transition region does not enter the key extraction process, and the attack defense against the avalanche transition region can be realized.

[0119] Due to device differences, the distribution of the avalanche transition region of each device on the time axis is different. Therefore, the coincidence gate width will be confirmed by calibration before leaving the factory. In view of this, another embodiment of the present application further provides a method for calibrating the gate width of a coincidence gate, and this method can be as Figure 5 shown, and is used for calibrating the gate width of the coincidence gate in the signal receiving end of the above embodiment.

[0120] Refer to Figure 5 shown, Figure 5 which is a schematic flow diagram of a method for calibrating the gate width of a coincidence gate provided by the embodiment of the present application. This calibration method includes:

[0121] Step S11: Configure a signal source, which is used to output a first signal C1, a second signal C2, and a third signal C3.

[0122] Among them, the first signal C1 is used to trigger a weak coherent pulse light source, and the frequency is f L ; the second signal C2 is used as a gate signal for controlling the working state of the second detection unit, and the frequency is f g ; the third signal C3 is used to provide a synchronization signal for the time-to-digital converter.

[0123] Step S12: Based on the first signal C1, the second signal C2, and the third signal C3, draw a first curve and a second curve; among them, the first curve and the second curve are respectively the relationship curves between multiple different delay positions and the corresponding counts when different pulse lights are emitted by the weak coherent pulse light source.

[0124] Step S13: Based on the first curve and the second curve, draw a third curve, and the third curve is the relationship curve between multiple different delay positions and the corresponding count ratio;

[0125] Step S14: Based on the third curve, determine the gate width of the coincidence gate, and store the gate width of the coincidence gate at the signal receiving end.

[0126] Among them, the time-to-digital converter records the counts of the second detection unit at different delay positions. A time-to-digital converter with a precision of η can be used to record the counts of the second detection unit at different delay positions, and η can be selected based on requirements.

[0127] Based on Figure 5 The calibration method shown can calibrate the gate width of the coincidence gate in the above embodiments, and solve the difference in the distribution of the avalanche transition region of each device on the time axis due to device differences.

[0128] In the above calibration method, the weak coherent pulse light source selected for calibration has two optical powers with a luminous power of 625 KHz, a pulse width of 55 ps (full width at half maximum), and average photon numbers of 100 and 50 respectively. Among them, the selection of the luminous power, pulse width, and average photon number can all be set based on requirements, and is not limited to the above values.

[0129] In the above step S12, the method for drawing the first curve is as Figure 6 shown.

[0130] Refer to Figure 6 shown, Figure 6 which is a flowchart of a method for drawing the first curve provided by an embodiment of the present application. The method includes:

[0131] Step S21: Adjust and calibrate the first pulse light emitted by the weak coherent pulse light source through an optical power meter and an adjustable optical attenuator.

[0132] When the weak coherent pulse light source emits the first pulse light, the pulse intensity can be that the average number of photons per pulse is μ1, and μ1 = 100.

[0133] Step S22: Traverse the relative delay between the first signal C1 and the second signal C2, and the delay range is the period T of the gate signal.

[0134] Based on the weak coherent pulse light source calibrated with an average number of photons per pulse of μ1 and the detection result of the second detection unit in the signal receiving end, it is possible to traverse the relative delay between the first signal C1 and the second signal C2 and determine the delay range.

[0135] Step S23: Through the time-to-digital converter, record the counts corresponding to the second detection unit at N different delay positions respectively.

[0136] When the average number of photons per pulse is μ1, record the counts corresponding to the second detection unit at different delay positions. It is set that there are N different delay positions, and these N delay positions are successively the 1st delay position to the Nth delay position. When the average number of photons per pulse is μ1, the count corresponding to the ith delay position is Q i . N is a positive integer set greater than 1, and i is a positive integer not greater than N.

[0137] Step S24: Draw the first curve based on the N delay positions and the counts corresponding to them.

[0138] When the average number of photons per pulse is μ1, the host can automatically draw the first curve based on each delay position and the count corresponding to it. The first curve is a relationship curve between the delay position and the count of the second detection unit when the average number of photons per pulse is calibrated to μ1.

[0139] In the above step S12, the method of drawing the second curve is as Figure 7 shown.

[0140] Refer to Figure 7 shown, Figure 7 which is a flowchart of a method for drawing the second curve provided by an embodiment of the present application. The method includes:

[0141] Step S31: Adjust and calibrate the weak coherent pulse light source to emit the second pulse light through an optical power meter and an adjustable optical attenuator; the second pulse light has a different average number of photons per pulse from the first pulse light.

[0142] When the weak coherent pulse light source emits the second pulse light, the pulse intensity can be that the average number of photons per pulse is μ2, and μ2 = 50.

[0143] Step S32: Traverse the relative delay between the first signal C1 and the second signal C2, and the delay range is the period T of the gate signal. When the average number of photons per pulse is calibrated to μ2, this step is the same as the above step S22.

[0144] Step S33: Record the counts corresponding to the N delay positions of the second detection unit through a time-to-digital converter. When calibrating the average number of photons per pulse as μ2, this step is the same as the above Step S23. When the average number of photons per pulse is μ2, the count corresponding to the i-th delay position is P i 。

[0145] Step S34: Draw a second curve based on the N delay positions and the counts corresponding thereto. When calibrating the average number of photons per pulse as μ2, this step is the same as the above Step S24. The second curve is the relationship curve between the delay position and the count of the second detection unit when calibrating the average number of photons per pulse as μ2.

[0146] As described above, both the first curve and the second curve include the counts corresponding to the 1st to N-th delay positions; in the above Step S13, the method of drawing the third curve is as Figure 8 shown.

[0147] Reference Figure 8 shown, Figure 8 is a flowchart of a method for drawing a third curve provided by an embodiment of the present application. The method includes:

[0148] Step S41: Determine the count ratio of the first curve and the second curve at the same delay position.

[0149] As described above, when the average number of photons per pulse is μ1, the count corresponding to the i-th delay position is Q i . When the average number of photons per pulse is μ2, the count corresponding to the i-th delay position is P i . Therefore, the count ratio at the i-th delay position is Q i / P i 。

[0150] Step S42: Draw a relationship curve between different delay positions and the corresponding count ratios based on the count ratios of the 1st to N-th delay positions.

[0151] The host can automatically draw the third curve based on each delay position and the corresponding count ratio. The horizontal axes of the first curve and the second curve are both delay positions, and the vertical axes are counts; in the third curve, the horizontal axis is the delay position, and the vertical axis is the count ratio.

[0152] In the embodiment of the present application, in the third curve, the duration of the delay position interval corresponding to the counting ratio interval greater than the set protection threshold χ on the vertical axis on the horizontal axis is T'. That is to say, the duration of the delay position interval corresponding to the interval with a counting ratio greater than χ on the vertical axis on the horizontal axis is T'. Among them, the protection threshold χ is a constant that can be set based on requirements. At this time, in the above step S14, based on the third curve, determining the gate width of the coincidence gate includes: setting the gate width of the coincidence gate as τ = T - 2T'; where T is the period of the gate signal. After determining the gate width τ of the coincidence gate, write the gate width τ into the storage module of the signal receiving end to complete the factory setting.

[0153] In the above calibration method, based on the set μ1 and μ2, the signal light and the attack pulse in the normal operation of the QKD system can be respectively simulated, and based on the counting results of the second detection unit, the counting ratio at different delay positions can be plotted to determine the required gate width of the coincidence gate.

[0154] In the embodiment of the present application, it can be based on Figure 9 the calibration system shown to implement the above calibration method.

[0155] Refer to Figure 9 shown, Figure 9 which is a schematic structural diagram of a calibration system provided by an embodiment of the present application, used to execute the above calibration method to calibrate the gate width of the coincidence gate in the signal receiving end. The shown calibration system includes:

[0156] Weak coherent pulse light source 32;

[0157] Signal source 31, the signal source 31 is used to output the first signal C1, the second signal C2 and the third signal C3; among them, the first signal C1 is used to trigger the weak coherent pulse light source 32; the second signal C2 is used as the gate signal for controlling the working state of the second detection unit 36; the third signal C3 is used to provide a synchronization signal for the time-to-digital converter 37;

[0158] Optical power meter 34 and variable optical attenuator 33, the optical power meter 34 and the variable optical attenuator 33 are used to calibrate the pulse intensity of the light pulse emitted by the weak coherent pulse light source 32 to set the average number of photons per pulse;

[0159] Host 35, the host 35 is used to plot the first curve and the second curve based on the first signal C1, the second signal C2 and the third signal C3, plot the third curve based on the first curve and the second curve, determine the gate width of the coincidence gate based on the third curve, and store the gate width of the coincidence gate in the signal receiving end.

[0160] Among them, the first curve and the second curve are respectively the relationship curves between multiple different delay positions and the corresponding counts when the weak coherent pulse light source 32 emits different pulsed lights, and the third curve is the relationship curve between multiple different delay positions and the corresponding count ratios; the time-to-digital converter 37 is used to record the counts of the second detection unit 36 at different delay positions.

[0161] The calibration system provided by the embodiments of the present application can execute the above-mentioned calibration method to complete the gate width calibration of the coincidence gate in the signal receiving end before leaving the factory, and solve the difference in the distribution of the avalanche transition region of each device on the time axis caused by device differences.

[0162] In this specification, the various embodiments are described in a progressive, or parallel, or a combination of progressive and parallel manners. The key point of each embodiment is to describe the differences from other embodiments. The same or similar parts among the various embodiments can be referred to each other. The technical solutions provided by the embodiments of the present application are applicable to high-speed QKD systems and also to low-speed QKD systems.

[0163] It should be noted that in the description of the present application, it should be understood that the descriptions of the drawings and embodiments are illustrative rather than restrictive. The same reference numerals throughout the embodiments of the specification identify the same structures. Additionally, for the sake of understanding and ease of description, the drawings may exaggerate the thickness of some layers, films, panels, regions, etc. At the same time, it can be understood that when an element such as a layer, film, region, or substrate is referred to as being "on" another element, the element can be directly on the other element or there may be intermediate elements. Additionally, "on..." means positioning the element on or below another element, but essentially does not mean positioning on the upper side of another element according to the direction of gravity.

[0164] The orientation or positional relationship indicated by the terms "upper", "lower", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be construed as a limitation of the present application. When a component is considered to be "connected" to another component, it can be directly connected to the other component or there may be intermediate components present.

[0165] It should also be noted that, in this text, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that an article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such article or device. Without further limitation, an element defined by the statement "comprising an..." does not exclude the presence of additional identical elements in the article or device comprising the above elements.

[0166] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A signal receiving end of a QKD system, wherein the signal transmitting end of the QKD system is used to emit synchronous light and signal light, and is characterized in that, The signal receiving end includes: A first detection unit for detecting synchronous light and obtaining a first detection signal; A second detection unit for detecting signal light and obtaining a second detection signal; the second detection signal includes an initial avalanche signal obtained by the second detection unit in the avalanche transition region; A gating signal output circuit for outputting a first gating signal and a second gating signal based on the first detection signal; A first output circuit for outputting a gate signal for controlling the working state of the second detection unit based on the first gating signal; A second output circuit for filtering the initial avalanche signal through a coincidence gate based on the input second gating signal to form an effective avalanche signal.

2. The signal receiving end according to claim 1, wherein The gating signal output circuit includes: A frequency multiplier for performing frequency increase processing on the first detection signal; A filter amplification circuit for performing amplification processing and filtering processing on the output signal of the frequency multiplier; A first adjustable delay circuit for performing delay processing based on the output signal of the filter amplification circuit to perform delay calibration on the signal light and the synchronous light; A controllable attenuation circuit for performing intensity modulation on the output signal of the first adjustable delay circuit and outputting the first gating signal and the second gating signal.

3. The signal receiving end according to claim 1, wherein The first output circuit is used for performing amplification processing and power coupling processing on the first gating signal and outputting the stable gate signal.

4. The signal receiving end according to claim 3, characterized in that, The first output circuit includes: A gate signal amplification circuit for performing amplification processing on the first gating signal output by the gating signal output circuit; A power coupling circuit for performing power coupling processing on the amplified first gating signal to output the gate signal.

5. The signal receiving end according to claim 1, characterized in that, The second output circuit includes: A second adjustable delay circuit for performing delay processing based on the second gating signal to perform delay calibration on the initial avalanche signal and the coincidence gate signal; An adjustable discrimination circuit for discriminating the output signal of the second adjustable delay circuit to output a coincidence gate signal; A discriminator for discriminating the amplified initial avalanche signal based on a set avalanche discrimination threshold and outputting an avalanche discrimination signal; A coincidence and pulse width shaping circuit for performing coincidence on the avalanche discrimination signal and the coincidence gate signal to filter out interference pulses in the avalanche transition region and perform pulse width shaping to output the effective avalanche signal.

6. A QKD system, characterized in that, Includes: A signal transmitting end for emitting synchronous light and signal light; The signal receiving end according to any one of claims 1-5 for detecting the synchronous light and the signal light.

7. A calibration method for calibrating the gate width of the coincidence gate in the signal receiver according to any one of claims 1-5, characterized in that, Includes: A configured signal source for outputting a first signal, a second signal, and a third signal; wherein, the first signal is used to trigger a weak coherent pulse light source; the second signal is used as a gate signal for controlling the working state of the second detection unit; the third signal is used to provide a synchronous signal for a time-to-digital converter; Based on the first signal, the second signal, and the third signal, a first curve and a second curve are plotted; wherein, the first curve and the second curve are respectively relationship curves between multiple different delay positions and corresponding counts when the weak coherent pulse light source emits different pulse lights. Based on the first curve and the second curve, a third curve is plotted, and the third curve is a relationship curve between multiple different delay positions and corresponding count ratios; Based on the third curve, the gate width of the coincidence gate is determined, and the gate width of the coincidence gate is stored in the signal receiving end; Wherein, the time-to-digital converter is used to record the counts of the second detection unit at different delay positions.

8. The calibration method according to claim 7, wherein The method for plotting the first curve includes: Adjusting and calibrating the weak coherent pulse light source to emit the first pulsed light through an optical power meter and a tunable optical attenuator; Traversing the relative delay between the first signal and the second signal, and the delay range is the period of the gate signal; Through the time-to-digital converter, recording the counts respectively corresponding to the second detection unit at N different delay positions; Based on the N delay positions and the counts corresponding thereto, a first curve is plotted.

9. The calibration method according to claim 8, characterized in that, The method for plotting the second curve includes: Adjusting and calibrating the weak coherent pulse light source to emit the second pulsed light through an optical power meter and a tunable optical attenuator; the second pulsed light has a different average number of photons per pulse from the first pulsed light; Traversing the relative delay between the first signal and the second signal, and the delay range is the period of the gate signal; Through the time-to-digital converter, recording the counts respectively corresponding to the second detection unit at N of the delay positions; Based on the N delay positions and the counts corresponding thereto, a second curve is plotted.

10. The calibration method according to claim 7, wherein Both the first curve and the second curve include the counts corresponding to the 1st delay position to the Nth delay position; The method for plotting the third curve includes: Determining the count ratio of the first curve and the second curve at the same delay position; Based on the count ratios of the 1st delay position to the Nth delay position respectively, a relationship curve between different delay positions and corresponding count ratios is plotted.

11. The calibration method according to claim 7, characterized in that, In the third curve, the duration of the delay position interval on the horizontal axis corresponding to the count ratio interval greater than the set protection threshold on the vertical axis is T'; Based on the third curve, determining the gate width of the coincidence gate includes: Setting the gate width of the coincidence gate as τ = T - 2T'; Wherein, T is the period of the gate signal.

12. A calibration system for calibrating the gate width of a coincidence gate in a signal receiver as described in any one of claims 1-5, characterized in that, Includes: Weak coherent pulse light source; A signal source for outputting a first signal, a second signal, and a third signal; wherein, the first signal is used to trigger the weak coherent pulse light source; the second signal is used as a gate signal for controlling the working state of the second detection unit; the third signal is used to provide a synchronization signal for the time-to-digital converter; An optical power meter and a tunable optical attenuator, and the optical power meter and the tunable optical attenuator are used to calibrate the pulse intensity of the light pulse emitted by the weak coherent pulse light source to set the average number of photons per pulse; A host computer, which is used to plot the first curve and the second curve based on the first signal, the second signal, and the third signal, plot the third curve based on the first curve and the second curve, determine the gate width of the coincidence gate based on the third curve, and store the gate width of the coincidence gate in the signal receiving end; Among them, the first curve and the second curve are respectively the relationship curves between multiple different delay positions and corresponding counts when the weak coherent pulse light source emits different pulsed lights, and the third curve is the relationship curve between multiple different delay positions and corresponding count ratios; the time-to-digital converter is used to record the counts of the second detection unit at different delay positions.

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