A TF-QKD method and device using an avalanche detector

Abstract

The present invention proposes a TF-QKD method and device using an avalanche detector, which relates to the field of quantum communication technology. Alice and Bob each use single-wavelength optical time division multiplexing to periodically encode phase reference light and quantum signal light, and then transmit them to Charlie. Before the start of TF-QKD, a synchronous clock sends a clock signal to Alice and Bob. Alice and Bob send test pulse signals at preset times. Charlie calibrates and delay-compensates the synchronous clock based on the detection results of the test pulse signals, so that the arrival times of the test pulse signals emitted by Alice and Bob are consistent. The output signal delay from the synchronous clock to the optical switch is adjusted to align the switching time of the optical switch with the encoding period of Alice and Bob. The optical switch controls the phase reference light and quantum signal light to enter different single-photon detectors for detection according to the switching time.

Description

Technical Field

[0001] The present invention relates to the field of quantum communication technology, and in particular to a TF-QKD method and device using an avalanche detector. Background Art

[0002] TF-QKD (Twin-Field Quantum Key Distribution) is a new quantum key distribution protocol that boasts a longer transmission distance and higher coding rate over long distances compared to previous protocols. The TF-QKD protocol consists of two transmitters, Alice and Bob, and a detector, Charlie. The transmitters, Alice and Bob, use encoding modules to encode their respective light sources—randomly encoding the "0" and "1" bits of information into weakly coherent single-photon-level signal light pulses of varying intensities and random phases. These are then transmitted via a channel to the measurement terminal, Charlie. Upon reaching Charlie's optical interferometer module, the two signal light pulses generate single-photon interference, which is detected by the single-photon detector.

[0003] Commonly used single-photon detectors include superconducting nanowire detectors (SNSPDs) and semiconductor avalanche detectors (such as InGaAs detectors). Compared to SNSPDs, avalanche detectors are cheaper and consume less power, and do not require very low operating temperatures (for example, InGaAs detectors can operate from -30°C to near room temperature). Therefore, compared to SNSPDs, avalanche detectors do not require bulky cryogenic refrigeration equipment, reducing the complexity and size of TF-QKD systems.

[0004] However, compared to SNSPDs, avalanche detectors have lower detection efficiency and higher noise caused by dark counts. Furthermore, the most significant and critical impact on the TF-QKD protocol is the afterpulse effect of avalanche detectors. This refers to the probability of additional noise being generated after a detection event, and the probability of this noise decays over time. Therefore, when using semiconductor detectors, a dead time must be set, meaning that the detection data for a period of time after the detection event is discarded.

[0005] In TF-QKD, to achieve stable interference at Charlie, Alice and Bob need to transmit phase reference light. Charlie then provides feedback or compensation for the phase difference based on the detection results of the phase reference light. The phase reference light must accumulate a high count rate within a short period of time. Therefore, the count rate of the reference light, after link transmission losses, must reach a certain level at the detector end. Meanwhile, the quantum signal light at the transmitter exits at a level below the single-photon level. Therefore, the phase reference light pulse is stronger than the quantum signal light pulse, and the intensity ratio between the phase reference light and the quantum signal light increases with increasing communication distance. Typically, the phase reference light and the quantum signal light are encoded using time-division multiplexing. When using an avalanche detector, there is still a certain probability of post-pulse noise outside the dead time. Because the count rate of the phase reference light is much higher than that of the quantum signal light, even with a dead time, the post-pulse noise of the phase reference light will still cause significant interference with the quantum signal light. Furthermore, the longer the signal transmission distance, the more stable the noise count rate generated by the reference light post-pulse, while the signal count rate generated by the signal light decreases. This results in a reduced signal-to-noise ratio for long-distance communication, making coding impossible over long distances. For example, in an experiment, the link length is 200 km, the probability of post-pulse noise outside the dead time is approximately 1%, and the encoding period is 100 ns. In this case, the post-pulse noise generated by the phase reference beam can be equivalently considered to be uniformly distributed in the time domain. In the experiment, the count rate of the phase reference beam when it reaches the detector is approximately 2 MHz, and the post-pulse noise outside the dead time is 20 kHz. We reduce this noise through time-domain filtering. For a 1 GHz system, the filtering system opens a 200 ps gate in the time domain, reducing the noise count rate to 1 / 5, or 4 kHz. Given an average quantum signal photon number of 0.1, a system frequency of 1 GHz, and a link and system loss of 40 dB, the signal light count rate is estimated to be approximately 10 kHz. At this point, the signal-to-noise ratio at the detector is extremely low, making coding impossible.

[0006] Existing solutions (such as patent CN116260508A) use dual-wavelength feedback to encode the phase reference light and quantum signal light using different wavelengths. At the detection end, DWDM is used to separate the reference light and signal light into different detectors according to wavelength for detection, so that the noise of the reference light does not affect the detection of the signal light. However, dual-wavelength has the following disadvantages: (1) Dual-wavelength feedback requires an additional light source for frequency locking, and the frequency-locked light source is sent to the channels at Alice and Bob's ends. In addition, to ensure the interference quality, there are additional requirements for line width and link, which increases the complexity of the system; (2) Dual-wavelength occupies more channels than a single wavelength, increasing the system cost. Moreover, when using DWDM for wavelength division, dual-wavelength will halve the number of multiplexes in DWDM, thereby reducing the number of available quantum channels; (3) The link phase difference of different wavelengths of light is different, and the resulting error reduces the coding rate. Summary of the Invention

[0007] To solve the above technical problems, the present invention proposes a TF-QKD device using an avalanche detector, comprising: an Alice terminal, a Bob terminal, and a Charlie terminal, wherein the Alice terminal and the Bob terminal serve as the transmitting terminal of the TF-QKD device, and the Charlie terminal serves as the detecting terminal of the TF-QKD device;

[0008] Alice and Bob each periodically encode the phase reference light and quantum signal light with the same wavelength by using single-wavelength optical time division multiplexing, and then transmit them to Charlie;

[0009] The Charlie end includes: single photon detector 1, single photon detector 2, single photon detector 3, single photon detector 4, an optical switch and a synchronous clock;

[0010] Before TF-QKD starts, the synchronous clock sends a clock signal to Alice and Bob. Alice and Bob send test signals at a preset time. Charlie calibrates and delay-compensates the synchronous clock based on the detected test signal, so that the arrival time of the test pulse signals emitted by Alice and Bob are consistent and the rising edges are aligned. The output signal delay from the synchronous clock to the optical switch is adjusted so that the switching time of the optical switch is aligned with the coding period of Alice and Bob. The optical switch controls the phase reference light and quantum signal light to enter different single-photon detectors for detection according to the switching time.

[0011] Furthermore, the Charlie end further includes: a phase feedback / compensation module and a synchronous counter;

[0012] The synchronous counter synchronously records the detection results of the single photon detector 1, the single photon detector 2, the single photon detector 3 and the single photon detector 4;

[0013] The phase feedback / compensation module includes a phase modulator and an operation processing module. The operation processing module calculates the phase difference between the optical signals at both ends of Alice and Bob based on the detection results of the single-photon detector 1 and the single-photon detector 2, and generates a feedback electrical signal. The phase modulator performs phase modulation in real time based on the feedback electrical signal to achieve phase feedback of the quantum signal light.

[0014] The detection results of the single-photon detector three and the single-photon detector four are directly used to generate quantum keys.

[0015] Furthermore, the Charlie end further comprises: a phase feedback / compensation module and a synchronous counter, wherein the phase feedback / compensation module comprises an operation processing module and does not require a phase modulator;

[0016] The operation processing module calculates the phase difference between the optical signals at both ends of Alice and Bob based on the detection results of single-photon detector 1 and single-photon detector 2, and performs phase post-processing on the detection results of single-photon detector 3 and single-photon detector 4, and uses the phase difference to generate the quantum key after compensating for it.

[0017] Furthermore, Alice and Bob encode the phase reference light and quantum signal light of the same wavelength through time division multiplexing. In the first half of the encoding period, the phase modulator uses a fixed phase to encode and modulate the phase reference light; in the second half of the encoding period, the phase modulator uses a random one of four phases to encode and modulate the quantum signal light.

[0018] Furthermore, at the moment when the phase reference light starts in the coding period, the voltage signal sent by the synchronous clock controls the optical switch to switch to detect the phase reference light through single-photon detector one and single-photon detector two; at the moment when the quantum signal light coding starts in the coding period, the voltage signal sent by the synchronous clock controls the optical switch to switch to detect the quantum signal light through single-photon detector three and single-photon detector four.

[0019] Furthermore, the single-photon detector 1, the single-photon detector 2, the single-photon detector 3 and the single-photon detector 4 are avalanche detectors.

[0020] Furthermore, the Charlie end also includes a polarization module and an optical interference module. The polarization module is used to modulate the polarization of the optical signal so that the polarization directions of the two optical signals entering the optical interference module are consistent; the optical interference module is used to cause the two optical signals to interfere and output the interference result.

[0021] The present invention also proposes a TF-QKD method using an avalanche detector, which is implemented using the above-mentioned TF-QKD device and is characterized in that:

[0022] The synchronized clock sends a clock signal to Alice and Bob before TF-QKD starts.

[0023] Alice and Bob send a test signal at a preset time. At the preset time, Alice and Bob each periodically encode the phase reference light and quantum signal light of the same wavelength using single-wavelength optical time division multiplexing and then transmit them to Charlie.

[0024] Charlie calibrates the synchronous clock and compensates for delay based on the detected test signal, ensuring that the test signals transmitted by Alice and Bob arrive at the same time and their rising edges are aligned.

[0025] Adjust the delay of the output signal from the synchronous clock to the optical switch so that the switching time of the optical switch is aligned with the coding period of Alice and Bob.

[0026] The optical switch controls the phase reference light and the quantum signal light to enter different single-photon detectors for detection according to the switching time.

[0027] Furthermore, at the moment when the phase reference light starts in the coding period, the voltage signal sent by the synchronous clock controls the optical switch to switch to detect the phase reference light through single-photon detector one and single-photon detector two; at the moment when the quantum signal light coding starts in the coding period, the voltage signal sent by the synchronous clock controls the optical switch to switch to detect the quantum signal light through single-photon detector three and single-photon detector four.

[0028] Compared with the prior art, the present invention has the following beneficial technical effects:

[0029] The TF-QKD method and device using an avalanche detector in the present application can generate codes when using an avalanche detector for detection at the Charlie end, while using the same wavelength to encode phase reference light and quantum signal light, avoiding the increase in system complexity caused by the use of additional wavelength light and reducing the resources used by TF-QKD.

[0030] At the same time, different types of avalanche detectors are selected to detect the phase reference light and quantum signal light respectively, and optical switches are used to switch different detectors to avoid mutual interference of post-pulse noise.

[0031] Single-wavelength light is used to time-division multiplex the phase reference light and quantum signal light. A synchronous clock is used to control the optical switch, and the time-division multiplexed reference light and signal light are time-divided and sent to different detectors for detection, avoiding mutual interference of post-pulse noise. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0033] Figure 1 Schematic diagram of the structure of the transmitting end of the present invention;

[0034] Figure 2 Schematic diagram of phase reference light and quantum signal light encoding;

[0035] Figure 3A schematic diagram of the positions of the light switches when the single-photon detectors 1 and 2 of the present invention detect the phase reference light;

[0036] Figure 4 A schematic diagram of the positions of the light switches when the single-photon detectors three and four of the present invention detect quantum signal light;

[0037] Figure 5 Schematic diagram of the phase feedback device structure of the present invention;

[0038] Figure 6 Schematic diagram of the phase compensation device structure of the present invention. DETAILED DESCRIPTION

[0039] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0040] In the drawings of the specific embodiments of the present invention, in order to better and more clearly describe the working principles of the various components in the system, the connection relationship of the various parts in the device is shown, which only clearly distinguishes the relative position relationship between the various components, and does not constitute a limitation on the signal transmission direction, connection sequence and size, dimension and shape of the components or structures.

[0041] The TF-QKD device using an avalanche detector of the present application includes a transmitting end and a detecting end.

[0042] The device settings of the transmitter are the same as those of the single wavelength transmitter when SNSPD detection is used. The transmitter includes Alice and Bob. The transmitter structure is as follows: Figure 1 As shown, it includes: a laser, an encoding module, an optical attenuator (ATT), and an optical fiber link. The encoding module includes a modulation channel, which uses time-division multiplexing to modulate quantum signal light pulses and phase reference light pulses, allowing the transmitter to emit periodic quantum signal light and phase reference light.

[0043] The modulation channel includes a phase modulator PM and an intensity modulator IM. In order to modulate the quantum signal light pulse to the single photon level, there can be multiple intensity modulators IM.

[0044] Before TF-QKD begins, Charlie's synchronized clock begins sending clock signals to Alice and Bob. Alice and Bob then send test pulses at fixed times. Charlie calibrates the clock and compensates for delay based on the test pulses, ensuring that the signals arrive at the same time and that the rising edges of the pulses align. The delay from the synchronized clock to the optical switch output is adjusted to align the switching time of the optical switch with the encoding period.

[0045] In TF-QKD, Alice and Bob encode the phase reference light and quantum signal light with the same wavelength through time division multiplexing. The encoding method is as follows: Figure 2 As shown in Figure 1. In the first half of the coding period, the phase reference light is code-modulated. The phase modulator PM uses fixed phase encoding, and the intensity modulator IM is set so that the count rate at the Charlie end is on the order of MHz. In the second half of the coding period, the quantum signal light is code-modulated. The phase modulator PM uses a random one of four phases for phase encoding, and the intensity modulator IM is set so that the average intensity upon entering the optical fiber is less than the single-photon level.

[0046] Optionally, in a preferred embodiment, the encoding period is 1 ms, the first 400 ns encodes the phase reference light so that Alice and Bob have four different phase differences within 400 ns (a phase change period of one 100 ns); the last 600 ns encodes the quantum signal light.

[0047] like Figure 3 As shown, the detection end (Charlie end) includes: a phase feedback / compensation module, a polarization module, multiple single-photon detectors, an optical switch, a synchronous clock, an optical interference module and a synchronous counter.

[0048] The polarization module can modulate the polarization of the optical signal so that the polarization directions of the two optical signals entering the optical interference module are consistent; the optical interference module is used to allow the two optical signals to interfere with each other and output the interference result.

[0049] An optical switch is used for time division, allowing the reference light and signal light to enter different single-photon detectors for detection. The solid line represents light transmission, and the dotted line represents signal transmission.

[0050] The synchronous clock sends a clock signal, which is calibrated and delayed before QKD to ensure that the pulses on Alice and Bob arrive at the same time.

[0051] In this invention, the synchronous clock adds two additional output voltage signals to control the optical switch. These two voltage signals are synchronized with the optical pulse transmission cycle of the transmitter, and the delay is adjusted so that the time when the optical pulse at the beginning of the cycle reaches the optical switch coincides with the time when the voltage signal reaches the optical switch.

[0052] The optical interferometer module has two outputs. Depending on the phase difference, a light pulse has different probabilities of exiting one of the two outputs. Therefore, an optical switch is connected between each output and the corresponding two single-photon detectors. The optical switch, controlled by an external voltage, selects a different single-photon detector to detect the input light. The optical switch has a fast switching function, enabling quick switching.

[0053] Single-photon detectors A1, A2, B1, and B2 are avalanche detectors. Detected data is collected by a synchronous counter. Optionally, A1 and B1 are high-saturation counting detectors, while A2 and B2 are low-noise detectors (e.g., cryogenic avalanche detectors).

[0054] The synchronous counter simultaneously records the detection results of single-photon detectors A1, B1, A2, and B2, adding timestamps based on a clock signal. The detection results of phase reference light from A1 and B1 are used to calculate the phase difference, while the detection results of quantum signal light from A2 and B2 are used to generate the quantum key.

[0055] At the beginning of a new encoding cycle, the voltage signal sent by the synchronous clock reaches the optical switch, and the voltage signal controls the optical switch to switch to Figure 3 At the position shown, the single-photon detector A1 and the single-photon detector B1 perform detection of the phase reference light.

[0056] At the moment when the quantum signal optical encoding begins in the encoding cycle, the voltage signal sent by the synchronous clock reaches the optical switch, and the voltage signal controls the optical switch to switch to Figure 4 At the position shown, the single-photon detector three A2 and the single-photon detector four B2 perform detection of the quantum signal light.

[0057] The phase feedback / compensation module performs phase difference feedback or compensation based on the detection result sequence recorded by the synchronous counter.

[0058] like Figure 5 The figure shows an embodiment of a phase feedback / compensation module. In this embodiment, the phase feedback / compensation module includes a phase modulator and an operation processing module. The operation processing module is connected to the phase modulator PM. The phase modulator PM performs real-time phase difference feedback based on the feedback electrical signal by changing the phase of the quantum signal light. The synchronous counter is connected to the operation processing module.

[0059] The calculation processing module can calculate the phase difference of the optical signals at both ends of Alice and Bob based on the detection results of single-photon detector A1 and single-photon detector B1, generate a feedback electrical signal, and the phase modulator PM performs phase difference feedback in real time based on the feedback electrical signal to modulate the phase of the quantum signal light.

[0060] The detection results of single-photon detectors single-photon detector three A2 and single-photon detector four B2 are directly used to generate quantum keys.

[0061] like Figure 6 The figure shows another embodiment of a phase feedback / compensation module. In this embodiment, the phase feedback / compensation module includes an operation processing module. There is no need to add a phase modulator PM to the detection end optical path to change the phase of the quantum signal light. This embodiment can use phase compensation to compensate for the phase difference of the optical signals at both ends of Alice and Bob. The operation processing module can perform phase compensation of the quantum signal light only through a phase post-processing program.

[0062] The calculation and processing module can calculate the phase difference of the optical signals at both ends of Alice and Bob based on the detection result sequence of single-photon detector 1 A1 and single-photon detector 2 B1, and compensate the phase difference of the detection results of single-photon detection 3 A2 and single-photon detection 4 B2 through phase post-processing to generate quantum keys.

[0063] In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware or any combination thereof. When implemented using software, it can be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the process or function described in the embodiment of the present application is generated in whole or in part. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted via the computer-readable storage medium. The computer-readable storage medium can be any available medium that a computer can access or a data storage device such as a server or a data center that includes one or more available media integrations. The available medium can be a magnetic medium (e.g., a floppy disk, a hard disk, a tape), an optical medium (e.g., a DVD), or a semiconductor medium (e.g., a solid-state drive (SSD)).

[0064] The above description is merely a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present application, and such modifications or substitutions should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. A TF-QKD device using an avalanche detector, characterized in that: include: Alice, Bob, and Charlie: Alice and Bob act as transmitters of the TF-QKD device, while Charlie acts as the detector. Alice and Bob each periodically encode the phase reference light and quantum signal light with the same wavelength by using single-wavelength optical time division multiplexing, and then transmit them to Charlie; The Charlie end includes: single photon detector 1, single photon detector 2, single photon detector 3, single photon detector 4, an optical switch and a synchronous clock; Before TF-QKD starts, the synchronous clock sends a clock signal to Alice and Bob. Alice and Bob send test pulse signals at a preset time. Charlie calibrates and delay-compensates the synchronous clock based on the detection results of the test pulse signals, so that the arrival time of the test signals transmitted by Alice and Bob are consistent and the rising edges are aligned. The output signal delay from the synchronous clock to the optical switch is adjusted so that the switching time of the optical switch is aligned with the coding period of Alice and Bob. The optical switch controls the phase reference light and quantum signal light to enter different single-photon detectors for detection according to the switching time.

2. The TF-QKD device using an avalanche detector according to claim 1, characterized in that: The Charlie end also includes: a phase feedback / compensation module and a synchronous counter; The synchronous counter synchronously records the detection results of the single photon detector 1, the single photon detector 2, the single photon detector 3 and the single photon detector 4; The phase feedback / compensation module includes a phase modulator and an operation processing module. The operation processing module calculates the phase difference between the optical signals at both ends of Alice and Bob based on the detection results of the single-photon detector 1 and the single-photon detector 2, and generates a feedback electrical signal. The phase modulator performs phase modulation in real time based on the feedback electrical signal to achieve phase feedback of the quantum signal light. The detection results of the single-photon detector three and the single-photon detector four are directly used to generate quantum keys.

3. The TF-QKD device using an avalanche detector according to claim 1, characterized in that: The Charlie end further includes: a phase feedback / compensation module and a synchronous counter, wherein the phase feedback / compensation module includes an operation processing module and does not require a phase modulator; The operation processing module calculates the phase difference between the optical signals at both ends of Alice and Bob based on the detection results of single-photon detector 1 and single-photon detector 2, and performs phase post-processing on the detection results of single-photon detector 3 and single-photon detector 4, and uses the phase difference to generate the quantum key after compensating for it.

4. The TF-QKD device using an avalanche detector according to claim 1, wherein: Alice and Bob encode the phase reference light and quantum signal light of the same wavelength through time division multiplexing. In the first half of the encoding period, the phase modulator uses a fixed phase to modulate the phase reference light; in the second half of the encoding period, the phase modulator uses a random one of four phases to modulate the quantum signal light.

5. The TF-QKD device using an avalanche detector according to claim 4, characterized in that: At the moment when the phase reference light starts in the coding period, the voltage signal sent by the synchronous clock controls the optical switch to switch to detect the phase reference light through single-photon detector one and single-photon detector two; at the moment when the quantum signal light coding starts in the coding period, the voltage signal sent by the synchronous clock controls the optical switch to switch to detect the quantum signal light through single-photon detector three and single-photon detector four.

6. The TF-QKD device using an avalanche detector according to claim 1, wherein: The single photon detector 1, the single photon detector 2, the single photon detector 3 and the single photon detector 4 are avalanche detectors.

7. The TF-QKD device using an avalanche detector according to claim 2 or 3, characterized in that: The Charlie end also includes a polarization module and an optical interference module. The polarization module is used to modulate the polarization of the optical signal so that the polarization directions of the two optical signals entering the optical interference module are consistent; the optical interference module is used to interfere the two optical signals and output the interference result.

8. A TF-QKD method using an avalanche detector, implemented using the TF-QKD apparatus according to any one of claims 1 to 7, characterized in that: The synchronized clock sends a clock signal to Alice and Bob before TF-QKD starts. Alice and Bob send a test signal at a preset time. At the preset time, Alice and Bob each periodically encode the phase reference light and quantum signal light of the same wavelength using single-wavelength optical time division multiplexing and then transmit them to Charlie. Charlie calibrates the synchronous clock and compensates for delay based on the detected test signal, ensuring that the test signals transmitted by Alice and Bob arrive at the same time and their rising edges are aligned. Adjust the delay of the output signal from the synchronous clock to the optical switch so that the switching time of the optical switch is aligned with the coding period of Alice and Bob. The optical switch controls the phase reference light and the quantum signal light to enter different single-photon detectors for detection according to the switching time.

9. The TF-QKD method using an avalanche detector according to claim 8, wherein: At the moment when the phase reference light starts in the coding period, the voltage signal sent by the synchronous clock controls the optical switch to switch to detect the phase reference light through single-photon detector one and single-photon detector two; at the moment when the quantum signal light coding starts in the coding period, the voltage signal sent by the synchronous clock controls the optical switch to switch to detect the quantum signal light through single-photon detector three and single-photon detector four.

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