A local oscillator continuous variable quantum key distribution system and method based on frequency locking technology

Abstract

The present invention relates to the field of quantum secure communication, and discloses a local local oscillator continuous variable quantum key distribution system and method based on frequency locking technology, wherein the system includes an Alice end, an optical fiber link, and a Bob end; first, a first continuous laser source at the Alice end and a second continuous laser source at the Bob end complete frequency locking through frequency locking technology, and Alice then sends a modulated signal to the Bob end through time division multiplexing and polarization multiplexing, and the Bob end performs polarization compensation and polarization demultiplexing on the received signal, extracts the synchronization signal and the balance detection result of the modulated signal, and performs post-processing. The present invention also discloses a specific feasibility method of the system, which effectively solves the problem of low efficiency in recovering data phase and inability to process data in real time in traditional local local oscillator systems, and provides a new solution for the practical application of local local oscillator systems.

Description

Technical Field

[0001] The present invention belongs to the field of quantum secure communication, and in particular relates to a local oscillator continuous variable quantum key distribution system and method based on frequency locking technology. Background Art

[0002] With the advent of quantum computers, traditional classical encryption is under threat. This has led to the emergence of quantum key distribution (QKD), a quantum secure communication technology based on the quantum no-cloning theorem. QKD is mainly divided into discrete variable quantum key distribution (DV-QKD) and continuous variable quantum key distribution (CV-QKD). Compared with DV-QKD, CV-QKD modulates information based on the orthogonal components of the electromagnetic field and uses coherent detection techniques (balanced homodyne and heterodyne detection) to demodulate the information at the receiver. This modulation and demodulation method is similar to classical communication, allowing for better integration with existing classical communications. It also eliminates the need for expensive equipment such as single-photon sources and detectors, and offers advantages such as higher security rates over short distances. Consequently, it has garnered extensive research and attention in recent years.

[0003] CV-QKD systems can be divided into local oscillator (LO) systems and local LO systems, depending on whether the LO signal is located at the transmitter or the receiver. In most LO systems, the LO signal and the modulated signal are transmitted to the channel through a co-fiber method using time division multiplexing and polarization multiplexing, and the modulated signal is then obtained by the receiver through balanced homodyne detection (YC Zhang, et al., Continuous-variable QKD over 50km commercial fiber, Quantum Sci. Technol. 4, 035006 (2019).). However, due to the related attacks proposed on the LO in recent years, such as LO jitter attacks and LO calibration attacks, the LO system has security vulnerabilities and reduced system performance. The introduction of a local oscillator system perfectly addresses these vulnerabilities in local oscillator attacks. In a local oscillator system, the local oscillator is placed at the receiving end of the system without passing through the channel. This not only avoids attacks on the local oscillator, but also ensures the strength of the local oscillator. However, this also creates new problems. The frequency drift of the lasers in the two locations will increase additional phase drift. This phase drift changes rapidly and is difficult to compensate for using the modulation signal. Therefore, most current local oscillator systems generate an additional pilot signal at the transmitting end and multiplex it with the modulation signal into the channel. Common multiplexing methods include time division multiplexing + polarization multiplexing or frequency division multiplexing + polarization multiplexing. The phase change of the pilot signal is extracted to compensate for the phase of the modulation signal. However, this will greatly slow the phase compensation rate and is easily limited by the hardware processing rate. It is also impossible to extract the synchronization signal from it. Therefore, there is still no local oscillator CV-QKD system that can process it in real time. Summary of the Invention

[0004] In order to solve the above problems, the present invention proposes a local local oscillator continuous variable quantum key distribution system and method based on frequency locking technology. The frequency locking technology is used to control the beat frequency of the lasers in the two locations within a certain small range, so that no additional pilot signal is required to compensate for the fast phase drift. Instead, phase compensation can be completed by compensating for the slow phase drift in the channel. At the same time, the clock synchronization and data synchronization information can be extracted to realize real-time calculation, providing a feasible solution for promoting the practical application of local local oscillator systems.

[0005] A local local oscillator continuous variable quantum key distribution system based on frequency locking technology of the present invention includes an Alice end, an optical fiber link and a Bob end. The Alice end transmits an optical signal through the optical fiber link and is received by the Bob end.

[0006] The Alice end includes a first continuous laser source, a first polarization-maintaining beam splitter, a pulse modulation module, a first control module, a second polarization-maintaining beam splitter, a quantum modulation module, a delay module, a power attenuation module and a polarization combiner; the output end of the first continuous laser source is connected to the input end of the first polarization-maintaining beam splitter, one port of the output end of the first polarization-maintaining beam splitter is connected to the pulse modulation module, and the other port is input into the optical fiber link, the pulse modulation module is connected to the second polarization-maintaining beam splitter, one port of the output end passes through the quantum modulation module to generate a quantum signal, and the other port passes through the delay module and the power attenuation module to generate a reference signal, and the two signals are coupled together through the polarization combiner and sent to the optical fiber link.

[0007] The optical fiber link includes two parts: a classical channel and a quantum channel.

[0008] The Bob end includes a second continuous laser source, a frequency shifting module, a third polarization-maintaining beam splitter, a first polarization-maintaining beam combiner, a first power amplifier module, a second control module, a first detection module, a power attenuation module, a second polarization-maintaining beam combiner, a polarization compensation module, a polarization beam splitter, a second power amplifier module, a second detection module, a balanced detection module and a third control module; the first power amplifier module is used for optical signals passing through the classical channel and is connected to the first polarization-maintaining beam combiner, one port of the first polarization-maintaining beam combiner is connected to the power attenuation module and the first detection module in sequence, the output end of the second continuous laser source is connected to the frequency shifting module and the third polarization-maintaining beam splitter in sequence, one output port of the third polarization-maintaining beam splitter is connected to the first polarization-maintaining beam combiner, and the other port is connected to the second polarization-maintaining beam combiner, the polarization compensation module is used to compensate for the polarization of the optical signal passing through the quantum channel, and is connected to the polarization beam splitter to be divided into a synchronization signal and a quantum signal, wherein the quantum signal is connected to the second polarization-maintaining beam combiner, the reference signal is connected to the second power amplifier module and the second detection module in sequence, and the output end of the second polarization-maintaining beam combiner is connected to the balanced detection module.

[0009] Furthermore, the pulse modulation module and the quantum modulation module are controlled by the electrical signal generated by the first control module, wherein the modulation mode in the quantum modulation module includes modulation modes of related continuous variables such as Gaussian modulation and discrete modulation.

[0010] Furthermore, the second control module receives the electrical signal obtained from the first detection module and obtains a corresponding feedback electrical signal through a relevant data processing algorithm, and acts on the frequency shift module.

[0011] Furthermore, the second detection module sends an electrical signal to the third control module for extracting a clock synchronization signal and a data synchronization signal, and the balance detection module sends an electrical signal to the third control module for extracting a security code rate.

[0012] A method of a local local oscillator continuous variable quantum key distribution system based on frequency locking technology of the present invention comprises the following steps:

[0013] Step 1: The first continuous laser source sends a continuous light through the first polarization-maintaining beam splitter and the classical channel to the receiving end. The second continuous laser source sends a continuous light through the frequency shifting module and the third polarization-maintaining beam splitter. The light reaches the receiving end and is amplified by the first power amplifier module, and interferes with the other light on the first polarization-maintaining beam splitter to generate a beat signal with a frequency of fp. The beat signal is converted into an electrical signal after attenuation through the power attenuation module and the first detection module and sent to the second control module. The second control module uses a machine learning algorithm to predict the frequency drift based on a portion of training data (fp1, fp2, ..fpn), and feeds the predicted data back to the frequency shifting module, so that the lasers at the two locations are frequency locked, and the locked frequency center is fs.

[0014] Step 2: The first control module generates a pulse modulation signal and a quantum modulation signal respectively, wherein the pulse modulation signal contains clock synchronization information and data synchronization information. After pulse modulation, it is split into a synchronization signal and a quantum signal by a second polarization-maintaining beam splitter. The reference signal is attenuated to a lower power by a delay module and a power attenuation module. The quantum modulation signal contains the modulation data and the reference data, and the two are arranged in a certain ratio. The quantum signal and the synchronization signal are time-division multiplexed and polarization-multiplexed.

[0015] Step 3: Use the polarization compensation module to compensate for the polarization drift of the signal transmitted in the quantum channel, and use the polarization beam splitter to separate the quantum signal from the synchronization signal. The synchronization signal is amplified by the second power amplifier module. The second detection module converts the amplified optical signal into an electrical signal and transmits it to the third control module for extracting clock synchronization information and data synchronization information. The quantum signal interferes with the light split by the third polarization-maintaining beam splitter of the second continuous laser source, and the interference signal is sent to the balanced detection module. The third control module performs post-processing phase compensation and security code rate calculation.

[0016] Furthermore, the frequencies fs, fp, fp1, fp2, ..., fpn of the beat signals should be smaller than the maximum frequency shifting range fmax of the frequency shifting module.

[0017] Furthermore, the phase compensation is to calculate the slow phase drift in the channel using a part of the reference data in the quantum signal, and compensate it to the modulated data arranged in a certain order.

[0018] The beneficial effects of the present invention are:

[0019] A local local oscillator continuous variable quantum key distribution system and method based on frequency locking technology are proposed, which effectively solves the problems of low efficiency in recovering data phase and inability to process data in real time in traditional local local oscillator systems, and provides a new solution for the practical application of local local oscillator systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the system structure of the present invention

[0021] Figure 2 Schematic diagram of modulation signal in a specific embodiment of the present invention DETAILED DESCRIPTION

[0022] In order to facilitate those skilled in the art to understand and implement the present invention, the specific implementation methods of the present invention will be further described in detail below.

[0023] like Figure 1 As shown, this embodiment provides a local local oscillator continuous variable quantum key distribution system based on frequency locking technology, including Alice end, optical fiber link and Bob end. Alice end transmits an optical signal through the optical fiber link and is received by Bob end.

[0024] The Alice end includes a first continuous laser source, a first polarization-maintaining beam splitter, a pulse modulation module, a first control module, a second polarization-maintaining beam splitter, a quantum modulation module, a delay module, a power attenuation module and a polarization combiner; the output end of the first continuous laser source is connected to the input end of the first polarization-maintaining beam splitter, one port of the output end of the first polarization-maintaining beam splitter is connected to the pulse modulation module, and the other port is input into the optical fiber link. The pulse modulation module is connected to the second polarization-maintaining beam splitter, one port of the output end passes through the quantum modulation module to generate a quantum signal, and the other port passes through the delay module and the power attenuation module to generate a reference signal. The two signals are coupled together through the polarization combiner and sent to the optical fiber link.

[0025] The optical fiber link consists of two parts: classical channel and quantum channel.

[0026] The Bob end includes a second continuous laser source, a frequency shifting module, a third polarization-maintaining beam splitter, a first polarization-maintaining beam combiner, a first power amplifier module, a second control module, a first detection module, a power attenuation module, a second polarization-maintaining beam combiner, a polarization compensation module, a polarization beam splitter, a second power amplifier module, a second detection module, a balanced detection module and a third control module; the first power amplifier module is used for the optical signal passing through the classical channel and is connected to the first polarization-maintaining beam combiner, one port of the first polarization-maintaining beam combiner is connected to the power attenuation module and the first detection module in sequence, the output end of the second continuous laser source is connected to the frequency shifting module and the third polarization-maintaining beam splitter in sequence, one of the output ports of the third polarization-maintaining beam splitter is connected to the first polarization-maintaining beam combiner, and the other port is connected to the second polarization-maintaining beam combiner, the polarization compensation module is used to compensate for the polarization of the optical signal passing through the quantum channel, and is connected to the polarization beam splitter to be divided into a synchronization signal and a quantum signal, wherein the quantum signal is connected to the second polarization-maintaining beam combiner, the reference signal is connected to the second power amplifier module and the second detection module in sequence, and the output end of the second polarization-maintaining beam combiner is connected to the balanced detection module.

[0027] In one embodiment of the present invention, the pulse modulation module and the quantum modulation module are controlled by the electrical signal generated by the first control module, wherein the modulation mode in the quantum modulation module includes modulation modes of related continuous variables such as Gaussian modulation and discrete modulation.

[0028] In one embodiment of the present invention, the second control module receives the electrical signal obtained from the first detection module and obtains a corresponding feedback electrical signal through a relevant data processing algorithm, and acts on the frequency shift module.

[0029] In one embodiment of the present invention, the second detection module sends an electrical signal to the third control module for extracting the clock synchronization signal and the data synchronization signal, and the balance detection module sends an electrical signal to the third control module for extracting the security code rate.

[0030] This embodiment also provides a local local oscillator continuous variable quantum key distribution method based on frequency locking technology, the steps of which are as follows:

[0031] Step 1: The first continuous laser source sends a continuous light through the first polarization-maintaining beam splitter and the classical channel to reach the receiving end. The second continuous laser source sends a continuous light through the frequency shifting module and the third polarization-maintaining beam splitter. The light that reaches the receiving end and is amplified by the first power amplifier module interferes with the first polarization-maintaining beam splitter to generate a beat signal with a frequency of fp. The beat signal is converted into an electrical signal after attenuation through the power attenuation module and the first detection module and sent to the second control module. The second control module uses a machine learning algorithm to predict the frequency drift based on a part of the training data (fp1, fp2, ..fpn), and feeds the predicted data back to the frequency shifting module to complete the frequency locking of the lasers at the two locations, and the locked frequency center is fs;

[0032] Step 2: The first control module generates a pulse modulation signal and a quantum modulation signal respectively, wherein the pulse modulation signal contains clock synchronization information and data synchronization information. After pulse modulation, it is split into a synchronization signal and a quantum signal by a second polarization-maintaining beam splitter. The reference signal is attenuated to a lower power by a delay module and a power attenuation module. The quantum modulation signal contains the modulation data and the reference data, and the two are arranged in a certain ratio. The quantum signal and the synchronization signal are time-division multiplexed and polarization-multiplexed.

[0033] Step 3: Use the polarization compensation module to compensate for the polarization drift of the signal transmitted in the quantum channel, and use the polarization beam splitter to separate the quantum signal from the synchronization signal. The synchronization signal is amplified by the second power amplifier module. The second detection module converts the amplified optical signal into an electrical signal and transmits it to the third control module for extracting clock synchronization information and data synchronization information. The quantum signal interferes with the light split by the third polarization-maintaining beam splitter of the second continuous laser source, and the interference signal is sent to the balanced detection module. The third control module performs post-processing phase compensation and security code rate calculation.

[0034] In one embodiment of the present invention, the frequencies fs, fp, fp1, fp2, ..., fpn of the beat signals should be smaller than the maximum frequency shifting range fmax of the frequency shifting module.

[0035] In one embodiment of the present invention, the phase compensation is to calculate the slow phase drift in the channel using a portion of reference data in the quantum signal, and compensate it for the modulated data arranged in a certain order.

[0036] In specific implementation, the central wavelength of the continuous light emitted by the first continuous laser source is 1550.12 + Δλ1(t) nm, and the central wavelength of the continuous light emitted by the second continuous laser source is 1550.12 + Δλ2(t) nm, where Δλ1(t) and Δλ2(t) are functions of the central wavelength of the laser varying with time. First, drive the frequency shift module with a maximum frequency shift of fmax, and set an initial value f0 < fmax. Then, after the first continuous laser source passes through the first polarization-maintaining beam splitter and the classical channel, the optical signal that reaches the receiving end and is amplified by the first power amplification module and the optical signal of the second continuous laser source passing through the frequency shift module and the third polarization-maintaining beam splitter interfere on the first polarization-maintaining beam splitter to generate a beat frequency fp, which will be expressed as

[0037]

[0038] where c is the speed of light. Fine-tune the central wavelength of any one of the lasers to make fp < fmax. The beat frequency signal passes through the power attenuation module and the first detection module to convert the attenuated optical signal into an electrical signal and send it to the second control module. The second control module uses a machine learning algorithm to predict the frequency drift situation based on a part of the training data (fp1, fp2,..fpn), and feedbacks the predicted data to the frequency shift module to complete the frequency locking of the lasers at both locations and control the frequency within f s ±Δf, where Δf satisfies 2πΔfT < 1°, and T is the signal pulse period. It should be ensured that the frequencies f0, fs, fp, fp1, fp2,…fpn that appear above should be less than the maximum frequency shift range fmax that the frequency shift module can perform frequency shift.

[0039] Immediately afterwards, the first control module at the Alice end generates a pulse modulation signal and a quantum modulation signal respectively, as Figure 2 shown. Among them, the pulse modulation signal contains 2 types of pulses. One is a pulse with a 20% duty cycle, and the other is a pulse with a 50% duty cycle. In this embodiment, the pulse generation order is to first generate 23 pulses with a 50% duty cycle, and then all are pulses with a 20% duty cycle. The former contains clock synchronization information and data synchronization information. After the pulse modulation and passing through the second polarization-maintaining beam splitter, it is split into a synchronization signal and a quantum signal. The reference signal passes through the delay module and the power attenuation module to be attenuated to a smaller power. One is to prevent being attacked by eavesdroppers, and the other is to reduce channel crosstalk. The quantum modulation signal contains modulated data and reference data, and the ratio of the two is arranged in a certain order. The specific arrangement method is related to the frequency locking effect. The quantum signal and the synchronization signal enter the quantum channel using time division multiplexing and polarization multiplexing.

[0040] On Bob's end, a polarization compensation module compensates for polarization drift during signal transmission in the quantum channel. A polarization beam splitter separates the quantum signal from the synchronization signal. The synchronization signal is amplified by a second power amplifier module. The second detection module converts the amplified optical signal into an electrical signal and transmits it to the third control module. The third control module extracts clock and data synchronization information by counting 23 pulse signals with a 50% duty cycle. The quantum signal interferes with the continuous light from the second continuous laser source, which is split by the third polarization-maintaining beam splitter. The interference signal is sent to the balanced detection module. The passband range of the balanced detection module should be greater than or equal to the spectrum of the interference signal. Finally, the third control module performs post-processing phase compensation. Since the laser is frequency-locked, fast phase drift can be ignored. The main phase drift of the system comes from the slow phase drift of the channel. Therefore, only a portion of reference data is required to calculate the slow phase drift in the channel and compensate it for the modulated data arranged in a certain order. The security code rate is calculated using common post-processing methods.

[0041] Although the present invention has been described in detail through the above preferred embodiments, it should be understood that the above description is not intended to limit the present invention. After reading the above description, various modifications and substitutions of the present invention will become apparent to those skilled in the art. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A local local oscillator continuous variable quantum key distribution system based on frequency locking technology, comprising an Alice end, an optical fiber link and a Bob end, wherein the Alice end transmits an optical signal through the optical fiber link and is received by the Bob end, characterized in that: The Alice end includes a first continuous laser source, a first polarization-maintaining beam splitter, a pulse modulation module, a first control module, a second polarization-maintaining beam splitter, a quantum modulation module, a delay module, a power attenuation module and a polarization beam combiner; the output end of the first continuous laser source is connected to the input end of the first polarization-maintaining beam splitter, one port of the output end of the first polarization-maintaining beam splitter is connected to the pulse modulation module, and the other port is input to an optical fiber link for transmitting classical signals, the pulse modulation module is connected to the second polarization-maintaining beam splitter, one port of the output end passes through the quantum modulation module to generate a quantum signal, and the other port passes through the delay module and the power attenuation module to generate a reference signal, and the two signals are coupled together through the polarization beam combiner and sent to the optical fiber link for transmitting quantum signals; The optical fiber link includes two parts: a classical channel and a quantum channel; The Bob end includes a second continuous laser source, a frequency shift module, a third polarization-maintaining beam splitter, a first polarization-maintaining beam combiner, a first power amplifier module, a second control module, a first detection module, a power attenuation module, a second polarization-maintaining beam combiner, a polarization compensation module, a polarization beam splitter, a second power amplifier module, a second detection module, a balanced detection module and a third control module; the first power amplifier module is used to amplify the power of the optical signal passing through the classical channel, and is connected to the first polarization-maintaining beam combiner, one port of the first polarization-maintaining beam combiner is connected to the power attenuation module and the first detection module in sequence, the output end of the second continuous laser source is connected to the frequency shift module and the third polarization-maintaining beam splitter in sequence, one output port of the third polarization-maintaining beam splitter is connected to the first polarization-maintaining beam combiner, and the other port is connected to the second polarization-maintaining beam combiner, the polarization compensation module is used to compensate for the polarization of the optical signal passing through the quantum channel, and is connected to the polarization beam splitter to be divided into a synchronization signal and a quantum signal, wherein the quantum signal is connected to the second polarization-maintaining beam combiner, the synchronization signal is connected to the second power amplifier module and the second detection module in sequence, and the output end of the second polarization-maintaining beam combiner is connected to the balanced detection module.

2. According to claim 1, a local oscillator continuous variable quantum key distribution system based on frequency locking technology is characterized in that: The pulse modulation module and the quantum modulation module are controlled by the electrical signal generated by the first control module, wherein the modulation mode in the quantum modulation module includes two modulation modes: Gaussian modulation and discrete modulation.

3. According to claim 1, a local oscillator continuous variable quantum key distribution system based on frequency locking technology is characterized in that: The second control module receives the electrical signal obtained from the first detection module and obtains a corresponding feedback electrical signal through a relevant data processing algorithm, and acts on the frequency shift module.

4. According to claim 1, a local oscillator continuous variable quantum key distribution system based on frequency locking technology is characterized in that: The second detection module sends an electrical signal to the third control module for extracting a clock synchronization signal and a data synchronization signal, and the balance detection module sends an electrical signal to the third control module for extracting a safety code rate.

5. A method for a local local oscillator continuous variable quantum key distribution system based on frequency locking technology according to claim 1, 2, 3 or 4, characterized in that: The following steps are involved: Step 1: The first continuous laser source sends a continuous light which passes through the first polarization-maintaining beam splitter and the classical channel and reaches the receiving end. The second continuous laser source sends a continuous light which passes through the frequency shifting module and the third polarization-maintaining beam splitter and interferes with another light which reaches the receiving end and is amplified by the first power amplifier module on the first polarization-maintaining beam splitter to generate a beat signal with a frequency of fp. The beat signal is converted into an electrical signal after attenuation through the power attenuation module and the first detection module and sent to the second control module. The second control module uses a machine learning algorithm to predict the frequency drift according to a part of the training data (fp1, fp2, .. fpn), and feeds back the predicted data to the frequency shifting module, so that the lasers at the two locations complete frequency locking, and the locked frequency center is fs. Step 2: The first control module generates a pulse modulation signal and a quantum modulation signal respectively, wherein the pulse modulation signal includes clock synchronization information and data synchronization information, and after pulse modulation, the pulse modulation signal is split into a synchronization signal and a quantum signal by a second polarization-maintaining beam splitter, and the synchronization signal is attenuated to a smaller power by a delay module and a power attenuation module, and the quantum modulation signal includes modulation data and reference data, and the two are arranged in a certain proportion in a cross order of modulation data, reference data, modulation data, and reference data, and the quantum signal and the synchronization signal are time-division multiplexed and polarization-multiplexed; Step 3: Use the polarization compensation module to compensate for the polarization drift of the signal transmitted in the quantum channel, and use the polarization beam splitter to separate the quantum signal from the synchronization signal. The synchronization signal is amplified by the second power amplifier module. The second detection module converts the amplified optical signal into an electrical signal and transmits it to the third control module for extracting clock synchronization information and data synchronization information. The quantum signal interferes with the light split by the third polarization-maintaining beam splitter of the second continuous laser source, and the interference signal is sent to the balanced detection module, which performs post-processing phase compensation and security code rate calculation by the third control module.

6. The method of a local oscillator continuous variable quantum key distribution system based on frequency locking technology according to claim 5, characterized in that: The frequencies fs, fp, fp1, fp2, ... fpn of the beat signals should be smaller than the maximum frequency shifting range fmax of the frequency shifting module.

7. The method of a local oscillator continuous variable quantum key distribution system based on frequency locking technology according to claim 5, characterized in that: The phase compensation is to calculate the slow phase drift in the channel by using a part of reference data in the quantum signal, and compensate it to the modulation data arranged in a certain order.

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