Signal processing methods and systems in continuous-variable quantum key distribution

Abstract

This disclosure provides a signal processing method and system for continuous-variable quantum key distribution, relating to the field of communication technology. The method includes: up-converting two double-sideband quadrature phase-shift keying (PSK) signals to a first center frequency and a second center frequency, respectively, to form a first quantum signal in the X-polarization direction and a first quantum signal in the Y-polarization direction; adding a first pilot signal in the X-polarization direction to the first quantum signal in the X-polarization direction to obtain a first optical signal in the X-polarization direction; adding a first pilot signal in the Y-polarization direction to the first quantum signal in the Y-polarization direction to obtain a first optical signal in the Y-polarization direction; using a polarization coupler to combine the first optical signal in the X-polarization direction and the first optical signal in the Y-polarization direction into a polarization multiplexed signal; and transmitting the polarization multiplexed signal to a receiver through a quantum channel. According to embodiments of this disclosure, the security key rate of continuous-variable quantum key distribution can be improved.

Description

Technical Field

[0001] This disclosure relates to the field of communication technology, and in particular to a signal processing method and system for continuous variable quantum key distribution. Background Technology

[0002] Quantum key distribution (QKD) primarily utilizes quantum states for key transmission. Based on the different quantum state carriers used, it can be divided into two technical routes: Discrete-Variable Quantum Key Distribution (DV-QKD) and Continuous-Variable Quantum Key Distribution (CV-QKD). DV-QKD uses discrete variables in a finite Hilbert space, such as the polarization of light, to achieve quantum key distribution, offering advantages in long-distance transmission scenarios and currently being used in backbone networks and metropolitan area networks (MANs). CV-QKD utilizes the continuous canonical components of a coherent optical field to achieve quantum key distribution, offering advantages such as high key generation rates over short distances and high compatibility with traditional coherent optical transmission systems, making it a promising candidate for applications in MANs and access networks. However, CV-QKD has a relatively low secure key rate, requiring further research and improvement.

[0003] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0004] This disclosure provides a signal processing method and system for continuous variable quantum key distribution, which improves the security key rate of continuous variable quantum key distribution to at least a certain extent.

[0005] Other features and advantages of this disclosure will become apparent from the following detailed description, or may be learned in part by practice of this disclosure.

[0006] According to one aspect of this disclosure, a signal processing method for continuous-variable quantum key distribution is provided, comprising:

[0007] Two double-sideband quadrature phase shift keying signals are up-converted to the first center frequency and the second center frequency, respectively, to form a first quantum signal in the X-polarization direction and a first quantum signal in the Y-polarization direction. The first center frequency and the second center frequency are different.

[0008] A first pilot signal in the X-polarization direction is added to the first quantum signal in the X-polarization direction to obtain the first optical signal in the X-polarization direction.

[0009] A first pilot signal in the Y-polarization direction is added to a first quantum signal in the Y-polarization direction to obtain a first optical signal in the Y-polarization direction; wherein, the center frequencies of the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction are the third center frequencies, and the third center frequency is different from both the first center frequency and the second center frequency;

[0010] A polarization coupler is used to combine the first optical signal in the X-polarization direction and the first optical signal in the Y-polarization direction into a polarization multiplexed signal;

[0011] The polarization multiplexed signal is transmitted to the receiving end via a quantum channel.

[0012] In one embodiment of this disclosure, the first quantum signal in the X-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a first center frequency and a bandwidth of a first bandwidth; the first quantum signal in the Y-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a second center frequency and a bandwidth of a first bandwidth; the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction are single-frequency signals with a center frequency of a third center frequency;

[0013] Among them, the difference between the first center frequency and the third center frequency is greater than half of the first bandwidth, and the difference between the second center frequency and the first center frequency is greater than the first bandwidth.

[0014] In one embodiment of this disclosure, before upconverting the two double-sideband quadrature phase shift keying signals, the method further includes:

[0015] A quantum random number generator is used to generate a first bit sequence and a second bit sequence, which are randomly distributed binary bit sequences.

[0016] Based on the preset mapping rules, the first bit sequence and the second bit sequence are mapped to the first orthogonal phase shift keying symbol sequence and the second orthogonal phase shift keying symbol sequence, respectively;

[0017] The first and second quadrature phase shift keying symbol sequences are upsampled and pulse shaping filters are applied to obtain two double-sideband quadrature phase shift keying signals with a bandwidth of the first bandwidth.

[0018] In one embodiment of this disclosure, transmitting a polarization multiplexed signal to a receiver via a quantum channel includes:

[0019] An optical attenuator is used to attenuate the quantum signal portion of a polarization-multiplexed signal to a quantum state.

[0020] The polarization multiplexed signal, decayed to a quantum state, is sent to the receiving end.

[0021] According to another aspect of this disclosure, a continuous-variable quantum key distribution system is provided, comprising:

[0022] At the transmitting end, two double-sideband quadrature phase-shift keying (QPSK) signals are up-converted to a first center frequency and a second center frequency, respectively, to form a first quantum signal in the X-polarization direction and a first quantum signal in the Y-polarization direction, with different center frequencies. A first pilot signal in the X-polarization direction is added to the first quantum signal in the X-polarization direction to obtain a first optical signal in the X-polarization direction. A first pilot signal in the Y-polarization direction is added to the first quantum signal in the Y-polarization direction to obtain a first optical signal in the Y-polarization direction. The center frequencies of the first pilot signals in the X-polarization and Y-polarization directions are a third center frequency, which is different from both the first and second center frequencies. A polarization coupler is used to combine the first optical signals in the X-polarization and Y-polarization directions into a polarization multiplexed signal. The polarization multiplexed signal is then transmitted to the receiving end through a quantum channel.

[0023] The receiving end receives polarization multiplexed signals.

[0024] In one embodiment of this disclosure, the first quantum signal in the X-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a first center frequency and a bandwidth of a first bandwidth; the first quantum signal in the Y-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a second center frequency and a bandwidth of a first bandwidth; the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction are single-frequency signals with a center frequency of a third center frequency;

[0025] Among them, the difference between the first center frequency and the third center frequency is greater than half of the first bandwidth, and the difference between the second center frequency and the first center frequency is greater than the first bandwidth.

[0026] In one embodiment of this disclosure, the transmitting end further uses a quantum random number generator to generate a first bit sequence and a second bit sequence, wherein the first bit sequence and the second bit sequence are randomly distributed binary bit sequences; based on a preset mapping rule, the first bit sequence and the second bit sequence are mapped to a first orthogonal phase shift keying symbol sequence and a second orthogonal phase shift keying symbol sequence, respectively; the first orthogonal phase shift keying symbol sequence and the second orthogonal phase shift keying symbol sequence are upsampled and pulse shaping filtered to obtain two double-sideband orthogonal phase shift keying signals with a bandwidth of a first bandwidth.

[0027] In one embodiment of this disclosure, the receiving end further divides the polarization multiplexed signal into a second optical signal in the X-polarization direction and a second optical signal in the Y-polarization direction using a polarization beam splitter; performs photoelectric conversion based on the second optical signal in the X-polarization direction and the local oscillator signal to obtain a first electrical signal; performs photoelectric conversion based on the second optical signal in the Y-polarization direction and the local oscillator signal to obtain a second electrical signal; and performs analog-to-digital conversion on the first electrical signal and the second electrical signal to obtain a first baseband digital electrical signal and a second baseband digital electrical signal.

[0028] The first baseband digital signal and the second baseband digital signal are used in the key generation process.

[0029] In one embodiment of this disclosure, the receiving end further performs bandpass filtering on the first baseband digital electrical signal and the second baseband digital electrical signal to obtain a second quantum signal in the X-polarization direction and a second quantum signal in the Y-polarization direction; and performs a down-conversion operation on the second quantum signal in the X-polarization direction and the second quantum signal in the Y-polarization direction corresponding to the up-conversion operation at the transmitting end to convert the second quantum signal in the X-polarization direction and the second quantum signal in the Y-polarization direction into a double-sideband quantum signal.

[0030] In one embodiment of this disclosure, the receiving end performs bandpass filtering on the first baseband digital electrical signal and the second baseband digital electrical signal to obtain a second pilot signal in the X-polarization direction and a second pilot signal in the Y-polarization direction; based on the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction, as well as the second pilot signal in the X-polarization direction and the second pilot signal in the Y-polarization direction, an error estimate of the pilot signal is calculated; and the error estimate is used to compensate for the second quantum signal in the X-polarization direction and the second quantum signal in the Y-polarization direction using a constant modulus algorithm.

[0031] In one embodiment of this disclosure, the receiver further utilizes frequency offset estimation and phase recovery algorithms to recover the phases of the second pilot signal in the X-polarization direction, the second pilot signal in the Y-polarization direction, the second quantum signal in the X-polarization direction, and the second quantum signal in the Y-polarization direction.

[0032] In one embodiment of this disclosure, the receiving end further downsamples the double-sideband quantum signal to a single sampling rate to obtain a third orthogonal phase shift keying (QPSK) symbol sequence and a fourth QPSK symbol sequence; the third and fourth QPSK symbol sequences are normalized to shot noise units, and channel parameters are estimated; based on the estimated signal parameters, the secure key rate is calculated using the theoretical formula for secure key rate; if the secure key rate is greater than 0, post-processing operations are performed to obtain a binary quantum key sequence.

[0033] According to another aspect of this disclosure, an electronic device is provided, comprising: a memory for storing instructions; and a processor for calling the instructions stored in the memory to implement the signal processing method in continuous variable quantum key distribution described above.

[0034] According to another aspect of this disclosure, a computer-readable storage medium is provided that stores computer instructions thereon, which, when executed by a processor, implement the signal processing method in continuous-variable quantum key distribution described above.

[0035] According to another aspect of this disclosure, a computer program product is provided, which stores instructions that, when executed by a computer, cause the computer to perform the signal processing method described above in continuous variable quantum key distribution.

[0036] According to yet another aspect of this disclosure, a chip is provided, including at least one processor and an interface;

[0037] An interface is used to provide program instructions or data to at least one processor;

[0038] At least one processor is used to execute program instructions to implement the signal processing method in continuous variable quantum key distribution described above.

[0039] The signal processing method and system for continuous variable quantum key distribution provided in this disclosure employs frequency division multiplexing (FDM) between the quantum signal and pilot signal at the transmitting end, which can reduce interference to the quantum signal; polarization multiplexing is used between the pilot signals, which can accelerate the convergence speed of the digital signal processing algorithm at the receiving end; and frequency division multiplexing and polarization multiplexing are used between the quantum signals, which can better eliminate polarization crosstalk between them.

[0040] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Attached Figure Description

[0041] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure.

[0042] Obviously, the accompanying drawings described below are merely some embodiments of this disclosure. Those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0043] Figure 1 This diagram illustrates a signal processing method in continuous variable quantum key distribution according to an embodiment of the present disclosure.

[0044] Figure 2This diagram illustrates a CV-QKD signal polarization multiplexing technique according to an embodiment of the present disclosure.

[0045] Figure 3 This diagram illustrates another CV-QKD signal polarization multiplexing technique in an embodiment of this disclosure.

[0046] Figure 4 This diagram illustrates yet another CV-QKD signal polarization multiplexing technique in an embodiment of the present disclosure.

[0047] Figure 5 This diagram illustrates a flowchart of the double-sideband quadrature phase shift keying signal generation process in an embodiment of this disclosure.

[0048] Figure 6 This diagram illustrates a continuous-variable quantum key distribution system according to an embodiment of the present disclosure.

[0049] Figure 7 This diagram illustrates a digital signal processing flowchart for a transmitting end in an embodiment of the present disclosure.

[0050] Figure 8 This diagram illustrates a digital signal processing flowchart for a receiver in an embodiment of the present disclosure.

[0051] Figure 9 This diagram illustrates the architecture of a continuous-variable quantum key distribution system according to an embodiment of the present disclosure.

[0052] Figure 10 A structural block diagram of an electronic device according to an embodiment of the present disclosure is shown. Detailed Implementation

[0053] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this disclosure, and not all of them. The components of the embodiments of this disclosure described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of this disclosure provided in the accompanying drawings is not intended to limit the scope of the claimed disclosure, but merely represents selected embodiments of this disclosure. All other embodiments obtained by those skilled in the art based on the embodiments of this disclosure without inventive effort are within the scope of protection of this disclosure.

[0054] Continuous-Variable Quantum Key Distribution (CV-QKD) has great development potential, but its practical application still faces many challenges. On the one hand, the current secret key ratio (SKR) of QKD is typically only on the order of Kbps, a significant gap compared to classical communication. This is mainly because CV-QKD systems usually operate in environments with very low signal-to-noise ratios and are highly sensitive to noise; therefore, improving the secret key ratio is crucial. On the other hand, the compatibility of current CV-QKD systems with traditional coherent optical communication systems has not yet reached the expected level, hindering effective cost reduction. This is primarily because current system designs are difficult to adapt effectively to traditional coherent optical communication devices and traditional digital signal processing algorithms; more comprehensive solutions are needed.

[0055] To improve the secure key rate (key generation rate) of CV-QKD, the current mainstream approach is to send a classical pilot signal at the transmitting end in addition to the quantum signal used to generate the key. The power of the pilot signal is usually much greater than that of the quantum signal, and it is used to assist in the recovery of the quantum signal. It is usually transmitted together with the quantum signal in the form of time division multiplexing, frequency division multiplexing, polarization multiplexing, etc.

[0056] Furthermore, similar to classical coherent optical communication, two quantum signals can be transmitted simultaneously using polarization multiplexing, theoretically achieving a significant increase in the secure key rate. However, these schemes suffer from two problems: first, they cannot effectively resolve crosstalk between the two polarization-multiplexed quantum signals or between the pilot signal and the quantum signal in time-division multiplexing, thus hindering the improvement of the secure key rate; second, these existing schemes typically require the receiver to employ complex digital signal processing algorithms, making them incompatible with classical communication systems.

[0057] The following detailed description of this exemplary implementation method is provided in conjunction with the accompanying drawings and embodiments.

[0058] Figure 1 This diagram illustrates a signal processing method for continuous variable quantum key distribution according to an embodiment of the present disclosure, as shown below. Figure 1 As shown, the signal processing method in continuous variable quantum key distribution provided in this embodiment includes S101-S105.

[0059] In S101, two double-sideband quadrature phase shift keying (QPSK) signals are up-converted to a first center frequency and a second center frequency, respectively, to form a first quantum signal in the X-polarization direction and a first quantum signal in the Y-polarization direction. The first center frequency and the second center frequency are different.

[0060] In some embodiments, a double-sideband QPSK signal can be upconverted to a first center frequency by multiplying it with a single-frequency signal output from a local oscillator using a mixer. Similarly, a double-sideband QPSK signal can be upconverted to a second center frequency.

[0061] The first quantum signal in the X-polarization direction and the first quantum signal in the Y-polarization direction mentioned above are both single-sideband QPSK signals.

[0062] In S102, a first pilot signal in the X-polarization direction is added to the first quantum signal in the X-polarization direction to obtain the first optical signal in the X-polarization direction.

[0063] In S103, a first pilot signal in the Y-polarization direction is added to the first quantum signal in the Y-polarization direction to obtain a first optical signal in the Y-polarization direction; wherein, the center frequency of the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction is a third center frequency, and the third center frequency is different from both the first center frequency and the second center frequency.

[0064] In S104, a polarization coupler is used to combine the first optical signal in the X-polarization direction and the first optical signal in the Y-polarization direction into a polarization multiplexed signal.

[0065] In S105, the polarization multiplexed signal is transmitted to the receiver via a quantum channel.

[0066] Figure 2 This paper presents a CV-QKD signal polarization multiplexing technique. Figure 2 The scheme shown employs polarization multiplexing between quantum signals, resulting in significant polarization crosstalk; time-division multiplexing of quantum signals and pilot signals also leads to large time-domain crosstalk, and the improvement in secure key rate is not significant. Furthermore, Figure 2 The solution requires the introduction of complex digital signal processing algorithms at the receiver to recover the quantum signal, which has low compatibility with traditional coherent optical communication systems and is not conducive to reducing costs.

[0067] Figure 3 The above is shown Figure 1 The CV-QKD signal polarization multiplexing technology solution corresponding to the embodiment is as follows: Figure 3The scheme shown employs frequency division multiplexing between quantum signals and pilot signals, polarization and frequency division multiplexing between quantum signals, and polarization multiplexing between pilot signals. This reduces interference from pilot signals to quantum signals while simultaneously reducing polarization crosstalk between the two quantum signals, thereby improving the system's security key rate. It is understood that the aforementioned quantum signals are... Figure 1 The first quantum signal in the X-polarization direction and the first quantum signal in the Y-polarization direction in the embodiment, the aforementioned pilot signal is Figure 1 The embodiments include a first pilot signal in the X-polarization direction and a first pilot signal in the Y-polarization direction. Furthermore, the embodiments disclosed herein can be adapted to mature algorithms in traditional coherent optical communication systems without introducing additional complex algorithms, thereby helping to reduce costs.

[0068] In some embodiments, the first quantum signal in the X-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a first center frequency and a bandwidth of a first bandwidth; the first quantum signal in the Y-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a second center frequency and a bandwidth of a first bandwidth; and the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction are single-frequency signals with a center frequency of a third center frequency.

[0069] Among them, the difference between the first center frequency and the third center frequency is greater than half of the first bandwidth, and the difference between the second center frequency and the first center frequency is greater than the first bandwidth.

[0070] The following is for reference. Figure 4 The process of multiplexing quantum signals and pilot signals in the above embodiments is described in detail.

[0071] The multiplexed signal is composed of X-polarized optical signal S x and Y-polarized optical signal S y Composition, S x and S y It is composed of quantum signal components and pilot signal components, respectively, and can be expressed as:

[0072] S x =S qx +S px

[0073] S y =S qy +S py

[0074] Among them, S qx The quantum signal representing the X-polarization direction, S qy The quantum signal representing the Y-polarization direction, S px S represents the pilot signal in the X-polarization direction. py This represents the pilot signal in the Y-polarization direction.

[0075] S qx The center frequency is f q1 The bandwidth is B q A single-sideband QPSK signal; S qy The center frequency is f q2 The bandwidth is B q The single-sideband QPSK signal, S px and S py All are center frequencies f p A single-frequency signal.

[0076] In this embodiment, the quantum signal uses a single-sideband signal to avoid interference from low-frequency noise. Furthermore, the relationships between the various frequencies in this disclosure satisfy the following:

[0077] f q1 -f p >B q / 2

[0078] f q2 -f q1 >B q

[0079] The above relationship can avoid crosstalk between the pilot signal and the quantum signal, as well as crosstalk between the two quantum signals.

[0080] In some embodiments, the above Figure 1 In this embodiment, the first quantum signal in the X-polarization direction can be the aforementioned S qx The first quantum signal in the Y-polarization direction can be the aforementioned S qy The first pilot signal in the X-polarization direction can be the aforementioned S px The first pilot signal in the Y-polarization direction can be the aforementioned S py The first center frequency can be the above f q1 The second center frequency can be the aforementioned f q2 The third center frequency can be the aforementioned f p .

[0081] In some embodiments, before performing the up-conversion operation on the two double-sideband quadrature phase-shift keying signals in S101 as described above, the method provided in this disclosure may further include... Figure 5 S501-S503.

[0082] In S501, a quantum random number generator is used to generate a first bit sequence and a second bit sequence, which are randomly distributed binary bit sequences.

[0083] In S502, based on a preset mapping rule, the first bit sequence and the second bit sequence are mapped to the first orthogonal phase shift keying symbol sequence and the second orthogonal phase shift keying symbol sequence, respectively.

[0084] As an example, the mapping rule could be to map every two bits to one QPSK symbol. For the four possible bit combinations (00, 01, 10, 11), each is mapped to one of four different complex values:

[0085] 00→1+j

[0086] 01→-1+j

[0087] 10→-1-j

[0088] 11→1-j

[0089] In S503, the first quadrature phase shift keying symbol sequence and the second quadrature phase shift keying symbol sequence are upsampled and pulse shaping filtering is performed to obtain two double-sideband quadrature phase shift keying signals with a bandwidth of the first bandwidth.

[0090] The above upsampling of QPSK symbols makes the system sampling rate greater than or equal to the Nyquist rate.

[0091] The embodiments disclosed herein not only ensure signal quality and bandwidth limitations, but also provide the necessary foundation for subsequent modulation, transmission and reception.

[0092] In some embodiments, the above-mentioned S105 transmits the polarization multiplexed signal to the receiving end through the quantum channel, which may be achieved by applying an optical attenuator to attenuate the quantum signal portion of the polarization multiplexed signal to a quantum state; and then transmitting the polarization multiplexed signal attenuated to a quantum state to the receiving end.

[0093] By using optical attenuators to attenuate the quantum signal portion of polarization-multiplexed signals to the single-photon level or a weakly coherent state (i.e., a quantum state), information-theoretical security can be provided using the quantum no-cloning theorem. Any eavesdropping introduces detectable interference, which can then be detected by both communicating parties; attenuating to the single-photon level reduces the likelihood of transmitting multiple photons per transmission, thus lowering the potential risk of information leakage.

[0094] In some embodiments, the optical attenuator can be a variable optical attenuator (VOA) or a fixed optical attenuator (FOA) that can precisely control the attenuation amount to ensure that the output signal reaches the required single-photon level or weak coherence state.

[0095] Figure 6This illustration shows a continuous variable quantum key distribution system provided by an embodiment of the present disclosure, including a transmitter 601 and a receiver 602. It is understood that the transmitter 601 and the receiver 602 each have their own separate hardware.

[0096] Transmitter 601 upconverts two double-sideband quadrature phase-shift keying (QPSK) signals to a first center frequency and a second center frequency, respectively, to form a first quantum signal in the X-polarization direction and a first quantum signal in the Y-polarization direction, with different center frequencies. A first pilot signal in the X-polarization direction is added to the first quantum signal in the X-polarization direction to obtain a first optical signal in the X-polarization direction. A first pilot signal in the Y-polarization direction is also added to the first quantum signal in the Y-polarization direction to obtain a first optical signal in the Y-polarization direction. The center frequencies of the first pilot signals in the X-polarization and Y-polarization directions are a third center frequency, which is different from both the first and second center frequencies. A polarization coupler is used to combine the first optical signals in the X-polarization and Y-polarization directions into a polarization multiplexed signal. This polarization multiplexed signal is then transmitted to receiver 602 through a quantum channel.

[0097] Receiver 602 receives polarization multiplexed signals.

[0098] In some embodiments, the first quantum signal in the X-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a first center frequency and a bandwidth of a first bandwidth; the first quantum signal in the Y-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a second center frequency and a bandwidth of a first bandwidth; and the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction are single-frequency signals with a center frequency of a third center frequency.

[0099] Among them, the difference between the first center frequency and the third center frequency is greater than half of the first bandwidth, and the difference between the second center frequency and the first center frequency is greater than the first bandwidth.

[0100] In some embodiments, the transmitter 601 further uses a quantum random number generator to generate a first bit sequence and a second bit sequence, wherein the first bit sequence and the second bit sequence are randomly distributed binary bit sequences; based on a preset mapping rule, the first bit sequence and the second bit sequence are mapped to a first orthogonal phase shift keying symbol sequence and a second orthogonal phase shift keying symbol sequence, respectively; the first orthogonal phase shift keying symbol sequence and the second orthogonal phase shift keying symbol sequence are upsampled and pulse shaping filtered to obtain two double-sideband orthogonal phase shift keying signals with a bandwidth of the first bandwidth.

[0101] In some embodiments, the receiver 602 further divides the polarization multiplexed signal into a second optical signal in the X-polarization direction and a second optical signal in the Y-polarization direction using a polarization beam splitter; performs photoelectric conversion based on the second optical signal in the X-polarization direction and the local oscillator signal to obtain a first electrical signal; performs photoelectric conversion based on the second optical signal in the Y-polarization direction and the local oscillator signal to obtain a second electrical signal; and performs analog-to-digital conversion on the first electrical signal and the second electrical signal to obtain a first baseband digital electrical signal and a second baseband digital electrical signal; wherein the first baseband digital electrical signal and the second baseband digital electrical signal are used in the key generation process.

[0102] In some embodiments, the receiver 602 further performs bandpass filtering on the first baseband digital electrical signal and the second baseband digital electrical signal to obtain a second quantum signal in the X-polarization direction and a second quantum signal in the Y-polarization direction; and performs downconversion operation on the second quantum signal in the X-polarization direction and the second quantum signal in the Y-polarization direction, corresponding to the upconversion operation at the transmitter, to convert the second quantum signal in the X-polarization direction and the second quantum signal in the Y-polarization direction into a double-sideband quantum signal.

[0103] In some embodiments, the receiver 602 performs bandpass filtering on the first baseband digital electrical signal and the second baseband digital electrical signal to obtain a second pilot signal in the X-polarization direction and a second pilot signal in the Y-polarization direction; based on the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction, as well as the second pilot signal in the X-polarization direction and the second pilot signal in the Y-polarization direction, it calculates an error estimate of the pilot signal; and uses the error estimate to compensate for the second quantum signal in the X-polarization direction and the second quantum signal in the Y-polarization direction using a constant modulus algorithm.

[0104] In some embodiments, the receiver 602 further utilizes frequency offset estimation and phase recovery algorithms to recover the phases of the second pilot signal in the X-polarization direction, the second pilot signal in the Y-polarization direction, the second quantum signal in the X-polarization direction, and the second quantum signal in the Y-polarization direction.

[0105] In some embodiments, the receiver further downsamples the double-sideband quantum signal to a single sampling rate to obtain a third orthogonal phase shift keying (QPSK) symbol sequence and a fourth QPSK symbol sequence; normalizes the third and fourth QPSK symbol sequences to shot noise units and estimates the channel parameters; calculates the secure key rate based on the estimated signal parameters using the theoretical formula for secure key rate; and performs post-processing operations when the secure key rate is greater than 0 to obtain a binary quantum key sequence.

[0106] This disclosure, while implementing the proposed signal multiplexing method, adapts to mature algorithms in traditional coherent optical communication systems without introducing additional complex algorithms, thus helping to reduce QKD costs and promote the future development of integrated communication and security technologies. Compared to existing solutions, it can reduce the interference of pilot signals on quantum signals while simultaneously reducing polarization crosstalk between the two quantum signals, thereby improving the system's security key rate. Furthermore, the digital signal processing method for the transceiver end of the CV-QKD system proposed in this invention can adapt to mature algorithms in traditional coherent optical communication systems while implementing the proposed signal multiplexing method, without introducing additional complex algorithms, thus helping to reduce QKD costs and promote the future development of integrated communication and security technologies.

[0107] The methods and systems provided in this disclosure can be applied to QKD devices and networks, which helps to improve the code generation rate of the devices, while also improving the compatibility of existing QKD systems with classic optical communication systems. This will promote the future cost reduction of QKD devices and the development of integrated communication and encryption.

[0108] The following is for reference. Figure 7 The digital signal processing flow at the transmitting end in the embodiments of this specification is described.

[0109] Quantum random number generation: used to generate the original quantum key bit sequence to be sent, which is required to be truly random.

[0110] Bit-symbol mapping: Mapping a random bit sequence to a symbol sequence using a specific encoding method. In this invention, mapping methods such as QPSK and QAM can be used.

[0111] Upsampling & Pulse Shaping Filtering: Upsampling makes the system sampling rate greater than or equal to the Nyquist rate, and pulse shaping filtering is used to convert the above symbol sequence into a double-sideband baseband digital signal.

[0112] Up-conversion: This involves frequency shifting a double-sideband digital signal to a single-sideband signal. This is because noise is usually high near zero frequency, so the signal needs to be shifted to a frequency band with less noise.

[0113] Add pilot signal: Generate a single-frequency signal of a specific frequency as an auxiliary pilot signal.

[0114] The digital signal processing flow at the transmitting end is as follows:

[0115] 1. Use a quantum random number generator to generate two randomly distributed binary bit sequences, and use a bit-symbol mapping module to map the binary bits into two QPSK symbol sequences;

[0116] 2. After upsampling to ensure the system sampling rate is greater than or equal to the Nyquist rate, a pulse shaping filter is applied to convert the two QPSK symbol sequences into two sequences with a bandwidth of B. q Double-side QPSK baseband digital signal;

[0117] 3. Perform upconversion on two double-sideband QPSK signals, converting one of them to the center frequency f. q1 Another frequency converter to the center frequency f q2 This forms two single-sideband QPSK baseband digital signals E. qx and E qy ;

[0118] 4. Add the pilot signal E to each of the two single-sideband QPSK baseband digital signals. px and E py The pilot signal is the center frequency f. p The single-frequency pilot signal, whose power is typically 10 dB greater than that of the quantum signal, is represented as:

[0119]

[0120] Where i is the imaginary unit, f p t is the center frequency of the pilot signal, and t is the time variable. Both pilot signals are single-frequency sine waves of the same frequency, with the same amplitude and phase.

[0121] The two baseband digital electrical signals after adding the pilot signal can be represented as:

[0122] E x =E qx +E px

[0123] E y =E qy +E py

[0124] The following is for reference. Figure 8 The digital signal processing flow at the receiving end in the embodiments of this specification is described.

[0125] Bandpass filtering & downconversion: The received baseband digital electrical signal containing noise is bandpass filtered to obtain the noisy pilot signal and quantum signal respectively, and then a downconversion operation corresponding to the upconversion at the transmitting end is performed to convert the single-sideband quantum signal into a double-sideband quantum signal.

[0126] Equalization & Demultiplexing: Primarily used to separate two quantum signals with polarization aliasing and eliminate inter-symbol interference generated during channel transmission.

[0127] Frequency offset estimation & phase recovery: Estimating and compensating for the center frequency difference between the transmitting and receiving light sources, as well as the phase noise caused by the linewidth of the two light sources.

[0128] Resampling: Resample the signal to a single sampling rate, consistent with the symbol of the transmitting end.

[0129] Normalization & Parameter Estimation: Estimate the system shot noise, normalize the signal to shot noise units, and estimate the channel parameters.

[0130] Security key rate calculation: Based on the estimated signal parameters, the security key rate is calculated using the theoretical formula, and the value of the security key rate is obtained, which serves as the basis for determining the system's security.

[0131] The digital signal processing flow at the receiving end is as follows:

[0132] 1. Perform analog-to-digital conversion on the demodulated electrical signal to obtain a baseband digital electrical signal E′ containing noise. x and E′ y Bandpass filtering is performed to obtain the noisy pilot signals E′. px E′ py and quantum signal E′ qx E′ qy And perform down-conversion operation corresponding to the up-conversion at the transmitting end to convert the single-sideband quantum signal into a double-sideband quantum signal;

[0133] 2. Enter the channel equalization & polarization demultiplexing module. The channel equalization & polarization demultiplexing module algorithm can adopt the constant mode algorithm most commonly used in classical coherent optical communication. In this algorithm, the error estimate of the pilot signal is used to compensate for the quantum signal, thereby eliminating the polarization rotation and other damages generated during the channel transmission process.

[0134] 3. Using frequency offset estimation and phase recovery algorithms from classical coherent optical communication, the phases of pilot signals and quantum signals are recovered;

[0135] 4. Downsample the quantum signal to a single sampling rate to obtain a symbol sequence corresponding to the QPSK symbol sequence transmitted from the transmitter;

[0136] 5. Normalize the downsampled symbol sequence to shot noise units and estimate the channel parameters;

[0137] 6. Based on the estimated signal parameters, the theoretical calculation formula for the security key rate is used to obtain the value of the security key rate, which serves as the basis for determining the system's security.

[0138] 7. If the security key rate is greater than 0, post-processing is performed to obtain the final binary quantum key sequence, thus achieving key distribution.

[0139] Figure 9 The present invention discloses a schematic diagram of the hardware structure of a transmitter and a receiver according to an embodiment of the present invention. The transmitter and receiver are described in detail below.

[0140] Transmitter digital signal processing module: used to generate the original quantum key bits to be transmitted, and to encode and modulate them in a specific way to form a baseband electrical signal.

[0141] IQ modulator: mainly used to modulate the baseband electrical signal onto the optical carrier emitted by the laser to achieve electro-optic conversion.

[0142] Adjustable attenuator: used to attenuate the power of the modulated optical signal to a quantum state, so that the canonical component of the optical field has quantum mechanical properties.

[0143] Polarization coupler: A device used to achieve polarization multiplexing.

[0144] Quantum channels: Generally, fiber optic channels or free-space channels can be selected.

[0145] Polarization controller: Through hardware control, it initially eliminates the polarization state rotation of signals caused during channel transmission;

[0146] Heterodyne detector: used to recover the original baseband electrical signal from the received optical signal;

[0147] The receiver digital signal processing module performs a series of noise compensation processes on the electrical signal output by the detector and recovers the quantum key bit data generated by the transmitter.

[0148] The hardware processing flow of the system transceiver is as follows:

[0149] 1. The digital signal processing module at the transmitting end generates two polarization-state optical signals S. x and S y The corresponding baseband digital complex signal E x and E y , respectively represented as:

[0150] E x =E x-real +iE x-imag

[0151] E y =E y-real +iE y-imag

[0152] 2. The digital-to-analog converter will convert E x-real E x-imag E y-real E y-imag Four digital electrical signals are converted into analog electrical signals, where Ex-real E x-imag E is the electrical drive signal for the first IQ modulator. y-real E y-imag As the electrical drive signal for the second IQ modulator;

[0153] 3. The optical signal emitted by the first continuous laser is split into two paths by the first beam splitter, which are used as the input optical carrier signals of the first IQ modulator and the second IQ modulator, respectively.

[0154] 4. After passing through two IQ modulators, E... x and E y The signals are modulated onto their respective optical carrier signals to form two optical signals. These two signals are then combined into two X and Y vertical polarization states multiplexed signals via a polarization coupler.

[0155] 5. The polarization multiplexed signal described above is attenuated to a quantum state by a tunable optical attenuator and then transmitted through a quantum channel;

[0156] 6. The received signal, after transmission through the quantum channel, is split into two mutually perpendicular optical signals by a polarization beam splitter, denoted as S′. x and S′ y Simultaneously, the second continuous laser is split into two signals by the second beam splitter, which are used as the local oscillator signals of the two heterodyne detectors respectively.

[0157] 7. Place S′ x and S′ y The two local oscillator signals are respectively input to the first and second heterodyne detectors to achieve photoelectric conversion and obtain electrical signals.

[0158] 8. Perform analog-to-digital conversion on the demodulated electrical signal to obtain a baseband digital electrical signal E′ containing noise. x and E′ y , respectively represented as:

[0159] E′ x =E′ x-real +iE′ x-imag

[0160] E′ y =E′ y-real +iE′ y-imag

[0161] 9. E′ x-real E′ x-imag E′ y-real E′ y-imag The signal is input to the receiving end's digital signal processing stage for subsequent noise compensation and key generation.

[0162] In embodiments of this disclosure, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0163] In this disclosure, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0164] Furthermore, although the steps of the method in this disclosure are described in a specific order in the accompanying drawings, this does not require or imply that the steps must be performed in that specific order, or that all the steps shown must be performed to achieve the desired result.

[0165] In some embodiments, certain steps may be omitted, multiple steps may be combined into one step for execution, and / or one step may be broken down into multiple steps for execution.

[0166] The concepts of "first" and "second" mentioned in this disclosure are used only to distinguish different devices, modules or units, and are not used to define the order of functions performed by these devices, modules or units or their interdependencies.

[0167] It should be noted that although several modules or units of the device used for action execution are mentioned in the detailed description above, this division is not mandatory.

[0168] In fact, according to embodiments of this disclosure, the features and functions of two or more modules or units described above can be embodied in one module or unit. Conversely, the features and functions of one module or unit described above can be further divided and embodied by multiple modules or units.

[0169] Some of the block diagrams shown in the accompanying drawings are functional entities and do not necessarily correspond to physically or logically independent entities. These functional entities can be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor devices and / or microcontroller devices.

[0170] The following reference Figure 10 This describes the electronic device provided in the embodiments of this disclosure. Figure 10 The electronic device 1000 shown is merely an example and should not impose any limitation on the functionality and scope of use of the embodiments disclosed herein.

[0171] Figure 10 This diagram illustrates the architecture of an electronic device 1000 provided in an embodiment of the present invention. Figure 10 As shown, the electronic device 1000 includes, but is not limited to, at least one processor 1010 and at least one memory 1020.

[0172] Memory 1020 is used to store instructions.

[0173] In some embodiments, memory 1020 may include a readable medium in the form of volatile storage cells, such as random access memory (RAM) 10201 and / or cache memory 10202, and may further include read-only memory (ROM) 10203.

[0174] In some embodiments, the memory 1020 may also include a program / utility 10204 having a set (at least one) of program modules 10205, such program modules 10205 including but not limited to: an operating system, one or more application programs, other program modules, and program data, each or some combination of these examples may include an implementation of a network environment.

[0175] In some embodiments, the memory 1020 may store an operating system. This operating system may be a real-time operating system (RTX), such as Linux, UNIX, Windows, or OS X.

[0176] In some embodiments, the memory 1020 may also store data.

[0177] As an example, processor 1010 can read data stored in memory 1020, which may be stored at the same memory address as the instruction, or the data may be stored at a different memory address than the instruction.

[0178] Processor 1010 is configured to invoke instructions stored in memory 1020 to implement the steps described in the "Exemplary Methods" section above, according to various exemplary embodiments of this disclosure. For example, processor 1010 can execute the steps of the signal processing method embodiments in continuous variable quantum key distribution described above.

[0179] It should be noted that the processor 1010 described above can be a general-purpose processor or a special-purpose processor. The processor 1010 may include one or more processing cores, and the processor 1010 executes various functional applications and data processing by running instructions.

[0180] In some embodiments, processor 1010 may include a central processing unit (CPU) and / or a baseband processor.

[0181] In some embodiments, the processor 1010 may determine an instruction based on the priority identifier and / or function category information carried in each control instruction.

[0182] In this disclosure, the processor 1010 and the memory 1020 can be configured separately or integrated together.

[0183] As an example, the processor 1010 and memory 1020 can be integrated on a single board or a system on chip (SOC).

[0184] like Figure 10 As shown, the electronic device 1000 is embodied in the form of a general-purpose computing device. The electronic device 1000 may also include a bus 1030.

[0185] Bus 1030 can represent one or more of several types of bus structures, including a memory bus or memory controller, peripheral bus, graphics acceleration port, processor, or a local bus using any of the various bus structures.

[0186] The electronic device 1000 can also communicate with one or more external devices 1040 (e.g., keyboard, pointing device, Bluetooth device, etc.), and with one or more devices that enable a user to interact with the electronic device 1000, and / or with any device that enables the electronic device 1000 to communicate with one or more other computing devices (e.g., router, modem, etc.). Such communication can be performed through the input / output (I / O) interface 1050.

[0187] Furthermore, the electronic device 1000 can also communicate with one or more networks (such as local area networks (LANs), wide area networks (WANs), and / or public networks, such as the Internet) via the network adapter 1060.

[0188] like Figure 10 As shown, network adapter 1060 communicates with other modules of electronic device 1000 via bus 1030.

[0189] It should be understood that, although not shown in the figure, other hardware and / or software modules may be used in conjunction with the electronic device 1000, including but not limited to: microcode, device drivers, redundant processing units, external disk drive arrays, RAID systems, tape drives, and data backup storage systems.

[0190] It is understood that the structures illustrated in the embodiments of this disclosure do not constitute a specific limitation on the electronic device 1000. In other embodiments of this disclosure, the electronic device 1000 may include... Figure 10This may involve more or fewer components, or combining certain components, or splitting certain components, or different component arrangements. Figure 10 The components shown can be implemented in hardware, software, or a combination of both.

[0191] This disclosure also provides a computer-readable storage medium storing computer instructions thereon, which, when executed by a processor, implement the signal processing method in continuous variable quantum key distribution described in the above method embodiments.

[0192] In this embodiment of the disclosure, the computer-readable storage medium is a computer instruction that can be sent, propagated, or transmitted for use by or in conjunction with an instruction execution system, apparatus, or device.

[0193] As an example, a computer-readable storage medium is a non-volatile storage medium.

[0194] In some embodiments, more specific examples of computer-readable storage media in this disclosure may include, but are not limited to: electrical connections having one or more wires, portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), optical fiber, portable compact disk read-only memory (CD-ROM), optical storage devices, magnetic storage devices, USB flash drives, portable hard drives, or any suitable combination of the foregoing.

[0195] In this embodiment of the disclosure, the computer-readable storage medium may include data signals propagated in baseband or as part of a carrier wave, wherein computer instructions (readable program code) are carried.

[0196] The transmitted data signal can take many forms, including but not limited to electromagnetic signals, optical signals, or any suitable combination thereof.

[0197] In some examples, computational instructions contained on a computer-readable storage medium may be transmitted using any suitable medium, including but not limited to wireless, wired, optical fiber, RF, etc., or any suitable combination thereof.

[0198] This disclosure also provides a computer program product that stores instructions that, when executed by a computer, cause the computer to implement the signal processing method in continuous variable quantum key distribution described in the above method embodiments.

[0199] The aforementioned instructions can be program code. In practice, the program code can be written using any combination of one or more programming languages.

[0200] Programming languages ​​include object-oriented programming languages—such as Java and C++—as well as conventional procedural programming languages—such as the "C" language or similar programming languages.

[0201] The program code can be executed entirely on the user's computing device, partially on the user's computing device, as a standalone software package, partially on the user's computing device and partially on a remote computing device, or entirely on a remote computing device or server.

[0202] In cases involving remote computing devices, the remote computing devices can be connected to user computing devices via any type of network, including local area networks (LANs) or wide area networks (WANs), or they can be connected to external computing devices (e.g., via the Internet using an Internet service provider).

[0203] This disclosure also provides a chip, including at least one processor and an interface;

[0204] An interface is used to provide program instructions or data to at least one processor;

[0205] At least one processor is used to execute program instructions to implement the signal processing method in continuous variable quantum key distribution described in the above method embodiments.

[0206] In some embodiments, the chip may further include a memory for storing program instructions and data, the memory being located within or outside the processor.

[0207] Those skilled in the art will understand that all or part of the steps of the above embodiments can be specifically implemented in the following forms: a completely hardware implementation, a completely software implementation (including firmware, microcode, etc.), or a combination of hardware and software implementations, which can be collectively referred to as "circuit", "module" or "system".

[0208] Other embodiments of this disclosure will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein.

[0209] This disclosure is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not disclosed herein. The description and examples are to be considered exemplary only, and the true scope and spirit of this disclosure are indicated by the appended claims.

Claims

1. A signal processing method for continuous-variable quantum key distribution, characterized in that, include: Two double-sideband quadrature phase shift keying signals are up-converted to a first center frequency and a second center frequency, respectively, to form a first quantum signal in the X-polarization direction and a first quantum signal in the Y-polarization direction. The first center frequency and the second center frequency are different. A first pilot signal in the X-polarization direction is added to the first quantum signal in the X-polarization direction to obtain a first optical signal in the X-polarization direction. A first pilot signal in the Y-polarization direction is added to the first quantum signal in the Y-polarization direction to obtain a first optical signal in the Y-polarization direction; wherein, the center frequencies of the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction are a third center frequency, and the third center frequency is different from both the first center frequency and the second center frequency; A polarization coupler is used to combine the first optical signal in the X-polarization direction and the first optical signal in the Y-polarization direction into a polarization multiplexed signal; The polarization multiplexed signal is transmitted to the receiving end via a quantum channel.

2. The method according to claim 1, characterized in that, The first quantum signal in the X-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a first center frequency and a bandwidth of a first bandwidth; the first quantum signal in the Y-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a second center frequency and a bandwidth of a first bandwidth; the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction are single-frequency signals with a center frequency of a third center frequency; Wherein, the difference between the first center frequency and the third center frequency is greater than half of the first bandwidth, and the difference between the second center frequency and the first center frequency is greater than the first bandwidth.

3. The method according to claim 2, characterized in that, Before performing the upconversion operation on the two double-sideband quadrature phase shift keying signals, the method further includes: A quantum random number generator is used to generate a first bit sequence and a second bit sequence, wherein the first bit sequence and the second bit sequence are randomly distributed binary bit sequences; Based on a preset mapping rule, the first bit sequence and the second bit sequence are mapped to a first orthogonal phase shift keying symbol sequence and a second orthogonal phase shift keying symbol sequence, respectively; The first quadrature phase shift keying symbol sequence and the second quadrature phase shift keying symbol sequence are upsampled and pulse shaping filtering is performed to obtain two double-sideband quadrature phase shift keying signals with a bandwidth of the first bandwidth.

4. The method according to claim 1, characterized in that, The step of transmitting the polarization multiplexed signal to the receiving end via a quantum channel includes: An optical attenuator is used to attenuate the quantum signal portion of the polarization multiplexed signal to a quantum state; The polarization multiplexed signal, decayed to a quantum state, is sent to the receiving end.

5. A continuous-variable quantum key distribution system, characterized in that, include: At the transmitting end, two double-sideband quadrature phase-shift keying (QPSK) signals are up-converted to a first center frequency and a second center frequency, respectively, to form a first quantum signal in the X-polarization direction and a first quantum signal in the Y-polarization direction, wherein the first center frequency and the second center frequency are different. A first pilot signal in the X-polarization direction is added to the first quantum signal in the X-polarization direction to obtain a first optical signal in the X-polarization direction. A first pilot signal in the Y-polarization direction is added to the first quantum signal in the Y-polarization direction to obtain a first optical signal in the Y-polarization direction. The center frequencies of the first pilot signals in the X-polarization direction and the first pilot signals in the Y-polarization direction are a third center frequency, which is different from both the first and second center frequencies. A polarization coupler is used to combine the first optical signals in the X-polarization direction and the first optical signals in the Y-polarization direction into a polarization multiplexed signal. The polarization multiplexed signal is then transmitted to the receiving end through a quantum channel. The receiving end receives the polarization multiplexed signal.

6. The system according to claim 5, characterized in that, The first quantum signal in the X-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a first center frequency and a bandwidth of a first bandwidth; the first quantum signal in the Y-polarization direction is a single-sideband quadrature phase shift keying signal with a center frequency of a second center frequency and a bandwidth of a first bandwidth; the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction are single-frequency signals with a center frequency of a third center frequency; Wherein, the difference between the first center frequency and the third center frequency is greater than half of the first bandwidth, and the difference between the second center frequency and the first center frequency is greater than the first bandwidth.

7. The system according to claim 6, characterized in that, The transmitting end also uses a quantum random number generator to generate a first bit sequence and a second bit sequence, which are randomly distributed binary bit sequences. Based on a preset mapping rule, the first bit sequence and the second bit sequence are mapped to a first orthogonal phase shift keying symbol sequence and a second orthogonal phase shift keying symbol sequence, respectively. The first orthogonal phase shift keying symbol sequence and the second orthogonal phase shift keying symbol sequence are upsampled and pulse shaping filtered to obtain two double-sideband orthogonal phase shift keying signals with a bandwidth equal to the first bandwidth.

8. The system according to claim 7, characterized in that, The receiving end further splits the polarization multiplexed signal into a second optical signal in the X-polarization direction and a second optical signal in the Y-polarization direction using a polarization beam splitter; performs photoelectric conversion based on the second optical signal in the X-polarization direction and the local oscillator signal to obtain a first electrical signal; performs photoelectric conversion based on the second optical signal in the Y-polarization direction and the local oscillator signal to obtain a second electrical signal; and performs analog-to-digital conversion on the first electrical signal and the second electrical signal to obtain a first baseband digital electrical signal and a second baseband digital electrical signal. The first baseband digital signal and the second baseband digital signal are used in the key generation process.

9. The system according to claim 8, characterized in that, The receiving end also performs bandpass filtering on the first baseband digital electrical signal and the second baseband digital electrical signal to obtain the second quantum signal in the X polarization direction and the second quantum signal in the Y polarization direction; A down-conversion operation corresponding to the up-conversion at the transmitting end is performed on the second quantum signal in the X-polarization direction and the second quantum signal in the Y-polarization direction to convert the second quantum signal in the X-polarization direction and the second quantum signal in the Y-polarization direction into a double-sideband quantum signal.

10. The system according to claim 9, characterized in that, The receiving end performs bandpass filtering on the first baseband digital electrical signal and the second baseband digital electrical signal to obtain a second pilot signal in the X polarization direction and a second pilot signal in the Y polarization direction; Based on the first pilot signal in the X-polarization direction and the first pilot signal in the Y-polarization direction, as well as the second pilot signal in the X-polarization direction and the second pilot signal in the Y-polarization direction, the error estimate of the pilot signal is calculated; the error estimate is then used to compensate for the second quantum signal in the X-polarization direction and the second quantum signal in the Y-polarization direction using a constant modulus algorithm.

11. The system according to claim 10, characterized in that, The receiving end also uses frequency offset estimation and phase recovery algorithms to recover the phases of the second pilot signal in the X-polarization direction, the second pilot signal in the Y-polarization direction, the second quantum signal in the X-polarization direction, and the second quantum signal in the Y-polarization direction.

12. The system according to claim 11, characterized in that, The receiving end also downsamples the double-sideband quantum signal to a single sampling rate to obtain a third orthogonal phase shift keying (QPSK) symbol sequence and a fourth QPSK symbol sequence; the third and fourth QPSK symbol sequences are normalized to shot noise units, and channel parameters are estimated. Based on the estimated signal parameters, the secure key rate is calculated using the theoretical formula for the secure key rate. If the secure key rate is greater than 0, post-processing is performed to obtain a binary quantum key sequence.

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