Method for preparing tri-state quantum signal, signal encoder and key distribution device

Abstract

The present application belongs to the field of quantum communication, and particularly relates to a preparation method of a three-state quantum signal, a signal encoder and a key distribution device. The method utilizes two beam splitters, two phase modulators, two loopers and a plurality of polarization maintaining optical fibers to form a signal encoder of Sagnac-MZ-Sagnac structure, then sets the beam splitting ratio of the two beam splitters to 1:1, and utilizes two unequal-length optical fibers to connect the Sagnac interference rings on both sides, so as to realize that the signal encoder can modulate the input pulse signal into a quantum signal of any quantum state by adjusting the phase difference state of the two phase modulators. The method adopts a new Sagnac and unequal-arm MZ structure to prepare the required quantum signal, has phase stability and polarization independence, and can be used for a quantum key distribution system against polarization interference; overcomes the problems of existing schemes, such as being easily disturbed by the environment, unstable phase and insufficient reliability, and guarantees the key rate and security of the quantum communication system.

Description

Technical Field

[0001] The present invention belongs to the field of quantum communication, and specifically relates to a method for preparing a three-state quantum signal, and a corresponding signal encoder, key distribution equipment and secure communication system. Background Art

[0002] Quantum key distribution is to encode the key in different dimensions of photons to achieve remote distribution. To ensure security, taking the typical BB84 protocol as an example, it is necessary to use two sets of mutually unbiased basis states Z and X, and encode 0 and 1 bits on the two eigenstates of each set of basis respectively. The actual channel will interfere with the transmission of quantum states. In order to reduce the impact of channel disturbances, the time slice-phase dimension is usually used for encoding. In this dimension, there are three sets of mutually unbiased bases, and the quantum state is specifically manifested as the two pulses L and E before and after and their relative phases. Specifically, the L pulse and the E pulse correspond to the two eigenstates of the Z basis. and , Corresponding to X-base and Y-base, where is the relative phase.

[0003] In practical applications, the Z-basis quantum state is highly stable, while the X- and Y-basis quantum states are prone to phase drift. Furthermore, a larger number of quantum states increases the complexity of the encoding scheme and reduces stability. The three-state protocol utilizes two Z-basis states and one phase state for encoding, which theoretically reduces the complexity of the encoding system and maximizes the use of stable quantum states. The prerequisite for optical communication based on the three-state protocol is to achieve stable preparation of the three states. Taking Chinese invention patent application publication number CN113872701A as an example, existing schemes mainly modulate the phase and time dimensions based on the Mach-Zehnder structure and intensity modulator. However, since the phase modulator needs to switch between four different phase states during the signal modulation stage and uses unstable phase states such as 1 / 2π and 3 / 2π to complete the signal modulation, the final output quantum signal is easily affected by environmental interference, and suffers from phase instability and inconsistent pulse intensities, affecting the system's key rate and security. Summary of the Invention

[0004] In order to solve the problems of phase instability, inconsistent pulse intensity, and significant influence of environmental interference in three-state quantum signals encoded by existing schemes, the present invention provides a method for preparing three-state quantum signals and its corresponding signal encoder, key distribution device and secure communication system.

[0005] The technical solution provided by the present invention is:

[0006] A method for preparing a three-state quantum signal, comprising:

[0007] The two ends of the first phase modulator are connected to the beam splitting ratio or The two ports on one side of the first beam splitter are connected to form a first Sagnac interference ring; the two ports on the other side of the first beam splitter serve as two external ports of the first Sagnac interference ring.

[0008] The two ends of the second phase modulator are connected to the beam splitting ratio or The two ports on one side of the second beam splitter are connected to form a second Sagnac interference ring; the two ports on the other side of the second beam splitter serve as two external ports of the second Sagnac interference ring.

[0009] The original pulse signal is input from any external port of the first Sagnac interferometer ring; and two optical signals generated by phase modulation and interference of the original pulse signal are obtained through the two external ports of the first Sagnac interferometer ring.

[0010] One of the optical signals output from the first Sagnac interferometer loop is selected as the E pulse, and the other is delayed according to a preset time length as the L pulse; the E pulse and the L pulse are input respectively from the two external ports of the second Sagnac interferometer loop.

[0011] The signal output from the input port of the E pulse in the second Sagnac interferometer ring is obtained and used as the required quantum signal; the two consecutive time slices in the quantum signal contain optical signals generated by phase modulation and interference of the E pulse and the L pulse respectively.

[0012] In which, by setting the phase difference of the first Sagnac interferometer ring and the phase difference of the second Sagnac interference ring At 0 and Switching is performed between the two so that the output quantum signal is encoded into the desired quantum state.

[0013] As a further improvement of the present invention, the encoding rule of the quantum signal is:

[0014] when =0, =0, the output quantum signal is in the X ground state;

[0015] when = , = 0, the output quantum signal is Z-based state;

[0016] when =0, = , then the output quantum signal is Z-based state;

[0017] when = , = , then the output quantum signal is in vacuum state .

[0018] As a further improvement of the present invention, the two optical signals output by the first Sagnac interferometer ring are transmitted to the second Sagnac interferometer ring through two polarization-maintaining optical fibers of unequal lengths, so that the two optical signals are input into the second Sagnac interferometer ring successively according to a preset delay.

[0019] Among them, the optical signal transmitted through the short optical fiber is used as the L pulse; the optical signal transmitted through the long optical fiber is used as the L pulse.

[0020] As a further improvement of the present invention, the intensity of the original pulse signal is set to twice the intensity of the quantum signals of the X ground state and the Z ground state to be modulated.

[0021] The present invention also includes a signal encoder, which is used to modulate the input original pulse signal using the above-mentioned preparation method of the three-state quantum signal, and then output an encoded quantum signal.

[0022] The signal encoder includes a first beam splitter, a second beam splitter, a first phase modulator, a second phase modulator, a first circulator, a second circulator and a plurality of polarization-maintaining optical fibers. The beam splitting ratios of the first beam splitter and the second beam splitter are both or .

[0023] The two ends of the first phase modulator are connected to the two ports on the left side of the first beam splitter via polarization-maintaining fiber. One of the ports on the right side of the first beam splitter is connected to the bidirectional port of the first circulator via polarization-maintaining fiber. The other port on the right side of the first beam splitter is connected to the input end of the second circulator via polarization-maintaining fiber. The output end of the first circulator is connected to one of the ports on the left side of the second beam splitter via polarization-maintaining fiber. The bidirectional port of the second circulator is connected to the other port on the left side of the second beam splitter via polarization-maintaining fiber. The two ports on the right side of the second beam splitter are connected to the two ends of the second phase modulator via polarization-maintaining fiber. The length L1 of the polarization-maintaining fiber between the output end of the first circulator and the second beam splitter is greater than the length L2 of the polarization-maintaining fiber between the bidirectional port of the second circulator and the second beam splitter.

[0024] The input end of the first circulator is used to input the original pulse signal; the output end of the second circulator is used to output the modulated quantum signal; the phase difference of the first Sagnac interference ring is converted to and the phase difference of the second Sagnac interference ring At 0 and Switching is performed between the two so that the output quantum signal is encoded into the desired quantum state.

[0025] As a further improvement of the present invention, the encoding rule of the quantum signal is:

[0026] when =0, =0, the output quantum signal is in the X ground state;

[0027] when = , = 0, the output quantum signal is Z-based state;

[0028] when =0, = , then the output quantum signal is Z-based state;

[0029] when = , = , then the output quantum signal is in vacuum state .

[0030] As a further improvement of the present invention, the signal modulation process of the quantum signal is as follows:

[0031] After the original pulse signal enters the signal encoder from the incident end of the first circulator, it first transmits to the left and enters the first Sagnac interferometer ring composed of the first phase modulator and the first beam splitter. After phase modulation and interference processing in the first Sagnac interferometer ring, two optical signals are emitted from the first beam splitter.

[0032] Two optical signals, traveling along two sections of polarization-maintaining fiber of different lengths, enter the second Sagnac interferometer ring on the right, acting as the E pulse and the L pulse, respectively. The E pulse undergoes phase modulation and interference processing in the second Sagnac interferometer ring before exiting the second beam splitter in two paths. The L pulse also undergoes phase modulation and interference processing in the second Sagnac interferometer ring before exiting the second beam splitter in two paths.

[0033] Among them, the optical signals of the E pulse and the L pulse output along the long optical path after being processed by the second Sagnac interferometer ring are intercepted by the first circulator; the optical signals of the E pulse and the L pulse output along the short optical path after being processed by the second Sagnac interferometer ring are output successively along the second circulator and constitute the required quantum signal.

[0034] As a further improvement of the present invention, the delay Δt of the E pulse and the L pulse input into the second Sagnac interferometer is related to the length difference ΔL between the long optical path and the short optical path, and satisfies the following formula:

[0035] Δt=ΔL / C,

[0036] In the above formula, C represents the transmission rate of the optical signal in the polarization-maintaining optical fiber.

[0037] The present invention also includes a key distribution device comprising: a light source, a signal encoder, a signal receiver, and a signal decoder. The light source is used to generate an original pulse signal; the signal encoder is a signal encoder as described above; the signal encoder is used to convert the original pulse signal into a corresponding quantum signal according to a preset encoding rule; the signal receiver is used to receive the quantum signal; and the signal decoder is used to identify the quantum state of the quantum signal received by the signal receiver and decode the corresponding key based on the quantum state of the quantum signal.

[0038] The present invention also includes a secure communication system, which uses the key distribution device as described above to implement key distribution based on quantum signals.

[0039] The technical solution provided by the present invention has the following beneficial effects:

[0040] The present invention uses two beam splitters, two phase modulators, two circulators and multiple polarization-maintaining optical fibers to construct a new signal encoder with a Sagnac-MZ-Sagnac structure (wherein Sagnac refers to a Sagnac interference ring structure and MZ refers to a structure similar to Mach-Zenhder interference composed of unequal length optical fibers). The signal encoder uses the unequal length optical fibers connected between the two Sagnac interference rings to introduce the delay difference required for the L pulse and the E pulse, and the beam splitting ratio of the beam splitters in the two Sagnac interference rings is set to or When the phase modulator is used, the Quantum signals of arbitrary quantum states are modulated between two phase states.

[0041] On the one hand, thanks to the natural phase stability of the Sagnac structure, the signal modulation scheme provided by the present invention can effectively overcome the phase instability problem caused by environmental interference. On the other hand, the scheme modulates the X ground state and the Z ground state. states and Z basis When the two phase modulators are at 0 and There are two phase states. The rate of change of signal intensity in this state is the lowest. Even if the accuracy of the phase modulator is not high, it can still modulate a quantum signal that meets the requirements; this can further improve the output stability of the signal encoder and avoid the problem of inconsistent pulse intensities before and after.

[0042] The present invention only utilizes the two phase states 0 and The preparation of a three-state quantum signal is now complete. In contrast, in the solution of the aforementioned Chinese invention patent application, publication number CN113872701A, preparing the required three-state quantum signal requires at least two phase modulators to be in at least four different phase states: 0, 1 / 2π, π, and 3 / 2π. This is clearly one of the advantages of the technical solution provided by this embodiment. Specifically, the solution of the present invention achieves simpler encoding logic for three-state quantum signal encoding, lowers operational difficulty, and increases efficiency and signal stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a flow chart of the method for preparing a three-state quantum signal provided in Example 1 of the present invention.

[0044] Figure 2 Schematic diagram of the structure of the Sagnac interference ring formed by the beam splitter and the phase modulator in Example 1 of the present invention.

[0045] Figure 3 This is a schematic diagram of the signal encoder provided in Example 2 of the present invention.

[0046] Figure 4 This is a system architecture diagram of the key distribution device provided in Example 3 of the present invention. DETAILED DESCRIPTION

[0047] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0048] Example 1

[0049] Three-state quantum key distribution refers to a quantum key distribution protocol that uses three of the four BB84 states for encoding. The encoding scheme is simple and suitable for high-speed quantum key distribution systems. However, existing three-state encoding schemes are significantly affected by environmental interference and suffer from problems such as phase instability and inconsistent pulse intensities, which affect the system's key rate and security. To address the problem of three-state stable preparation, this embodiment provides a new method for preparing three-state quantum signals. This method can use two Sagnac-structured optical devices to process the input original pulse signal, thereby preparing a quantum signal with phase stability and polarization independence. The quantum signal prepared by this scheme can be applied to various quantum key distribution systems that require resistance to polarization interference.

[0050] like Figure 1 As shown, the process of the method for preparing the three-state quantum signal provided in this embodiment is as follows:

[0051] The two ends of the first phase modulator are connected to the beam splitting ratio or The two ports on one side of the first beam splitter are connected to form the first Sagnac interference ring; the two ports on the other side of the first beam splitter serve as the two external ports of the first Sagnac interference ring. The two ends of the second phase modulator are connected to the beam splitting ratio of or The two ports on one side of the second beam splitter are connected to form a second Sagnac interference ring; the two ports on the other side of the second beam splitter serve as two external ports of the second Sagnac interference ring.

[0052] In this embodiment, the structure of the Sagnac interference ring composed of a beam splitter and a phase modulator is as follows: Figure 2 As shown, the two ports of the phase modulator are connected to the two ports on the left side of the beam splitter via polarization-maintaining optical fibers. When any optical signal enters from any external port on the right side of the beam splitter, it is first split into two optical signals of different intensities in the beam splitter according to the preset splitting ratio. The two optical signals pass through the phase modulator along the clockwise and counterclockwise optical paths for phase modulation, respectively. They then pass through the beam splitter again for beam splitting processing, and are output from the two ends of the right side of the beam splitter. Since the phase-modulated optical signals that arrive at the beam splitter along different optical paths are each split into two signals of different intensities according to the preset splitting ratio, the signals ultimately output from the two ports of the beam splitter actually contain both partial components of the optical signal that enters the beam splitter along the clockwise optical path and components of the optical signal that enters the beam splitter along the counterclockwise optical path.

[0053] In this embodiment, after the original pulse signal is input from any external port of the first Sagnac interferometer ring, two optical signals generated by phase modulation and interference of the original pulse signal can be obtained through the two external ports of the first Sagnac interferometer ring.

[0054] One of the optical signals output from the first Sagnac interferometer loop is selected as the E pulse, and the other is delayed by a preset time length to serve as the L pulse. The E pulse and L pulse are then input into the two external ports of the second Sagnac interferometer loop, respectively. The desired quantum signal is obtained by obtaining the optical signal generated by phase modulation and interference of the E pulse and the L pulse, which is output from the input port of the second Sagnac interferometer loop.

[0055] In this embodiment, after obtaining the two optical signals output by the first Sagnac interferometer loop, the two optical signals are sequentially input into a second Sagnac interferometer loop comprised of a second beam splitter and a second phase modulator, according to the desired preset delays for the E pulse and L pulse. For example, if one of the two optical signals output by the first Sagnac interferometer loop is selected as the L pulse, it should be input into the second Sagnac interferometer loop first; the other should be input into the second Sagnac interferometer loop after the preset delay.

[0056] The second Sagnac interferometer loop can process the input E pulse and L pulse using the same signal processing strategy as the first Sagnac interferometer loop. Then, two signals, one from the E pulse and the other from the L pulse, are output from the two external ports of the second beam splitter.

[0057] In practical applications, this embodiment can transmit the two optical signals output by the first beam splitter to the second beam splitter through polarization-maintaining fibers of unequal lengths, thereby achieving a specified delay time for the optical signal transmitted by the longer polarization-maintaining fiber. For example, one of the optical signals output by the first beam splitter is transmitted to the upper port on the left side of the second beam splitter through a longer polarization-maintaining fiber, and the other optical signal output by the first beam splitter is transmitted to the lower port on the left side of the second beam splitter through a shorter polarization-maintaining fiber. In this case, the optical signal transmitted by the shorter polarization-maintaining fiber has a shorter optical path and a shorter signal transmission time, so it will reach the second beam splitter first and be treated as an E pulse; while the optical signal transmitted by the longer polarization-maintaining fiber has a longer optical path and a longer signal output time, so it will reach the second beam splitter later and be treated as an L pulse.

[0058] The delay length Δt of the L pulse and the E pulse is controlled by the length difference ΔL of the two polarization-maintaining fibers connecting the two Sagnac interferometer rings, and the two satisfy the following numerical relationship:

[0059] Δt=ΔL / C,

[0060] In the above formula, C represents the transmission rate of the optical signal in the polarization-maintaining optical fiber.

[0061] That is, in one solution provided in this embodiment, the first Sagnac interferometer loop outputs two optical signals. The signal transmitted via the shorter optical fiber to the second Sagnac interferometer loop serves as the L pulse, while the signal transmitted via the longer optical fiber to the third Sagnac interferometer loop serves as the E pulse. Of course, in other embodiments, other feasible methods can be considered to delay one of the two optical signals output by the first Sagnac interferometer loop to generate the desired L and E pulses.

[0062] Next, the E and L pulses sequentially enter the second Sagnac interferometer ring from the two external ports of the second beam splitter. They are then split into two paths, each traveling clockwise and counterclockwise through the second beam splitter. They then exit from the two external ports of the second beam splitter. Consequently, the two external ports of the second beam splitter each output a unique signal containing both the E pulse signal component processed by the second Sagnac interferometer ring and the L pulse signal component processed by the second Sagnac interferometer ring.

[0063] Finally, this embodiment obtains the signal output from the input port of the E pulse in the second Sagnac interferometer ring and uses it as the required quantum signal; the signal components in two consecutive time slices contained in the quantum signal are respectively composed of optical signals generated by phase modulation and interference of the E pulse and the L pulse.

[0064] Combined with the above content, it can be seen that when the input original pulse signal is processed through two Sagnac interferometer rings according to the above signal processing flow; assuming that the beam splitting ratios of the beam splitters used in the two Sagnac interferometer rings are both set to or At this time, when the first phase modulator and the second phase modulator are used to introduce 0 or 0 into the optical signals transmitted clockwise and counterclockwise in the Sagnac interference ring respectively, When the phase difference is , the optical signals output by the first phase modulator and the second phase modulator will correspond to two output states: one is that the optical pulse intensity is halved and then emitted from both paths at the same time; the other is that it is emitted only from the other path corresponding to the input port.

[0065] Therefore, from the perspective of quantitative analysis, assuming t is the beam splitter ratio, and the phase difference introduced by the first Sagnac interference ring is , the phase difference introduced by the second Sagnac interference ring is , the intensity of the original input pulse signal is recorded as , then according to the principles of quantum optics, the quantum state of the output quantum signal is:

[0066]

[0067] In the above formula, , , Substituting into the above formula, we can find:

[0068] (1) When =0, =0, then the quantum state of the output quantum signal is , showing the X ground state.

[0069] (2) When =0, = , then the output quantum signal is: , Z-based state.

[0070] (3) When = , =0, then the output quantum signal is: , Z-based state.

[0071] (4) When = , = , then the output quantum signal is: , in a vacuum state.

[0072] In summary, the technical solution provided by this embodiment can be realized by the phase difference between the first Sagnac interference ring and the second Sagnac interference ring. 、 The quantum state of the output quantum signal is modulated. states and Z basis The state can be used as the triplet state required for quantum key distribution, while the vacuum state is the ground state (zero-point energy state) of the quantum system.

[0073] It should be emphasized that: the present embodiment scheme finally modulates the X ground state and the Z ground state. states and Z basis When the two phase modulators are at 0 and There are two phase states. The rate of change of signal intensity in this state is the lowest. Even if the accuracy of the phase modulator is not high, it can still modulate a quantum signal that meets the requirements; this can further improve the output stability of the signal encoder and avoid the problem of inconsistent pulse intensities before and after.

[0074] In addition, considering that the signal processing process of the two Sagnac interference loops used in this embodiment will cause the signal intensity of the output quantum signal to be significantly lower than the signal intensity of the input original pulse signal, in order to ensure that the signal intensity of the output quantum signal in the three states meets the requirements, it is necessary to set the intensity of the input original pulse signal to twice the target signal intensity of the desired modulated quantum signal.

[0075] Example 2

[0076] Based on the solution in Example 1, this embodiment further provides an optical device that can use the preparation method of the three-state quantum signal in Example 1 to directly convert the input original pulse signal into the required quantum signal of different quantum states; that is, a signal encoder.

[0077] like Figure 3 As shown, the signal encoder provided in this embodiment includes a first beam splitter, a second beam splitter, a first phase modulator, a second phase modulator, a first circulator, a second circulator, and a plurality of polarization-maintaining optical fibers. The splitting ratios of the first beam splitter and the second beam splitter are both set to or The optical connection between each device is as follows:

[0078] The two ends of the first phase modulator are connected to the two ports on the left side of the first beam splitter via polarization-maintaining fiber. One of the ports on the right side of the first beam splitter is connected to the bidirectional port of the first circulator via polarization-maintaining fiber. The other port on the right side of the first beam splitter is connected to the input end of the second circulator via polarization-maintaining fiber. The output end of the first circulator is connected to one of the ports on the left side of the second beam splitter via polarization-maintaining fiber. The bidirectional port of the second circulator is connected to the other port on the left side of the second beam splitter via polarization-maintaining fiber. The two ports on the right side of the second beam splitter are connected to the two ends of the second phase modulator via polarization-maintaining fiber. The length L1 of the polarization-maintaining fiber between the output end of the first circulator and the second beam splitter is greater than the length L2 of the polarization-maintaining fiber between the bidirectional port of the second circulator and the second beam splitter.

[0079] In practical applications, the input end of the first circulator is used to input the original pulse signal; the output end of the second circulator is used to output the modulated quantum signal. The quantum state of the output quantum signal is modulated by the phase difference of the first Sagnac interference ring. and the phase difference of the second Sagnac interference ring The phase state of is adjusted. and At 0 and Switching is performed between the two so that the output quantum signal is encoded into the desired quantum state.

[0080] During the actual operation of the signal encoder provided in this embodiment, the two phase modulators only need to operate in two phase states: one phase state is 0, and the other is either π or -π. Because each phase modulator has two phase state options, the signal encoder containing two phase modulators can be freely combined to produce four different operating modes. The quantum signals output in these four operating modes also have four different quantum states, including the three states used for quantum communication encoding and a redundant vacuum state.

[0081] The signal encoder of this embodiment can utilize the two phase states 0 and The preparation of a three-state quantum signal is now complete. In contrast, in the solution of the aforementioned Chinese invention patent application, publication number CN113872701A, preparing the required three-state quantum signal requires at least two phase modulators to be in at least four different phase states: 0, 1 / 2π, π, and 3 / 2π. This is clearly one of the advantages of the technical solution provided by this embodiment. Specifically, the solution of this embodiment achieves simpler encoding logic for three-state quantum signal encoding, lowers operational complexity, and increases efficiency.

[0082] In the aforementioned signal encoder, the first beam splitter and the first phase modulator form the first Sagnac interferometer ring on the left, while the second beam splitter and the second phase modulator form the second Sagnac interferometer ring on the right. The upper and lower external ports of the two Sagnac interferometer rings are connected by optical fibers of unequal lengths. The first circulator is located on the long optical path, providing a unidirectional input port for the original pulse signal while maintaining an unobstructed optical path from the first Sagnac interferometer ring to the second Sagnac interferometer ring. The second circulator is located on the short optical path, providing a unidirectional output port for the quantum signal while maintaining an unobstructed optical path from the first Sagnac interferometer ring to the second Sagnac interferometer ring.

[0083] It can be seen from this that the signal encoder provided in this embodiment actually forms a Sagnac-MZ-Sagnac structure, in which the delay introduced by the unequal length optical fibers connecting the two Sagnac interference rings is the delay of the L pulse and the E pulse in the quantum signal.

[0084] On this basis, combined with Figure 3 It can also be found that the process of modulating the original pulse signal F1 into the desired quantum signal F4 in this embodiment is:

[0085] After the original pulse signal enters the signal encoder from the incident end of the first circulator, it first transmits to the left and enters the first Sagnac interferometer ring composed of the first phase modulator and the first beam splitter. After phase modulation and interference processing in the first Sagnac interferometer ring, it is emitted from the first beam splitter as two optical signals.

[0086] Two optical signals, traveling along two sections of polarization-maintaining fiber of different lengths, enter the second Sagnac interferometer ring on the right, acting as the E pulse and the L pulse, respectively. The E pulse undergoes phase modulation and interference processing in the second Sagnac interferometer ring before exiting the second beam splitter in two paths. The L pulse also undergoes phase modulation and interference processing in the second Sagnac interferometer ring before exiting the second beam splitter in two paths.

[0087] Among them, the optical signals of the E pulse and the L pulse output along the long optical path after being processed by the second Sagnac interferometer ring are intercepted by the first circulator; the optical signals of the E pulse and the L pulse output along the short optical path after being processed by the second Sagnac interferometer ring are output successively along the second circulator and constitute the required quantum signal.

[0088] Specifically, when the phase difference of the first Sagnac interferometer ring is When the phase difference of the second Sagnac interferometer ring is 0, the input pulse signal F1 intensity is halved and then emitted from two paths respectively, forming L pulse and E pulse after passing through unequal length optical fibers. If it is also 0, the L pulse and E pulse are halved and emitted from two paths respectively. The final output signal contains both L pulse and E pulse components, forming state, corresponding to the X ground state. If the phase difference of the second Sagnac ring is If π is π, then the L pulse and E pulse are emitted from the opposite exits respectively, where the E pulse is transmitted along the uplink and eventually lost and discarded, and the L pulse is transmitted along the downlink and output, thus forming the Z-based state.

[0089] On the contrary, when the phase difference of the first Sagnac interferometer ring is When the phase difference of the second Sagnac interference ring is π, the light pulse enters the second Sagnac interference ring along the lower path and forms an E pulse. If the E pulse is 0, the E pulse is halved and emitted from two paths. The part transmitted along the upper path is eventually lost and discarded, and the part transmitted along the lower path is used as the final output, forming the Z-based If the phase difference of the second Sagnac interference ring is If it is also π, then the E pulse is emitted from the opposite exit, transmitted along the upper path and eventually lost, forming a vacuum state. .

[0090] In summary, in the signal encoder provided in this embodiment, the strategy for implementing quantum state modulation of quantum signals is shown in the following table:

[0091] Table 1: Encoding rules of quantum signals output by signal encoder

[0092]

[0093] Therefore, in the signal encoder provided in this embodiment, if it is necessary to modulate the quantum signal of the X ground state, it is only necessary to control the two-phase modulator to =0, =0; if you need to modulate the Z basis To obtain the quantum signal of the state, we only need to control the two phase modulators to = , =0; if you need to modulate the Z basis To obtain the quantum signal of the state, we only need to control the two phase modulators to =0, = .

[0094] In particular, the working principle of the signal encoder provided in this embodiment is analyzed to show that the device, based on the Sagnac-MZ-Sagnac structure, adopts the signal modulation strategy of "time slice + phase dimension" to prepare the required three-state signal; rather than the traditional modulation strategy of "phase + time dimension", it can effectively overcome the problems of environmental interference and phase instability. In addition, in the solution provided in this embodiment, the X ground state and Z ground state are modulated. states and Z basis PM1 and PM2 are just at 0 and There are two phase states. The rate of change of signal intensity in this state is the lowest. Therefore, even if the accuracy of the phase modulator is not high, it can still modulate a quantum signal that meets the requirements; this can further improve the reliability of the device and avoid the problem of inconsistent pulse intensities before and after.

[0095] Example 3

[0096] Based on the solution of embodiment 1, this embodiment further provides a key distribution device, such as Figure 4 As shown, it includes: a light source, a signal encoder, a signal receiver, and a signal decoder. The light source and the signal encoder are located at the transmitting end. The light source is used to generate the original pulse signal. The signal encoder adopts the signal encoder of Example 2; the signal encoder is used to convert the control phase of the key code to be sent into the signal code. and , and then in the corresponding state, the original pulse signal is converted into the corresponding quantum signal according to the preset coding rules.

[0097] The signal receiver and signal decoder are located at the receiving end. The transmitting and receiving ends are connected via a channel. The signal receiver is used to receive quantum signals. The signal decoder is used to identify the quantum state of the quantum signal received by the signal receiver and decode the corresponding key based on the quantum state of the quantum signal.

[0098] Furthermore, this embodiment also provides a secure communication system, which uses the aforementioned key distribution device to implement key distribution based on quantum signals.

[0099] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a three-state quantum signal, characterized in that: It includes: The two ends of the first phase modulator are connected to the beam splitting ratio or The two ports on one side of the first beam splitter are connected to form a first Sagnac interference ring; the two ports on the other side of the first beam splitter serve as two external ports of the first Sagnac interference ring; The two ends of the second phase modulator are connected to the beam splitting ratio or The two ports on one side of the second beam splitter are connected to form a second Sagnac interference ring; the two ports on the other side of the second beam splitter serve as two external ports of the second Sagnac interference ring; Inputting the original pulse signal from any external port of the first Sagnac interferometer ring; and obtaining two optical signals generated by phase modulation and interference of the original pulse signal through the two external ports of the first Sagnac interferometer ring; One of the optical signals output from the first Sagnac interferometer loop is used as an E pulse, and the other is delayed by a preset time length and used as an L pulse; the E pulse and the L pulse are input into the two external ports of the second Sagnac interferometer loop respectively; Obtaining a signal output from an input port of the E pulse in the second Sagnac interferometer ring and using it as the desired quantum signal; two consecutive time slices of the quantum signal respectively contain optical signals generated by phase modulation and interference of the E pulse and the L pulse; In which, by setting the phase difference of the first Sagnac interferometer ring and the phase difference of the second Sagnac interference ring At 0 and Switching is performed between the two so that the output quantum signal is encoded into the desired quantum state.

2. The method for preparing a three-state quantum signal according to claim 1, wherein: The encoding rule of the quantum signal is: when =0, =0, the output quantum signal is in the X ground state; when = , = 0, the output quantum signal is Z-based state; when =0, = , then the output quantum signal is Z-based state; when = , = , then the output quantum signal is in vacuum state .

3. The method for preparing a three-state quantum signal according to claim 1, wherein: The two optical signals outputted from the first Sagnac interferometer loop are transmitted to the second Sagnac interferometer loop through two polarization-maintaining optical fibers of unequal lengths, so that the two optical signals are inputted into the second Sagnac interferometer loop in sequence according to a preset time delay; Among them, the optical signal transmitted through the short optical fiber is used as the L pulse; the optical signal transmitted through the long optical fiber is used as the L pulse.

4. The method for preparing a three-state quantum signal according to claim 1, wherein: The intensity of the original pulse signal is set to twice the intensity of the quantum signals of the desired modulated X ground state and Z ground state.

5. A signal encoder, characterized in that: It is used to modulate the input original pulse signal using the preparation method of the three-state quantum signal according to any one of claims 1 to 4, and then output an encoded quantum signal; The signal encoder includes a first beam splitter, a second beam splitter, a first phase modulator, a second phase modulator, a first circulator, a second circulator and a plurality of polarization-maintaining optical fibers; the beam splitting ratios of the first beam splitter and the second beam splitter are both set to or ; The two ends of the first phase modulator are respectively connected to the two ports on the left side of the first beam splitter through polarization-maintaining optical fibers; one of the ports on the right side of the first beam splitter is connected to the bidirectional port of the first circulator through polarization-maintaining optical fibers; the other port on the right side of the first beam splitter is connected to the input end of the second circulator through polarization-maintaining optical fibers; the output end of the first circulator is connected to one of the ports on the left side of the second beam splitter through polarization-maintaining optical fibers; the bidirectional port of the second circulator is connected to the other port on the left side of the second beam splitter through polarization-maintaining optical fibers; the two ports on the right side of the second beam splitter are respectively connected to the two ends of the second phase modulator through polarization-maintaining optical fibers; wherein, the length of the polarization-maintaining optical fiber between the output end of the first circulator and the second beam splitter is greater than the length of the polarization-maintaining optical fiber between the bidirectional port of the second circulator and the second beam splitter; The input end of the first circulator is used to input the original pulse signal; the output end of the second circulator is used to output the modulated quantum signal; the phase difference of the first Sagnac interference ring is converted to and the phase difference of the second Sagnac interference ring At 0 and Switching is performed between the two so that the output quantum signal is encoded into the desired quantum state.

6. The signal encoder according to claim 5, wherein: The encoding rule of the quantum signal is: when =0, =0, the output quantum signal is in the X ground state; when = , = 0, the output quantum signal is Z-based state; when =0, = , then the output quantum signal is Z-based state; when = , = , then the output quantum signal is in vacuum state .

7. The signal encoder according to claim 6, wherein The signal modulation process of quantum signal is: After the original pulse signal enters the signal encoder from the incident end of the first circulator, it first transmits to the left and enters the first Sagnac interferometer ring composed of the first phase modulator and the first beam splitter. After phase modulation and interference processing in the first Sagnac interferometer ring, two optical signals are emitted from the first beam splitter. Two optical signals enter the second Sagnac interferometer ring on the right along two sections of polarization-maintaining optical fiber of different lengths, and are respectively emitted as E pulse and L pulse. After phase modulation and interference processing in the second Sagnac interferometer ring, the E pulse is emitted from the second beam splitter in two paths. The L pulse is also emitted from the second beam splitter in two paths after phase modulation and interference processing in the second Sagnac interferometer ring. The optical signals of the E pulse and the L pulse output along the long optical path after being processed by the second Sagnac interferometer ring are intercepted by the first circulator; the optical signals of the E pulse and the L pulse output along the short optical path after being processed by the second Sagnac interferometer ring are output successively along the second circulator and constitute the required quantum signal.

8. The signal encoder according to claim 5, wherein: The delay Δt of the E pulse and L pulse input to the second Sagnac interferometer is related to the length difference ΔL between the long optical path and the short optical path, and satisfies the following formula: Δt=ΔL / C, In the above formula, C represents the transmission rate of the optical signal in the polarization-maintaining optical fiber.

9. A key distribution device, characterized in that: It includes: a light source for generating an original pulse signal; A signal encoder, comprising the signal encoder according to any one of claims 5 to 8; The signal encoder is used to convert the original pulse signal into a corresponding quantum signal according to a preset encoding rule; a signal receiver, configured to receive the quantum signal; A signal decoder is used to identify the quantum state of the quantum signal received by the signal receiver and decode the corresponding key according to the quantum state of the quantum signal.

10. A secure communication system, characterized in that: It uses the key distribution device as described in claim 9 to realize key distribution based on quantum signals.

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