Polarization encoding device, method and quantum communication system

Abstract

The present application provides a polarization encoding device, method and quantum communication system, which includes: a polarization beam splitter, a first polarizer, a second polarizer, a transmission optical path, a first phase modulator and a first driver. The polarization beam splitter is used to receive an input optical pulse and polarization-split it into two sub-optical pulses; the first polarizer is used to polarize the first optical pulse and output the first polarized sub-optical pulse; the second polarizer is used to polarize the second optical pulse and output the second polarized sub-optical pulse. The first phase modulator is arranged in the transmission optical path and is used to phase-modulate at least one of the two polarized sub-optical pulses based on the modulation signal applied by the first driver, and then combine and output them through the polarization beam splitter. The present application realizes highly stable polarization quantum state encoding at different rates.

Description

Technical Field

[0001] The present application relates to the field of optical quantum coding technology, and in particular to a polarization coding device, method and quantum communication system. Background Art

[0002] Quantum communication technology is a cutting-edge field at the intersection of quantum physics and information science. Current applications primarily include quantum key distribution (QKD) and quantum direct communication (QDC). Based on physical principles such as the Heisenberg uncertainty relation in quantum mechanics and the quantum no-cloning theorem, QKD enables real-time and secure key sharing between communicating parties. Quantum direct communication ensures secure information transmission. Quantum communication can detect potential eavesdropping on communication channels and has applications in defense, government, finance, electricity, and other sectors requiring high-security information transmission.

[0003] The encoding of quantum states is a crucial step in the physical implementation of quantum communication systems. In particular, precise manipulation of quantum states is the cornerstone for ensuring secure and efficient communication in quantum key distribution and direct quantum communication.

[0004] Currently, the encoding and decoding rates of quantum communication systems in practical applications are mainly concentrated in the range of 100 megahertz (Mhz) to 1 gigahertz (GHz). However, with the continuous development of quantum communication technology and the diversification of user needs, the requirements for encoding rates are also constantly increasing, such as 10GHz or even higher.

[0005] Polarization quantum state encoding is a common method in quantum communication. As a fundamental property of photons, polarization can be precisely manipulated and measured, making it well-suited for encoding quantum information.

[0006] However, one common method for encoding polarization quantum states is through an optical path containing a phase modulator, but the phase drift caused by its sensitivity to temperature is a challenge. Therefore, how to achieve highly stable polarization quantum state encoding at different rates has become a key issue that needs to be addressed in quantum communication applications. Summary of the Invention

[0007] In view of this, the present application provides a polarization encoding device, method and quantum communication system to solve the technical problems raised in the background technology.

[0008] In a first aspect, an embodiment of the present application provides a polarization encoding device, which includes: a polarization beam splitter, a first polarizer, a second polarizer, a transmission optical path, a first phase modulator and a first driver; the first driver is connected to the first phase modulator for applying a modulation signal to the first phase modulator; the polarization beam splitter includes a first port, a second port, a third port and a fourth port; the first port is an input port for receiving an input optical pulse, polarizing it into two sub-optical pulses, and outputting them through the second port and the third port respectively, and the polarization states of the two sub-optical pulses are respectively related to the polarization state of the first and second sub-optical pulses. The two intrinsic polarization states of the beam splitter are the same; the second port and the third port are connected to the port on the first side of the first polarizer and the port on the first side of the second polarizer respectively; the port on the second side of the first polarizer and the port on the second side of the second polarizer are connected through a transmission optical path; the first polarizer is used to polarize the first path of light pulses output from the second port, and output the first path of polarized light pulses to the transmission optical path; the second polarizer is used to polarize the second path of light pulses output from the third port, and output the second path of polarized light pulses to the transmission optical path; the polarization direction of the first polarizer is the same as that of the second end of the polarization beam splitter The angle between the polarization state of the first path light pulse output from the first port is θ, the angle between the polarization direction of the second polarizer and the polarization state of the second path light pulse output from the third port of the polarization beam splitter is δ, the angle between the polarization direction of the first polarizer and the polarization direction of the second polarizer is ω, wherein θ, δ≠n·90°, n is an integer, 0≤ω≤2π, a first phase modulator is provided in the transmission optical path, and is used to perform phase modulation on the first path polarized light pulse and / or the second path polarized light pulse based on the modulation signal, so as to obtain the phase of the first path modulated light pulse and the second path modulated light pulse. optical pulses; a specific phase difference is caused between a first modulated sub-optical pulse and a second modulated sub-optical pulse; the first modulated sub-optical pulse and the second modulated sub-optical pulse are output from different ports of a first phase modulator, transmitted along different directions of a transmission optical path, respectively passed through a second polarizer and a first polarizer, and input to a third port and a second port of a polarization beam splitter; the polarization beam splitter is further configured to combine a component of the first modulated sub-optical pulse input from the third port with a component of the second modulated sub-optical pulse input from the second port, and the obtained combined optical pulse is output through the first port or the fourth port.

[0009] In a second aspect, an embodiment of the present application provides a polarization encoding method, which uses the above-mentioned polarization encoding device to implement quantum state polarization encoding.

[0010] In a third aspect, an embodiment of the present application provides a quantum communication system, which may include the polarization encoding device described in any embodiment of the present application.

[0011] In summary, the polarization encoding device, method and quantum communication system provided by the embodiments of the present application have at least the following beneficial effects: the present application uses a polarization beam splitter to polarize a received input optical pulse into two sub-optical pulses, which are input into corresponding polarizers through corresponding ports, and the two sub-optical pulses are polarized respectively by two polarizers. In this way, the polarization states of the two sub-optical pulses can be accurately controlled by the polarizer, thereby obtaining two polarized sub-optical pulses with stable relative phases and different polarization states. The two polarized sub-optical pulses are transmitted to the first phase modulator along different directions of the transmission optical path. The first phase modulator phase-modulates the first polarized sub-optical pulse and / or the second polarized sub-optical pulse based on the modulation signal, so that a first modulated sub-optical pulse and a second modulated sub-optical pulse with a specific phase difference can be obtained. The first modulated sub-optical pulse and the second modulated sub-optical pulse are transmitted in different directions in the transmission optical path through the corresponding polarizers and enter different ports of the polarization beam splitter. The polarization beam splitter combines the components of the two modulated sub-optical pulses and outputs the combined optical pulse through the first port or the fourth port. In this way, the two sub-light pulses of the polarization beam splitter take exactly the same optical path when they are combined, thereby having a self-compensation function for environmental interference, the advantages of high anti-interference stability, and realizing highly stable polarization quantum state encoding at different rates. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific implementation methods or the description of the prior art. Obviously, the drawings described below are some implementation methods of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without any creative work.

[0013] Figure 1 A schematic structural diagram of a polarization encoding device provided in an embodiment of the present application is shown;

[0014] Figure 2 A schematic structural diagram of another polarization encoding device provided in an embodiment of the present application is shown;

[0015] Figure 3 A schematic structural diagram of another polarization encoding device provided in an embodiment of the present application is shown;

[0016] Figure 4 A schematic diagram showing a process flow of a polarization encoding method provided in an embodiment of the present application is shown;

[0017] Figure 5 A schematic structural diagram of a quantum communication system provided in an embodiment of the present application is shown.

[0018] The reference numerals are as follows:

[0019] 101, polarization beam splitter; 102, first polarizer; 103, second polarizer; 104, transmission optical path; 105, first phase modulator; 106, first driver; 107, polarization state rotator; A, first port; B, second port; C, third port; D, fourth port; E, fifth port; F, sixth port; G, seventh port; 301, polarization beam splitter; 302, first polarizer; 303, second polarizer; 304, transmission optical path; 305, first phase modulator; 306, first driver; 307, second phase modulator; 308, second driver 308; H, eighth port; I, ninth port; J, tenth port. DETAILED DESCRIPTION

[0020] In order to make the above and other features and advantages of the present application more clear, the present application is further described below with reference to the accompanying drawings. It should be understood that the specific embodiments given herein are for the purpose of explaining to those skilled in the art and are only exemplary and not restrictive.

[0021] In the following description, many specific details are set forth to provide a thorough understanding of the present application. However, it will be apparent to those skilled in the art that it is not necessary to adopt the specific details to practice the present application. In other cases, well-known steps or operations are not described in detail to avoid obscuring the present application.

[0022] On the one hand, an embodiment of the present application provides a polarization encoding device. Figure 1 A schematic diagram of the structure of a polarization encoding device provided in an embodiment of the present application is shown in FIG. Figure 1 As shown, the polarization encoding device may include a polarization beam splitter 101, a first polarizer 102, a second polarizer 103, a transmission optical path 104, a first phase modulator 105, and a first driver 106. The polarization beam splitter 101 and the transmission optical path 104 may form a Sagnac interferometer.

[0023] like Figure 1 As shown, the first driver 106 is connected to the first phase modulator 105 . The first driver 106 is used to apply a modulation signal to the first phase modulator 105 .

[0024] The modulation signal may be a modulated voltage signal or a modulated current signal. In embodiments of the present application, the modulation signal may be set based on the user's requirements for the quantum communication system. Optionally, the modulation signal may be a low-frequency modulation signal with a frequency in the range of 100 MHz to 1 gigahertz (GHz), or a high-frequency modulation signal not less than a specified frequency. Optionally, the specified frequency may be 10 GHz.

[0025] The polarization beam splitter 101 includes a first port, a second port, a third port, and a fourth port. The first port is an input port for receiving an input optical pulse, polarization-splitting it into two sub-optical pulses, which are output through the second port and the third port, respectively. The polarization states of the two sub-optical pulses are the same as the two intrinsic polarization states of the polarization beam splitter.

[0026] In one embodiment, the input optical pulse may be a high repetition rate pulse light with a pulse repetition rate higher than 10 GHz.

[0027] Each output port of the polarization beam splitter 101 corresponds to a specific polarization state. The polarization beam splitter 101 has two intrinsic polarization states, and the two intrinsic polarization states are represented as and and The intrinsic polarization state can be linear polarization. The intrinsic polarization state can be understood as the polarization state that remains unchanged after transmission through the polarization beam splitter.

[0028] The polarization states of the two optical pulses are respectively identical to the two intrinsic polarization states of the polarization beam splitter 101. This can be understood as follows: the second port of the polarization beam splitter 101 outputs a first optical pulse having a polarization state identical to one intrinsic polarization state of the polarization beam splitter 101, and the third port of the polarization beam splitter 101 outputs a second optical pulse having a polarization state identical to the other intrinsic polarization state of the polarization beam splitter 101. In other words, the polarization beam splitter 101 splits the input optical pulse into two optical pulses having polarization states identical to the two intrinsic polarization states.

[0029] The second and third ports of the polarization beam splitter 101 are connected to the first port of the first polarizer 102 and the first port of the second polarizer 103 respectively. The second port of the first polarizer 102 and the second port of the second polarizer 103 are connected via a transmission optical path 104.

[0030] That is, the second port of the polarization beam splitter 101 is connected to the port on the first side of the first polarizer 102, and the third port is connected to the port on the first side of the second polarizer 103. The sub-light pulses output from the second port and transmitted in the clockwise direction (such as the first path of sub-light pulses) reach the first polarizer 102, and the sub-light pulses output from the third port and transmitted in the counterclockwise direction (such as the second path of sub-light pulses) reach the second polarizer 103.

[0031] The first polarizer 102 is used to polarize the first path of sub-optical pulses output from the second port, and output the first path of polarized sub-optical pulses to the transmission optical path 104 .

[0032] The second polarizer 103 is used to polarize the second path of sub-light pulses output from the third port, and output the second path of polarized sub-light pulses to the transmission optical path 104 .

[0033] Because the device needs to meet the requirements that the light pulse output from the second port of the polarization beam splitter 101 to the first polarizer 102 has a component output to the transmission optical path 104 after being polarized by the first polarizer 102, and the light pulse output from the transmission optical path 104 to the first polarizer 102 has a component output to the fourth port of the polarization beam splitter 101 after being polarized by the first polarizer 102, and the light pulse output from the third port of the polarization beam splitter 101 to the second polarizer 103 has a component output to the transmission optical path 104 after being polarized by the second polarizer 103, and the light pulse output from the transmission optical path 104 to the second polarizer 103 has a component output to the fourth port of the polarization beam splitter 101 after being polarized by the second polarizer 103. That is, the angle between the polarization direction of the first polarizer 102 and the polarization direction of the second polarizer 103 can be arbitrary, that is, the angle between the polarization direction of the first polarizer 102 and the polarization direction of the second polarizer 103 is ω, 0≤ω≤2π.

[0034] The angle between the polarization direction of the first polarizer 102 and the polarization state of the first path of light pulses output from the second port of the polarization beam splitter 101 is θ, where θ≠n·90°, and n is an integer.

[0035] The angle between the polarization direction of the second polarizer 103 and the polarization state of the second path optical pulse output from the third port of the polarization beam splitter 101 is δ, δ≠n·90°, where n is an integer.

[0036] That is, the polarization direction of the first polarizer 102 is consistent with an intrinsic polarization state of the polarization beam splitter 101. The polarization direction of the second polarizer 103 is not orthogonal nor different from the other intrinsic polarization state of the polarization beam splitter 101. Not orthogonal nor identical.

[0037] This is because when the polarization direction of the first-path optical pulse is orthogonal to the polarization direction of the first polarizer 102, the first-path optical pulse is blocked and cannot pass through the first polarizer 102; when the polarization direction of the first-path optical pulse is the same as the polarization direction of the first polarizer 102, the optical pulse output from the transmission optical path 104 to the first polarizer 102 has no component output at the fourth port of the polarization beam splitter 101. Furthermore, when the polarization direction of the second-path optical pulse is orthogonal to the polarization direction of the second polarizer 103, the second-path optical pulse is blocked and cannot pass through the second polarizer 103; when the polarization direction of the second-path optical pulse is the same as the polarization direction of the second polarizer 103, the optical pulse output from the transmission optical path 104 to the second polarizer 103 has no component output at the fourth port of the polarization beam splitter 101. Therefore, in order to allow the two light pulses to pass through their respective polarizers and have a component output at the fourth port of the polarization beam splitter 101, the polarization direction of the first polarizer 102 is consistent with the intrinsic polarization state of the polarization beam splitter 101. The polarization direction of the second polarizer 103 is not orthogonal or different (ie not 90 degrees or multiples of 90), and the polarization direction of the second polarizer 103 is not orthogonal to the intrinsic polarization state of the polarization beam splitter 101. Not orthogonal or different (ie not 90 degrees or a multiple of 90).

[0038] It should be noted that the intensity of the first polarized light pulse and the intensity of the second polarized light pulse are related to the angle ω. The polarization direction of the first polarized light pulse is the same as the polarization direction of the first polarizer 102, and the polarization direction of the second polarized light pulse is the same as the polarization direction of the second polarizer 103.

[0039] In one embodiment, when ω is 90 degrees, that is, the polarization direction of the second polarizer 103 is orthogonal to the polarization direction of the first polarizer 102, the polarization state direction of the first polarized light pulse output from the second side of the first polarizer 102 is orthogonal to the polarization state direction of the second polarized light pulse output from the second side of the second polarizer 103.

[0040] In another embodiment, when ω is 180 degrees, that is, the polarization direction of the second polarizer 103 is the same as the polarization direction of the first polarizer 102, the polarization state direction of the first polarized light pulse output from the second side of the first polarizer 102 is the same as the polarization state direction of the second polarized light pulse output from the second side of the second polarizer 103.

[0041] like Figure 1As shown, the first phase modulator 105 is disposed in the transmission optical path 104. The first polarizer 102 is connected to the first phase modulator 105 via the transmission optical path 104, and the second polarizer 103 is connected to the first phase modulator 105 via the transmission optical path 104. The first phase modulator 105 includes three ports: a fifth port, a sixth port, and a seventh port. The fifth port and the sixth port are the forward and reverse optical input ports of the first phase modulator 105, respectively, and the seventh port is an electrical port. In other words, the first phase modulator 105 is coupled to the transmission optical path 104 via the fifth port and the sixth port, which are connected to the port on the second side of the first polarizer 102 and the port on the second side of the second polarizer 103, respectively. It should be noted that the fifth port can be either the forward or reverse optical input port, and the sixth port can be either the reverse or forward optical input port.

[0042] The seventh port of the first phase modulator 105 is connected to the first driver 106. The first phase modulator 105 receives a modulation signal applied by the first driver 106 via the seventh port. The first phase modulator 105 is configured to phase-modulate the first polarized optical pulse and / or the second polarized optical pulse based on the modulation signal, thereby generating a first modulated optical sub-pulse and a second modulated optical sub-pulse. In this manner, the modulation signal can be used to create a specific phase difference between the first modulated optical sub-pulse and the second modulated optical sub-pulse. The specific phase difference can be determined based on the requirements of the quantum communication protocol.

[0043] In the embodiment of the present application, the polarized optical pulses passing through the phase modulator are referred to as modulated optical pulses. The first modulated optical pulse is the first polarized optical pulse after passing through the first phase modulator 105, and the second modulated optical pulse is the second polarized optical pulse after passing through the first phase modulator 105.

[0044] In some embodiments, when the modulation signal is a low-frequency modulation signal, the first phase modulator 105 performs time-sharing phase modulation on the first polarization optical pulse and the second polarization optical pulse, that is, different phase modulation is performed on the first polarization optical pulse and the second polarization optical pulse at different time points.

[0045] In this way, time-division modulation ensures that both optical sub-pulses can obtain corresponding phase modulation according to the modulation signal, while avoiding interference between the two.

[0046] In some embodiments, when the modulation signal has a frequency higher than a predetermined threshold, the first phase modulator 105 operates in a non-reciprocal state and effectively phase-modulates one of the first and second polarized optical pulses. Specifically, the ratio of the modulation efficiency of the polarized optical pulses simultaneously input from the forward optical input port to the modulation efficiency of the polarized optical pulses input from the reverse optical input port is no less than a predetermined threshold. The predetermined threshold is 10 decibels.

[0047] In some embodiments, when the first phase modulator 105 applies a high-frequency modulation signal having a frequency higher than a specified threshold, the first phase modulator 105 may have multiple optical sub-pulses inputted from the forward input port and optical sub-pulses inputted from the reverse input port at the same time.

[0048] In an embodiment of the present application, the first phase modulator 105 can be made of a lithium niobate crystal and can be a single polarization phase modulator or a birefringence phase modulator. The first phase modulator 105 can precisely control the refractive index of the crystal by applying an electric field, thereby modulating the phase of the light wave. For example, when a high-frequency modulation signal of 10 GHz or higher is applied, the first phase modulator 105 has a significant difference in phase modulation efficiency between the optical pulse input to the forward input optical port (e.g., the first polarization optical pulse) and the optical pulse input to the reverse input optical port (e.g., the second polarization optical pulse), which is at least 10 times or more than 100 times.

[0049] In other words, since the modulation efficiency of the first phase modulator 105 for the first polarized optical pulse is significantly different from the modulation efficiency of the first phase modulator 105 for the second polarized optical pulse, it can be considered that the first phase modulator 105 can effectively modulate the polarized optical pulse for forward transmission input from the fifth port (i.e., the forward input optical port) during each modulation, but cannot effectively phase modulate the polarized optical pulse for reverse transmission input from the sixth port (i.e., the reverse input optical port).

[0050] The first modulated optical sub-pulse and the second modulated optical sub-pulse are output from different ports of the first phase modulator 105, transmitted in different directions along the transmission optical path 104, respectively pass through the second polarizer 103 and the first polarizer 102, and are input to the third port and the second port of the polarization beam splitter 101.

[0051] In one embodiment, a first path of optical sub-pulses is output from the second port of the polarization beam splitter 101, polarized by the first polarizer 102, and converted into a first path of polarized optical sub-pulses. The first path of polarized optical sub-pulses is transmitted to the first phase modulator 105 through the fifth port of the first phase modulator 105 in the clockwise direction of the transmission optical path 104. After phase modulation by the first phase modulator 105, a first path of modulated optical sub-pulses is obtained. The first path of modulated optical sub-pulses is output through the sixth port of the first phase modulator 105 and transmitted clockwise along the transmission optical path 104 to the port on the second side of the second polarizer 103. The first path of modulated optical sub-pulses is input to the third port of the polarization beam splitter 101 through the port on the first side of the second polarizer 103.

[0052] The second optical sub-pulse is output from the third port of the polarization beam splitter 101, polarized by the second polarizer 103, and converted into a second polarized optical sub-pulse. The second polarized optical sub-pulse is transmitted counterclockwise along the transmission optical path 104 through the sixth port of the first phase modulator 105 to the first phase modulator 105. After phase modulation by the first phase modulator 105, a second modulated optical sub-pulse is obtained. The second modulated optical sub-pulse is output from the fifth port of the first phase modulator 105 and transmitted counterclockwise along the transmission optical path 104 to the port on the second side of the first polarizer 102. The second modulated optical sub-pulse is input to the second port of the polarization beam splitter 101 through the port on the first side of the first polarizer 102. When the second polarized optical sub-pulse reaches the first phase modulator 105, the first polarized optical sub-pulse has already been phase modulated by the first phase modulator 105, achieving time-sharing modulation of the two polarized optical sub-pulses, thereby generating a phase difference between the two polarized optical sub-pulses.

[0053] In another embodiment, the first path of optical pulses is output from the second port of the polarization beam splitter 101, polarized by the first polarizer 102, and converted into the first path of polarized optical pulses. The first path of polarized optical pulses is transmitted to the first phase modulator 105 through the fifth port of the first phase modulator 105 in the clockwise direction of the transmission optical path 104. After being effectively phase modulated by the first phase modulator 105, the first path of modulated optical pulses is obtained. The first path of modulated optical pulses is output through the sixth port of the first phase modulator 105 and transmitted clockwise along the transmission optical path 104 to the port on the second side of the second polarizer 103. The first path of modulated optical pulses is input to the third port of the polarization beam splitter 101 through the port on the first side of the second polarizer 103.

[0054] The second optical sub-pulse is output from the third port of the polarization beam splitter 101, polarized by the second polarizer 103, and converted into a second polarized optical sub-pulse. The second polarized optical sub-pulse is transmitted counterclockwise along the transmission optical path 104 through the sixth port of the first phase modulator 105 to the first phase modulator 105. After being ineffectively phase modulated by the first phase modulator 105, a second modulated optical sub-pulse is obtained. The second modulated optical sub-pulse is output through the fifth port of the first phase modulator 105 and transmitted counterclockwise along the transmission optical path 104 to the port on the second side of the first polarizer 102. It is then input into the second port of the polarization beam splitter 101 through the port on the first side of the first polarizer 102.

[0055] In the embodiment of the present application, when two polarized optical pulses simultaneously pass through the first phase modulator 105 from the forward and reverse input optical ports, respectively, applying the same high-frequency modulation pulse to the first polarized optical pulse and the second polarized optical pulse can create a desired phase difference between the first modulated optical pulse and the second modulated optical pulse output by the first phase modulator 105. Furthermore, applying high-frequency modulation pulses of different magnitudes can result in different phase differences between the first modulated optical pulse and the second modulated optical pulse after being modulated by the first phase modulator 105.

[0056] In one embodiment, the first phase modulator 105 may include a fast axis and a slow axis. The transmission optical path 104 may be coupled to either the fast axis or the slow axis of the first phase modulator 105. The two polarized optical pulses may enter and exit the first phase modulator 105 via the slow axis of the first phase modulator 105, or the two polarized optical pulses may enter and exit the first phase modulator 105 via the fast axis of the first phase modulator 105. This ensures that the modulation environment of the two polarized optical pulses is identical, reducing the impact of environmental interference.

[0057] The first port or the fourth port of the polarization beam splitter 101 is an output port. The polarization beam splitter 101 is also used to combine the components of the first modulated optical sub-pulse with the components of the second modulated optical sub-pulse, and output the obtained combined optical pulse through the first port or the fourth port.

[0058] In one embodiment, the phase of the first modulated optical sub-pulse is different from the phase of the second modulated optical sub-pulse. The polarization beam splitter 101 can combine the components of the first modulated optical sub-pulse input from the third port with the components of the second modulated optical sub-pulse input from the second port, and output the combined optical pulse from the first port or the fourth port. During the combining process, the polarization beam splitter 101 can combine the two modulated optical sub-pulses into one optical pulse while ensuring that the specific phase difference is maintained.

[0059] It should be noted that the optical pulse output from the first port of the polarization beam splitter 101 is a combination of the component of the first modulated sub-optical pulse input from the third port and the component reflected by the polarization beam splitter 101 via the second port; the optical pulse output from the fourth port of the polarization beam splitter 101 is a combination of the component of the first modulated sub-optical pulse input from the third port and the component reflected by the polarization beam splitter 101 via the second port.

[0060] In the above embodiment, a polarization beam splitter 101 is used to split a received input optical pulse into two sub-optical pulses with the same polarization state as its intrinsic polarization state. These pulses are then fed into corresponding polarizers through corresponding ports. The two polarizers then polarize the two sub-optical pulses, yielding two polarized sub-optical pulses. In this manner, the polarization states of the two sub-optical pulses can be precisely controlled by the two polarizers, resulting in two polarized sub-optical pulses with stable relative phases and polarization states having a specific angle. The two polarized sub-optical pulses are then transmitted along different directions of the optical transmission path to a first phase modulator 105. The first phase modulator 105, to which a modulation signal is applied, phase modulates the first polarized sub-optical pulse and / or the second polarized sub-optical pulse, yielding first and second modulated sub-optical pulses with a specific phase difference. The first and second modulated optical sub-pulses travel in different directions along the optical transmission path, passing through corresponding polarizers and entering different ports of polarization beam splitter 101. Polarization beam splitter 101 combines the components of the two modulated optical sub-pulses and outputs the combined optical pulse through the first or fourth port. This ensures that the two optical sub-pulses of the polarization beam splitter travel along identical optical paths when combined, thus providing self-compensation for environmental interference, high stability against interference, and highly stable polarization quantum state encoding at various rates.

[0061] In one embodiment, the relationship between the input optical pulse and the combined optical pulse can be represented by the angles between the two intrinsic polarization states of the polarization beam splitter 101 and the two polarizers.

[0062] The two intrinsic polarization states of the polarization beam splitter 101, namely the polarization states of the transmitted and reflected light pulses, are and and the polarization direction of the first polarizer 102 The angle between them is θ, and θ≠n·90°, and the polarization direction of the second polarizer 103 The angle between the two is δ, and δ≠n·90°, where n is an integer. In other words, the polarization state of the combined optical pulse can be determined based on the angle.

[0063] The polarization state of the input light pulse is 0≤β≤2π, and are the directions of the two eigenpolarization states of the polarization beam splitter 101 .

[0064] The polarization state of the input light pulse is output from the second port through the polarization beam splitter 101 as The polarization state of the first polarized light pulse outputted from the port on the second side of the first polarizer 102 is The first polarized optical pulse is phase modulated by the first phase modulator 105, and the modulation phase is When the first modulated sub-light pulse reaches the port on the second side of the second polarizer 103, the polarization state becomes The polarization state of the first modulated sub-light pulse outputted from the port on the first side of the second polarizer 103 is The component of the first modulated light sub-pulse inputted through the third port of the polarization beam splitter 101 and transmitted to the first port can be expressed as The component reflected to the fourth port can be expressed as

[0065] The polarization state of the input light pulse is output from the third port through the polarization beam splitter 101 as The polarization state of the second polarized sub-light pulse outputted through the port on the second side of the second polarizer 103 can be The second polarized sub-light pulse is phase modulated by the first phase modulator 105 to obtain a second modulated sub-light pulse with a modulation phase of φ. When the second modulated sub-light pulse reaches the port on the second side of the first polarizer 102, the polarization state becomes The polarization state of the second modulated sub-light pulse outputted from the port on the first side of the first polarizer 102 is The component of the second modulated light sub-pulse inputted through the second port of the polarization beam splitter 101 and transmitted to the fourth port can be expressed as The component reflected to the first port can be expressed as

[0066] When the two components are combined at the fourth port, the polarization state of the combined optical pulse outputted through the fourth port can be

[0067] When the two components are combined at the first port, the polarization state of the combined optical pulse outputted from the first port can be

[0068] The phase is modulated by the first phase modulator 105 The phase difference between the two components of the first or fourth port is achieved by the angles θ and δ, and the intensity relationship between the two components of the first or fourth port can be determined by the angles θ and δ. In general, in applications, to maintain the intensities of the two output polarization components to be equal, the angles θ and δ can be set equal. For example, the polarization direction of the first polarizer 102 and the polarization direction of the second polarizer 103 are orthogonal to each other. Alternatively, the sum of the angles θ and δ can be set to 180° and one of θ and δ can be set to 45°, that is, the polarization direction of the first polarizer 102 and the polarization direction of the second polarizer 103 are the same.

[0069] In one embodiment, when the modulation signal is a high-frequency modulation signal, the position of the first phase modulator 105 in the transmission optical path 104 can be adjusted so that the first polarized optical pulse and the second polarized optical pulse pass through the first phase modulator 105 simultaneously. In other words, at a specific point in time, both polarized optical pulses enter the first phase modulator 105 and are simultaneously affected by the modulation signal.

[0070] In this way, since the first polarized optical pulse and the second polarized optical pulse pass through the same modulation environment at the same time, not only the consistency and accuracy of phase modulation can be ensured, but also the phase difference between the two optical pulses can be precisely controlled.

[0071] In one embodiment, the distance between the first phase modulator 105 and the second port and the third port of the polarization beam splitter 101 can be adjusted to control whether the first polarized light pulse and the second polarized light pulse pass through the first phase modulator 105 at the same time.

[0072] Specifically, when the distance between the first phase modulator 105 and the second port and the third port of the polarization beam splitter 101 is equal, that is, it is set in the middle position of the two polarizers, the first polarized light pulse and the second polarized light pulse can be controlled to pass through the first phase modulator 105 at the same time.

[0073] When the distances between the first phase modulator 105 and the second port and the third port of the polarization beam splitter 101 are not equal, the first polarized light pulse and the second polarized light pulse can be controlled to pass through the first phase modulator 105 in a time-sharing manner.

[0074] In one embodiment, the specific phase difference can be determined based on the polarization encoding requirements of the quantum communication protocol. For example, in the quantum communication protocol BB84, the required modulation phase is 0°, 90°, 180°, or 270°. The first phase modulator 105 randomly modulates the phase to 0°, 90°, 180°, or 270°. In other words, the modulation results in a phase difference of 0°, 90°, 180°, or 270° between the first modulated optical sub-pulse and the second modulated optical sub-pulse.

[0075] In some embodiments, the first port of the polarization beam splitter can be connected to a front free space optical path or a front polarization maintaining fiber. The first port of the polarization beam splitter can receive an input light pulse input from the front free space optical path or the front polarization maintaining fiber. The polarization state of the input light pulse can be and are respectively the directions of the two eigenpolarization states of the polarization beam splitter.

[0076] In some embodiments, the polarization encoding device further comprises: when the first port of the polarization beam splitter 101 is connected to the front polarization-maintaining fiber, setting the slow axis and fast axis of the front polarization-maintaining fiber to be the same as the two intrinsic polarization states of the polarization beam splitter 101 respectively.

[0077] The embodiment of the present application involves setting the slow axis and fast axis of the front polarization-maintaining fiber to be the same as the two intrinsic polarization states of the polarization beam splitter 101. This can be considered as aligning the fast axis and slow axis of the front polarization-maintaining fiber with the two intrinsic polarization states of the polarization beam splitter 101. The above setting can be achieved by adjusting the connection angle between the front polarization-maintaining fiber and the polarization beam splitter 101.

[0078] and are the directions of the two intrinsic polarization states of the polarization beam splitter 101, and are also the slow axis and fast axis of the front polarization-maintaining fiber. That is, the polarization state of the input light pulse can be expressed as a combination of two polarization components on the slow and fast axes of the front polarization-maintaining fiber. In other words, the polarization state of the input light pulse can also be expressed as a combination of two linear polarization states aligned with the slow and fast axes of the polarization-maintaining fiber.

[0079] When the fast axis and slow axis of the front polarization-maintaining optical fiber are aligned with the two intrinsic polarization states of the polarization beam splitter 101, the polarization beam splitter 101 can effectively separate the different polarization components of the input light pulse (i.e., the first path light pulse and the second path light pulse) to different output ports (i.e., the second port and the third port).

[0080] In some embodiments, when β = 0, the polarization state of the input light pulse can be the same as the slow axis or the fast axis of the front polarization-maintaining fiber. In order to enable the polarization beam splitter to split the input light pulse into two components with equal intensities but different polarization states, the angle between the slow axis or the fast axis of the front polarization-maintaining fiber and the direction of an intrinsic polarization state of the polarization beam splitter 101 is set to 45 degrees.

[0081] Specifically, when the polarization state of the input optical pulse is the same as the slow axis of the front polarization-maintaining fiber, the input optical pulse is transmitted along the slow axis of the front polarization-maintaining fiber. Because the slow axis of the front polarization-maintaining fiber and the direction of one of the intrinsic polarization states of the polarization beam splitter 101 are at an angle of 45°, when the optical pulse passes through the polarization beam splitter, it is decomposed into two sub-optical pulses with equal intensity but different polarization states.

[0082] When the polarization state of the input optical pulse is identical to the fast axis of the polarization-maintaining fiber, the input optical pulse is transmitted along the fast axis of the polarization-maintaining fiber. Because the fast axis of the polarization-maintaining fiber and the direction of one of the intrinsic polarization states of the polarization beam splitter 101 are at a 45° angle, when the optical pulse passes through the polarization beam splitter, it is split into two sub-optical pulses with equal intensity but different polarization states.

[0083] Thus, by setting the angle of 45°, the polarization beam splitter 101 can split the input optical pulse having the same axis as the front polarization-maintaining optical fiber into two sub-optical pulses having the same intensity but different polarization states.

[0084] In one embodiment, the transmission optical path 104 may include a wired channel and a wireless channel. The wired channel may include a polarization-maintaining optical fiber, and the wireless channel may include a free-space optical path.

[0085] Polarization-maintaining fiber is a specialized fiber that maintains the polarization of light while transmitting along its slow or fast axis. A free-space optical path refers to the path of light traveling in free space, independent of optical fiber or other physical media.

[0086] In some embodiments, when the transmission optical path 104 is a polarization-maintaining fiber with ω twist, both the first polarizer 102 and the second polarizer 103 are coupled to the fast axis or the slow axis of the polarization-maintaining fiber 104 .

[0087] Polarization-maintaining fiber can have two transmission axes: a fast axis and a slow axis. Polarization-maintaining fiber with ω twist refers to a fiber whose fast and slow axes are twisted at an angle of ω. When a light pulse propagates along the fast or slow axis of a polarization-maintaining fiber with ω twist, its polarization state undergoes an ω-shift.

[0088] In some embodiments, the transmission optical path 104 is a free-space optical path. Figure 2 FIG. 2 shows a schematic structural diagram of another polarization encoding device provided in an embodiment of the present application. Figure 2 The polarization encoding device shown is Figure 1 The polarization encoding device shown is slightly different. Figure 2 As shown, a polarization rotator 107 is provided in the transmission optical path 104. The polarization rotator 107 can be used to rotate the polarization states of the polarized sub-light pulses and modulated sub-light pulses passing therethrough by ω. The polarization rotator 107 can be a half-wave plate or a Faraday rotator.

[0089] It should be noted that the polarization state rotator 107 can be arranged between the second polarizer 103 and the first phase modulator 105, such as Figure 2 Alternatively, the polarization state rotator 107 may be disposed between the first polarizer 102 and the first phase modulator 105. In addition, no matter whether the sub-light pulse passes through the polarization state rotator 107 clockwise or counterclockwise, it will be rotated by 90 degrees.

[0090] In some embodiments, the transmission optical path 104 is a polarization-maintaining optical fiber. When the first polarizer 102 and the second polarizer 103 are coupled to different axes of the polarization-maintaining optical fiber, the polarization encoding device may further include a polarization state rotator 107, such as Figure 2 shown.

[0091] Taking a high-frequency modulated signal as an example, the first polarizer 102 can be coupled to the fast axis of the polarization-maintaining fiber, and the second polarizer 103 can be coupled to the slow axis of the polarization-maintaining fiber. Specifically, the first optical pulse output from the second port of the polarization beam splitter 101 is polarized by the first polarizer 102 and converted into a first polarized optical pulse. The first polarized optical pulse propagates clockwise along the fast axis of the polarization-maintaining fiber, enters the first phase modulator 105 through the fifth port for effective phase modulation, and generates a first modulated optical pulse. The first modulated optical pulse is output through the sixth port of the first phase modulator 105 and propagates clockwise along the fast axis of the polarization-maintaining fiber to the polarization rotator 107. The polarization rotator 107 rotates the polarization state of the first modulated optical pulse by 90° and outputs the pulse clockwise along the slow axis of the polarization-maintaining fiber to the second polarizer 103. The pulse is then input into the third port of the polarization beam splitter 101 through the second polarizer 103.

[0092] The second optical pulse output from the third port of the polarization beam splitter 101 is polarized by the second polarizer 103 and converted into a second polarized optical pulse. The second polarized optical pulse is transmitted counterclockwise along the slow axis of the polarization-maintaining fiber to the polarization rotator 107. The polarization rotator 107 rotates the polarization state of the second polarized optical pulse by 90° and transmits it counterclockwise along the fast axis of the polarization-maintaining fiber to the first phase modulator 105. The second polarized optical pulse enters the first phase modulator 105 through the sixth port of the first phase modulator 105 for ineffective phase modulation, thereby obtaining a second modulated optical pulse. The second modulated optical pulse is output through the fifth port of the first phase modulator 105 and transmitted counterclockwise along the fast axis of the polarization-maintaining fiber to the first polarizer 102. The second polarized optical pulse is then input into the second port of the polarization beam splitter 101 through the first polarizer 102.

[0093] In another embodiment, the first polarizer 102 may be coupled to the slow axis of the polarization-maintaining fiber, and the second polarizer 103 may be coupled to the fast axis of the polarization-maintaining fiber.

[0094] Taking a high-frequency modulated signal as an example, the first optical pulse output from the second port of the polarization beam splitter 101 is polarized by the first polarizer 102 and converted into a first polarized optical pulse. The first polarized optical pulse propagates clockwise along the slow axis of the polarization-maintaining fiber, enters the first phase modulator 105 through the fifth port for effective phase modulation, and obtains a first modulated optical pulse. The first modulated optical pulse is output through the sixth port of the first phase modulator 105 and propagates clockwise along the slow axis of the polarization-maintaining fiber to the polarization rotator 107. The polarization rotator 107 rotates the polarization state of the first modulated optical pulse by 90° and outputs the first modulated optical pulse clockwise along the fast axis of the polarization-maintaining fiber to the second polarizer 103. The second polarizer 103 then inputs the third port of the polarization beam splitter 101.

[0095] The second optical pulse output from the third port of the polarization beam splitter 101 is polarized by the second polarizer 103 and converted into a second polarized optical pulse. The second polarized optical pulse is transmitted counterclockwise along the fast axis of the polarization-maintaining fiber to the polarization rotator 107. The polarization rotator 107 rotates the polarization state of the second polarized optical pulse by 90° and transmits it counterclockwise along the slow axis of the polarization-maintaining fiber to the first phase modulator 105. The second modulated optical pulse enters the first phase modulator 105 through the sixth port of the first phase modulator 105 for ineffective phase modulation, thereby obtaining a second modulated optical pulse. The second modulated optical pulse is output through the fifth port of the first phase modulator 105 and transmitted counterclockwise along the slow axis of the polarization-maintaining fiber to the first polarizer 102. The second modulated optical pulse is then input into the second port of the polarization beam splitter 101 through the first polarizer 102.

[0096] In this way, by setting the polarization state rotator 107, the polarization states of one polarized sub-light pulse and one modulated sub-light pulse can be accurately deflected in the transmission optical path, so that the transmission paths of the two sub-light pulses in the device are the same, reducing the errors caused by different transmission paths.

[0097] In one embodiment, the first phase modulator 105 can be set in reverse in the transmission optical path 104, that is, the fifth port of the first phase modulator 105 can be connected to the second polarizer 103 through the transmission optical path 104, and the sixth port of the first phase modulator 105 can be connected to the first polarizer 102 through the transmission optical path 104.

[0098] In one embodiment, in order to reduce the types of phase modulation by a single phase modulator, multiple phase modulators may be used for combined modulation to achieve polarization state control. Figure 3 A schematic structural diagram of another polarization encoding device provided in an embodiment of the present application is shown in FIG. Figure 3As shown, the polarization encoding device may include a polarization beam splitter 301 , a first polarizer 302 , a second polarizer 303 , a transmission optical path 304 , a first phase modulator 305 , a first driver 306 , a second phase modulator 307 and a second driver 308 .

[0099] compared to Figure 1 The polarization encoding device shown, Figure 3 The polarization encoding device shown is further provided with a second phase modulator 307 and a second driver 308 .

[0100] Figure 3 The polarization beam splitter 301 in Figure 1 The polarization beam splitter 301 is similar to the polarization beam splitter 101 in FIG. The polarization beam splitter 301 includes a first port, a second port, a third port, and a fourth port. The first port is the input port of the polarization beam splitter 301, and the fourth port is the output port of the polarization beam splitter 301. The polarization beam splitter 301 receives an input optical pulse through the first port and splits the input optical pulse into a first optical pulse and a second optical pulse. The first optical pulse and the second optical pulse are output through the second port and the third port, respectively.

[0101] The polarization states of the first path optical pulse and the second path optical pulse are respectively the same as the two intrinsic polarization states of the polarization beam splitter 301 .

[0102] Figure 3 The first polarizer 302 and Figure 1 The structure and function of the first polarizer 102 are the same as those of Figure 3 The second polarizer 303 in Figure 1 The structure and function of the second polarizer 103 are the same.

[0103] The second phase modulator 307 can be identical to the first phase modulator 305. The second phase modulator 307 can include three ports, namely, an eighth port, a ninth port, and a tenth port. The eighth port and the ninth port are the forward optical input port and the reverse optical input port, respectively, and the tenth port is an electrical port.

[0104] That is, the sixth port of the first phase modulator 305 is connected to the eighth port of the second phase modulator 307 via the transmission optical path 304, and the ninth port of the second phase modulator 307 is connected to the second polarizer 303 via the transmission optical path 304. Alternatively, the sixth port of the first phase modulator 305 is connected to the ninth port of the second phase modulator 307 via the transmission optical path 304, and the eighth port of the second phase modulator 307 is connected to the second polarizer 303 via the transmission optical path 304.

[0105] The second phase modulator 307 can be used to cooperate with the first phase modulator 305 to perform phase modulation on the first polarization optical pulse and the second polarization optical pulse.

[0106] In some embodiments, after applying a modulation signal having a frequency higher than a specified threshold, the second phase modulator 307 operates in a non-reciprocal state, and the ratio of the modulation efficiency of the polarized optical pulse input from the forward input optical port to the modulation efficiency of the polarized optical pulse input from the reverse input optical port is not less than a preset threshold.

[0107] In some embodiments, when the second phase modulator 307 applies a high-frequency modulation signal having a frequency higher than a specified threshold, the second phase modulator 307 may have multiple optical sub-pulses inputted from the forward input port and multiple optical sub-pulses inputted from the reverse input port at the same time.

[0108] In one embodiment, when the modulation signal is a high-frequency modulation signal, the first polarization optical pulse and the second polarization optical pulse are configured to be transmitted simultaneously through the first phase modulator 305 .

[0109] In another embodiment, when the modulation signal is a high-frequency modulation signal, the first polarization optical pulse and the second polarization optical pulse are configured to be transmitted simultaneously through the second phase modulator 307 .

[0110] Specifically, by setting the positions of the first phase modulator 305 and the second phase modulator 307 in the transmission optical path 304, it is possible to control whether the first polarized optical pulse and the second polarized optical pulse are simultaneously transmitted through the first phase modulator 305 or the second phase modulator 307. The setting method is similar to the setting method of the first phase modulator in the aforementioned embodiment and will not be repeated here.

[0111] It should be noted that in the implementation of the two phase modulators cooperative modulation, the two phase modulators are connected in series in the transmission optical path, and the same Figure 3 Taking the structure shown in as an example, if the two sub-light pulses are allowed to pass through the first phase modulator 305 at the same time, then for the first phase modulator 305, when the second polarized light pulse is input, the second polarized light pulse has already passed through the second phase modulator 307. At this time, the second polarized light pulse input to the first phase modulator 305 is the second modulated sub-light pulse after phase modulation; similarly, if the two sub-light pulses are allowed to pass through the second phase modulator 307 at the same time, then for the second phase modulator 307, when the first polarized light pulse is input, the first polarized light pulse has already passed through the first phase modulator 305. At this time, the first polarized light pulse input to the second phase modulator 307 is the first modulated sub-light pulse after phase modulation.

[0112] In other words, the second phase modulator 307 has two functions. One is to serve as a backup phase modulator. If the first phase modulator 305 fails to operate, the second phase modulator operates to phase-modulate the first and second polarization optical pulses based solely on the modulation signal applied by the second driver 308. The other is to serve as a cooperative phase modulator. That is, if the first phase modulator 305 operates, the second phase modulator 307 also operates to phase-modulate the first and second polarization optical pulses based on the modulation signals applied to them.

[0113] In one embodiment, the modulation modes of the first phase modulator 305 and the second phase modulator 307 may include a separate modulation mode and a combined modulation mode. The separate modulation mode may refer to the first phase modulator 305 or the second phase modulator 307 performing phase modulation independently. The combined modulation mode may refer to the first phase modulator 305 and the second phase modulator 307 performing phase modulation in coordination.

[0114] The second driver 308 is connected to the second phase modulator 307. That is, the second phase modulator 307 is connected to the second driver 308 via the tenth port. The second driver 308 is configured to apply a modulation signal to the second phase modulator 307. It should be noted that the second driver 308 and the first driver 306 can be of the same type and have the same function.

[0115] It should be noted that the modulation signal applied by the second driver 308 can be a low-frequency signal or a high-frequency signal. The second phase modulator 307 can phase-modulate the first polarized optical pulse and / or the second polarized optical pulse based on the modulation signal. The modulation principle of the second phase modulator 307 is similar to that of the first phase modulator 305 and will not be further described here for the sake of brevity.

[0116] In one embodiment, the first path light pulse is output from the second port of the polarization beam splitter 301, polarized by the first polarizer 302, transmitted in the clockwise direction along the transmission optical path 304 through the first phase modulator 305 and the second phase modulator 307, modulated by the first phase modulator 305 and / or the second phase modulator 307, transmitted in the clockwise direction along the transmission optical path 304 to the port on the second side of the second polarizer 303, and output to the third port of the polarization beam splitter 301 through the port on the first side of the second polarizer 303.

[0117] The second path of light pulses is output from the third port of the polarization beam splitter 301, polarized by the second polarizer 303, transmitted counterclockwise along the transmission optical path 304 through the first phase modulator 305 and the second phase modulator 307, modulated by the first phase modulator 305 and / or the second phase modulator 307, transmitted counterclockwise along the transmission optical path 304 to the port on the second side of the first polarizer 302, and output to the second port of the polarization beam splitter 301 through the port on the first side of the first polarizer 302.

[0118] In the above embodiment, the addition of the second phase modulator 307 increases device redundancy. The first phase modulator 305 and the second phase modulator 307 collaboratively modulate two polarized optical pulses, thereby achieving a more flexible phase modulation mode and improving the accuracy and speed of phase control. Furthermore, if the first phase modulator 305 fails to operate due to a malfunction, the second phase modulator 307 is activated to perform phase modulation, thereby ensuring the normal operation of the phase modulation process and preventing device interruption.

[0119] In addition, two phase modulators are configured in the transmission optical path 304. By combining the two phase modulators to which a modulation signal with a frequency higher than a specified threshold is applied, the first driver 306 and the second driver 308 can generate different polarization states at different rates through digital modulation.

[0120] In some embodiments, the random modulation phase of the second phase modulator 307 can be determined based on the polarization encoding requirements of the quantum communication protocol. For example, in the quantum communication protocol BB84, the modulation requirements are two sets of four basis quantum states, and the random modulation phase of the second phase modulator 307 is 0°, 90°, 180°, or 270°.

[0121] In some embodiments, the modulation phases of the first polarized optical pulse and the second polarized optical pulse by the first phase modulator 305 and the second phase modulator 307 are determined according to the requirements of polarization encoding of the quantum communication protocol.

[0122] In other words, whether in single or combined modulation mode, the phase difference / phase sum between the first modulated optical sub-pulse and the second modulated optical sub-pulse, after modulation by the first phase modulator and / or the second phase modulator, meets the modulation requirements of the quantum communication protocol. This ensures high-precision implementation of the quantum communication protocol. Furthermore, by providing two phase modulators, the number of phase modulation states of a single phase modulator is reduced, thereby improving the flexibility and adaptability of the device.

[0123] In some embodiments, when the first phase modulator 305 and the second phase modulator 307 are connected in forward series or reverse series, the phase difference between the first polarized optical pulse and the second polarized optical pulse after phase modulation is equal to the sum or difference of the phase modulated by the first phase modulator 305 and the phase modulated by the second phase modulator 307.

[0124] Specifically, when the first phase modulator 305 and the second phase modulator 307 both modulate the sub-optical pulse of the same path, the phase difference between the first polarized sub-optical pulse and the second polarized sub-optical pulse after phase modulation is equal to the sum of the phase modulated by the first phase modulator 305 and the phase modulated by the second phase modulator 307.

[0125] When the first phase modulator 305 performs phase modulation on one of the two polarized optical pulses and the second phase modulator 307 performs phase modulation on the other polarized optical pulse, the phase difference between the first polarized optical pulse and the second polarized optical pulse after phase modulation is equal to the phase difference between the phase modulated by the first phase modulator 305 and the phase modulated by the second phase modulator 307.

[0126] In one embodiment, when the modulation signal is a high-frequency modulation signal, the first phase modulator 305 and the second phase modulator 307 are connected in series in a forward direction, that is, the first phase modulator 305 and the second phase modulator 307 both perform phase modulation on the first polarized optical pulse signal, and the phase difference between the first modulated optical pulse and the second modulated optical pulse obtained after phase modulation is equal to the sum of the phase modulated by the first phase modulator 305 and the phase modulated by the second phase modulator 307.

[0127] The first phase modulator 305 and the second phase modulator 307 are connected in reverse series. That is, when the first phase modulator 305 phase-modulates one of the two polarized optical pulses, and the second phase modulator 307 phase-modulates the other polarized optical pulse, the phase difference between the first modulated optical pulse and the second modulated optical pulse obtained after phase modulation is equal to the phase difference between the modulations of the first phase modulator 305 and the modulations of the second phase modulator 307.

[0128] The phases modulated by the first phase modulator and the second phase modulator on the first polarized optical pulse and the second polarized optical pulse can be determined according to the requirements of polarization encoding of the quantum communication protocol.

[0129] Based on this, in the combined modulation mode, the types of phases modulated by the first phase modulator 305 can be less than the required phases modulated in the quantum communication protocol. The types of phases modulated by the second phase modulator 307 can be less than the required phases modulated in the quantum communication protocol.

[0130] For example, in the quantum communication protocol BB84, the modulation phase requirements are 0°, 90°, 180°, or 270°. In forward series connection, the first phase modulator 305 can modulate the phase to 0° or 90°, and the second phase modulator 307 can modulate the phase to 0° or 180°. In reverse series connection, the first phase modulator 305 can modulate the phase to 0° or 180°, and the second phase modulator 307 can modulate the phase to 0° or 90°.

[0131] In one embodiment, the phase modulated by the first phase modulator 305 and the phase modulated by the second phase modulator 307 are both determined by the magnitude of the modulation signals applied by their respective drivers.

[0132] In this way, by combining the modulation of the first phase modulator 305 and / or the second phase modulator 307 , not only polarization state control can be achieved, but also the types of phases modulated by a single phase modulator can be reduced, thereby reducing the difficulty of implementing the phase modulator driver.

[0133] In some embodiments, the first polarized optical pulse and the second polarized optical pulse are simultaneously transmitted through the first phase modulator 305, or the first polarized optical pulse and the second polarized optical pulse are simultaneously transmitted through the second phase modulator 307. In this way, by simultaneously passing through the same phase modulator, both optical pulses are modulated by the same modulation signal, thereby helping to reduce phase drift caused by environmental interference and improving the accuracy and reliability of the device.

[0134] also, Figure 3 The polarization encoding device shown may also be provided with a polarization state rotator. The polarization state rotator may be provided between the second phase modulator 307 and the third port of the polarization beam splitter 301. The role of the polarization state rotator is similar to Figure 2 The functions of the polarization rotators shown are the same and are not described here for brevity.

[0135] In one embodiment, the polarization encoding device can also be used for polarization decoding.

[0136] Another aspect of the embodiments of the present application provides a polarization encoding method, which is applied to any polarization encoding device in the embodiments of the present application to implement quantum state polarization encoding. Figure 4 A schematic diagram of a polarization encoding method provided in an embodiment of the present application is shown in FIG. Figure 4 As shown, the polarization encoding method may include the following steps.

[0137] S41 , receiving an input optical pulse through the first port of the polarization beam splitter 101 .

[0138] S42 , polarization-splitting the received input optical pulse into a first path optical pulse and a second path optical pulse by the polarization beam splitter 101 .

[0139] The polarization states of the first path optical pulses and the second path optical pulses are respectively the same as the two intrinsic polarization states of the polarization beam splitter 101 .

[0140] S43, the first path of light pulses is output from the second port of the polarization beam splitter 101, and polarized by the first polarizer 102 to obtain a first path of polarized light pulses. Also, the second path of light pulses is output from the third port of the polarization beam splitter 101, and polarized by the second polarizer 103 to obtain a second path of polarized light pulses.

[0141] The angle between the polarization direction of the first polarizer 102 and the polarization state of the first light pulse output from the second port of the polarization beam splitter 101 is θ, the angle between the polarization direction of the second polarizer 103 and the polarization state of the second light pulse output from the third port of the polarization beam splitter 101 is δ, and the angle between the polarization direction of the first polarizer 102 and the polarization direction of the second polarizer 103 is ω, wherein θ, δ≠n·90°, n is an integer, and 0≤ω≤2π.

[0142] S44, using different ports of the first phase modulator 105 to receive the first polarized optical pulse and the second polarized optical pulse, and phase modulating the first polarized optical pulse and / or the second polarized optical pulse based on the modulation signal to obtain a first modulated optical pulse and a second modulated optical pulse.

[0143] There is a specific phase difference between the first modulated optical sub-pulse and the second modulated optical sub-pulse.

[0144] S45, the first modulated sub-optical pulse and the second modulated sub-optical pulse are respectively output from different ports of the first phase modulator 105, transmitted along different directions in the transmission optical path 104, respectively pass through the second polarizer 103 and the first polarizer 102, and are input to the third port and the second port of the polarization beam splitter 101.

[0145] S46, combining the components of the first modulated sub-optical pulse input from the third port and the components of the second modulated sub-optical pulse input from the second port through the polarization beam splitter 101, and outputting the obtained combined optical pulse through the first port or the fourth port.

[0146] In the above embodiment, a polarization beam splitter 101 is used to split a received input optical pulse into two sub-optical pulses with the same polarization state as its intrinsic polarization state. These pulses are then fed into corresponding polarizers through corresponding ports, and the two sub-optical pulses are polarized separately to produce two polarized sub-optical pulses. In this way, the polarization states of the two sub-optical pulses can be precisely controlled by the two polarizers, thereby producing two polarized sub-optical pulses with stable relative phases and different polarization states. The two polarized sub-optical pulses are then transmitted along different directions of the transmission optical path 104 to the first phase modulator 105. The first phase modulator 105, to which a modulation signal is applied, phase modulates the first polarized sub-optical pulse and / or the second polarized sub-optical pulse, thereby producing first and second modulated sub-optical pulses with a specific phase difference. The first and second modulated optical sub-pulses travel in different directions along the optical transmission path, passing through corresponding polarizers and entering different ports of polarization beam splitter 101. Polarization beam splitter 101 combines the components of the two modulated optical sub-pulses and outputs the combined optical pulse through the first or fourth port. This ensures that the two optical sub-pulses of the polarization beam splitter travel along identical optical paths during their combined transmission, thus achieving self-compensation for environmental interference, high anti-interference stability, and highly stable polarization quantum state encoding at various rates.

[0147] In some embodiments, the method may further include: both the first optical pulse and the second optical pulse are transmitted to the first phase modulator 105 from the fast axis of the first phase modulator 105 , or both are transmitted to the first phase modulator 105 from the slow axis of the first phase modulator 105 .

[0148] In some embodiments, the method may include: based on a setting position of the first phase modulator, controlling the first polarized optical pulse and the second polarized optical pulse to pass through the first phase modulator simultaneously.

[0149] In some embodiments, the method may include: when the frequency of the modulation signal is higher than a specified threshold, the first phase modulator 105 operates in a non-reciprocal state, and the ratio of the modulation efficiency of the polarized optical pulse input by the forward input optical port to the modulation efficiency of the polarized optical pulse input by the reverse input optical port is not less than a preset threshold.

[0150] In some embodiments, the polarization state of the input light pulse is and are the directions of the two eigenpolarization states of the polarization beam splitter.

[0151] In some embodiments, the method may further include: when the first port of the polarization beam splitter is connected to the front polarization-maintaining fiber, setting the slow axis and the fast axis of the front polarization-maintaining fiber to be respectively the same as the two intrinsic polarization states of the polarization beam splitter.

[0152] In some embodiments, the method may further include: when the first port of the polarization beam splitter is connected to the front polarization-maintaining optical fiber and the polarization state of the input light pulse is the same as the slow axis or the fast axis of the front polarization-maintaining optical fiber, setting the angle between the slow axis or the fast axis of the front polarization-maintaining optical fiber and the direction of an intrinsic polarization state of the polarization beam splitter to 45°.

[0153] In some embodiments, the method may further include: the transmission optical path is a free space optical path or a polarization-maintaining optical fiber.

[0154] In some embodiments, the method may further include: when the transmission optical path is a free-space optical path, rotating the polarization states of the passing polarized sub-optical pulse and the modulated sub-optical pulse by ω using a polarization rotator.

[0155] In some embodiments, the method may further include: when the transmission optical path is a polarization-maintaining optical fiber, and the first polarizer and the second polarizer are respectively coupled to the fast axis and slow axis of the polarization-maintaining optical fiber, rotating the polarization state of the passing polarized sub-light pulse and the modulated sub-light pulse by 90° through a polarization state rotator.

[0156] In some embodiments, the method may further include: performing phase modulation on the first polarized optical pulse and the second polarized optical pulse by the second phase modulator in cooperation with the first phase modulator.

[0157] In some embodiments, the method may further include: after the second phase modulator applies a modulation signal with a frequency higher than a specified threshold, the second phase modulator operates in a non-reciprocal state, and the ratio of the modulation efficiency of the polarized light pulse input by the forward input optical port to the modulation efficiency of the polarized light pulse input by the reverse input optical port is not less than a preset threshold.

[0158] In some embodiments, the method may further include: arranging the first polarized optical pulse and the second polarized optical pulse to be transmitted simultaneously through the first phase modulator, or arranging the first polarized optical pulse and the second polarized optical pulse to be transmitted simultaneously through the second phase modulator.

[0159] Another aspect of the present application provides a quantum communication system. Figure 5 A schematic diagram of the structure of a quantum communication system provided by an embodiment of the present application is shown as follows: Figure 5 As shown, the quantum communication system 500 may include the polarization encoding device 501 described in any embodiment of the present application.

[0160] It should be understood that the specific features, operations, and details described hereinabove with respect to the apparatus of the present application may also be similarly applied to the method and system of the present application, or vice versa. In addition, each step of the method of the present application described above may be performed by a corresponding component or unit of the apparatus or system of the present application.

[0161] It should be understood that the various modules / units of the apparatus of the present application may be implemented in whole or in part by software, hardware, firmware, or a combination thereof. Each module / unit may be embedded in the processor of the electronic device in the form of hardware or firmware or may be independent of the processor, or may be stored in the memory of the electronic device in the form of software for the processor to call to execute the operation of each module / unit. Each module / unit may be implemented as an independent component or module, or two or more modules / units may be implemented as a single component or module.

[0162] The various technical features described above can be combined arbitrarily. Although not all possible combinations of these technical features are described, any combination of these technical features should be considered to be covered by this specification as long as such combination does not conflict.

[0163] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A polarization encoding device, characterized in that: include: A polarization beam splitter, a first polarizer, a second polarizer, a transmission optical path, a first phase modulator, and a first driver; The first driver is connected to the first phase modulator and is used to apply a modulation signal to the first phase modulator; The polarization beam splitter includes a first port, a second port, a third port, and a fourth port; the first port is an input port, used to receive an input light pulse, polarization-split it into two sub-light pulses, and output them through the second port and the third port, respectively, and the polarization states of the two sub-light pulses are respectively the same as the two intrinsic polarization states of the polarization beam splitter; the second port and the third port are respectively connected to the port on the first side of the first polarizer and the port on the first side of the second polarizer; the port on the second side of the first polarizer and the port on the second side of the second polarizer are connected through the transmission optical path; The first polarizer is used to polarize the first path of sub-light pulses output from the second port, and output the first path of polarized sub-light pulses to the transmission optical path; The second polarizer is used to polarize the second path of sub-light pulses output from the third port, and output the second path of polarized sub-light pulses to the transmission optical path; The angle between the polarization direction of the first polarizer and the polarization state of the first path light pulse output from the second port of the polarization beam splitter is θ, the angle between the polarization direction of the second polarizer and the polarization state of the second path light pulse output from the third port of the polarization beam splitter is δ, and the angle between the polarization direction of the first polarizer and the polarization direction of the second polarizer is ω, where θ, δ≠n·90°, n is an integer, and 0≤ω≤2π; The first phase modulator is provided in the transmission optical path, and is used to perform phase modulation on the first polarized optical pulse and / or the second polarized optical pulse based on the modulation signal to obtain a first modulated optical sub-pulse and a second modulated optical sub-pulse; so that a specific phase difference exists between the first modulated optical sub-pulse and the second modulated optical sub-pulse; The first modulated optical sub-pulse and the second modulated optical sub-pulse are output from different ports of the first phase modulator, transmitted along different directions of the transmission optical path, respectively pass through the second polarizer and the first polarizer, and are input to the third port and the second port of the polarization beam splitter; The polarization beam splitter is further configured to combine a component of a first modulated optical sub-pulse input from the third port with a component of a second modulated optical sub-pulse input from the second port, and output the obtained combined optical pulse through the first port or the fourth port.

2. The device according to claim 1, characterized in that The first phase modulator includes: a fifth port, a sixth port and a seventh port, the fifth port and the sixth port are respectively a forward input optical port and a reverse input optical port of the first phase modulator, and the seventh port is an electrical port connected to the first driver and is used to receive the modulation signal applied by the first driver.

3. The device according to claim 2, characterized in that When the frequency of the modulation signal is higher than a specified threshold, the first phase modulator operates in a non-reciprocal state, and the ratio of the modulation efficiency of the polarization optical pulse input from the forward input optical port to the modulation efficiency of the polarization optical pulse input from the reverse input optical port is not less than a preset threshold.

4. The device according to claim 3, characterized in that include: Based on the setting position of the first phase modulator, the first polarized optical pulse and the second polarized optical pulse are controlled to pass through the first phase modulator at the same time.

5. The device according to claim 1, characterized in that The polarization state of the input light pulse is 0≤β≤2π, and are respectively the directions of the two eigenpolarization states of the polarization beam splitter.

6. The device according to claim 5, characterized in that Also includes: When the first port of the polarization beam splitter is connected to the front polarization-maintaining optical fiber, the slow axis and the fast axis of the front polarization-maintaining optical fiber are respectively set to be the same as the two intrinsic polarization states of the polarization beam splitter.

7. The device according to claim 5, characterized in that Also includes: When the first port of the polarization beam splitter is connected to a front polarization-maintaining optical fiber, and the polarization state of the input light pulse is the same as the slow axis or the fast axis of the front polarization-maintaining optical fiber, the angle between the slow axis or the fast axis of the front polarization-maintaining optical fiber and the direction of an intrinsic polarization state of the polarization beam splitter is set to 45°.

8. The device according to claim 1, characterized in that The transmission optical path is a free space optical path or a polarization-maintaining optical fiber.

9. The device according to claim 8, characterized in that When the transmission optical path is the free space optical path, the device further comprises a polarization state rotator, which is used to rotate the polarization states of the passing polarized sub-light pulses and modulated sub-light pulses by ω.

10. The device according to claim 8, characterized in that When the transmission optical path is a polarization-maintaining optical fiber and the first polarizer and the second polarizer are respectively coupled to the fast axis and slow axis of the polarization-maintaining optical fiber, the device also includes a polarization state rotator, which is used to rotate the polarization state of the passing polarized sub-light pulse and the modulated sub-light pulse by 90°.

11. The device according to any one of claims 1 to 10, characterized in that: Also includes: a second phase modulator and a second driver, The second phase modulator is provided in the transmission optical path, and is connected in series with the first phase modulator, and is used to cooperate with the first phase modulator to phase modulate the first polarized optical pulse and the second polarized optical pulse; The second phase modulator includes: an eighth port, a ninth port, and a tenth port, the eighth port and the ninth port being a forward input optical port and a reverse input optical port of the second phase modulator, respectively; the tenth port being an electrical port connected to the second driver for receiving a modulation signal applied by the second driver.

12. The device according to claim 11, characterized in that After applying a modulation signal having a frequency higher than a specified threshold, the second phase modulator operates in a non-reciprocal state, and a ratio of a modulation efficiency of a polarized optical pulse input from a forward input optical port to a modulation efficiency of a polarized optical pulse input from a reverse input optical port is not less than a preset threshold.

13. The device according to claim 12, characterized in that The first polarized optical pulse and the second polarized optical pulse are configured to be transmitted simultaneously through the first phase modulator, or the first polarized optical pulse and the second polarized optical pulse are configured to be transmitted simultaneously through the second phase modulator.

14. A polarization encoding method, characterized in that: The polarization encoding device according to any one of claims 1 to 13 is used to implement quantum state polarization encoding.

15. A quantum communication system, characterized in that: The polarization encoding device comprises the polarization encoding device according to any one of claims 1 to 13.

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