Polarization encoding device and quantum key distribution light source

Abstract

A polarization encoding device and a quantum distribution light source based on the device, the device comprises: an optical beam splitter, a first polarizer, a second polarizer and a phase modulator; the optical beam splitter comprises an input port, a reflection output port, a transmission output port and an output port; the first polarizer and the second polarizer are connected to the phase modulator through a polarization-maintaining optical fiber to form a bidirectional ring optical path that satisfies the Sagnac effect; the input port receives input light through a polarization-maintaining optical fiber. The device can avoid the problems of polarization mode delay and poor stability caused by the difference in optical fiber paths of traditional polarization modulators, and the device is simplified and easy to operate. The quantum distribution light source based on the device can quickly, accurately and stably realize quantum key distribution.

Description

Technical Field

[0001] The present invention relates to the field of quantum communication, and in particular to a polarization encoding device and a quantum key distribution light source. Background Art

[0002] At present, there are mainly two encoding methods for polarization state modulation of quantum key distribution (QKD) light sources.

[0003] First, a polarization modulator is used to achieve this. The polarization modulator is equivalent to an equal-arm interferometer, and the lengths of the two arms of the interferometer must be strictly equal to ensure the interference effect. However, due to the different refractive indices of the two polarization components H and V, polarization mode delay (PMD) will occur after passing through the modulator. This delay is generally on the order of 10ps for modulators based on lithium niobate crystals. The existence of PMD will greatly limit the performance of the modulator, and generally requires compensation with solutions such as high refractive index optical fiber. The system is complex and the performance is limited. In addition, since the two arms of the interferometer are affected by changes in external environmental conditions such as temperature and mechanics, the impact on the two arms is not the same, causing the polarization modulation results to gradually drift and the long-term stability is poor.

[0004] Second, it is implemented based on the polarization modulation method of circulator + sagnac ring. This method is generally implemented using the structure of circulator + Sagnac ring. Specifically, this method uses a polarization controller to control the polarization state of the incident linear polarized light, uses single-mode optical fiber transmission, and transmits it to the polarization beam splitter (PBS) after passing through the circulator, ensuring that the light incident on the PBS is 45° linearly polarized light and is evenly output from the two arms of the PBS. The light components of the two arms are re-interfered at the PBS through a Sagnac ring structure, and an additional phase component is added on one of the arms using a phase modulator. The light after interference on the PBS produces different polarization states, and then returns to the circulator and is output. Since a single-mode optical fiber is used for transmission between the circulator and the PBS, a polarization controller needs to be used to modulate the polarization state of the incident light, resulting in a more complex system and poor integration. At the same time, the solution needs to be calibrated before implementation to ensure that the 45° linearly polarized light is evenly projected onto the two arms of the PBS. The calibration process is cumbersome, and a calibration process is required before each polarization encoding device is used, which has poor practicality. Summary of the invention

[0005] 1. Technical issues

[0006] In view of the problems existing in the prior art, the present invention proposes a polarization encoding device and a quantum key distribution light source, which are used to at least partially solve the above technical problems.

[0007] (II) Technical solution

[0008] The present invention provides a polarization encoding device, comprising: an optical beam splitter, a first polarization plate, a second polarization plate and a phase modulator; the optical beam splitter comprises an input port, a reflection output port, a transmission output port and an output port; the first polarization plate and the second polarization plate are connected to the phase modulator through a polarization-maintaining optical fiber to form a bidirectional ring optical path that satisfies the Sagnac effect; the input port receives input light through the polarization-maintaining optical fiber; wherein the input light enters the optical beam splitter after being rotated by the polarization-maintaining optical fiber, and the optical beam splitter splits the rotated input light into a first light component and a second light component, the first light component is transmitted to the first polarizer through the reflection output port, the fast axis is cut off through the first polarizer and coupled to the slow axis of the polarization-maintaining fiber, the first light component is transmitted to the phase modulator in a clockwise direction for phase modulation and then transmitted back to the optical beam splitter, the second light component is transmitted to the second polarizer through the transmission output port, the fast axis is cut off through the second polarizer and coupled to the slow axis of the polarization-maintaining fiber, the second light component is transmitted back to the optical beam splitter in a counterclockwise direction, and the two beams of light transmitted back to the optical beam splitter are output through the output port after interference.

[0009] Optionally, the angle between the first polarization plate and the horizontal direction is 0°, and the angle between the second polarization plate and the horizontal direction is 90°.

[0010] Optionally, the length of the polarization-maintaining optical fiber between the first polarization plate, the second polarization plate and the phase modulator is adjustable.

[0011] Optionally, the input light is horizontal linear polarized light, and the polarization-maintaining optical fiber of the input port is further used to rotate the horizontal linear polarized light by 45° to obtain 45° linear polarized light.

[0012] Optionally, an optical signal is transmitted between the reflective output port and the first polarizer through free space, and an optical signal is transmitted between the transmissive output port and the second polarizer through free space.

[0013] Optionally, after the phase modulator modulates the first light component, the phase added is 0 or π / 2 or π or 3π / 2.

[0014] Optionally, the output port is connected to a single-mode optical fiber for outputting an optical signal obtained after interference between the two beams of light transmitted back to the optical beam splitter.

[0015] The present invention provides another aspect of a quantum key distribution light source based on the above-mentioned polarization encoding device, including: a laser, used to generate a narrow light pulse signal; an intensity modulator, used to perform intensity modulation on the above-mentioned narrow light pulse signal to generate an intensity state signal required for quantum key distribution; a polarization encoding device, used to perform polarization encoding on the above-mentioned intensity state signal to generate a polarization state signal required for quantum key distribution; an attenuator, used to output the above-mentioned polarization state signal after attenuating it to the single photon level required for quantum key distribution.

[0016] Optionally, the quantum key distribution light source further includes: a pulse generator for emitting a pulse signal to drive the laser, the intensity modulator and the polarization encoding device.

[0017] Optionally, the pulse generator generates a periodic electrical pulse signal to drive the laser; the pulse generator generates a random pulse signal to drive the intensity modulator and the polarization encoding device.

[0018] (III) Beneficial effects

[0019] The present invention provides a polarization encoding device and a quantum key distribution light source, which adopts a polarization modulation scheme based on a Sagnac loop. Since the forward and reverse light components have the same polarization state in the phase modulator and have traveled through the same length of optical fiber, the fiber path difference is eliminated, thereby avoiding the problems of polarization mode delay and poor stability caused by the fiber path difference of the traditional polarization modulator. At the same time, compared with the polarization modulation method based on a circulator + sagnac loop, the polarization encoding device simplifies the use of the circulator and the polarization controller, uses polarization-maintaining optical fiber to achieve 45° incidence of linear polarized light, avoids the complicated initial polarization state calibration process, and simplifies the system implementation device, which can quickly, accurately and stably realize quantum key distribution. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] For a more complete understanding of the present invention and its advantages, reference will now be made to the following description taken in conjunction with the accompanying drawings, in which:

[0021] Figure 1 A schematic diagram showing a structure of a polarization encoding device according to an embodiment of the present invention;

[0022] Figure 2 The structure of a quantum key distribution light source according to an embodiment of the present invention is schematically shown. DETAILED DESCRIPTION

[0023] Below, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present invention. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of embodiments of the present invention. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of concepts of the present invention.

[0024] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. The terms "comprise", "include", etc. used herein indicate the existence of the features, steps, operations and / or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

[0025] All terms (including technical and scientific terms) used herein have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification, and should not be interpreted in an idealized or overly rigid manner.

[0026] Figure 1 The structure diagram of the polarization encoding device according to an embodiment of the present invention is schematically shown. Figure 1 As shown, the polarization encoding device includes:

[0027] An optical beam splitter (BS), a first polarization plate (Pola1), a second polarization plate (Pola2) and a phase modulator (PM).

[0028] The optical beam splitter (BS) includes an input port, a reflection output port, a transmission output port, and an output port. The input light (Input) is connected to the input port through the input optical fiber, which is a polarization-maintaining optical fiber. The output light (Output) is connected to the output port through the output optical fiber, which is a single-mode optical fiber.

[0029] like Figure 1 As shown, the optical beam splitter (BS) includes a first port A, a second port B, a third port C and a fourth port D. The specific input port, reflection output port, transmission output port and output port can be determined according to actual needs. For example:

[0030] When the first port A is an input port, the second port B is a reflection output port, and the third port C is a transmission output port.

[0031] When the second port B is an input port, the first port A is a reflection output port, and the fourth port D is a transmission output port.

[0032] When the third port C is an input port, the fourth port D is a reflection output port, and the first port A is a transmission output port.

[0033] The first polarizer (Pola1), the second polarizer (Pola2) and the phase modulator (PM) are connected by a polarization-maintaining fiber based on a Sagnac effect (Sagnac) ring to form a bidirectional ring optical path. The length of the polarization-maintaining fiber between the first polarizer (Pola1), the second polarizer (Pola2) and the phase modulator (PM) is adjustable. By adjusting the length of the polarization-maintaining fiber from the phase modulator (PM) to the first polarizer (Pola1) and the second polarizer (Pola2), the delay of the Sagnac ring reaching the phase modulator (PM) in the clockwise direction and the counterclockwise direction can be different. Among them, the angle between the first polarizer (Pola1) and the horizontal direction is 0°, and the angle between the second polarizer (Pola2) and the horizontal direction is 90°.

[0034] The polarization encoding process of the polarization encoding device is as follows: the input light is rotated by the polarization-maintaining optical fiber and then enters the optical beam splitter (BS), the optical beam splitter (BS) evenly splits the rotated input light into a first light component and a second light component, the first light component is transmitted to the first polarizer (Pola1) through the reflection output port, the fast axis is cut off by the first polarizer (Pola1) and coupled to the slow axis of the polarization-maintaining optical fiber, the first light component is transmitted clockwise to the phase modulator (PM) for phase modulation and then transmitted back to the optical beam splitter (BS), the second light component is transmitted to the second polarizer (Pola2) through the transmission output port, the fast axis is cut off by the second polarizer (Pola2) and coupled to the slow axis of the polarization-maintaining optical fiber, the second light component is transmitted counterclockwise to the optical beam splitter (BS), and the two beams of light transmitted back to the optical beam splitter (BS) are output through the output port after interference.

[0035] Specifically, the first port A is the input port, the second port B is the reflection output port, the third port C is the transmission output port, and the fourth port D is the output port. The polarization encoding process is as follows:

[0036] S1, linear polarized light is input horizontally, and the input optical fiber is rotated 45° through the polarization-maintaining optical fiber, so that the input horizontal linear polarized light becomes 45° linear polarized light, which is incident on port A of the BS. The BS evenly divides the 45° linear polarized light into the first light component and the second light component, which are output from the reflection output port B and the transmission output port C respectively. Among them, the polarization state of the incident polarized light can be expressed as:

[0037]

[0038] The reflection output port B of S2 and BS outputs the first light component to Pola1 through free space. Pola1 is placed at 0°, and the fast axis cutoff is achieved through Pola1, so that the first light component is coupled to the slow axis of the polarization-maintaining fiber and propagates clockwise in the bidirectional ring optical path.

[0039] The transmission output end C of S3 and BS outputs the second light component to Pola2 through free space. Pola2 is placed at 90°, and the fast axis is cut off by Pola2. The second light component is coupled to the slow axis of the polarization-maintaining fiber after passing through the polarization plate, and propagates counterclockwise in the bidirectional ring optical path. At this time, the corresponding polarization state is:

[0040]

[0041] Here, |S> refers to polarization along the slow axis in the polarization-maintaining light, and the subscripts c and a refer to clockwise and counterclockwise propagation, respectively.

[0042] S4, the first light component output clockwise is affected by the electrical signal loaded on PM when passing through PM, adding an additional phase When the second light component outputted counterclockwise passes through the PM, the electrical signal loaded on the PM is zero, and no additional phase is superimposed. At this time, the corresponding polarization state is:

[0043]

[0044] S5, the first light component reaches Pola2 clockwise and is transmitted to the transmission output port C of the BS through free space, and the second component reaches Pola1 counterclockwise and is transmitted to the reflection output port B of the BS through free space. The two pulse components interfere on the BS and are output through the output port D. At this time, the corresponding polarization state is:

[0045]

[0046] Among them, the additional phase It can be 0 or π / 2 or π or 3π / 2, and the corresponding output quantum states are The quantum states of these two sets of basis vectors can be directly used for BB84 protocol encoding.

[0047] In addition, a polarization controller is added after the output port D to apply a unitary transformation, adjusting the |L> and |R> quantum states to |H> and |V> quantum states while keeping the |+> and |-> quantum states unchanged, thereby realizing the original BB84 protocol encoding.

[0048] The polarization encoding device provided in this embodiment adopts a polarization modulation scheme based on a Sagnac loop. Since the forward and reverse light components have the same polarization state in the phase modulator and travel through the same length of optical fiber, the fiber path difference is eliminated, thereby avoiding the problems of polarization mode delay and poor stability caused by the fiber path difference of the traditional polarization modulator. At the same time, compared with the polarization modulation method based on a circulator + sagnac loop, the polarization encoding device simplifies the use of the circulator and the polarization controller, uses polarization-maintaining optical fiber to achieve 45° incidence of linearly polarized light, avoids the complicated initial polarization state calibration process, and simplifies the system implementation device.

[0049] Figure 2 The structure of a quantum key distribution light source according to an embodiment of the present invention is schematically shown. Figure 2 As shown, the quantum key distribution light source includes a laser, an intensity modulator, the polarization encoding device mentioned above, an attenuator and a pulse generator.

[0050] The laser is used to generate a narrow optical pulse signal. Specifically, the laser emits a periodic narrow optical pulse signal under the drive of the periodic electrical pulse generated by the pulse generator.

[0051] The intensity modulator is used to modulate the intensity of the narrow optical pulse signal to generate the intensity state signal required for quantum key distribution. Specifically, the intensity modulator modulates the input narrow optical pulse signal under the action of the random pulse generated by the pulse generator to generate the three intensity states required for quantum key distribution, namely the signal state, the decoy state, and the vacuum state.

[0052] The polarization encoding device is used to polarization encode the intensity state signal to generate the polarization state signal required for quantum key distribution. Specifically, the optical signal after intensity modulation enters the polarization encoding device. Under the action of the random pulse generated by the pulse generator, the polarization encoding device modulates the input narrow optical pulse signal to generate the four polarization states of +, -, L, and R required for quantum key distribution.

[0053] The attenuator is used to attenuate the polarization state signal to the single photon level required for quantum key distribution and then output it.

[0054] The quantum key distribution light source provided in this embodiment can avoid the problems of polarization mode delay and poor stability caused by the difference in optical fiber paths of the traditional polarization modulator due to the use of the polarization encoding device described in the above embodiment. In addition, the operation is simple, and the four polarization states of +, -, L, and R required for quantum key distribution can be quickly generated, thereby achieving accurate, stable and rapid distribution of quantum keys.

[0055] It will be appreciated by those skilled in the art that, although the present invention has been shown and described with reference to specific exemplary embodiments of the present invention, it will be appreciated by those skilled in the art that, without departing from the spirit and scope of the present invention as defined by the appended claims and their equivalents, various changes in form and detail may be made to the present invention. Therefore, the scope of the present invention should not be limited to the above-described embodiments, but should be determined not only by the appended claims, but also by the equivalents of the appended claims.

Claims

1. A polarization encoding device, comprising: An optical beam splitter, a first polarization plate, a second polarization plate and a phase modulator, wherein the angle between the first polarization plate and the horizontal direction is 0°, and the angle between the second polarization plate and the horizontal direction is 90°; The optical beam splitter comprises an input port, a reflection output port, a transmission output port and an output port, wherein an optical signal is transmitted between the reflection output port and the first polarizer through free space, and an optical signal is transmitted between the transmission output port and the second polarizer through free space; The first polarizer and the second polarizer are connected to the phase modulator via a polarization-maintaining optical fiber to form a bidirectional ring optical path that satisfies the Sagnac effect, wherein the length of the polarization-maintaining optical fiber between the first polarizer, the second polarizer and the phase modulator is adjustable; The input port receives input light through a polarization-maintaining optical fiber, wherein the input light is horizontal linear polarized light, and the polarization-maintaining optical fiber of the input port is also used to rotate the horizontal linear polarized light by 45° to obtain 45° linear polarized light; Wherein, the input light enters the optical beam splitter after being rotated by the polarization-maintaining fiber, and the optical beam splitter splits the rotated input light into a first light component and a second light component. The first light component is transmitted to the first polarizer through the reflection output port, and is coupled to the middle edge of the slow axis of the polarization-maintaining fiber through the first polarizer, and is transmitted to the phase modulator clockwise for phase modulation, and then transmitted back to the optical beam splitter. The second light component is transmitted to the second polarizer through the transmission output port, and is coupled to the middle edge of the slow axis of the polarization-maintaining fiber through the second polarizer, and is transmitted back to the optical beam splitter counterclockwise. The two beams of light transmitted back to the optical beam splitter are output through the output port after interference; The phase added by the phase modulator after modulating the first light component is: 0 or π / 2 or π or 3π / 2, and the four quantum states correspondingly output by the polarization encoding device are: ， ， ， ； in, represents the horizontal polarization state, Indicates the vertical polarization state.

2. The polarization encoding device according to claim 1, wherein: The output port is connected to a single-mode optical fiber for outputting an optical signal obtained after interference between two beams of light transmitted back to the optical beam splitter.

3. A quantum key distribution light source system based on the polarization encoding device according to any one of claims 1 to 2, comprising: Lasers, used to generate narrow optical pulse signals; An intensity modulator, used to perform intensity modulation on the narrow optical pulse signal to generate an intensity state signal required for quantum key distribution; A polarization encoding device, used for performing polarization encoding on the intensity state signal to generate a polarization state signal required for quantum key distribution; The attenuator is used to attenuate the polarization state signal to the single photon level required for quantum key distribution and then output it.

4. The quantum key distribution light source system according to claim 3, further comprising: A pulse generator is used to send out a pulse signal to drive the laser, the intensity modulator and the polarization encoding device.

5. The quantum key distribution light source system according to claim 4, wherein: The pulse generator generates a periodic electrical pulse signal to drive the laser; The pulse generator generates a random pulse signal to drive the intensity modulator and the polarization encoding device.

Images