Time phase hybrid coding device and quantum key distribution system

Abstract

The invention discloses a time phase hybrid coding device and a quantum key distribution system, and the device enables an optical pulse with the central wavelength of 1550nm to pass through two phase coding modules and a time coding module, and the two phase coding modules respectively generate a group of orthogonal normalization bases with the phase difference of 0 and pi. And the time coding module generates an orthogonal ground state through time domain difference. According to the structural design, the transmitting end performance of the QKD system is improved, the reliability and safety of quantum communication are enhanced, the influence of optical fiber polarization change on the quantum state is eliminated, the influence of optical fiber phase change on the quantum state is eliminated, and the quantum key distribution system is suitable for long-distance quantum key distribution.

Description

Technical Field

[0001] The present invention belongs to the field of quantum communication technology, and more specifically, the present invention relates to a time-phase hybrid encoding device and a quantum key distribution system. Background Art

[0002] Quantum key distribution (QKD) technology is suitable for communication fields with high security and confidentiality. By combining quantum key distribution with "one-time-one-pad" encryption technology, secure information transmission can be achieved. Compared with classical communication, a major advantage of quantum communication is that it can achieve security under strict mathematical proof. The theoretical basis of quantum key distribution is quantum mechanics. The no-cloning theorem of unknown quantum states ensures that quantum states cannot be copied during transmission. The uncertainty principle ensures that when a third party steals information, it will cause the destruction of information and thus be detected by both parties to the communication, which is used to achieve secure confidential communication.

[0003] Quantum key distribution technology has broad application prospects. In the era of rapid development of information technology, quantum communication, with its high security characteristics, has important value and application prospects in military defense, financial information, government agencies and other occasions. In the future, with the continuous development of quantum communication technology, quantum key distribution technology will be gradually promoted.

[0004] In the current fiber-optic quantum key distribution system, photons are used as carriers of quantum information to distribute keys through optical fiber channels. There are two main quantum coding schemes for this technology: polarization coding and phase coding. Although both schemes can achieve accurate transmission and measurement of single photon states, they each face specific technical challenges. Specifically, polarization coding is susceptible to polarization mode dispersion, noise interference, and nonlinear optical effects in optical fiber media, all of which can lead to increased bit error rates. Phase coding uses the phase difference between quantum bits to encode information, and it is extremely sensitive to phase fluctuations and drifts caused by the environment, which may also cause an increase in bit error rates. Summary of the invention

[0005] The present invention provides a time-phase hybrid encoding device, aiming to improve the above-mentioned problem.

[0006] The present invention is implemented as follows: a time phase hybrid encoding device, the device comprising:

[0007] A 1×3 optical switch, two phase encoding modules and a time encoding module,

[0008] The 1×3 optical switch divides the input laser pulse into three optical signals, which are input into two phase encoding modules and one time encoding module respectively;

[0009] The two phase encoding modules use the electro-optical effect of the phase modulator to generate a set of orthogonal normalized bases with a phase difference of 0 and a phase difference of π respectively; the time encoding module generates an orthogonal basis state through time domain differences.

[0010] Furthermore, it is characterized in that the two phase encoding modules are respectively a first phase encoding module and a second phase encoding module, the first phase encoding module is composed of a first spectroscope, a first phase modulator and a second phase modulator, and the first spectroscope is respectively connected to the first phase modulator and the second phase modulator through optical fibers.

[0011] Furthermore, the optical signal output from the first output channel of the 1×3 optical switch enters the first beam splitter, is equally divided into two optical signals in the first beam splitter, and the two optical signals enter the first phase modulator and the second phase modulator respectively, and the voltages of the first phase modulator and the second phase modulator are adjusted so that the first phase modulator and the second phase modulator generate a set of optical signals with a phase difference of 0, corresponding to the encoding state |+〉.

[0012] Furthermore, the second phase encoding module is composed of a second beam splitter, a third phase modulator and a fourth phase modulator, and the second beam splitter is connected to the third phase modulator and the fourth phase modulator respectively through optical fibers.

[0013] Furthermore, the optical signal output from the second output channel of the 1×3 optical switch enters the second beam splitter, is equally divided into two optical signals in the second beam splitter, and the two optical signals enter the third phase modulator and the fourth phase modulator respectively, and the voltages of the third phase modulator and the fourth phase modulator are adjusted so that the third phase modulator and the fourth phase modulator generate a set of optical signals with a phase difference of π, corresponding to the encoding state |-〉.

[0014] Furthermore, the time coding module includes a third beam splitter, a total internal reflection type Y-branch optical switch and a delay line. The third beam splitter is respectively connected to the total internal reflection type Y-branch optical switch and the output optical fiber. The total internal reflection type Y-branch optical switch is respectively connected to the delay line and the output optical fiber. The delay line is connected to the output optical fiber.

[0015] Further, the path between the total internal reflection type Y-branch optical switch and the output optical fiber is the first path, and the path between the internal reflection type Y-branch optical switch and the output optical fiber through the delay line is the second path; the optical signal output from the third output channel of the 1×3 optical switch enters the third beam splitter, and is equally divided into the first optical signal in the third beam splitter, the first optical signal is transmitted to the output end through a single optical fiber, the second optical signal enters the total internal reflection type Y-branch optical switch, an external voltage is applied to the total internal reflection type Y-branch optical switch, and under the action of different external voltages, the second optical signal enters the first path and the second path respectively, the optical path of the first path and the second path are different, the third optical signal is output from the first path, and the fourth optical signal is output from the second path, the encoding state corresponding to the first optical signal and the third optical signal is |1〉, and the encoding state corresponding to the first optical signal and the fourth optical signal is |0〉.

[0016] Furthermore, the central wavelength of the laser pulse is 1550 nm.

[0017] The present invention is implemented as follows: a quantum key distribution system, the system comprising:

[0018] The transmitter and the receiver communicate with each other through a quantum channel;

[0019] The transmitting end randomly modulates the quantum light signal based on the above-mentioned time phase hybrid encoding device, decays the modulated quantum light into a single photon, and sends the single photon to the receiving end through the quantum channel;

[0020] The receiving end randomly demodulates the received single photon in quantum state and detects the demodulated photon.

[0021] The present invention provides a time-phase hybrid coding scheme, which adopts time and phase coding to avoid the problems caused by polarization coding. Compared with a single phase coding scheme, this scheme is mainly based on time coding, and the time coding is mainly related to the time when the photon arrives at the detector, has no phase information, is insensitive to phase fluctuations and drifts caused by the environment, and the system stability is greatly improved.

[0022] In addition, the hardware module that implements this solution has the advantages of simple structure, low cost, and small size, and is compatible with the existing fiber-optic communication infrastructure, which can lay the foundation for the realization of miniaturized and integrated QKD systems in the future. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments are briefly introduced below. The drawings are used to provide a further understanding of the present invention and constitute a part of the specification. They are used together with the embodiments of the present invention to explain the present invention and do not constitute a limitation of the present invention.

[0024] Figure 1 A framework diagram of a time-phase hybrid encoding device provided by an embodiment of the present invention;

[0025] Figure 2 A schematic diagram of the structure of a first phase encoding module provided in an embodiment of the present invention;

[0026] Figure 3 A schematic diagram of the structure of a second phase encoding module provided by an embodiment of the present invention;

[0027] Figure 4 A schematic diagram of the structure of a time coding module provided by an embodiment of the present invention;

[0028] Figure 5 A schematic diagram of the structure of a phase modulator provided in an embodiment of the present invention;

[0029] Figure 6 A schematic diagram of the adjustment relationship between voltage and phase in a phase modulator provided in an embodiment of the present invention;

[0030] Figure 7 A schematic diagram of the structure of a total internal reflection type Y-branch optical switch provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0031] The specific implementation modes of the present invention are further explained in detail below by describing the embodiments with reference to the accompanying drawings, so as to help those skilled in the art to have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention.

[0032] Figure 1 A schematic diagram of the structure of a quantum key distribution system provided in an embodiment of the present invention. For the sake of convenience, only the part related to the embodiment of the present invention is shown. The quantum key distribution system includes:

[0033] Transmitter, and receiver; quantum communication is performed between the transmitter and the receiver via a quantum channel;

[0034] The transmitting end randomly modulates the quantum light based on the time phase hybrid coding device described below, decays the modulated quantum light into a single photon, and sends the single photon to the receiving end through the quantum channel;

[0035] The receiving end randomly demodulates the received single photons and detects the demodulated quantum light.

[0036] Figure 2 A schematic diagram of the structure of a time-phase hybrid encoding device provided in an embodiment of the present invention. For ease of description, only parts related to the embodiment of the present invention are shown. The device includes:

[0037] A 1×3 optical switch, two phase encoding modules and a time encoding module,

[0038] The 1×3 optical switch divides the input laser pulse into three optical signals, which are input into two phase encoding modules and one time encoding module respectively;

[0039] The two phase encoding modules use the electro-optical effect of the phase modulator to generate a set of orthogonal normalized bases with a phase difference of 0 and a phase difference of π respectively; the time encoding module generates an orthogonal basis state through time domain differences.

[0040] The present invention adopts an MZI type 1×3 electro-optical switch, which is based on silicon-based photonics and can meet the requirement of crosstalk below -20dB, while maintaining the loss of a single switch at about 2dB. At a wavelength of 1550nm, it is driven by a scheme of DC voltage Vdc=-3.5V and square wave voltage peak-to-peak value Vpp=4.0V.

[0041] A laser pulse signal with a central wavelength of 1550nm enters a 1×3 optical switch, changing the voltages of the three channels. Under the action of an external electric field, the pulsed light signal enters the three output channels of the optical switch respectively. The output signal of the first output channel is recorded as optical signal a, the output signal of the second output channel is recorded as optical signal b, and the output signal of the third output channel is recorded as optical signal c.

[0042] The two phase encoding modules are respectively a first phase encoding module and a second phase encoding module, the first phase encoding module uses the electro-optical effect of the phase modulator to generate a group of optical signals with a phase difference of 0, and the second phase encoding module uses the electro-optical effect of the phase modulator to generate a group of optical signals with a phase difference of π;

[0043] Figure 2 A structural schematic diagram of a first phase encoding module provided in an embodiment of the present invention, wherein the first phase encoding module is composed of a first beam splitter, a first phase modulator and a second phase modulator, wherein the first beam splitter is connected to the first phase modulator and the second phase modulator respectively through optical fibers, wherein the first beam splitter is a 50:50 directional coupler having a size of 60μm×30μm, and at a wavelength of 1550nm, the energy ratio between the coupling end and the through end is 50:50.

[0044] Optical signal a is output from the first output channel of the 1×3 optical switch and enters the first phase encoding module. Optical signal a first enters the first beam splitter, where it is equally divided into two optical signals, namely optical signal a1 and optical signal a2. Optical signal a1 enters the first phase modulator, and optical signal a2 enters the second phase modulator. The first phase modulator and the second phase modulator use the linear electro-optic effect principle of the electro-optic crystal in the phase modulator to adjust the voltages of the first phase modulator and the second phase modulator respectively, so that the first phase modulator and the second phase modulator generate a set of optical signals a3 and a4 with a phase difference of 0, and the corresponding encoding state is |+〉.

[0045] Figure 3 A schematic diagram of the structure of a second phase encoding module provided in an embodiment of the present invention, wherein the second phase encoding module is composed of a second beam splitter, a third phase modulator and a fourth phase modulator, and the second beam splitter is connected to the third phase modulator and the fourth phase modulator respectively through optical fibers.

[0046] The optical signal b is output from the second output channel of the 1×3 optical switch and enters the second phase encoding module. The optical signal b first enters the second beam splitter, where it is equally divided into two optical signals, namely, optical signal b1 and optical signal b2. The optical signal b1 enters the third phase modulator, and the optical signal b2 enters the fourth phase modulator. The third phase modulator and the fourth phase modulator respectively adjust the voltages of the third phase modulator and the fourth phase modulator using the linear electro-optic effect principle of the electro-optic crystal in the phase modulator, so that the third phase modulator and the fourth phase modulator generate a set of optical signals b3 and b4 with a phase difference of π, and the corresponding encoding state is |-〉.

[0047] The first phase modulator to the fourth phase modulator have the same structure, including: an electrode, a waveguide, and an optoelectronic substrate; Figure 5 The specific workflow is as follows: Input optical signal E in (t) enters the phase modulator and is transmitted along the waveguide. The waveguide is composed of an electro-optic crystal. The refractive index of the electro-optic crystal changes with the size of the externally applied electric field. Using the electro-optic effect principle of the electro-optic crystal, the external circuit applies voltage to the electrode, thereby changing the refractive index of the electro-optic crystal in the waveguide. The phase of the light pulse passing through the waveguide changes, thereby realizing phase modulation of the optical signal. Figure 6 is the relationship between external voltage and optical phase.

[0048] Figure 4 A structural schematic diagram of a time coding module provided in an embodiment of the present invention, wherein the time coding module includes a third beam splitter, a total internal reflection type Y-branch optical switch and a delay line, wherein the third beam splitter is respectively connected to the total internal reflection type Y-branch optical switch and the output optical fiber, the total internal reflection type Y-branch optical switch is respectively connected to the delay line and the output optical fiber, and the delay line is connected to the output optical fiber.

[0049] The optical signal c enters the time encoding module and is equally divided into two optical signals by the third beam splitter, namely, optical signal c1 and optical signal c2. The optical signal c1 directly outputted through the output optical fiber is defined as the reference light. The optical signal c2 enters the total internal reflection type Y-branch optical switch. An external voltage is applied to the total internal reflection type Y-branch optical switch. Under the action of different external voltages, the optical signal c2 enters path 1 and path 2 respectively. Since path 2 has a delay line, the optical path lengths of path 1 and path 2 are different. The signal outputted from path 1 is marked as optical signal c3, and the signal outputted from path 2 is marked as optical signal c4. The encoding state corresponding to the optical signal c1 and the optical signal c3 is |1〉, and the encoding state corresponding to the optical signal c1 and the optical signal c4 is |0〉.

[0050] The total internal reflection type Y-branch optical switch includes an input waveguide, an output waveguide, a control unit, and a semiconductor medium. For the convenience of intuitive expression, refer to Figure 7 , Figure 7 The external control unit is not shown. The Y-branch optical switch is a commonly used optical path switching device in optical fiber communication. Based on the principle of total internal reflection, it has the advantages of low insertion loss, high isolation, fast switching speed, miniaturization and good stability, making it widely used in optical fiber communication networks, optical fiber test equipment and optical sensor systems. Figure 7 The shaded area is the injection reflection area, with a thickness of d. When entering the total reflection state, the refractive index of this area is n2, the refractive index of the waveguide area is n1, and the refractive index of other areas is n0. The control unit applies an external voltage to the Y-branch optical switch, and uses the plasma effect caused by carrier injection of semiconductor materials and the quantum confinement Stark effect in the quantum well to control the change of the refractive index, so that the c signal enters the two output waveguides respectively under different external voltages.

[0051] A set of orthogonal normalized bases output by two phase encoding modules correspond to the encoding states |+〉 and |-〉, respectively, and a set of orthogonal normalized bases output by the time encoding module correspond to the encoding states |1〉 and |0〉, respectively. The output end of the time encoding module and the output end of the phase encoding module converge to complete the modulation of the signal state, and then perform operations such as optical signal attenuation to single photons, and finally the optical signal is transmitted to the receiving end through the optical fiber channel. The present invention proposes to complete different information encodings through time-phase hybrid encoding, which improves the problems of high bit error, low security and slow transmission efficiency of traditional single encoding.

[0052] After the laser pulse signal with a central wavelength of 1550nm arrives at the receiving end from the transmitting end, the receiving end detects two sets of orthogonal normalization bases, which are the orthogonal normalization bases |+〉 and |-〉 with phase differences of 0 and π formed by the optical signal a and the optical signal b after passing through the first phase encoding module and the second phase encoding module, and the orthogonal normalization bases |1〉 and |0〉 generated by the optical signal c after passing through the time encoding module due to different time domains. At the same time, since the time of the optical signal intensity modulation modules of the two sets of orthogonal normalization bases is separated in the time domain, the interference of optical signals arriving at the detector from other branches can be excluded by using the time window, thereby forming two sets of orthogonal normalization bases with phases of 0 and π and before and after in time, and realizing the completion of the BB84 protocol by time-phase encoding. At the same time, since the two optical signals encoded in phase and the two optical signals encoded in time all pass through the same common optical fiber channel, even if the optical fiber length is very long, the phase and polarization of the optical signal will be offset, but this effect is the same for the four optical signals and can offset each other. Therefore, this method is suitable for long-distance (long distance between the transmitter and the receiver) quantum key distribution systems.

[0053] The optical signal is composed of a pair of photons. The coding state corresponding to the optical signal a3 and the optical signal a4 is |+〉, the coding state corresponding to the optical signal b3 and the optical signal b4 is |-〉, the coding state corresponding to the optical signal c1 and the optical signal c3 is |1〉, and the coding state corresponding to the optical signal c1 and the optical signal c4 is |0〉. The first photon in each pair is a reference photon. In ultra-long distance transmission, the optical fiber has the same effect on the phase of the two photons. The receiving end compares the two photons to offset the effect of the optical fiber on the phase and improve the anti-interference ability. Since the optical signal c1 is used, it can be accurately judged whether it is the optical signal c3 or the optical signal c4 according to the time delay of the second photon arriving at the receiving end, thereby reducing the bit error rate. The present invention does not use polarization encoding, so there is no need to consider the effect of the optical fiber on polarization, thereby increasing the transmission distance.

[0054] Since a 1×3 optical switch is used and two phase encoding modules are connected, during operation, the two phase encoding modules work stably in their respective states and output optical signals a3 and a4 (corresponding encoding states |+〉) and optical signals b3 and b4 (corresponding encoding states |-〉), respectively. High-speed phase encoding can be completed without switching the working states of the phase encoding modules, thereby improving the speed of phase encoding. Precisely because there is no need to quickly switch the working states of the phase encoding modules, the control circuit of the phase encoding module is very simple, reducing costs and research and development difficulties, and also avoiding high-frequency interference caused by the control circuit of the phase encoding module.

[0055] The present invention has been described exemplarily. Obviously, the specific implementation of the present invention is not limited to the above-mentioned method. As long as various non-substantial improvements are made using the method concept and technical solution of the present invention, or the concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. A time-phase hybrid encoding device, characterized in that: The device comprises: A 1×3 optical switch, two phase encoding modules and a time encoding module, The 1×3 optical switch divides the input laser pulse into three optical signals, which are input into two phase encoding modules and one time encoding module respectively; The two phase encoding modules use the electro-optical effect of the phase modulator to generate a set of orthogonal normalized bases with a phase difference of 0 and a phase difference of π respectively; the time encoding module generates an orthogonal basis state through time domain differences.

2. The time-phase hybrid encoding device according to claim 1, characterized in that: The two phase encoding modules are respectively a first phase encoding module and a second phase encoding module. The first phase encoding module is composed of a first beam splitter, a first phase modulator and a second phase modulator. The first beam splitter is connected to the first phase modulator and the second phase modulator respectively through optical fibers.

3. The time-phase hybrid encoding device as claimed in claim 2, characterized in that: The optical signal output from the first output channel of the 1×3 optical switch enters the first beam splitter, and is equally divided into two optical signals in the first beam splitter. The two optical signals enter the first phase modulator and the second phase modulator respectively. The voltages of the first phase modulator and the second phase modulator are adjusted so that the first phase modulator and the second phase modulator generate a set of optical signals with a phase difference of 0, corresponding to the encoding state |+〉.

4. The time-phase hybrid encoding device as claimed in claim 2, characterized in that: The second phase encoding module is composed of a second beam splitter, a third phase modulator and a fourth phase modulator. The second beam splitter is connected to the third phase modulator and the fourth phase modulator respectively through optical fibers.

5. The time-phase hybrid encoding device as claimed in claim 4, characterized in that: The optical signal output from the second output channel of the 1×3 optical switch enters the second beam splitter, where it is equally divided into two optical signals. The two optical signals enter the third phase modulator and the fourth phase modulator respectively. The voltages of the third phase modulator and the fourth phase modulator are adjusted so that the third phase modulator and the fourth phase modulator generate a set of optical signals with a phase difference of π, corresponding to the encoding state |-〉.

6. The time-phase hybrid encoding device according to claim 1, characterized in that: The time encoding module includes a third beam splitter, a total internal reflection type Y-branch optical switch and a delay line. The third beam splitter is connected to the total internal reflection type Y-branch optical switch and the output optical fiber respectively. The total internal reflection type Y-branch optical switch is connected to the delay line and the output optical fiber respectively. The delay line is connected to the output optical fiber.

7. The time-phase hybrid encoding device as claimed in claim 7, characterized in that: The path between the total internal reflection type Y-branch optical switch and the output optical fiber is the first path, and the path between the internal reflection type Y-branch optical switch and the output optical fiber through the delay line is the second path; the optical signal output from the third output channel of the 1×3 optical switch enters the third beam splitter, and is equally divided into the first optical signal in the third beam splitter, the first optical signal is transmitted to the output end through a single optical fiber, the second optical signal enters the total internal reflection type Y-branch optical switch, an external voltage is applied to the total internal reflection type Y-branch optical switch, and under the action of different external voltages, the second optical signal enters the first path and the second path respectively, the optical path of the first path and the second path are different, the third optical signal is output from the first path, and the fourth optical signal is output from the second path, the encoding state corresponding to the first optical signal and the third optical signal is |1〉, and the encoding state corresponding to the first optical signal and the fourth optical signal is |0〉.

8. The time-phase hybrid encoding device according to claim 1, characterized in that: The central wavelength of the laser pulse is 1550nm.

9. A quantum key distribution system, characterized in that: The system comprises: The transmitter and the receiver communicate with each other through a quantum channel; At the transmitting end, the time phase hybrid encoding device according to any one of claims 1 to 7 is used to randomly modulate the quantum light signal, the modulated quantum light is decayed into a single photon, and the single photon is sent to the receiving end through a quantum channel; The receiving end randomly demodulates the received single photons and detects the demodulated quantum light.

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