WO2020151546A1

WO2020151546A1 - Sending end for decoy state encoding and polarization encoding, encoding method and quantum key distribution system

Info

Publication number: WO2020151546A1
Application number: PCT/CN2020/072213
Authority: WO
Inventors: 汤艳琳; 李东东
Original assignee: 科大国盾量子技术股份有限公司; 上海国盾量子信息技术有限公司
Priority date: 2019-01-23
Filing date: 2020-01-15
Publication date: 2020-07-30
Also published as: CN111478767A; CN111478767B

Abstract

The present application relates to a sending end for decoy state encoding and polarization encoding, an encoding method and a quantum key distribution system. In the present invention, the sending end may comprise a light source, an encoding module, and a light path fold-back module. The encoding module comprises a phase modulation unit, and is configured to perform first phase modulation on signal light to be encoded by means of the phase modulation unit and perform decoy state encoding on the signal light on the basis of the phase modulation; the light path fold-back module is configured to receive the signal light subjected to the decoy state encoding and fold back same to the encoding module; the encoding module is further configured to perform second phase modulation on the folded-back signal light subjected to the decoy state encoding by means of the phase modulation unit, and perform polarization encoding on the folded-back signal light subjected to the decoy state encoding on the basis of the phase modulation. Therefore, by reusing the encoding module by means of the light path fold-back module, it is possible to configure the encoding module to implement both decoy state encoding and polarization encoding, thereby greatly simplifying the structure of the sending end.

Description

Transmitting terminal, coding method and quantum key distribution system for decoy state coding and polarization coding

This application requires that a Chinese patent application be submitted to the Chinese Patent Office on January 23, 2019, the application number is 201910064043.9, and the invention title is "Sender, encoding method and quantum key distribution system for decoy state encoding and polarization encoding" Priority, the entire content of which is incorporated in this application by reference.

Technical field

The invention relates to the field of quantum secure communication, in particular to a transmitting end, an encoding method and a quantum key distribution (QKD) system for decoy state encoding and polarization encoding.

Background technique

The fundamental difference between quantum key distribution (QKD) and the classical key system is that it uses a single photon or entangled photon pair as the carrier of the key. The basic principles of quantum mechanics ensure that the process cannot be eavesdropped or deciphered, thereby providing A more secure key system is created.

The transmitting end of the QKD system often uses laser as the light source, but the multi-photon component of the laser light source will be attacked by the number of photons. In this regard, the light source of the QKD system often uses a decoy state modulation scheme to solve this problem. The decoy state modulation scheme needs to modulate different light intensities randomly, and the commonly used method is realized by an intensity modulator.

The transmitting end of the QKD system also needs to perform quantum state BB84 encoding on the decoy state light source, such as the commonly used polarization encoding. The main principle of polarization state encoding based on phase modulation is:

Among them, by dividing the optical pulse into two optical pulse components (|H> and |V>) whose polarization directions are perpendicular to each other, the phase difference of the two optical pulse components is adjusted

By modulating a specific phase difference between two optical pulse components

A specific polarization state can be achieved on the optical pulse formed by combining two optical pulse components, for example, when the phase difference

When the values are respectively 0, π/2, π, and 3π/2, the P, R, N, and L polarization states can be obtained by combining beams. In the prior art, a phase modulator is usually used to adjust the phase difference.

In short, the sending end of the QKD system not only needs to perform decoy state coding, but also needs to perform BB84 coding such as polarization coding. In the prior art, the optical paths of the two encodings are independent of each other, and each requires a modulator to be implemented. Due to the high cost of the modulator, the system cost is high, and it is not easy to achieve low cost and miniaturization.

Summary of the invention

In view of the above-mentioned problems in the prior art, the first aspect of the present invention proposes a transmitter for decoy state encoding and polarization encoding, which includes a light source 1, an encoding module 2 and an optical path reentry module 3. Wherein, the light source 1 is used to provide signal light to be encoded; the encoding module 2 includes

phase modulation units

211, 221, 231, and is configured to pass the

phase modulation units

211, 221, 231 to the signal light to be encoded The signal light is subjected to the first phase modulation, and based on the first phase modulation, the signal light to be encoded is decoy-state-encoded; the optical path return module 3 is configured to receive the decoy-state-encoded The signal light is turned back to the encoding module 2; and the encoding module 2 is also configured to perform the first step on the turned-back decoy state-encoded signal light through the

phase modulation units

211, 221, and 231 A secondary phase modulation is performed, and based on the second phase modulation, the signal light that has been returned and decoy-state encoded is polarization-encoded.

Preferably, the optical path folding module 3 may include a circulator 31 or reflective structures 32 and 33.

Preferably, the encoding module 2 may further include a polarization beam splitting unit 212 and a beam splitting unit 213. The polarization beam splitting unit 212 and the beam splitting unit 213 pass through a first optical path and a second optical path having the same optical length. The optical paths are connected to form an equal-arm MZ interferometer; and the phase modulation unit 211 is provided on the first optical path and/or the second optical path.

Preferably, the polarization beam splitting unit 212 receives the signal light to be coded, and outputs the decoy state-encoded and polarization-encoded signal light; and, the beam splitting unit 213 outputs the decoy state-encoded signal light Signal light, and receiving the signal light encoded by the decoy state that has been turned back.

Preferably, the transmitting end of the present invention may further include a phase detection feedback module to compensate for phase drift in the equal-arm MZ interferometer; and/or, the optical path reentry module 3 may be a circulator 31; and/or The polarization splitting unit 212 may be a polarization beam splitter; and/or, the beam splitting unit 213 may be a beam splitter.

Preferably, the encoding module 2 may further include an optical transmission unit 224, a polarization beam splitting unit 222, and an analyzer 223. The polarization beam splitting unit 222 is arranged in the optical loop to form a Sagnac interferometer, so The phase modulation unit 221 is provided in the optical loop.

Preferably, the encoding module 2 may further include an optical transmission unit 234, a polarization beam splitting unit 232, and an analyzer 233. The polarization beam splitting unit 232 is connected to the first optical path and the second optical path with the same optical path. A Faraday rotating mirror 235 and a second Faraday rotating mirror 236 form a Michelson interferometer, and the phase modulation unit 231 is arranged in the first optical path and/or the second optical path.

Preferably, the

optical transmission unit

224, 234 may be configured to receive the signal light to be encoded and make it propagate toward the

polarization splitting unit

222, 232, and to receive the decoy state coded and output from the

polarization splitting unit

222, 232. The polarization-encoded signal light is output outward.

Preferably, the

analyzer units

223 and 233 may be arranged between the polarization

beam splitting units

222 and 232 and the optical path folding module 3.

Preferably, the

polarization splitting units

222, 232 may be polarization beam splitters, and the

analyzers

223, 233 may be polarization beam splitters, polarizing plates, or a combination of polarizing plates and wave plates, and forming an angle of 45 degrees or -45 degree angle setting; and/or, the signal light to be encoded is one of 45° linearly polarized light, -45° linearly polarized light, left-handed circularly polarized light, and right-handed circularly polarized light.

Preferably, the

optical transmission unit

224, 234 may include a circulator; and/or, the optical path folding module 3 may include a mirror or a Faraday rotating mirror 32, 33.

The second aspect of the present invention relates to a quantum key distribution system, which includes the sender of the present invention.

The third aspect of the present invention relates to an encoding method for simultaneous decoy state encoding and polarization encoding, which includes the following steps: a decoy state encoding step, wherein the signal light to be encoded is passed through a phase modulation unit to perform first Sub-phase modulation, and intensity modulation of the signal light to be encoded based on the first phase modulation, thereby realizing decoy state encoding; a light path reentry step, wherein the decoy state-encoded signal light is reentered; And a polarization encoding step, wherein the signal light that has been folded and decoy-encoded passes through the phase modulation unit again to perform a second phase modulation on it, and the folded signal light is subjected to the second phase modulation based on the second phase modulation. The decoy state-encoded signal light undergoes polarization state modulation, thereby realizing polarization encoding.

Preferably, the decoy state encoding step further includes the following steps: dividing the signal light to be encoded equally into a first signal light component and a second signal light component whose linear polarization directions are perpendicular to each other; At least one of the first and second signal light components performs the first phase modulation to form a first phase difference θ ₁ therebetween; and makes the first phase difference θ ₁ The first and second signal light components interfere with each other.

Preferably, the polarization encoding step further includes the step of: dividing the signal light that has been turned back and the decoy state coded into a first decoy state-encoded signal light component and a second decoy state-encoded signal light component. Use the phase modulation unit to perform the second phase modulation on at least one of the first and second decoy state-encoded signal light components to form a second phase difference θ ₂ therebetween; and the signal light component having the second phase difference θ ₂ of the first and second encoded by the decoy-state interfering effect.

Preferably, in the encoding method of the present invention, a polarization beam splitting unit and a beam splitting unit can also be used to construct an equal-arm MZ interferometer, and the phase modulation unit is set in the equal-arm MZ interferometer, wherein, The interference effect in the decoy state encoding step occurs at the beam splitting unit, and the interference effect in the polarization encoding step occurs at the polarization beam splitting unit; alternatively, polarization beam splitting can also be used The unit and the optical loop construct a Sagnac interferometer, and the phase modulation unit is set in the Sagnac interferometer, wherein the interference effect and the polarization in the decoy state encoding step The interference effects in the encoding step all occur at the polarization beam splitting unit, and the decoy state encoding step further includes the step of using an analyzer to analyze the result of the interference effect; or, further A polarization beam splitting unit and two Faraday rotating mirrors can be used to construct a Michelson interferometer, and the phase modulation unit can be arranged in the Michelson interferometer, wherein the interference effect in the decoy state encoding step and The interference effects in the polarization encoding step all occur at the polarization beam splitting unit, and the decoy state encoding step further includes the step of using a polarization analyzer to analyze the result of the interference effect.

Preferably, in the decoy state encoding step, the intensity of the signal light encoded by the decoy state is (1+cosθ ₁ )/2 of the intensity of the signal light to be encoded; and/or, in the In the polarization encoding step, the polarization state of the polarization-encoded signal light is

Preferably, the first phase difference θ ₁ may be set to 180°.

Description of the drawings

In order to explain the embodiments of the present invention or the technical solutions of the prior art more clearly, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the drawings in the following description are merely For some of the embodiments of the present invention, for those of ordinary skill in the art, other drawings may be obtained based on these drawings without creative work.

Fig. 1 shows a first exemplary embodiment of the present invention for simultaneously realizing decoy state coding and polarization coding at the transmitting end and coding method;

FIG. 2 shows the timing relationship between the signal light pulse and the electrical pulse used for the phase modulation unit in the embodiment shown in FIG. 1 during two phase modulations;

Figure 3 shows a further embodiment of the embodiment shown in Figure 1;

Fig. 4 shows a second exemplary embodiment of the present invention for simultaneously realizing decoy state coding and polarization coding at the transmitting end and coding method;

FIG. 5 shows the timing relationship between the signal light pulse and the electrical pulse used in the phase modulation unit in the embodiment shown in FIG. 4 during two phase modulation;

Fig. 6 shows a third exemplary embodiment of the present invention for simultaneously realizing decoy state coding and polarization coding at the transmitting end and coding method; and

Fig. 7 and Fig. 8 respectively show two exemplary embodiments of the optical path reentry module.

detailed description

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided by way of example in order to fully convey the spirit of the present invention to those skilled in the art to which the present invention belongs. Therefore, the present invention is not limited to the embodiments disclosed herein.

In the method of the present invention, the decoy state encoding and polarization encoding of the signal light will be realized by the same phase modulation unit. The encoding process is basically as follows: first, the signal light is passed through the phase modulation unit to perform the first phase modulation on it, and The intensity modulation of the signal light is realized based on the first phase modulation, thereby realizing the decoy state encoding; then, the decoy state-encoded signal light is fold back through the phase modulation unit to perform the second phase modulation on it, and based on the first The secondary phase modulation realizes the modulation of the polarization state of the signal light, thereby realizing polarization encoding. That is, the signal light to be encoded is made to reciprocate in the same optical path, and the decoy state encoding and polarization encoding of the signal light are respectively realized based on the two successive phase modulations of the signal light by the same phase modulation unit.

Specifically, first, the signal light to be encoded is equally divided into a first signal light component and a second signal light component whose linear polarization directions are perpendicular to each other; at least one of the first and second signal light components is phased by the phase modulation unit Modulate to form a first phase difference between the two; and make the two components with the first phase difference interact (for example, interference) to form an optical signal whose intensity is related to the first phase difference, that is, encoded by a decoy state Signal light.

As an example, the above process can be realized by means of an equal-arm MZ interferometer composed of a polarization beam splitter and a beam splitter and a phase modulator set in the interferometer; or by means of a Sager composed of a polarization beam splitter and an optical loop Naike interferometer, phase modulator and analyzer in the interferometer; or by means of a Michelson interferometer composed of a polarization beam splitter and two Faraday rotators, and a phase modulation in the interferometer Detector, and analyzer implementation. For example, the signal light encoded by the decoy state can satisfy the relationship of (1+cosθ ₁ )/2 in intensity with the signal light to be encoded. As a special example, when θ ₁ =180 degrees, the intensity of the signal light encoded by the decoy state will be 0, and at this time, vacuum state encoding is implemented on the signal light.

Subsequently, the signal light encoded by the decoy state is turned back. As an example, a circulator or a reflecting unit can be used to realize the folding back of the signal light.

Then, the returned decoy state-encoded signal light is equally divided into the first decoy state-encoded signal light component and the second decoy state-encoded signal light component; the same phase modulation unit is used for the first and second decoy states. At least one of the state-encoded signal light components is phase modulated to form a second phase difference θ ₂ therebetween, and two components having the second phase difference θ ₂ interact to form a polarization direction and a second phase The optical signal related to the difference θ ₂ is the polarization-encoded signal light. Here, the polarization encoding process is realized when the signal light passes through the above-mentioned optical path for decoy state encoding in the reverse direction. Thus, by making the signal light refold, with the same optical path structure, the decoy state encoding and polarization encoding of the signal light are realized based on two phase modulations.

Those skilled in the art can easily understand that in the present invention, the first phase difference θ ₁ and the second phase difference θ ₂ can have any suitable values to provide the required decoy state encoding and polarization state encoding.

In order to further illustrate the principle of the encoding method of the present invention, FIG. 1 shows an exemplary implementation of the transmitting end of the present invention that realizes decoy state and polarization encoding at the same time.

As shown in the figure, the transmitting end of the present invention may include a light source 1, an encoding module 2 and an optical path reentry module 3.

In the present invention, the light source 1 is used to output signal light to be encoded, which may be in the form of a laser to output laser pulse signals, for example. The encoding module 2 receives the signal light output by the light source 1 and uses the phase modulation unit 211 to perform the first phase modulation on it, and performs intensity modulation on the signal light based on the first phase modulation to achieve decoy state encoding. The signal light encoded in the decoy state is output by the encoding module 2 and then enters the optical path reentry module 3. In the optical path turnback module 3, the propagation direction of the signal light is turned back, thereby entering the encoding module 2 again. The encoding module 2 receives the decoy state-encoded signal light turned back by the optical path turning module 3 and uses the phase modulation unit 211 to perform a second phase modulation on it, and modulates the polarization state of the signal light based on the second phase modulation to achieve Polarization coding. Finally, the encoding module 2 outputs the decoy state and polarization encoded signal light.

In this embodiment, as shown in FIG. 1, the encoding module 2 may include a phase modulation unit 211, a polarization beam splitting unit 212 and a beam splitting unit 213.

The polarization splitting unit 212 receives the signal light pulse output from the light source 1 and divides it equally into first and second signal light pulse components of linearly polarized light whose polarization directions are perpendicular to each other. As an example, the polarization beam splitting unit 212 may be in the form of a polarization beam splitter (PBS), and as shown in FIG. 1 specifically, the signal light pulse enters through the fourth port of the polarization beam splitting unit 212, and the first beam splitter And the second signal light pulse component are respectively output through its first port and second port.

The first and second signal light pulse components are output by the polarization beam splitting unit 212 and then propagate toward the two input ends of the beam splitting unit 213 along the first optical path and the second optical path, respectively. The first optical path and the second optical path are set to have the same optical length, so that the first and second signal light pulse components can reach the beam splitting unit 213 at the same time. Therefore, the polarization splitting unit 212, the splitting unit 213, and the first and second optical paths connecting the two substantially form an equal-arm MZ interferometer structure. Here, the beam splitting unit 213 may be in the form of a beam splitter (BS).

The phase modulation unit 211 is arranged on one of the first optical path and the second optical path, and is used to phase-modulate the passing optical pulse components, thereby forming a preset phase difference between the two optical pulse components. For example, as shown in FIG. 1, the phase modulation unit 211 is arranged on the first optical path, and is used to perform phase modulation with a magnitude of θ ₁ (that is, the first phase modulation) on the first signal light pulse component transmitted along the first optical path, Thus, a phase difference θ ₁ is formed between the first and second signal light pulse components.

The first and second signal light pulse components with a phase difference θ ₁ will reach the beam splitting unit 213 at the same time, thereby causing interference; at this time, the interference result (ie, the first interference light pulse) output by the beam splitting unit 213 is The intensity will be related to the first phase modulation amount θ ₁ provided by the phase modulation unit 211. For example, the intensity of the first interference light pulse may be (1+cosθ ₁ )/2 of the signal light pulse intensity. Obviously, in the encoding module 2, by controlling the first phase modulation amount θ ₁ can adjust the intensity of the first interference output light pulse, i.e., to achieve modulation of the intensity value of the optical pulse signal, whereby the signal light can be achieved Deception encoding process.

Subsequently, the decoy-encoded light pulse (ie, the first interfering light pulse) output by the encoding module 2 enters the optical path reentry module 3, and the propagation direction is folded back under the action of the encoding module 2 and returns to the encoding module 2. In the present invention, the optical path folding module 3 can be realized in the form of a circulator, or in the form of a reflective structure, such as a reflector or a Faraday rotating mirror. For example, as shown in FIG. 1, the optical path return module 3 may include a circulator 31.

When the decoy-encoded light pulse is returned to the beam splitting unit 213, it is divided into the first and second decoy-encoded light pulse components by the beam splitting unit 213, and is respectively directed toward the polarization beam splitting unit 212 along the first and second light paths. spread.

The phase modulation unit 211 further performs phase modulation (ie, the second phase modulation) on the decoy-encoded light pulse component, thereby forming a phase difference θ ₂ between the two components. For example, as shown in FIG. 1, the phase modulation unit 211 provided on the first optical path further modulates the first component of the decoy state-encoded optical pulse with a magnitude of θ ₂ so that the first sum of the decoy state-encoded optical pulse is A phase difference θ ₂ is formed between the second components.

The first and second components of the decoy-encoded light pulse with a phase difference θ ₂ return to the polarization beam splitting unit 212 at the same time, thereby causing interference; at this time, those skilled in the art can easily understand that the output of the polarization beam splitting unit 212 The polarization direction of the interference result (that is, the second interference light pulse, which is output through the third port in FIG. 1, for example) will be related to the second phase modulation amount θ ₂ provided by the phase modulation unit 211, for example, the second interference light pulse The polarization state can be

Obviously, in the encoding module 2, by controlling the second phase modulation amount [theta] ₂ can be adjusted second interference light pulse output from the polarization state thereof, i.e., to achieve modulation of the polarization state of the optical pulse signal, whereby the signal light can be achieved The decoy state coding process. At this time, the decoy state encoding and polarization encoding have been implemented on the signal light pulses output from the encoding module 2 (ie, the polarization beam splitting unit 212).

FIG. 2 shows the timing relationship between the signal light pulse and the electrical pulse used in the phase modulation unit 211 in the structure of the transmitting end shown in FIG. Realized decoy state and polarization encoding process.

In the transmitting end structure shown in FIG. 1-2, a polarization splitting unit 212 (for example, a polarization beam splitter), a splitting unit 213 (for example, a beam splitter), and a phase modulation unit 211 constitute an equal arm with a phase modulation function. The MZ interferometer first uses the polarization beam splitter 212 as the input terminal and the beam splitter 213 as the output terminal, and realizes the intensity modulation of the signal light pulse based on the first phase modulation provided by the phase modulation unit 211, thereby completing the decoy state encoding ; Then use the optical path reentry module to make the decoy state-encoded signal light pulses return to the above-mentioned equal-arm MZ interferometer again, with the beam splitter 213 as the input end and the polarization beam splitter 212 as the output end, based on the phase modulation unit 211 to provide The second phase modulation realizes the modulation of the polarization state of the signal light pulse, thereby completing the polarization state encoding, and finally completing the decoy state and polarization state encoding of the signal light pulse. With the help of this special transmitting end structure and encoding method, the multiplexing of the encoding module 2 is realized through the optical path reentry module, so that the same optical path structure (even the same phase modulation unit 211) is used to realize the decoy state and polarization encoding of the same signal light pulse Compared with the optical paths of the two encodings in the prior art that are independent of each other and each requires a modulator to be implemented, the present invention can effectively reduce the system complexity of the transmitting end, reduce the cost of the QKD transmitting end, and improve system integration and stability. Moreover, under this structure, the requirements for the phase-modulated electrical signal are relatively low, and it is only necessary to distinguish the two arrival times of the pulse in one cycle (that is, as long as the phase modulation is performed twice).

Furthermore, the inventor noticed that since the equal-arm MZ interferometer structure is adopted in the transmitting end structure of FIG. 1, the signal light pulse may shift in phase when propagating therein. Therefore, preferably, the phase detection feedback module can also be used in the transmitting end structure of FIG. 1.

FIG. 3 shows an example of a phase detection feedback module, which may include a light splitting unit, a phase detection unit, a phase feedback algorithm unit, a phase shifter driving unit, and a phase shifter. The optical splitting unit is configured to split a part of the optical pulse to the phase detection unit; the phase detection unit is used to detect the phase shift; the phase feedback algorithm unit is used to calculate the phase value that needs feedback compensation according to the detected phase shift; The sensor is set in the equal arm MZ interferometer, and the signal light pulse is phase compensated under the driving signal provided by the phase shifter driving unit. Since the functions of each unit of the phase detection feedback module have been explained, those skilled in the art can understand and implement the corresponding phase detection, calculation, and feedback processes, so this article will not repeat them.

Fig. 4 schematically shows another embodiment of the transmitting end structure of the present invention for simultaneously realizing the decoy state and polarization encoding.

As shown in the figure, the transmitting end may include a light source 1, an encoding module 2 and an optical path reentry module 3.

In this embodiment, the light source 1 preferably can output 45° linearly polarized light (which has

The polarization state), or -45° linearly polarized light, or left-handed circularly polarized light, or right-handed circularly polarized light, used as signal light.

The encoding module 2 may include an optical transmission unit 224, a phase modulation unit 221, a polarization beam splitting unit 222, and an analyzer 223.

Wherein, the optical transmission unit 224 is used to receive the signal light pulse output by the light source 1 to perform the subsequent encoding process, and to receive and output the signal light pulse in the decoy state and polarization encoding. In this embodiment, the optical transmission unit 224 may include three ports 1-3, and is configured such that the light entering from the first port 1 can exit from the second port 2, and the light entering from the second port 2 can pass from The third port 3 leaves. For example, as shown in FIG. 4, the signal light pulse output by the light source 1 propagates toward the polarization beam splitting unit 222 via the first port 1 and the second port 2 of the optical transmission unit 224, and the decoy state and polarization code output from the polarization beam splitting unit 222 The signal light pulses will be output through the second port 2 and the third port of the optical transmission unit 224. As an example, the optical transmission unit 224 may be in the form of a circulator.

The polarization splitting unit 222 is arranged in the optical loop to form a Sagnac interferometer structure, which receives the signal light pulse and divides it equally into first and second signal light pulses of linearly polarized light whose polarization directions are perpendicular to each other Weight. As an example, the polarization beam splitting unit 222 may be in the form of a polarization beam splitter (PBS). As shown in FIG. 4, the signal light pulse can be input through the fourth port of the polarization beam splitting unit 222, and the first and second signal light pulse components obtained by splitting are output through the first port and the second port respectively.

The first and second signal light pulse components respectively enter the optical loop and propagate along the loop in opposite directions. The phase modulation unit 221 is arranged on the loop, and is used for phase modulation (that is, the first phase modulation) on the passing optical pulse components, thereby forming a preset phase difference between the two optical pulse components. Preferably, the phase modulator 221 may be configured to make the two signal light pulse components propagating backward in the loop arrive at the phase modulation unit 221 at different times. Such as shown in FIG. 4, the phase modulation unit 221 may be a size of a phase modulation θ ₁ of the first pulse signal light component transmitted clockwise, thereby forming a phase difference θ between the first and second light pulse components of signal ₁ .

The first and second signal light pulse components with a phase difference θ ₁ will return to the polarization beam splitting unit 222 at the same time, thereby causing interference; at this time, the interference result output by the polarization beam splitting unit 222 (that is, the first interference light pulse , For example, the polarization direction of the output via the third port in FIG. 4 will be related to the first phase modulation amount θ ₁ provided by the phase modulation unit 221, for example, the polarization state of the first interference light pulse may be

The analyzer unit 223 receives the first interference light pulse, which is set to make the intensity of the light pulse output after the analyzer be (1+cosθ ₁ )/2 of the signal light pulse intensity. Obviously, in the encoding module 2, the same intensity value can be adjusted by controlling the light pulse first phase modulation amount θ _1, whereby the coding process can be realized decoy-state of signal light. As an example, as shown in FIG. 4, the analyzer 223 may be a polarization beam splitter, and when the polarization beam splitter 222 is a polarization beam splitter, the polarization beam splitter 223 may be set to be opposite to the polarization beam splitter 222. The angle is 45 degrees (or -45 degrees), so that the intensity of the light pulse output after the analyzer is (1+cosθ ₁ )/2 of the signal light pulse intensity. Alternatively, the analyzer 223 may be a polarization plate, or a combination of a polarization plate and a wave plate, to replace the polarization beam splitter.

Subsequently, the decoy-encoded light pulse output by the encoding module 2 enters the optical path reentry module 3, and the propagation direction is folded back under the action of the encoding module 2, and returns to the encoding module 2. Similarly, the optical path folding module 3 can be realized in the form of a circulator or a reflective structure (such as a reflector or a Faraday rotator). For example, as shown in FIG. 4, the optical path folding module 3 may include a mirror 32.

When returning to the encoding module 2 again, the decoy-encoded light pulse is returned to the polarization beam splitting unit 222 via the analyzer 223, where it is divided into the first and second decoy-encoded light pulse components, and is reversed along the loop spread.

The phase modulation unit 221 further performs phase modulation (ie, the second phase modulation) on the decoy state encoded light pulse component, thereby forming a phase difference θ ₂ between the two components. For example, as shown in FIG. 4, the phase modulation unit 221 further modulates the first component propagating clockwise with a magnitude of θ _{2 to} form a phase difference θ between the first and second components of the decoy-encoded optical pulse. ₂ .

The first and second components of the decoy state-encoded light pulse with a phase difference θ ₂ return to the polarization beam splitting unit 222 at the same time, thereby causing interference; similarly, the interference result output by the polarization beam splitting unit 222 (ie, the second interference For example, the polarization direction of the optical pulse, which is output via the 4th port in FIG. 4, will be related to the second phase modulation amount provided by the phase modulation unit 221. For example, the polarization state of the second interference optical pulse may be

Obviously, in the encoding module 2, the polarization state of the second interference light pulse output can be adjusted by controlling the amount of the second phase modulation, that is, the polarization state of the signal light pulse can be modulated, and thus the signal light can be deceived. State coding process. At this time, the decoy state encoding and polarization encoding have been implemented on the signal light pulses externally output by the polarization beam splitting unit 222. The second interference light pulse output via the polarization beam splitting unit 222 is output externally through the second and third ports of the optical transmission unit 224, and finally a signal light pulse that has been decoyed and polarization-encoded is provided.

FIG. 5 shows the timing relationship between the signal light pulse and the electrical pulse used in the phase modulation unit 221 in the structure of the transmitting end shown in FIG. Realized decoy state and polarization encoding process.

In the transmitting end structure and encoding method shown in Figure 4-5, since the Sagnac interferometer with self-stabilization function is used, there is no phase drift in it, so it can still achieve high performance without the phase detection feedback module. Coding efficiency.

FIG. 6 schematically shows another embodiment of the transmitting end structure of the present invention that simultaneously realizes the decoy state and polarization encoding. The transmitting end also includes a light source 1, an encoding module 2 and an optical path reentry module 3, and the light source 1 and The optical path turnback module 3 is similar to the embodiment shown in FIG. 4, so it will not be repeated.

Compared with the embodiment shown in FIG. 4, the encoding module 2 shown in FIG. 6 also includes an optical transmission unit 234, a phase modulation unit 321, a polarization splitting unit 232, and an analyzer 233, but the difference is that it also includes a second A Faraday rotator mirror 235 and a second Faraday rotator mirror 236, wherein: the first and second Faraday rotator mirrors 235, 236 are respectively connected to the first and second polarizing beam splitting units 232 through first and second optical paths having the same optical path. 2 ports, which constitutes the Michelson interferometer structure instead of the Sagnac interferometer. In addition, the optical transmission unit 234, the phase modulation unit 321, the polarization splitting unit 232, and the analyzer 233 are similar to the embodiment shown in FIG. 4, and therefore will not be described again.

As shown in FIG. 6, the signal light pulse output by the light source 1 propagates toward the polarization splitting unit 232 via the first port 1 and the second port 2 of the optical transmission unit 224; the polarization splitting unit 232 receives the signal light pulse via its fourth port. , And the signal light pulse is equally divided into first and second signal light pulse components whose linear polarization directions are perpendicular to each other to output to the first and second optical paths through the first and second ports thereof, respectively.

The phase modulation unit 231 is disposed on the first optical path to perform the first phase modulation on the first signal light pulse component, thereby forming a phase difference θ ₁ between the two signal light pulse components. First and second phase difference θ with the optical signal pulse component _1, respectively, after the first and second Faraday rotator mirror 235, and returns along the same route to the reflective polarization splitting unit 232, so as to interfere, via its action Terminal 3 port outputs the first interference light pulse, the first interference light pulse has

The polarization state. The analyzer 233 receives the first interference light pulse, and the intensity of the light pulse output by the analyzer is (1+cosθ ₁ )/2 of the signal light pulse intensity, so that the first phase modulation θ _{1 is used to} realize the signal light pulse Intensity modulation to achieve decoy state coding. Similarly, as shown in FIG. 6, the analyzer unit 233 may be a polarization beam splitter, and when the polarization beam splitter unit 232 is a polarization beam splitter, the polarization beam splitter 233 may be set relative to the polarization beam splitter 232. At a 45-degree angle (or -45-degree angle), so that the intensity of the light pulse output after the analyzer is (1+cosθ ₁ )/2 of the signal light pulse intensity. Alternatively, the analyzer 223 may be a polarization plate, or a combination of a polarization plate and a wave plate, to replace the polarization beam splitter.

Subsequently, the decoy-encoded light pulse output by the encoding module 2 enters the optical path reentry module 3, and the propagation direction is folded back under the action of the encoding module 2, and returns to the encoding module 2.

When returning to the encoding module 2 again, the decoy-encoded light pulse is returned to the polarization beam splitting unit 232 via the analyzer 233, where it is divided into the first and second decoy-encoded light pulse components, and passes through the first and second decoy-encoded light pulse components. The second port outputs to the first and second optical paths.

The phase modulation unit 231 performs phase modulation θ ₂ (that is, the second phase modulation) on the first component of the decoy-encoded light pulse, thereby forming a phase difference θ ₂ between the two components. The first and second components of the decoy state-encoded light pulse with a phase difference θ ₂ are also reflected and returned to the polarization beam splitting unit 232 at the same time and interfere, so that the polarization state is output from the fourth port of the polarization beam splitting unit 232 for

The second interference light pulse. Obviously, the polarization state of the second interference light pulse is related to the second phase modulation θ ₂ , that is, by controlling the second phase modulation amount θ ₂ , different output polarization states can be obtained at the second interference light pulse, thereby achieving Polarization encoding of signal light pulses. The second interference light pulse output through the polarization beam splitting unit 232 is output through the second and third ports of the optical transmission unit 234, and finally a signal light pulse that has been decoyed and polarization-encoded is provided.

In the transmitting end structure and encoding method shown in Fig. 6, the advantage is that the Michelson interferometer can be built with single-mode fiber devices, thereby eliminating the need for polarization-maintaining optical devices.

In addition, FIGS. 7 and 8 respectively show two implementations of the optical path folding back module 3 to further illustrate the working principle of the optical path folding back module 3.

The above description is not a limitation of the present invention, and the present invention is not limited to the above examples, and the above-mentioned various alternatives can be used in combination without any contradiction. Changes, modifications, additions or substitutions made by persons of ordinary skill in the art within the essential scope of the present invention shall also belong to the protection scope of the present invention, and the protection scope of the present invention shall be subject to the claims.

Claims

A transmitting terminal for decoy state encoding and polarization encoding, which includes a light source (1), an encoding module (2) and an optical path reentry module (3), wherein,

The light source (1) is used to provide signal light to be encoded;

The encoding module (2) includes a phase modulation unit (211, 221, 231), and is configured to perform the first phase modulation on the signal light to be encoded through the phase modulation unit (211, 221, 231) , And perform decoy state encoding on the signal light to be encoded based on the first phase modulation;

The optical path turning module (3) is configured to receive the signal light encoded in the decoy state and turn it back to the encoding module (2); and,

The encoding module (2) is also configured to perform a second phase modulation on the signal light that has been folded back and decoy-state encoded via the phase modulation unit (211, 221, 231), and based on the second phase modulation. The sub-phase modulation performs polarization encoding on the signal light that has been folded back and encoded in the decoy state.
The transmitting end according to claim 1, wherein the optical path return module (3) comprises a circulator (31) or a reflective structure (32, 33).
The transmitting end according to claim 1, wherein the encoding module (2) further comprises a polarization splitting unit (212) and a splitting unit (213), the polarization splitting unit (212) and the splitting unit (213) The units (213) are connected by a first optical path and a second optical path with the same optical path to form an equal-arm MZ interferometer; and the phase modulation unit (211) is arranged in the first optical path and/or the On the second light path.
The transmitting end according to claim 3, wherein the polarization beam splitting unit (212) receives the signal light to be encoded, and outputs the decoy state-encoded and polarization-encoded signal light; and, the splitting unit (212) The beam unit (213) outputs the signal light encoded by the decoy state, and receives the signal light encoded by the decoy state that is turned back.
The transmitter according to claim 3, further comprising a phase detection feedback module, which is used to compensate the phase drift in the equal arm MZ interferometer; and/or, the optical path reentry module (3) comprises a circulator (31 ); and/or, the polarization beam splitting unit (212) includes a polarization beam splitter; and/or, the beam splitting unit (213) includes a beam splitter.
The transmitter according to claim 1, wherein the encoding module (2) further comprises an optical transmission unit (224), a polarization splitting unit (222), and an analyzer (223), and the polarization splitting unit ( 222) is arranged in the optical loop to constitute a Sagnac interferometer, and the phase modulation unit (221) is arranged in the optical loop.
The transmitting end according to claim 1, wherein the encoding module (2) further comprises an optical transmission unit (234), a polarization splitting unit (232) and an analyzer (233), the polarization splitting unit ( 232) The first Faraday rotating mirror (235) and the second Faraday rotating mirror (236) are respectively connected through the first optical path and the second optical path having the same optical path to form a Michelson interferometer, and the phase modulation unit (231) is arranged In the first optical path and/or the second optical path.
The transmitting end according to claim 6 or 7, wherein the optical transmission unit (224, 234) is configured to receive the signal light to be encoded and make it propagate toward the polarization splitting unit (222, 232), and receive The decoy state-encoded and polarization-encoded signal light output by the polarization beam splitting unit (222, 232) is output outward.
The transmitting end according to claim 6 or 7, wherein the analyzer (223, 233) is arranged between the polarization beam splitting unit (222, 232) and the optical path folding module (3).
The transmitter according to claim 9, wherein the polarization splitting unit (222, 232) is a polarization beam splitter, and the analyzer (223, 233) is a polarization beam splitter, a polarizing plate or a polarizing plate and wave A combination of plates and arranged at an angle of 45 degrees or -45 degrees to each other; and/or, the signal light to be encoded is 45° linearly polarized light, -45° linearly polarized light, left-handed circularly polarized light and right-handed circularly polarized light A kind of light.
The transmitting end according to claim 6 or 7, wherein the optical transmission unit (224, 234) includes a circulator; and/or, the optical path return module (3) includes a mirror or a Faraday rotating mirror (32, 33) .
A quantum key distribution system, comprising the sender according to any one of claims 1-11.
An encoding method for simultaneous decoy state encoding and polarization encoding, which includes:

Decoy state encoding step: the signal light to be encoded is passed through a phase modulation unit to perform the first phase modulation on it, and the intensity of the signal light to be encoded is modulated based on the first phase modulation, thereby achieving deception State coding

Optical path reentry step: reentry the signal light encoded by the decoy state; and

Polarization encoding step: the signal light that has been reentered and decoy-state-encoded passes through the phase modulation unit again to perform a second phase modulation on it, and based on the second phase modulation, the signal light that has been reentered and decoy-state is The encoded signal light undergoes polarization state modulation, thereby realizing polarization encoding.
The encoding method according to claim 13, wherein the decoy state encoding step further comprises the step of dividing the signal light to be encoded into a first signal light component and a second signal light component whose linear polarization directions are perpendicular to each other. ; Use the phase modulation unit to perform the first phase modulation on at least one of the first and second signal light components to form a first phase difference θ 1 therebetween; The first and second signal light components with a phase difference θ 1 interfere.
The encoding method according to claim 14, wherein the polarization encoding step further comprises the step of dividing the signal light that has been folded back into the decoy state encoded signal light component and the second decoy state encoded signal light component. Two decoy state-encoded signal light components; using the phase modulation unit to perform the second phase modulation on at least one of the first and second decoy state-encoded signal light components to be between the two Forming a second phase difference θ 2 ; and causing the first and second decoy-encoded signal light components having the second phase difference θ 2 to interfere.
The encoding method according to claim 15, wherein:

The polarization beam splitting unit and the beam splitting unit are used to construct an equal arm MZ interferometer, and the phase modulation unit is set in the equal arm MZ interferometer, wherein the interference effect in the decoy state encoding step occurs in At the beam splitting unit, and the interference effect in the polarization encoding step occurs at the polarization beam splitting unit;

Alternatively, a polarization beam splitting unit and an optical loop are used to construct a Sagnac interferometer, and the phase modulation unit is arranged in the Sagnac interferometer, wherein the decoy state encoding step The interference effect and the interference effect in the polarization encoding step both occur at the polarization beam splitting unit, and the decoy state encoding step further includes using an analyzer to analyze the result of the interference effect A step of;

Alternatively, a polarization beam splitting unit and two Faraday rotating mirrors are used to construct a Michelson interferometer, and the phase modulation unit is set in the Michelson interferometer, wherein the interference effect in the decoy state encoding step And the interference effect in the polarization encoding step both occur at the polarization beam splitting unit, and the decoy state encoding step further includes the step of using an analyzer to analyze the result of the interference effect .
The encoding method according to any one of claims 13-16, wherein in the decoy state encoding step, the intensity of the signal light encoded by the decoy state is ( 1+cosθ 1 )/2; and/or, in the polarization encoding step, the polarization state of the polarization-encoded signal light is
The encoding method according to claim 17, wherein the first phase difference θ 1 =180°.