CN113708932A

CN113708932A - Phase encoding device for quantum key distribution and quantum key distribution system

Info

Publication number: CN113708932A
Application number: CN202111279253.3A
Authority: CN
Inventors: 齐若琳; 蒋荻; 席鹏
Original assignee: Zhejiang Quantum Technologies Co ltd
Current assignee: Zhejiang Quantum Technologies Co ltd
Priority date: 2021-11-01
Filing date: 2021-11-01
Publication date: 2021-11-26
Anticipated expiration: 2041-11-01
Also published as: CN113708932B

Abstract

The invention discloses a phase encoding device for quantum key distribution, which comprises a first laser L1, a second laser L2, an unequal arm polarization interferometer, a first optical path selection element OS1, a second optical path selection element OS2 and a quarter wave plate QWP. The first optical path selection element OS1 and the second optical path selection element OS2 form an equal-arm Mach-Zehnder interferometer, and the quarter-wave plate QWP is positioned on one arm of the equal-arm Mach-Zehnder interferometer. Compared with the prior art, the phase encoding device can prepare 4 phase encoding states required by the BB84 protocol without a phase modulator, and the polarizations of the two time modes are mutually perpendicular, so that the non-interference component is easily eliminated through polarization selection, and the energy utilization rate is improved. The cost and the complexity of the coding device can be reduced, and the safe code rate of the quantum key distribution system can be improved.

Description

Phase encoding device for quantum key distribution and quantum key distribution system

Technical Field

The invention relates to the technical field of quantum phase encoding, in particular to a phase encoding device for quantum key distribution and a quantum key distribution system.

Background

Quantum Key Distribution (QKD) can provide unconditionally secure key distribution for both parties in remote communications, and the information theoretical security of the QKD is guaranteed by the fundamental principles of quantum mechanics. At present, the quantum key distribution system of the BB84 protocol is mature and is already put into practical use. The encoding modes commonly used by the BB84 protocol are polarization encoding and phase encoding. Due to the birefringence effect of the optical fiber channel, the photon polarization state is easy to change randomly under the influence of the environment, and the polarization encoding mode is not stable enough, so that phase encoding is mostly adopted. As shown in fig. 1, the conventional phase encoding scheme employs an unequal arm mach-zehnder interferometer or a faraday michelson interferometer, in which a phase modulator is added to one arm to randomly modulate the phase difference between the long and short arms, thereby preparing 4 quantum states. The phase modulator has high insertion loss and high cost, and 4 voltages are needed to load the phase modulator for adjusting 4 phases, which increases the cost and complexity of the system.

In addition, in the scheme of adopting the conventional unequal arm interferometer, the light pulse at the transmitting end is divided into two sub-pulses after passing through the unequal arm interferometer, and the two sub-pulses are further changed into four pulses after reaching the receiving end and passing through the same interferometer, and the energy of each pulse is 1/4 of the total energy without considering the loss of devices. Wherein, the two pulses pass through the same optical path (the paths of the long arm + the short arm and the short arm + the long arm) and interfere with the second beam splitter of the interferometer at the receiving end, and the other two pulses respectively pass through the paths of the long arm + the long arm and the short arm + the short arm without participating in interference and are discarded. The energy of the interference peak is 1/2 as a total energy, i.e. the light energy utilization of the conventional scheme is 1/2, and the final security key rate is proportional thereto. To increase the security key rate of the system, the energy utilization of further interferometers needs to be increased.

Disclosure of Invention

Aiming at the defects in the prior art, the invention provides a phase coding device and a quantum key distribution system for quantum key distribution, which are used for solving the technical defects of high requirement, high cost, high complexity and the like of the quantum key distribution system on a phase modulator driving circuit in the prior art, improving the energy utilization rate of an interferometer by adopting polarization multiplexing and further improving the safe code yield rate of the system.

The invention provides a phase encoding device and a key distribution system for quantum key distribution, which are as follows:

the technical scheme of the invention is realized as follows:

a phase encoding device for quantum key distribution comprises a first laser L1, a second laser L2, an unequal arm polarization interferometer, a first optical path selection element OS1, a second optical path selection element OS2 and a quarter wave plate QWP, wherein the first laser L1 and the second laser L2 are respectively connected with a first port and a second port of the unequal arm polarization interferometer, a third port of the unequal arm polarization interferometer is connected with a first port of a first optical path selection element OS1, the first optical path selection element OS1 and the second optical path selection element OS2 form an equal arm Mach-Zehnder interferometer, the quarter wave plate QWP is positioned on one arm of the equal arm Mach-Zehnder interferometer, a third port of the second optical path selection element OS2 is used as an output port of the phase encoding device to output encoded optical pulses,

the light pulses generated by the first laser L1 and the second laser L2 respectively generate two front and back light pulses with a certain time difference through an unequal arm polarization interferometer, the front light pulse is horizontally polarized, the back light pulse is vertically polarized, the phase difference of the front and back light pulses generated by the first laser L1 is pi, the phase difference of the front and back light pulses generated by the second laser L2 is 0, an additional pi ⁄ 2 phase is introduced into the vertical polarization by the quarter wave plate QWP, and the front and back two pulses with mutually perpendicular polarization can be generated by combining the light path selection of the second light path selection element OS2 and the third light path selection element OS3 through whether the first laser L1 and the second laser L2 are triggered or not, and the phase difference 0, pi ⁄ 2, pi, 3 pi ⁄ 2 required by phase encoding can be prepared.

Preferably, the unequal arm polarization interferometer comprises a first beam splitter BS1 and a polarization beam combiner PBC, two input ports of the first beam splitter BS1 are used as a first port and a second port of the unequal arm polarization interferometer, two output ports are respectively connected with the two input ports of the polarization beam combiner PBC to be long and short arms of the unequal arm polarization interferometer, and an output port of the polarization beam combiner PBC is used as a third port of the unequal arm polarization interferometer.

Preferably, the unequal arm polarization interferometer comprises a second beam splitter BS2, a first polarizer POL1, a second polarizer POL2, a first mirror M1, a second mirror M2, and a first circulator CIR 1. The first port of the second beam splitter BS2 and the first port of the unequal arm polarization interferometer are welded at 45 degrees, the first port of the first circulator CIR1 and the polarization maintaining optical fiber between the second ports of the unequal arm polarization interferometers are welded at 45 degrees, the second port of the first circulator CIR1 is connected with the second port of the second beam splitter BS2, the third port of the first circulator CIR1 is used as the third port of the unequal arm polarization interferometer, the first polarizer 1 is placed at the third port of the second beam splitter BS2, the included angle between the polarization direction and the horizontal direction is 0 degree, the second polarizer POL2 is placed at the fourth port of the second beam splitter BS2, the included angle between the polarization direction and the horizontal direction is 90 degrees, the first polarizer 1 and the second polarizer 2 are respectively connected with the first reflector M1 and the second reflector M2 through the polarization maintaining optical fiber, and the lengths are respectively S2, And L.

Preferably, the first laser L1 and the second laser L2 are injection-locked by the same seed laser, so that the wavelengths of the first laser L1 and the second laser L2 are consistent, and the safety of the system is improved.

Preferably, the phase encoding apparatus further includes a third laser L3, a third beam splitter BS3, a second circulator CIR2 and a third circulator CIR3, the third laser L3 is used as a seed laser and is connected to an input port of the third beam splitter BS3, two output ports of the third beam splitter BS3 are respectively connected to first ports of the second circulator CIR2 and the third circulator CIR3, second ports of the second circulator CIR2 and the third circulator CIR3 are respectively connected to the first port L1 and the second laser L2, and third ports of the second circulator CIR2 and the third circulator CIR3 are respectively connected to the first port and the second port of the unequal arm polarization interferometer.

Preferably, the phase encoding apparatus further includes a fourth laser L4, the polarization beam combiner PBC further includes a fourth port, and the fourth laser L4 is used as a seed laser and connected to the fourth port of the polarization beam combiner PBC.

Preferably, the first optical path selection element OS1 is a first optical switch OSW1, and the second optical path selection element OS2 is a beam combiner BC.

Preferably, the first optical path selection element OS1 is a fourth beam splitter BS4, and the second optical path selection element OS2 is a second optical switch OSW 2.

Preferably, the unequal arm polarization interferometer further comprises a fourth circulator CIR4 and a fourth port, the first port of the fourth circulator CIR4 is fused at 45 degrees with the polarization maintaining fiber between the first ports of the unequal arm interferometers, the second port of the fourth circulator CIR4 is connected with the first port of the second beam splitter BS2, a third port of the fourth circulator CIR4 is used as a fourth port of the unequal arm polarization interferometer, is connected with a quarter wave plate QWP, and is connected to the second port of the third optical switch OSW3, and the third port of the unequal arm polarization interferometer is connected to the first port of the third optical switch OSW3, the first optical path selection element OS1 is omitted, the second beam splitter BS2, the first circulator CIR1, the fourth circulator CIR4, the quarter-wave plate QWP and the third optical switch OSW3 constitute an equal-arm mach-zehnder interferometer, the third port of the third optical switch OSW3 outputs an encoded optical pulse as an output port of the phase encoding device.

The invention also provides a phase coding quantum key distribution system, which comprises a transmitting end Alice and a receiving end Bob which are connected through an optical fiber channel, wherein the transmitting end Alice comprises a phase coding device, and the receiving end Bob comprises an unequal-arm polarization interferometer.

Compared with the prior art, the invention has the following beneficial effects:

the invention provides a phase encoding device, which can prepare 4 phase encoding states required by BB84 protocols without a phase modulator, and the polarization of two time modes is mutually vertical, so that the non-interference component is easily eliminated through polarization selection, and the energy utilization rate is improved. The cost and the complexity of the coding device can be reduced, and the safe code rate of the quantum key distribution system can be improved.

Drawings

FIG. 1 is a diagram of a typical prior art phase-encoded quantum key distribution scheme;

FIG. 2 is a schematic diagram of a phase encoding apparatus for quantum key distribution according to the present invention;

FIG. 3 is a schematic diagram of a first embodiment of the present invention;

FIG. 4 is a schematic diagram of a second embodiment of the present invention;

FIG. 5 is a schematic diagram of a third embodiment of the present invention;

FIG. 6 is a schematic diagram of a fourth embodiment of the present invention;

fig. 7 is a block diagram of a phase-encoded quantum key distribution system according to the present invention.

Detailed Description

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown.

As shown in fig. 2, a phase encoding apparatus for quantum key distribution (hereinafter referred to as a phase encoding apparatus) includes a first laser L1, a second laser L2, an unequal arm polarization interferometer, a first optical path selection element OS1, a second optical path selection element OS2, and a quarter wave plate QWP, the first laser L1 and the second laser L2 are respectively connected with a first port and a second port of the unequal arm polarization interferometer, the third port of the unequal arm polarization interferometer is connected with the first port of the first optical path selection element OS1, the first optical path selection element OS1 and the second optical path selection element OS2 form an equal arm Mach-Zehnder interferometer, the quarter wave plate is positioned on one arm of the equal arm Mach-Zehnder interferometer, the third port of the second optical path selection element OS2 outputs an encoded optical pulse as an output port of the phase encoding device.

The specific phase encoding process is as follows:

when the first laser L1 is triggered and the second laser L2 is not triggered, a horizontally polarized light pulse P1 is generated and enters the unequal arm polarization interferometer to generate two sub-pulses P11 and P12 with mutually perpendicular polarizations, wherein P11 passes through a short arm of the unequal arm polarization interferometer and is in a previous time window when being output from a third port of the unequal arm polarization interferometer, the polarization state is H, P12 passes through a long arm of the unequal arm polarization interferometer and is in a next time window when being output from the third port of the unequal arm polarization interferometer, and the polarization state is V. The phase difference between P11 and P12 output from the unequal arm polarization interferometer is pi, the polarizations are perpendicular to each other, then P11 and P12 enter the equal arm Mach Zehnder interferometer in sequence, and when the equal arm Mach Zehnder interferometer enters one arm with the quarter wave plate QWP, the included angle is 0 degree due to the fact that the optical axis of the quarter wave plate QWP is aligned with the slow axis of the polarization maintaining optical fiber. The Jones matrix of the quarter-wave plate QWP is

The horizontal polarization H and the vertical polarization V become after passing through QWP respectively

That is, the phase of the horizontally polarized pulse after passing through the quarter-wave plate QWP is not changed, but the phase of the vertically polarized pulse is increased by pi ⁄ 2, so that the phase difference between P11 and P12 after passing through the quarter-wave plate QWP becomes 3 pi ⁄ 2, and when P11 and P12 enter one arm of the equiarm mach-zehnder interferometer without the quarter-wave plate QWP, the phase difference between the two is not changed and still pi, and it can be seen that when the first laser L1 is triggered and the second laser L2 is not triggered, the encoded phase difference which can be output is 3 pi ⁄ 2 and pi.

When the first laser L1 is not triggered and the second laser L2 is triggered, a horizontally polarized light pulse P2 is generated, enters the unequal arm polarization interferometer, and two sub-pulses P21 and P22 with mutually perpendicular polarizations are generated, wherein P21 passes through the short arm of the unequal arm polarization interferometer and is in the previous time window when being output from the third port of the unequal arm polarization interferometer, and the polarization state is H. P22 passes through the long arm of the unequal arm polarization interferometer, is in the next time window when being output from the third port, the polarization state is V, the phase difference between P21 and P22 output from the unequal arm polarization interferometer is 0, the polarizations are perpendicular to each other, then P21 and P22 enter the equal arm Mach Zehnder interferometer in sequence, and when entering the arm with the quarter wave plate QWP, the phase difference of P21 and P22 after passing through the quarter wave plate QWP is changed into pi ⁄ 2. When the P11 and the P12 enter one arm of the equal-arm Mach-Zehnder interferometer without the quarter-wave plate QWP, the phase difference between the two is not changed and still is 0. It can be seen that when the first laser L1 is not triggered and the second laser L2 is triggered, the encoded phase difference that can be output is pi ⁄ 2 and 0.

From the above analysis, it can be seen that the 4 phase encoded states required by the BB84 protocol can be prepared without a phase modulator by only triggering the first laser L1 or the second laser L2 in combination with the first optical path selection element OS1 and the second optical path selection element OS 2.

As shown in fig. 3, a first embodiment of the phase encoding apparatus of the present invention:

the phase encoding device has the structure that: the unequal arm polarization interferometer comprises a first beam splitter BS1 and a polarization beam combiner PBC, the first optical path selection element OS1 is a first optical switch OSW1, the second optical path selection element OS2 is a beam combiner BC, and the phase encoding device further comprises a third laser L3, a third beam splitter BS3, a second circulator CIR2 and a third circulator CIR 3. The third laser L3 is used as a seed laser and is connected to an input port of a third beam splitter BS3, two output ports of the third beam splitter BS3 are respectively connected to first ports of a second circulator CIR2 and a third circulator CIR3, second ports of the second circulator CIR2 and a third circulator CIR3 are respectively connected to the first laser L1 and the second laser L2, and third ports of the second circulator CIR2 and the third circulator CIR3 are respectively connected to a first port and a second port of the unequal arm polarization interferometer.

An embodiment of a phase encoding process comprises:

the third laser L3 is used as a seed laser to perform injection locking on the first laser L1 and the second laser L2 through an optical path formed by the third beam splitter BS3, the second circulator CIR2 and the third circulator CIR 3. When the first laser L1 is triggered and the second laser L2 is not triggered, a horizontally polarized optical pulse P1 is generated, enters the first beam splitter BS1 through the second circulator CIR2, is divided into two pulse components P11 and P12 with equal amplitude, and the polarization state is still horizontally polarized, wherein the pulse component P11 is transmitted from the first beam splitter BS1, propagates along the slow axis of the polarization maintaining fiber in the short arm of the unequal arm polarization interferometer, reaches the polarization beam combiner PBC, is transmitted, and is output from the third port of the polarization beam combiner PBC, is horizontally polarized and is in the previous time window; the pulse component P12 is reflected by the first beam splitter BS1, propagates along the slow axis of the polarization maintaining fiber in the long arm of the unequal arm polarization interferometer, reaches the polarization beam combiner PBC, is reflected, is output from the third port of the polarization beam combiner PBC, is vertically polarized, and is in the latter time window. Since P11 is twice transmitted at the first beam splitter BS1 and the polarization beam combiner PBC, there is no phase jump, whereas P12 is twice reflected at the first beam splitter BS1 and the polarization beam combiner PBC, there are two phase jumps of pi ⁄ 2, and the phase difference between the two becomes pi. When the state of the first optical switch OSW1 is switched to 0, P11 and P12 enter one arm of the equal-arm mach-zehnder interferometer with a quarter-wave plate QWP, the phase difference between the two arms becomes 3 pi ⁄ 2, and the output is output through the beam combiner BC. When the state of the first optical switch OSW1 is switched to 1, P11 and P12 enter one arm of the equal-arm mach-zehnder interferometer without a quarter-wave plate QWP, the phase difference between the two is still pi, and finally the phase difference is also output from the beam combiner BC. That is, when the first laser L1 is triggered and the second laser L2 is not triggered, the encoded phase difference that can be output by switching the state of the first optical switch OSW1 is 3 pi ⁄ 2 and pi.

When the first laser L1 is not triggered and the second laser L2 is triggered, a horizontally polarized optical pulse P2 is generated, enters the first beam splitter BS1 through the third circulator CIR3, is divided into two pulse components P21 and P22 with equal amplitude, and the polarization state is still horizontally polarized, wherein the pulse component P21 is reflected from the first beam splitter BS1, propagates along the slow axis of the polarization maintaining fiber on the short arm of the unequal arm polarization interferometer, reaches the polarization beam combiner PBC, is transmitted, is output from the third port of the polarization beam combiner PBC, is horizontally polarized, and is in the previous time window; the pulse component P22 is transmitted by the first beam splitter BS1, propagates along the polarization maintaining fiber slow axis in the long arm of the unequal arm polarization interferometer, is reflected after reaching the polarization beam combiner PBC, and is output from the third port of the polarization beam combiner PBC as vertically polarized in the latter time window. Since P21 is reflected at the first beam splitter BS1 and transmitted at the polarization beam combiner PBC, there is a phase jump of pi ⁄ 2, whereas P22 is transmitted at the first beam splitter BS1 and reflected at the polarization beam combiner PBC, there is also a phase jump of pi ⁄ 2, so that the phase difference between the two is 0. When the state of the first optical switch OSW1 is switched to 0, P21 and P22 enter one arm of the equal-arm mach-zehnder interferometer with a quarter-wave plate QWP, the phase difference between the two arms becomes pi ⁄ 2, and the output is output through a beam combiner BC. When the state of the first optical switch OSW1 is switched to 1, P21 and P22 enter one arm of the equi-arm mach-zehnder interferometer without the quarter-wave plate QWP, the phase difference between the two is still 0, and finally the phase difference is also output from the beam combiner BC. That is, when the first laser L1 is not triggered and the second laser L2 is triggered, the encoded phase difference that can be output by switching the state of the first optical switch OSW1 is pi ⁄ 2 and 0.

During quantum state encoding, the first laser L1 and the second laser L2 are not triggered at the same time, and are triggered by on, and are not triggered by off, and 0 indicates that the first optical switch OSW1 is switched to the arm where the optical pulse only travels the quarter-wave plate QWP in the equal-arm mach-zehnder interferometer, and 1 indicates that the first optical switch OSW1 is switched to the arm where the optical pulse only travels the quarter-wave plate QWP in the equal-arm mach-zehnder interferometer. The code of the first embodiment of the phase encoding apparatus is shown in table 1.

Table 1: code table of first embodiment of phase encoding device

When the encoding device performs phase encoding randomly according to table 1, corresponding 4 quantum states can be obtained, and stable phase encoding is realized.

As shown in fig. 4, a second embodiment of the phase encoding apparatus of the present invention:

the phase encoding device has the structure that: the unequal arm polarization interferometer comprises a first beam splitter BS1 and a polarization beam combiner PBC, the first optical path selection element OS1 is a fourth beam splitter BS4, the second optical path selection element OS2 is a second optical switch OSW2, the polarization beam combiner PBC comprises four ports, and the phase encoding device further comprises a fourth laser L4, and is connected to the fourth port of the polarization beam combiner PBC.

An embodiment two-phase encoding process comprises:

the fourth laser L4 is used as a seed laser, and enters the first laser L1 and the second laser L2 from the opposite direction through the short arm of the unequal arm polarization interferometer by the polarization beam combiner PBC, so as to perform injection locking on the first laser L1 and the second laser L2 respectively. When the first laser L1 is triggered and the second laser L2 is not triggered, the horizontally polarized optical pulse P1 is generated, and the process of passing through the unequal arm polarization interferometer is the same as that of the first embodiment, that is, the P11 and the P12 are output from the polarization beam combiner PBC, the polarizations of the P11 and the P12 are perpendicular to each other, and the phase difference is pi. Then split into components of the same amplitude by the fourth beam splitter BS4 and enter both arms of the equal arm mach-zehnder interferometer simultaneously. Wherein the phase difference of the component passing through the QWP with the quarter-wave plate becomes 3 pi ⁄ 2, and the phase difference of the component passing through the QWP without the quarter-wave plate is still pi. When the state of the second optical switch OSW2 is switched to 0, only the component with one arm of the quarter-wave plate QWP in the equal-arm mach-zehnder interferometer may be output from the second optical switch OSW2, i.e., the phase difference between the output pulses is 3 pi ⁄ 2. When the state of the second optical switch OSW2 is switched to 1, only the component of the arm of the equal-arm mach-zehnder interferometer without the quarter-wave plate QWP can be output from the second optical switch OSW2, i.e., the phase difference between the output pulses is pi.

The first laser L1 is not triggered, and when the second laser L2 is triggered, the horizontally polarized optical pulse P2 is generated, and the process of passing through the unequal arm polarization interferometer is the same as that of the first embodiment, that is, the P21 and the P22 are output from the polarization beam combiner PBC, the polarizations of the P21 and the P22 are perpendicular to each other, and the phase difference is 0. Then split into components of the same amplitude by the fourth beam splitter BS4 and enter both arms of the equal arm mach-zehnder interferometer simultaneously. Wherein the phase difference of the component passing through the QWP with the quarter-wave plate becomes pi ⁄ 2, and the phase difference of the component passing through the QWP without the quarter-wave plate is still 0. When the state of the second optical switch OSW2 is switched to 0, only the component with one arm of the quarter-wave plate QWP in the equal-arm mach-zehnder interferometer may be output from the second optical switch OSW2, i.e., the phase difference between the output pulses is pi ⁄ 2. When the state of the second optical switch OSW2 is switched to 1, only the component of the one arm of the equal-arm mach-zehnder interferometer without the quarter-wave plate QWP can be output from the second optical switch OSW2, i.e., the phase difference between the output pulses is 0.

During quantum state encoding, the fourth laser L4 injection locks the first laser L1 and the second laser L2, the first laser L1 and the second laser L2 are not triggered at the same time, on is used for triggering, off is used for not triggering, 0 is used for switching the second optical switch OSW2 to output only the optical pulse of one arm with the quarter-wave plate QWP in the unequal arm mach-zehnder interferometer, and 1 is used for switching the second optical switch OSW2 to output only the optical pulse of one arm without the quarter-wave plate QWP in the unequal arm mach-zehnder interferometer. The encoding of the second phase encoding device embodiment is the same as table 1.

As shown in fig. 5, a third embodiment of the phase encoding apparatus of the present invention:

the phase encoding device has the structure that: the unequal arm polarization interferometer includes a second beam splitter BS2, a first polarizer POL1, a second polarizer POL2, a first mirror M1, a second mirror M2, and a first circulator CIR 1. The first port of the second beam splitter BS2 and the polarization maintaining optical fiber between the first laser L1 are fused at 45 °, the first port of the first circulator CIR1 and the polarization maintaining optical fiber between the second laser L2 are fused at 45 °, the second port of the first circulator CIR1 and the second port of the second beam splitter BS2 are connected, the third port of the first circulator CIR1 is used as the third port of the unequal arm polarization interferometer, the first polarizer POL1 is placed at the third port of the second beam splitter BS2, the polarization direction and the horizontal direction form an angle of 0 °, the second polarizer POL2 is placed at the fourth port of the second beam splitter BS2, the polarization direction and the horizontal direction form an angle of 90 °, the first polarizer POL1 and the second polarizer 2 are respectively connected with the first reflector M1 and the third reflector M2 through the polarization maintaining optical fiber, and the length of the reflectors is S, L.

An embodiment three phase encoding process comprises:

when the first laser L1 is triggered and the second laser L2 is not triggered, the horizontally polarized light pulse P1 is generated, and after being rotated by 45 °, the horizontally polarized light pulse P1 changes to a 45 ° polarization state, and includes a horizontally polarized component H and a vertically polarized component V, and then is split into two light pulses P11 and P12 with the same amplitude by the second beam splitter BS2, and the polarization states of the two light pulses are 45 °. The P11 is transmitted by the second beam splitter BS2, and the first polarizer POL1 is parallel to the horizontal direction and only can transmit a horizontal polarization component, so the light transmits through the first polarizer POL1 and then propagates along the slow axis of the polarization-maintaining optical fiber with the length S, after being reflected by the first reflector M1, the light passes through the first polarizer POL1 again, the polarization state is horizontal polarization, and half of the light intensity is reflected by the second beam splitter BS2 to the first circulator CIR1 for emergence. The P12 is reflected by the second beam splitter BS2, because the included angle between the second polarizer POL2 and the horizontal direction is 90 degrees, only the vertical polarization component can be transmitted, the light propagates along the slow axis of the polarization-maintaining optical fiber with the length L after passing through the second polarizer POL2, passes through the second polarizer POL2 after being reflected by the second reflector M2, the polarization state is vertical polarization, and half of the light intensity is transmitted to the first circulator CIR1 by the second beam splitter BS2 to be output. Since the optical paths traveled by P11 and P12 are different and have an optical path difference of 2 (L-S), P11 is horizontally polarized when it is output from first circulator CIR1 in the previous time window, and P12 is vertically polarized when it is output from first circulator CIR1 in the next time window. Since P11 is transmitted before reflected at the second beam splitter BS2, there is a phase jump of pi ⁄ 2, whereas P12 is reflected before transmitted at the second beam splitter BS2, there is also a phase jump of pi ⁄ 2, so the phase difference between the two is 0. When the state of the first optical switch OSW1 is switched to 0, P11 and P12 enter one arm of the equal-arm mach-zehnder interferometer with a quarter-wave plate QWP, the phase difference between the two arms becomes pi ⁄ 2, and the output is output through a beam combiner BC. When the state of the first optical switch OSW1 is switched to 1, P11 and P12 enter one arm of the equi-arm mach-zehnder interferometer without the quarter-wave plate QWP, the phase difference between the two is still 0, and finally the phase difference is also output from the beam combiner BC. That is, when the first laser L1 is triggered and the second laser L2 is not triggered, the encoded phase difference that can be output by switching the state of the first optical switch OSW1 is pi ⁄ 2 and 0.

When the first laser L1 is deactivated and the second laser L2 is activated, the horizontally polarized light pulse P2 is generated, which after 45 ° rotation becomes 45 ° polarization state, containing a horizontal polarization component H and a vertical polarization component V, and is then split by the first circulator CIR1 into two light pulses P21 and P22 of equal amplitude by the second beam splitter BS2, both polarization states being 45 °. The P21 is reflected by the second beam splitter BS2, the horizontal polarization component of the P21 propagates along the slow axis of the polarization maintaining fiber with the length S after passing through the first polarizer POL1, and passes through the first polarizer POL1 again after being reflected by the first mirror M1, the polarization state is horizontal polarization, and half of the light intensity is reflected by the second beam splitter BS2 to the first circulator CIR1 for emergence. The P22 is transmitted by the second beam splitter BS2, the vertical polarization component of the P22 is transmitted along the slow axis of the polarization-maintaining optical fiber with the length L after passing through the second polarizer POL2, the P22 passes through the second polarizer POL2 again after being reflected by the second reflecting mirror M2, the polarization state is vertical polarization, and half of the light intensity is transmitted to the first circulator CIR1 by the second beam splitter BS2 to be output. The optical path difference between P21 and P22 is 2 (L-S), so P21 is horizontally polarized when it is output from the first circulator CIR1 in the previous time window, and P22 is vertically polarized when it is output from the first circulator CIR1 in the next time window. Since P21 undergoes two reflections at the second beam splitter BS2, there are 2 phase jumps of pi ⁄ 2, while P22 undergoes two transmissions at the second beam splitter BS2, there are no phase jumps, so the phase difference between the two is pi. When the state of the first optical switch OSW1 is switched to 0, P21 and P22 enter one arm of the equal-arm mach-zehnder interferometer with a quarter-wave plate QWP, the phase difference between the two arms becomes 3 pi ⁄ 2, and the output is output through the beam combiner BC. When the state of the first optical switch OSW1 is switched to 1, P21 and P22 enter one arm of the equal-arm mach-zehnder interferometer without a quarter-wave plate QWP, the phase difference between the two is still pi, and finally the phase difference is also output from the beam combiner BC. That is, when the first laser L1 is not triggered and the second laser L2 is triggered, the encoded phase difference that can be output by switching the state of the first optical switch OSW1 is 3 pi ⁄ 2 and pi.

During quantum state encoding, the first laser L1 and the second laser L2 are not triggered at the same time, and are triggered by on, and are not triggered by off, and 0 indicates that the first optical switch OSW1 is switched to the arm where the optical pulse only travels the quarter-wave plate QWP in the equal-arm mach-zehnder interferometer, and 1 indicates that the first optical switch OSW1 is switched to the arm where the optical pulse only travels the quarter-wave plate QWP in the equal-arm mach-zehnder interferometer. The code of the third embodiment of the phase encoding apparatus is shown in table 2.

Table 2: code table of phase encoding device embodiment three

When the encoding device performs phase encoding randomly according to table 2, corresponding 4 quantum states can be obtained, and stable phase encoding is realized.

As shown in fig. 6, a fourth embodiment of the phase encoding apparatus of the present invention:

the phase encoding device has the structure that: the unequal arm polarization interferometer also comprises a polarization maintaining optical fiber between the first port of the fourth circulator CIR4 and the first port of the unequal arm interferometer for 45-degree fusion, the second port of the fourth circulator CIR4 is connected with the first port of the second beam splitter BS2, and the third port of the fourth circulator CIR4 is used as the third port of the fourth circulator CIR4

The unequal arm polarization interferometer has a first port, a second port, a third port, and a fourth port, and includes a second beam splitter BS2, a first polarizer POL1, a second polarizer POL2, a first mirror M1, a second mirror M2, a first circulator CIR1, and a fourth circulator CIR 4. The first port of the fourth circulator CIR4 is welded at 45 degrees with the polarization maintaining fiber between the first laser L1, the first port of the first circulator CIR1 is welded at 45 degrees with the polarization maintaining fiber between the second laser L2, the second port of the first circulator CIR1 is connected with the second port of the second beam splitter BS2, the third port of the first circulator CIR1 is used as the third port of the unequal arm polarization interferometer, the second port of the fourth circulator CIR4 is connected with the first port of the second beam splitter BS2, and the third port of the fourth circulator CIR4 is used as the fourth port of the unequal arm polarization interferometer. The first polarizer POL1 is placed at the third port of the second beam splitter BS2, the included angle between the polarization direction and the horizontal direction is 0 degree, the second polarizer POL2 is placed at the fourth port of the second beam splitter BS2, the included angle between the polarization direction and the horizontal direction is 90 degrees, the first polarizer POL1 and the second polarizer POL2 are respectively connected with the first reflector M1 and the second reflector M2 through polarization maintaining optical fibers, and the lengths of the first polarizer POL1 and the second polarizer POL 3538 are S, L respectively. The fourth port of the unequal arm polarization interferometer is connected with the quarter wave plate QWP and is connected with the second port of the third optical switch OSW3, the third port of the unequal arm polarization interferometer is connected with the first port of the third optical switch OSW3, the first optical path selecting element OS1 is omitted compared with the three-phase ratio of the embodiment, the second beam splitter BS2, the first circulator CIR1, the fourth circulator CIR4, the quarter wave plate QWP and the third optical switch OSW3 form an equal arm mach-zehnder interferometer, and the third port of the third optical switch OSW3 is used as an output port of the phase encoding device to output the encoded optical pulse.

An embodiment four-phase encoding process includes:

when the first laser L1 is triggered and the second laser L2 is not triggered, the horizontally polarized light pulse P1 is generated, and after being rotated by 45 °, the horizontally polarized light pulse P1 changes to a 45 ° polarization state, and includes a horizontally polarized component H and a vertically polarized component V, and after passing through the fourth circulator CIR4, the horizontally polarized light pulse L and the vertically polarized light pulse P4 are divided into two paths of light pulses P11 and P12 with the same amplitude by the second beam splitter BS2, and the polarization states of the two paths of light pulses are 45 °. Wherein P11 is transmitted by the second beam splitter BS2, passes through the first polarizer POL1, then propagates along the slow axis of the polarization maintaining fiber with the length S, is reflected by the first mirror M1, then passes through the first polarizer POL1 again, and is polarized horizontally, and then is split into pulses P111 and P112 with the same amplitude by the second beam splitter BS2, and both are horizontally polarized. Where P111 is reflected by the second beam splitter BS2 to exit the first circulator CIR1 and P112 is transmitted by the second beam splitter BS2 to exit the fourth circulator CIR 4. The P12 is reflected by the second beam splitter BS2, passes through the second polarizer POL2, then propagates along the slow axis of the polarization maintaining fiber with the length L, is reflected by the second mirror M2, then passes through the second polarizer POL2 again, is polarized in the vertical polarization state, and then is split into pulses P121 and P122 with the same amplitude by the second beam splitter BS2, and both are vertically polarized. Where P121 is transmitted by the second beam splitter BS2 to the first circulator CIR1 and P122 is reflected by the second beam splitter BS2 to the fourth circulator CIR 4. Since the optical paths traveled by P11 and P12 are different and the optical path difference is 2 (L-S), P111 and P112 are horizontally polarized and in the previous time window when output from the first circulator CIR1 and the fourth circulator CIR4, respectively; p121 and P122 are vertically polarized when output from the first circulator CIR1 and the fourth circulator CIR4, respectively, in the latter time window. And the phase difference between P111 and P121 is 0, and becomes pi ⁄ 2 after passing through the quarter wave plate QWP. The phase difference between P112 and P122 is pi. When the state of the third optical switch OSW3 is switched to 0, P111 and P121 are output from the third port of the third optical switch OSW 3. When the state of the third optical switch OSW3 is switched to 1, the phase difference between P112 and P122 is not changed, and the output from the third optical switch OSW3 is direct. That is, when the first laser L1 is triggered and the second laser L2 is not triggered, the encoded phase difference that can be output by switching the state of the third optical switch OSW3 is pi ⁄ 2 and pi.

When the first laser L1 is not triggered and the second laser L2 is triggered, the analysis method is similar, and the encoding phase difference that can be output by switching the state of the third optical switch OSW3 is 3 pi ⁄ 2 and 0.

In the case of quantum state coding, the first laser L1 and the second laser L2 are not activated simultaneously, and are on, off, 0 indicating that the second port to the third port of the third optical switch OSW3 are on, and 1 indicating that the first port to the third port of the third optical switch OSW3 are on. The code of the fourth embodiment of the phase encoding apparatus is shown in table 3.

Table 3: code table of the fourth embodiment of the phase encoding device

When the encoding device performs phase encoding randomly according to table 3, corresponding 4 quantum states can be obtained, and stable phase encoding is realized.

As shown in fig. 7, the present invention further discloses a phase-coded quantum key distribution system, which includes a transmitting end Alice and a receiving end Bob, where the transmitting end Alice may include any one of the above phase-coding devices, and in the transmitting end Alice, a fourth laser is connected to a fourth port of a polarization beam combiner PBC through an intensity modulator IM, and the second optical switch OSW2 is connected to the receiving end Bob through an adjustable attenuator VOA. The receiving end Bob comprises an unequal-arm polarization interferometer, so that non-interference components can be eliminated, the energy utilization rate of the interferometer is improved, and the safe code rate of the system is further improved.

According to the embodiments of the invention, the phase encoding device provided by the invention can prepare 4 phase encoding states required by the BB84 protocol without a phase modulator, and the polarizations of the two time modes are mutually perpendicular, so that the non-interference component is easily eliminated through polarization selection, and the energy utilization rate of the optical pulse is improved by 1 time. The cost and the complexity of the coding device can be reduced, and the safe code rate of the quantum key distribution system can be improved.

Claims

1. A phase encoding device for quantum key distribution is characterized by comprising a first laser L1, a second laser L2, an unequal arm polarization interferometer, a first optical path selection element OS1, a second optical path selection element OS2 and a quarter wave plate QWP, wherein the first laser L1 and the second laser L2 are respectively connected with a first port and a second port of the unequal arm polarization interferometer, a third port of the unequal arm polarization interferometer is connected with a first port of a first optical path selection element OS1, the first optical path selection element OS1 and the second optical path selection element OS2 form an equal arm Mach-Zehnder interferometer, the quarter wave plate QWP is positioned on one arm of the equal arm Mach-Zehnder interferometer, a third port of the second optical path selection element OS2 is used as an output port of the phase encoding device to output encoded optical pulses,

the optical pulses generated by the first laser L1 and the second laser L2 respectively generate front and rear two optical pulses with a certain time difference and mutually perpendicular polarization states through an unequal arm polarization interferometer, the phase difference between the front and rear two optical pulses generated by the first laser L1 is pi, the phase difference between the front and rear two optical pulses generated by the second laser L2 is 0, the quarter wave plate QWP introduces an extra pi/2 phase to the vertically polarized pulses, the front and rear two pulses with mutually perpendicular polarizations can be generated through whether the first laser L1 and the second laser L2 are triggered or not and combining the optical path selection of the first optical path selection element OS1 and the second optical path selection element OS2, and the phase difference 0, pi ⁄ 2, pi, 3 pi ⁄ 2 required by phase encoding can be prepared.

2. The phase encoding apparatus for quantum key distribution according to claim 1, wherein the unequal arm polarization interferometer comprises a first beam splitter BS1 and a polarization beam combiner PBC, two input ports of the first beam splitter BS1 are used as a first port and a second port of the unequal arm polarization interferometer, two output ports are respectively connected with two input ports of the polarization beam combiner PBC to be a long arm and a short arm of the unequal arm polarization interferometer, and an output port of the polarization beam combiner PBC is used as a third port of the unequal arm polarization interferometer.

3. The phase encoding apparatus for quantum key distribution according to claim 1, wherein the unequal arm polarization interferometer comprises a second beam splitter BS2, a first polarizer POL1, a second polarizer POL2, a first mirror M1, a second mirror M2, and a first circulator CIR1, a first port of the second beam splitter BS2 is fused at 45 ° to the polarization maintaining fiber between the first ports of the unequal arm polarization interferometer, a first port of the first circulator CIR1 is fused at 45 ° to the polarization maintaining fiber between the second ports of the unequal arm polarization interferometer, a second port of the first circulator CIR1 is connected to a second port of the second beam splitter BS2, a third port of the first circulator CIR1 is used as a third port of the unequal arm polarization interferometer, the first polarizer POL1 is placed at the second port of the second beam splitter BS2, the polarization direction is at an angle of 0 ° to the horizontal direction, the second polarizer POL2 is placed at the fourth port of the second beam splitter BS2, the included angle between the polarization direction and the horizontal direction is 90 degrees, the first polarizer POL1 and the second polarizer POL2 are respectively connected with the first reflector M1 and the second reflector M2 through polarization maintaining optical fibers, and the lengths of the first polarizer POL1 and the second polarizer POL2 are S, L.

4. Phase encoding apparatus for quantum key distribution according to any of claims 1 to 3, wherein the first laser L1 and the second laser L2 are injection locked by the same seed laser.

5. The phase encoding apparatus for quantum key distribution according to claim 4, further comprising a third laser L3, a third beam splitter BS3, a second circulator CIR2 and a third circulator CIR3, wherein the third laser L3 is used as a seed laser and is connected to the input port of the third beam splitter BS3, two output ports of the third beam splitter BS3 are respectively connected to the first ports of the second circulator CIR2 and the third circulator CIR3, the second ports of the second circulator CIR2 and the third circulator CIR3 are respectively connected to the first port L1 and the second laser L2, and the third ports of the second circulator CIR2 and the third circulator CIR3 are respectively connected to the first port and the second port of the unequal arm polarization interferometer.

6. The phase encoding apparatus for quantum key distribution according to claim 4, further comprising a fourth laser L4, wherein the polarization beam combiner PBC further comprises a fourth port, and wherein the fourth laser L4 is connected to the fourth port of the polarization beam combiner PBC as a seed laser.

7. Phase encoding apparatus for quantum key distribution according to any of claims 6, wherein the first optical path selection element OS1 is a first optical switch OSW1 and the second optical path selection element OS2 is a beam combiner BC.

8. Phase encoding apparatus for quantum key distribution according to any of claims 6, wherein the first optical path selection element OS1 is a fourth beam splitter BS4 and the second optical path selection element OS2 is a second optical switch OSW 2.

9. The phase encoding apparatus for quantum key distribution according to claim 3, wherein the unequal arm polarization interferometer further includes a fourth circulator CIR4 and a fourth port, the first port of the fourth circulator CIR4 is fused at 45 ° to the polarization maintaining fiber between the first ports of the unequal arm interferometers, the second port of the fourth circulator CIR4 is connected to the first port of the second beam splitter BS2, the third port of the fourth circulator CIR4 is used as the fourth port of the unequal arm polarization interferometer, a quarter wave plate QWP is connected and connected to the second port of the third optical switch OSW3, the third port of the unequal arm polarization interferometer is connected to the first port of the third optical switch OSW3, the first optical path selecting element OS1 is omitted, the second circulator BS2, the first circulator CIR1, the fourth circulator 4, the quarter wave plate CIR 63P, and the third optical switch OSW3 form the equal optical wave plate, the third port of the third optical switch OSW3 outputs an encoded optical pulse as an output port of the phase encoding device.

10. A phase coding quantum key distribution system is characterized by comprising a transmitting end Alice and a receiving end Bob which are connected through an optical fiber channel, wherein the transmitting end Alice comprises a phase coding device, and the receiving end Bob comprises an unequal-arm polarization interferometer.