CN111555863B - Sending end, encoding method and quantum key distribution system for time phase-polarization combined encoding - Google Patents
Sending end, encoding method and quantum key distribution system for time phase-polarization combined encoding Download PDFInfo
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Abstract
The invention relates to a sending end, an encoding method and a quantum key distribution system for time phase-polarization combined encoding. The encoding method of the present invention includes a time phase encoding step and a polarization encoding step. In the time phase encoding step, signal light to be encoded is subjected to first phase modulation by a phase modulation unit, and time phase encoding information is generated on the signal light by reflection of the light based on the first phase modulation. In the polarization encoding step, the reflected signal light having the time phase encoding information is passed through the phase modulation unit again to be subjected to the second phase modulation, and the polarization encoding information is generated on the reflected signal light having the time phase encoding information based on the second phase modulation.
Description
Technical Field
The invention relates to the field of quantum secret communication, in particular to a sending end, an encoding method and a Quantum Key Distribution (QKD) system for time phase-polarization combined encoding.
Background
Quantum Key Distribution (QKD) is fundamentally different from the classical key system in that it employs a single photon or entangled photon pair as a carrier for the key, and the fundamental principles of quantum mechanics ensure that the process is not eavesdroppable and indecipherable, thereby providing a more secure key system.
Fig. 1a shows a transmitting end for implementing time bit-phase encoding in the prior art. As shown, a laser pulse output from a light source produces two temporally separated pulse components via an unequal arm MZ interferometer. In order to obtain the phase code in the X and Y basis vectors, four phases 0, pi/2 and 3 pi/2 are applied between the two pulse components by means of a phase modulator to obtain the time bit code in the Z basis vector, the front or rear pulse components are modulated respectively by means of an Intensity Modulator (IM), the passing or extinction is controlled, and the front or rear pulse component is retained to obtain the time state | t0> or | t1 >. If it is an X or Y basis vector encoding, the intensity modulator passes light 1/2 for both pulse components.
Fig. 1b shows another prior art quantum key distribution system based on time phase encoding, for example, see chinese patent application 201610695435.1. As shown in the figure, the quantum key distribution system comprises a sending end and a receiving end which are mutually connected in an optical way, wherein an encoding unit in the sending end comprises a Z-basis vector time encoding module and a phase encoding module, and the phase encoding module is an X-basis vector phase encoding module or a Y-basis vector phase encoding module; the decoding unit in the receiving end comprises a Z-basis vector time measuring module and a phase measuring module, wherein the phase measuring module is an X-basis vector phase measuring module or a Y-basis vector phase measuring module and is adaptive to the phase encoding module.
Fig. 1c shows yet another prior art quantum key distribution system based on time-phase encoding. As shown, in this system, a laser pulse output from a light source passes through an equal arm MZ interferometer to generate two temporally separated pulse components, which then enter the equal arm interferometer. The equal-arm interferometer comprises two phase modulators, different interference output light intensity and phase results can be obtained by adjusting the relative phase difference of the two phase modulators, and different light intensity and phase results can be modulated by switching modulation voltage values for pulse components arriving at different times. At the transmitting end of the quantum key distribution system, encoding of 3 basis vectors is possible. For example, when the phase difference of the equal-arm interferometer is 0 and pi, the corresponding output is an extinction result and a light result, and the Z-basis vector coding is performed; the pulses are output when the phase difference is pi/2 and-pi/2, and the phase difference between the pulses determines X basis vector coding or Y basis vector coding.
The sending end of the QKD also needs quantum state BB84 encoding, such as commonly used polarization encoding, for the decoy state light source. The polarization state coding principle based on phase modulation is as follows:
by dividing the light pulse into two components of | H > and | V > perpendicular to each other, the phase difference between the two component light pulses is adjusted
For a particular
Obtaining a specific polarization state according to Euler's formula eixAfter being cosx + isinx, respectively to give
Such as
When the values of the components are respectively taken as 0,
such as
When the values are respectively taken as pi/2,
such as
When the values are respectively taken as pi, the alloy,
such as
When the ratio is 3 pi/2,
thereby obtaining P, R, N and L four polarization states.
However, in the prior art, whether polarization encoding or time phase encoding is performed by using 2-dimensional space for quantum key encoding and decoding, 1 photon can transmit 1 bit of information at most. In the long-distance transmission process, the photon loss is very serious, so that the key generation rate is low.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a high-dimensional joint coding mode, which simultaneously carries out time phase modulation and polarization modulation on 1 photon, so that 1 photon simultaneously carries 2 bits of information, and the key generation rate can be greatly improved.
Specifically, one aspect of the present invention discloses a transmitting end for implementing time phase-polarization joint encoding, which includes a light source 1, an encoding module 2, a first reflecting unit 3, and a second reflecting unit 4. Wherein the light source 1 is used for providing signal light to be encoded. The encoding module 2 comprises a phase modulation unit 211 and is arranged to: dividing the signal light to be encoded into a first signal light component and a second signal light component; performing a first phase modulation on at least one of the first and second signal light components by means of the phase modulation unit 211; and interfering the first and second signal light components and outputting one or two first interference light signals. The first reflecting unit 3 and the second reflecting unit 4 have different optical paths from each other and are arranged to receive the first interfering light signal and reflect it back to the encoding module 2. The encoding module 2 is further configured to: receiving a reflected optical signal and separating it into a first reflected optical signal component and a second reflected optical signal component, wherein the reflected optical signal component has time phase encoded information; performing a second phase modulation on at least one of said first and second reflected optical signal components by means of said phase modulation unit 211; and interfering the first and second reflected optical signal components and outputting a second interference optical signal, wherein the second interference optical signal has the time phase encoded information and polarization encoded information.
Preferably, the first reflection unit 3 includes an optical fiber and a reflection structure provided at an end of the optical fiber, wherein: the optical fiber is a polarization maintaining optical fiber, and the reflecting structure is a common reflector or a reflecting film; or, the optical fiber is a polarization maintaining optical fiber or a common optical fiber, and the reflecting structure is a Faraday reflector or a Faraday reflecting film. And, the second reflecting unit 4 includes an optical fiber and a reflecting structure provided at an end of the optical fiber, wherein: the optical fiber is a polarization maintaining optical fiber, and the reflecting structure is a common reflector or a reflecting film; or, the optical fiber is a polarization maintaining optical fiber or a common optical fiber, and the reflecting structure is a Faraday reflector or a Faraday reflecting film.
Preferably, the encoding module 2 further includes a polarization beam splitting unit 212 and a beam splitting unit 213, and the polarization beam splitting unit 212 and the beam splitting unit 213 are connected through a first optical path and a second optical path having the same optical path to form an equal-arm MZ interferometer; and, the phase modulation unit 211 is provided on at least one of the first and second optical paths.
Preferably, the polarization beam splitting unit 212 receives the signal light to be encoded and outputs the second interference light signal with the time phase encoding information and polarization encoding information; and/or, the beam splitting unit 213 connects the first reflection unit 3 and the second reflection unit 4, and outputs the first interference light signal and receives the reflected light signal.
Preferably, the transmitting end may further include a phase detection feedback module, configured to compensate for a phase drift in the equal-arm MZ interferometer; and/or, the polarization beam splitting unit 212 includes a polarization beam splitter; and/or, the beam splitting unit 213 includes a beam splitter.
Preferably, the first secondary phase modulation phase-difference θ formed between the first and second signal light components1Is one of 0, pi/2, pi and 3 pi/2; and/or the second time phase modulation forms a second phase difference θ between the first and second reflected light signal components2Is one of 0, pi/2, pi and 3 pi/2.
In another aspect of the present invention, a quantum key distribution system is disclosed, which includes the above transmitting end.
In yet another aspect of the invention, an encoding method for time phase-polarization joint encoding is disclosed, which comprises a time phase encoding step and a polarization encoding step. In the time phase encoding step, signal light to be encoded is subjected to first phase modulation by a phase modulation unit, and time phase encoding information is generated on the signal light by means of reflection of light based on the first phase modulation. In the polarization encoding step, the reflected signal light having the time phase encoding information is passed through the phase modulation unit again to be subjected to a second phase modulation, and polarization encoding information is generated on the reflected signal light having the time phase encoding information based on the second phase modulation.
Preferably, the time phase encoding step further comprises the steps of: splitting the signal light to be encoded into a first signal light component and a second signal light component according to two components of which the polarization directions are perpendicular to each other; performing the first phase modulation on at least one of the first and second signal light components using the phase modulation unit to form a first phase difference θ therebetween1(ii) a So as to have the first phase difference theta1The first and second signal light components generate a first interference effect and output one or two paths of first interference light signals; and according to the first phase difference theta1And enabling the first interference optical signal to enter at least one of a first reflection unit and a second reflection unit with different optical paths and to be reflected.
Preferably, the polarization encoding step further comprises the steps of: splitting the reflected signal light having the time phase encoded information into first and second reflected light signal components; performing the second phase modulation on at least one of the first and second reflected optical signal components with the phase modulation unit to form a second phase difference θ therebetween2(ii) a And having the second phase difference theta2And a second interference occurs with the first and second reflected optical signal components.
Preferably, an equal-arm MZ interferometer is constructed by utilizing a polarization beam splitting unit and a beam splitting unit, and the phase modulation unit is arranged in the equal-arm MZ interferometer; the first interference occurs at the beam splitting unit or the second interference occurs at the polarizing beam splitting unit.
Preferably, the time phase encoded information is different from the first phase difference θ1Related to the optical path of the reflection unit; or the polarization encoded information differs from the second phase by a value theta2It is related.
Preferably, the first phase difference θ1Is one of 0, pi/2, pi and 3 pi/2; or, the second phase difference θ2Is one of 0, pi/2, pi and 3 pi/2.
Drawings
FIG. 1a shows a prior art structure of a transmitting end based on time phase coding;
FIG. 1b illustrates another prior art quantum key distribution system based on time-phase encoding;
FIG. 1c illustrates yet another prior art quantum key distribution system based on time-phase encoding;
fig. 2 is a schematic diagram illustrating the structure of a transmitting end for time-phase polarization joint encoding according to the present invention; and
FIGS. 3a-3d illustrate the phase difference at the first phase difference θ, respectively1The invention relates to a coding principle of a transmitting end for time phase polarization combined coding, which is respectively 0 pi, pi/2 and 3 pi/2.
Detailed Description
Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided by way of illustration in order to fully convey the spirit of the invention to those skilled in the art to which the invention pertains. Accordingly, the present invention is not limited to the embodiments disclosed herein.
In the method of the present invention, the time phase encoding and the polarization encoding of the signal light are implemented by the same phase modulation unit, and the encoding process is basically as follows: firstly, enabling signal light to be coded to pass through a phase modulation unit to carry out first-time phase modulation on the signal light, and generating time phase coding on the signal light based on the first-time phase modulation; subsequently, the time-phase-encoded signal light is again passed through the same phase modulation unit to be subjected to the second phase modulation, and the polarization state modulation of the signal light is realized based on the second phase modulation, thereby realizing the polarization encoding. Namely, the signal light to be coded is made to reciprocate in the same optical path, and the time phase coding and the polarization coding of the signal light are respectively realized based on the two successive phase modulations of the same phase modulation unit on the signal light.
Specifically, signal light to be encoded is split into a first signal light component and a second signal light component which are respectively transmitted along a first optical path and a second optical path according to two components with polarization directions perpendicular to each other; phase-modulating at least one of the first and second signal light components with a phase modulation unit to form a first phase difference θ therebetween1(ii) a So as to have a first phase difference theta1And outputs a result of the first interference, which is out of phase with the first phase difference theta1It is related.
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 phase modulator provided in the interferometer. First phase difference theta1May be any of (0, π, π/2,3 π/2).
Then, the interference light pulse output as a result of the first interference enters at least one of the first reflecting unit and the second reflecting unit, and obtains a phase difference θ from the first phase after being reflected1The associated time phase encoding. The first reflection unit and the second reflection unit are arranged to have different return optical paths (respectively recorded as time t0 and time t1) for optical signals, namely, the optical signals entering the first reflection unit and the second reflection unit at the same time return to the input end at different times t0 and t1 after being reflected in the first reflection unit and the second reflection unit. As an example, the first (second) reflecting unit may comprise an optical fiber and a reflecting structure provided at an end of the optical fiber, wherein: the optical fiber is a polarization maintaining optical fiber, and the reflecting structure is a common reflector or a reflecting film; or, the optical fiber is a polarization maintaining optical fiber or a common optical fiber, and the reflecting structure is a Faraday reflector or a Faraday reflecting film.
Each of the reflected light pulses returning in the reflection unit is again split into two components, which re-enter the first and second optical paths, respectively, and now have time-phase encoded information.
As an example, when θ1At 0, the first interference will output an interference light pulse which enters the first reflection unit to propagate and return after time t0 and is split into two reflected light pulse components which re-enter the first optical path and the second optical path, respectively, at which time the reflected light pulse components on the first and second optical paths have time-encoded information corresponding to time t 0.
When theta is1At pi, the first interference will likewise output an interference light pulse, but it will travel into the second reflecting unit and return after time t1 and be split into two reflected light pulse components which re-enter the first and second optical paths, respectively, at which time the reflected light pulse components on the first and second optical paths have time-encoded information corresponding to time t 1.
When theta is1At pi/2, the first interference will output two interfering light pulses, which will propagate into the first and second reflecting elements, respectively, and return after times t0 and t1, respectively. The reflected light pulse returned at time t0 is divided into two reflected light pulse components which re-enter the first optical path and the second optical path, respectively; the reflected light pulse returning at time t1 is also divided into twoAnd a reflected light pulse component which re-enters the first and second optical paths, respectively. At this time, on both the first and second optical paths, there are two temporally separated reflected light pulse components (which correspond to times t0 and t1, respectively) with a phase difference of 0 or pi between them, for example, with a phase difference of 0 here, i.e., at this time, the two temporally separated reflected light pulses on the first (second) optical path carry phase-encoded information (i.e., are pairs of light pulses with a phase difference of 0).
When theta is1At 3 pi/2, the first interference will output two interference light pulses, which will enter the first and second reflection units, respectively, to propagate and return after times t0 and t1, respectively. The reflected light pulse returned at time t0 is divided into two reflected light pulse components which re-enter the first optical path and the second optical path, respectively; the reflected light pulse returning at time t1 is also split into two reflected light pulse components which re-enter the first and second optical paths, respectively. At this time, on both the first and second optical paths, there are two temporally separated reflected light pulse components (which correspond to times t0 and t1, respectively) with a phase difference of 0 or pi, for example, here with a phase difference of pi, i.e. when the two temporally separated reflected light pulses on the first (second) optical path carry phase-encoded information (i.e. are pairs of light pulses with a phase difference of pi).
It follows that, in the present invention, the first phase modulation (first phase difference θ)1) And the first and second reflection units may implement time phase encoding on the signal light to be encoded.
When the two reflected light pulse components having time-phase encoded information and originating from the same reflected light pulse propagate on the first and second optical paths, respectively, at least one of the two reflected light pulse components is second phase-modulated with the same phase modulation unit to form a second phase difference θ between the two2. To have a second phase difference theta2The two reflected light pulse components interfere with each other and output a second interference result, the polarization state of the light pulse output by the second interference will be in the second phaseA potential difference theta2It is related.
Therefore, in the present invention, the same phase modulation unit is used to perform two phase modulations on the signal light to be encoded (which respectively provide the first phase difference θ) in sequence during the round trip process of the same optical path1And a second phase difference theta2) Time phase encoding and polarization encoding of the signal light can be achieved.
It is easily understood by those skilled in the art that, in the present invention, the first phase difference θ1And a second phase difference theta2And may have any suitable value to provide the desired temporal phase encoding and polarization state encoding. As an example, when θ1And theta2When the value is taken in (0, pi/2, 3 pi/2), four time phase states and four polarization states, namely sixteen time phase-polarization states, can be provided for the coding of the quantum key.
To further illustrate the encoding method principle of the present invention, fig. 2 shows an exemplary embodiment of the transmitting end for time phase-polarization encoding of the present invention.
As shown in the figure, the transmitting end of the present invention may include a light source 1, an encoding module 2, a first reflecting unit 3, and a second reflecting unit 4.
The light source 1 is used to output signal light to be encoded, which may be in the form of a laser, for example, to output a laser pulse signal.
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 beam splitting unit 212 receives the signal light pulse output from the light source 1 and equally splits it into first and second signal light pulse components in two components 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 specifically, as shown in fig. 2, the signal optical pulse enters via the 3 rd port of the polarization beam splitting unit 212, and the split first and second signal optical pulse components are output via the 1 st port and the 2 nd port thereof, respectively.
The first and second signal optical pulse components are output by the polarization beam splitting unit 212 and propagate along the first optical path and the second optical path, respectively, towards two input ends (e.g., ports 8 and 7) of the beam splitting unit 213. The first optical path and the second optical path are set to have the same optical path length so that the first and second signal optical pulse components can reach the beam splitting unit 213 at the same time. Therefore, the polarization beam splitting unit 212, the beam 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).
Preferably, when the polarization beam splitting unit 212 and the beam splitting unit 213 are connected to construct the equal-arm MZ interferometer, two polarization directions at the beam splitting unit of the interferometer are the same, that is, the polarization beam splitting unit 212 of the equal-arm MZ interferometer splits the signal light to be encoded into a first signal light component and a second signal light component according to two components with polarization directions perpendicular to each other, and when the first interference is performed on the beam splitting unit 213 of the equal-arm MZ interferometer, the same polarization direction is maintained.
The phase modulation unit 211 is disposed on at least one of the first optical path and the second optical path, and is configured to perform phase modulation on the passed optical pulse component so as to form a preset phase difference between the two optical pulse components. For example, as shown in fig. 2, the phase modulation unit 211 may be disposed on the first optical path for performing a first signal optical pulse component of magnitude θ on the first signal optical pulse component transmitted along the first optical path1So as to form a phase difference theta between the first and second signal optical pulse components1。
With a phase difference theta1The first and second signal light pulse components arrive at the beam splitting unit 213 at the same time, it will be readily understood by those skilled in the art that under the structure disclosed in the present invention, the first and second signal light pulse components now appear to have the same polarization direction to the beam splitting unit 213, and therefore the two components will interfere for the first time here. Those skilled in the art will readily appreciate that the result of the first interference is out of phase with the first phase difference θ1Relating, for example, to the first phase difference θ1The interference light pulses determining the output of the first interference are divided into one or two, and the interference light pulses are output from the output port of the beam splitting unit 213. As an example, when θ1When equal to 0, firstThe result of the secondary interference will only be an interfering light pulse output at port 5 of beam splitting unit 213; when theta is1At pi, the result of the first interference will only output an interference light pulse at port 6 of beam splitting unit 213; when theta is1At pi/2 or 3 pi/2, the result of the first interference will output interfering light pulses at ports 5 and 6 of beam splitting unit 213 simultaneously.
Two output ports 5 and 6 of the beam splitting unit 213 are connected to the first reflection unit 3 and the second reflection unit 4, respectively. As an example, as shown in fig. 2, the first reflection unit 3 may include a third optical path and a reflection structure for reflecting light, the second reflection unit 4 may include a fourth optical path and a reflection structure for reflecting light, and the third optical path and the fourth optical path have different optical path length differences. For example, in the first reflection unit, the light pulse entering from port 5 of beam splitting unit 213 will return to port 5 after time t0 by reflection; in the second reflection unit, the light pulse entering through port 6 of beam splitting unit 213 is reflected and returns to port 6 after time t 1.
At the beam splitting unit 213, the reflected light pulse is also equally divided into two components, which propagate along the first and second optical paths, respectively, towards the polarizing beam splitting unit 212. To this end, a time-phase encoded information is formed on the reflected light pulse component propagating on the first and second optical paths, with a first phase difference θ1It is related.
Subsequently, on the first (second) optical path, the phase modulation unit 211 in turn phase modulates (i.e., second phase modulates) two reflected light pulse components having time phase encoded information and originating from the same reflected light pulse, thereby forming a second phase difference θ between the two components2. For example, as shown in fig. 2, the phase modulation unit 211 in the first optical path performs a reflected light pulse component of magnitude θ2So as to form a phase difference θ between two reflected light pulse components having time phase encoded information and originating from the same reflected light pulse2。
Having a second phase difference theta2And time phase encoded information (which is out of phase with the first phase difference θ)1Related) and two reflected lights originating from the same reflected light pulseThe pulse components are simultaneously returned to the polarization beam splitting unit 212 via the first and second optical paths, respectively, where a second interference occurs. Those skilled in the art will readily understand that the polarization direction of the second interference result output by the polarization beam splitting unit 212 (e.g., the interference light pulse output via the 4 th port in fig. 2) at this time will be equal to the second phase modulation amount θ provided by the phase modulating unit 2112It is related. For example, the polarization state of the interference light pulse output by the second interference may be
Obviously, in the encoding block 2, the second-order phase modulation amount θ is controlled2The polarization state of the interference light pulse in the second interference result output by the interference device can be adjusted, so that the modulation of the polarization state of the signal light pulse is realized. To this end, the encoding module 2 outputs the interference light pulse based on the second interference, which has the first phase difference θ1Associated time phase encoded information and a second phase difference theta2The relevant polarization encoding information, that is, the time phase encoding and the polarization encoding of the signal light to be encoded output from the light source 1 are realized.
The following is a table illustrating the time phase-polarization encoding results that can be achieved by the transmitting end and the encoding method of the present invention.
Phase modulation theta1And theta2Correspondence table with codes
In the transmitting end structure of the present invention, an equal-arm MZ interferometer having a phase modulation function is configured by a polarization beam splitter unit 212 (e.g., a polarization beam splitter), a beam splitter unit 213 (e.g., a beam splitter) and a phase modulation unit 211, and first, with the polarization beam splitter 212 as an input end and the beam splitter 213 as an output end, time phase encoding information is provided on a signal light pulse based on first time phase modulation and first and second reflection units provided by the phase modulation unit 211, thereby completing time phase encoding; meanwhile, the optical signal can be returned to the equal-arm MZ interferometer again, and the polarization state modulation of the signal optical pulse is realized based on the second phase modulation provided by the phase modulation unit 211 with the beam splitter 213 as an input end and the polarization beam splitter 212 as an output end, so that the polarization state encoding is completed, and finally the time phase encoding and the polarization state encoding of the signal optical pulse are completed. By means of the special transmitting end structure and the encoding method, the time phase encoding of the signal and the multiplexing of the encoding module 2 are realized through the two reflecting units, so that the time phase and the polarization encoding of the same signal light pulse are realized by using the same light path structure (even the same phase modulation unit 211). In addition, in such a structure, the requirement for the phase-modulated electrical signal is low, and only two arrival times of the pulse need to be distinguished (i.e., only two times of phase modulation need to be performed) in one cycle.
Further, the present inventors have noticed that, due to the use of the equiarm MZ interferometer structure in the transmitting-end structure of the present invention, a phase shift may occur in a signal optical pulse while propagating therein. Therefore, the phase detection feedback module is also preferably in the transmitting end in the present invention. As an example of the phase detection feedback module, it 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 light splitting unit is arranged for splitting a part of the light pulse to the phase detection unit; the phase detection unit is used for detecting the phase drift amount; the phase feedback algorithm unit is used for calculating the phase quantity needing feedback compensation according to the detected phase drift quantity; the phase shifter is arranged in the equal-arm MZ interferometer, and the signal light pulse is correspondingly phase-compensated under the driving signal provided by the phase shifter driving unit. The phase detection feedback module carries out phase detection, phase feedback and the like aiming at forward or backward modulation effects of the equal-arm MZ interferometer, and the equal-arm MZ interferometer has a forward and backward modulation effect in the patent, so that the forward or backward modulation effects can be kept stable by the phase stability of the interferometer. Since those skilled in the art can understand and implement the corresponding phase detection, calculation and feedback processes under the condition that the functions of the units of the phase detection feedback module have been described, details thereof are not repeated herein.
By way of example, FIGS. 3a-3d illustrate the phase difference θ at the first phase difference, respectively1When the time phase is 0, pi/2 and 3 pi/2, respectively, the time phase-polarization encoding method and the encoding process under the structure of the transmitting end are based on the invention.
FIG. 3a shows θ1Coding process when 0: the signal light to be encoded, which is generated by the light source 1, is input from port 3 of the polarization beam splitter 212, and is split into first and second signal light pulse components, which enter the first optical path and the second optical path via ports 1 and 2 of the polarization beam splitter 212, respectively. The phase modulator 211 on the first optical path does not modulate the phase of the first component, i.e. the first phase difference θ provided by the phase modulator 211 between the first and second components 10. The two optical pulse components with zero phase difference arrive at ports 8 and 7 of beam splitter 213 at the same time and interfere there, only interfering optical pulses will appear at output port 5 of the beam splitter. This first interference light pulse is reflected back to the beam splitter 213 after a time t0 in the first reflection unit 3 and is split by it into two reflected light pulse components, which at this time will have time bit information related to the time t 0. Two reflected light pulse components having time bit information t0 propagate via the first and second optical paths, respectively, and the phase modulator 211 may perform a second phase modulation on the reflected light pulse component on the first optical path to form a second phase difference θ between the two reflected light pulse components2. Having a second phase difference theta2And the two reflected light pulse components with time bit information t0 arrive at the polarization beam splitter 212 at the same time and generate interference, and the polarization state of the signal light output by the second interference can be expressed as
And has a phase difference theta with1Associated time bit informationAnd t 0. That is, the interference light pulse output at this time is a signal light carrying both time-encoded information and polarization-encoded information.
FIG. 3b shows θ1Coding process when pi: the signal light to be encoded, which is generated by the light source 1, is input from port 3 of the polarization beam splitter 212, and is split into first and second signal light pulse components, which enter the first optical path and the second optical path via ports 1 and 2 of the polarization beam splitter 212, respectively. The phase modulator 211 on the first optical path applies a modulation amount of pi to the phase of the first component, i.e., a first phase difference θ provided by the phase modulator 211 between the first and second components1Pi. The two optical pulse components with a phase difference of pi arrive at the ports 8 and 7 of the beam splitter 213 at the same time and interfere there, only interfering optical pulses will appear at the output port 6 of the beam splitter. This first interference light pulse is reflected back to the beam splitter 213 after a time t1 in the second reflecting unit 4 and is split by it into two reflected light pulse components, which now will have time bit information related to the time t 1. The two reflected light pulse components with the time bit information t1 propagate via the first and second optical paths, respectively, and the phase modulator 211 may perform a second phase modulation on the reflected light pulse component on the first optical path to form a second phase difference θ between the two components2. Having a second phase difference theta2And the two reflected light pulse components with time bit information t1 arrive at the polarization beam splitter 212 at the same time and generate interference, and at this time, the polarization state of the signal light output by the second interference can be expressed as
And has a phase difference theta with1The relevant time bit information t 1. That is, the interference light pulse output at this time is a signal light carrying both time-encoded information and polarization-encoded information.
FIG. 3c shows θ1And (3) coding process when the ratio is pi/2: the signal light to be encoded, generated by the light source 1, is input from port 3 of the polarization beam splitter 212 and is split into first and second signal light pulse componentsThe components enter the first and second optical paths via ports 1 and 2 of polarizing beam splitter 212, respectively. The phase modulator 211 on the first optical path applies a modulation amount of pi/2 to the phase of the first component, i.e., a first phase difference θ provided by the phase modulator 211 between the first and second components1Pi/2. The two optical pulse components with a phase difference of pi/2 arrive at the ports 8 and 7 of the beam splitter 213, respectively, simultaneously and interfere there, when interfering optical pulses appear at both output ports 5 and 6 of the beam splitter. The two interference light pulses return to the beam splitter 213 after times t0 and t1 in the first and second reflection units, respectively, and are separated into two reflected light pulse components by it at times t0 and t1, respectively, and propagate via the first and second optical paths. At this time, corresponding to the signal light pulse to be encoded, there are two reflected light pulse components separated in time on the first (second) optical path, which correspond to times t0 and t1, respectively, and have a phase difference of 0 from each other, whereby it is seen that the pair of reflected light pulse components carry phase-encoded information thereon. The phase modulator 211 phase modulates the two reflected light pulse components originating from the same reflected light pulse on the first and second optical paths a second time corresponding to times t0 and t1, respectively, and forms a second phase difference θ between them2. Having a second phase difference θ corresponding to times t0 and t1, respectively2The two reflected light pulse component pairs arrive at the polarization beam splitter 212 at the same time and generate interference, and the polarization state of the light pulse output by the second interference can be expressed as
It follows that the result of the second interference, corresponding to the signal light pulses to be encoded, comprises a phase difference θ from the second phase2A pair of interference light pulses (which correspond in time to times t0 and t1, respectively) of corresponding polarization states and separated in time, and a phase difference between the pair of interference light pulses and a first phase difference θ1Here, the value is 0, for example. That is, the interference light pulse pair output at this time carries both the phase encoding information and the polarization encoding information.
FIG. 3d shows θ1Coding at 3 pi/2The process, which is similar to that shown in fig. 3c, outputs interference light pulse pairs carrying both phase encoded information and polarization encoded information, but differs from that of fig. 3c in that the phase encoded information is carried differently, for example, when the phase difference between two interference light pulses is pi instead of 0.
In summary, with the time phase-polarization joint encoding method and the sending end structure of the present invention, it is possible to carry 2 kinds of bit information (time-polarization bits or phase-polarization bits) on 1 photon, thereby greatly increasing the key transmission rate of QKD, and increasing the key rate by 1 time under the same condition. In addition, the phase detection feedback module can also compensate the deviation of the phase modulation caused by the optical fiber environment.
In addition, the invention also discloses a quantum key distribution system comprising the sending end for the time phase-polarization combined coding and a quantum key distribution method comprising the time phase-polarization combined coding method.
The above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and the above alternatives may be used in combination with each other without contradiction. Those skilled in the art will also appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.
Claims (16)
1. A transmitting end for time phase-polarization joint encoding comprises a light source (1), an encoding module (2), a first reflecting unit (3) and a second reflecting unit (4),
the light source (1) is used for providing signal light to be coded;
the encoding module (2) comprises a phase modulation unit (211) and is arranged to: dividing the signal light to be encoded into a first signal light component and a second signal light component; -performing a first phase modulation of at least one of said first and second signal light components by means of said phase modulation unit (211); and interfering the first and second signal light components and outputting one or two first interference light signals;
the first reflecting unit (3) and the second reflecting unit (4) have different optical paths from each other and are arranged to receive the first interfering light signal and reflect it back to the encoding module (2);
the encoding module (2) is further configured to: receiving a reflected optical signal and separating it into a first reflected optical signal component and a second reflected optical signal component, wherein the reflected optical signal component has time phase encoded information; -second phase modulating at least one of the first and second reflected light signal components by means of the phase modulation unit (211); and interfering the first and second reflected optical signal components and outputting a second interference optical signal, wherein the second interference optical signal has the time phase encoded information and polarization encoded information.
2. The transmitting end according to claim 1,
the first reflection unit (3) comprises an optical fiber and a reflection structure arranged at the tail end of the optical fiber, wherein the optical fiber is a polarization maintaining optical fiber, and the reflection structure is a common reflector or a reflection film; or the optical fiber is a polarization maintaining optical fiber or a common optical fiber, and the reflecting structure is a Faraday reflector or a Faraday reflecting film; and the number of the first and second electrodes,
the second reflection unit (4) comprises an optical fiber and a reflection structure arranged at the tail end of the optical fiber, wherein the optical fiber is a polarization maintaining optical fiber, and the reflection structure is a common reflector or a reflection film; or, the optical fiber is a polarization maintaining optical fiber or a common optical fiber, and the reflecting structure is a Faraday reflector or a Faraday reflecting film.
3. The transmitting end according to claim 1, wherein the encoding module (2) further comprises a polarization beam splitting unit (212) and a beam splitting unit (213), and the polarization beam splitting unit (212) and the beam splitting unit (213) are connected through a first optical path and a second optical path having the same optical path to form an equal-arm MZ interferometer; and the phase modulation unit (211) is provided on at least one of the first and second optical paths.
4. The transmission end according to claim 3, wherein the polarization beam splitting unit (212) receives the signal light to be encoded and outputs the second interference light signal with the time phase encoding information and polarization encoding information; and/or the beam splitting unit (213) connects the first reflecting unit (3) and the second reflecting unit (4) and outputs the first interference light signal and receives the reflected light signal.
5. The transmitting end of claim 3, further comprising a phase detection feedback module for compensating for phase drift in the equal-arm MZ interferometer; and/or the polarizing beam splitting unit (212) comprises a polarizing beam splitter; and/or the beam splitting unit (213) comprises a beam splitter.
6. The transmission end of claim 1, wherein the first time phase modulation forms a first phase difference θ between the first and second signal light components1Is one of 0, pi/2, pi and 3 pi/2; and/or the second time phase modulation forms a second phase difference θ between the first and second reflected light signal components2Is one of 0, pi/2, pi and 3 pi/2.
7. A quantum key distribution system comprising the transmitting end of any one of claims 1-6.
8. An encoding method for temporal phase-polarization joint encoding, comprising,
a time phase encoding step: enabling signal light to be coded to pass through a phase modulation unit to perform first-time phase modulation on the signal light, and generating time phase coding information on the signal light by means of reflection of light based on the first-time phase modulation; and
a polarization encoding step: the reflected signal light having the time phase encoding information is subjected to the phase modulation unit again to perform a second phase modulation thereon, and polarization encoding information is generated on the reflected signal light having the time phase encoding information based on the second phase modulation.
9. The encoding method of claim 8, wherein the time phase encoding step further comprises the steps of: splitting the signal light to be encoded into a first signal light component and a second signal light component; performing the first phase modulation on at least one of the first and second signal light components using the phase modulation unit to form a first phase difference θ therebetween1(ii) a So as to have the first phase difference theta1The first and second signal light components generate a first interference effect and output one or two paths of first interference light signals; and according to the first phase difference theta1And enabling the first interference optical signal to enter at least one of a first reflection unit and a second reflection unit with different optical paths and to be reflected.
10. The encoding method of claim 8, wherein the polarization encoding step further comprises the steps of: splitting the reflected signal light having the time phase encoded information into first and second reflected light signal components; performing the second phase modulation on at least one of the first and second reflected optical signal components with the phase modulation unit to form a second phase difference θ therebetween2(ii) a And having the second phase difference theta2And a second interference occurs with the first and second reflected optical signal components.
11. The encoding method of claim 9, wherein:
the equal-arm MZ interferometer is constructed by utilizing a polarization beam splitting unit and a beam splitting unit, and the phase modulation unit is arranged in the equal-arm MZ interferometer, and the first interference effect occurs at the beam splitting unit.
12. The encoding method of claim 10, wherein: the equal-arm MZ interferometer is constructed by utilizing a polarization beam splitting unit and a beam splitting unit, the phase modulation unit is arranged in the equal-arm MZ interferometer, and the second interference effect occurs at the polarization beam splitting unit.
13. The encoding method of claim 9, wherein the time phase encoded information is out of phase with the first phase difference θ1And the optical path of the reflecting unit.
14. The encoding method of claim 10, wherein the polarization encoded information differs from the second phase by a difference θ2It is related.
15. The encoding method of claim 9, wherein the first phase difference θ1Is one of 0, pi/2, pi and 3 pi/2.
16. The encoding method of claim 10, wherein the second phase difference θ2Is one of 0, pi/2, pi and 3 pi/2.
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