CN106161011B - Plug-and-play quantum key distribution system and method based on time-phase coding, sending end and receiving end - Google Patents
Plug-and-play quantum key distribution system and method based on time-phase coding, sending end and receiving end Download PDFInfo
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
- H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
- H04L9/0852—Quantum cryptography
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
- H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
- H04L9/0819—Key transport or distribution, i.e. key establishment techniques where one party creates or otherwise obtains a secret value, and securely transfers it to the other(s)
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
- H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
- H04L9/0852—Quantum cryptography
- H04L9/0858—Details about key distillation or coding, e.g. reconciliation, error correction, privacy amplification, polarisation coding or phase coding
Abstract
The invention discloses a plug-and-play quantum key distribution system and method based on time-phase coding, a sending end and a receiving end. The plug-and-play quantum key distribution system comprises a sending end and a receiving end which are mutually connected in an optical mode, wherein an encoding module in the sending end comprises a Z-base vector time encoding module and a phase encoding module which perform encoding processing on optical signals in any order, and the phase encoding module is an X-base vector phase encoding module; the decoding module in the receiving end is adapted to the encoding module. The plug-and-play quantum key distribution method comprises the steps of carrying out Z-basis vector time coding and phase coding after receiving and reflecting an optical signal from a Bob end at an Alice end, and then sending the optical signal to the Bob end for decoding and detecting, wherein the phase coding is X-basis vector phase coding. The invention can realize the coding and decoding with ultrahigh contrast ratio and improve the code rate by using the improved time-phase coding.
Description
Technical Field
The invention relates to the field of quantum key distribution, in particular to a plug-and-play quantum key distribution system based on time-phase coding.
Background
Quantum secure communication has been receiving attention from people in various fields as a novel research field combining quantum mechanics, electronic information science, and computer technology. In order to ensure the secure transmission of data, secure key exchange must be ensured between users. Quantum key distribution has now become an efficient way of absolutely secure key distribution over public channels. Different from the traditional confidentiality principle of classical cryptographic communication, quantum key distribution utilizes the basic principle of quantum mechanics as a support, such as the heisenberg inaccuracy-measuring principle, quantum unclonable theorem and the like, and realizes unconditionally safe random key sharing.
Many QKD devices can operate in the field of free space or optical fiber, planar optical waveguide, and other transmission media. The QKD system includes light source generation, encoding and decoding, detection, data acquisition and extraction, data post-processing, etc. In fiber QKD systems, both polarization and phase encoding formats are typically employed. Under long-distance transmission, the stability of the phase encoding QKD is higher than that of a polarization encoding QKD system, and the phase encoding is more advantageous than the polarization encoding. Phase encoded QKD systems are typically built using an unequal-arm mach-zehnder interferometer or an unequal-arm michelson interferometer using a 90-degree rotating faraday mirror.
In general, in order to obtain a better interference contrast in a quantum channel formed by unequal arm Mach-Zehnder interferometers, it is required that the two Mach-Zehnder interferometers are as symmetrical as possible, which is difficult to achieve in practical operation, so Gisin et al proposed a "plug and play" type experimental scheme in 1997 (documents: muller A, herzog T, huttnerB, et al, "plug and play" systems for quaternary cryptography [ J ]. Applied physics Letters,1997, 793-795). The scheme mainly adopts three 90-degree rotating Faraday reflectors, a pulse light source is divided into two paths through a coupler, light pulses are reflected back and forth in the three 90-degree rotating Faraday reflectors, each light pulse passes through all light paths once, polarization is automatically compensated, and the optical fiber birefringence influence is eliminated. The working frequency of the system reaches 1KHZ, and when the working frequency is increased, the error rate is also increased. To solve this problem and to achieve a higher key generation rate, gisin et al have improved the "plug and play" type of experimental protocol in 1998 (literature: ribordy G, gautier J D, gisin N, et al, automated "plug and play" quality key distribution [ J ]. Electronics Letters,1998, 34. An unequal arm Mach-Zehnder interferometer and a 90-degree rotating Faraday reflector are mainly adopted to form a reciprocating optical path. The Faraday mirror is mainly used for reflecting incident light by rotating 90 degrees, and the polarization of the incident light is just rotated by 90 degrees, so that polarization drift caused by the birefringence effect of the optical fiber during long-distance transmission is overcome. Meanwhile, the 90-degree rotating Faraday reflector and the unequal arm Mach-Zehnder interferometer form a reciprocating loop, so that the self-compensation effect of the phase is realized.
Referring to fig. 1, the phase-coded quantum key distribution system includes a sending end (Alice end) and a receiving end (Bob end) that are optically connected to each other, where all devices and optical fibers at the Bob end are polarization-maintaining, a pulse light source 100 of a light source module in the Bob end emits a linearly polarized light pulse, enters a coupler 107 through a circulator 101, and is divided into two horizontal polarized light pulses (| H >), the two light pulses respectively pass through a long arm and a short arm in an unequal arm interference module, where a light pulse a passing through the long arm is processed by a phase modulator 106, reflected by a polarization-maintaining polarization beam splitter 105, and then output in the form of a vertically polarized light pulse (| V >), and a light pulse B passing through the short arm is directly transmitted and output in the form of a horizontally polarized light pulse (| H >) by the polarization beam splitter 105. The polarization-maintaining polarization beam splitter has the function of converting one path of horizontal polarized light pulse (| H >) at the reflection end into vertical polarized light pulse (| V >) for transmission or converting the vertical polarized light pulse (| V >) into the horizontal polarized light pulse (| H >) for transmission.
The two light pulses reach a 90-degree rotating Faraday mirror 103 at the Alice end through a quantum channel 102, and after reflection, the polarization directions of the two light pulses are respectively rotated by 90 degrees.
The light pulse a is reflected to be a horizontal polarized light pulse (| H >) and is transmitted into the coupler 107 through the polarization-maintaining polarization beam splitter 105; the light pulse B is reflected and then changed into a vertical polarized light pulse (| V >) which is processed by a phase modulator 104 of a transmitting end coding module, and then is reflected by a polarization-maintaining polarization beam splitter 105 and changed into a horizontal polarized light pulse (| H >) which enters a coupler 107 to interfere with the reflected light pulse A; and respectively reach the single-photon detector 108 and the single-photon detector 109 according to different interference information.
The system needs to add a phase modulator in a measurement part of a receiving end to select an X measurement basis vector or a Y measurement basis vector, the added phase modulator brings extra insertion loss and reduces the code rate, and the device has inherent problems, so that the interference contrast is not high enough, and the code rate is reduced.
Chinese patent ZL200610170557.5 proposes quantum key distribution using a pair of faraday michelson interferometers with unequal arms based on a four-port polarization beam splitter to implement polarization-controlled phase encoding and polarization detection. The above patent adopts a pair of one-to-one corresponding encoding and decoding devices, i.e. a pair of unequal arm interferometers, at the sender and the receiver, which requires that the arm length difference (difference between the long arm and the short arm) of the unequal arm interferometers at both communication sides is equal, but the prior art and the external environment influence hardly ensure the phase stability of the arm length difference of the two unequal arm mach-zehnder interferometers for a long time.
In summary, the prior art mainly has the following problems:
1. the conventional phase encoding QKD system using an unequal arm mach-zehnder interferometer requires that the arm length difference (difference between the long arm and the short arm) of the unequal arm mach-zehnder interferometers of both communication sides be equal. However, due to the influence of the existing process and the external environment, the stability of the arm length difference phase of the two unequal arm Mach-Zehnder interferometers is difficult to ensure for a long time.
2. The existing phase coding system needs to add a phase modulator in a measurement part of a receiving end, extra attenuation is brought, and the code rate is reduced.
3. The existing phase coding system needs to generate a high-speed random electric signal to actively control the selection of unbalanced basis vectors, thereby influencing the repetition frequency of the system and the precise modulation of the phase so as to reduce the code rate.
Disclosure of Invention
The invention provides a sending end of a quantum key distribution system, wherein the sending end adopts an improved time-phase coding mode to improve the code rate, and correspondingly, the hardware requirement of a Bob end decoding process can be simplified.
A sending end of a plug-and-play quantum key distribution system based on time-phase coding comprises a 90-degree rotary Faraday reflector for forming a reflection light path and a coding module arranged on the reflection light path, wherein the coding module comprises a Z basis vector time coding module and a phase coding module which correspondingly code optical signals, and the phase coding module is an X basis vector phase coding module.
The invention combines a time-phase coding mode into an instant-plug and-play system, a circuit part for sending a control signal to a coding module can adopt the existing hardware architecture, only adopts X-base vector phase coding in the phase coding process, only uses one phase modulator to carry out phase coding at a sending end, and does not arrange the phase modulator at a receiving end, thereby reducing the insertion loss caused by the device of the phase modulator, simultaneously avoiding the condition of low code rate caused by the external hardware control of the phase modulator, and further improving the code rate during decoding.
Preferably, the X-basis vector phase encoding module is a phase modulator, and the Z-basis vector time encoding module is an intensity modulator.
Existing hardware architectures, optical systems, and encoding procedures can be employed for X-basis vector phase encoding as well as Z-basis vector temporal encoding per se.
Regarding the relationship between the Z-basis vector time coding module and the phase coding module, the existing method is also adopted, namely, the basis vectors are randomly selected during coding, the Z-basis vector time coding is selected according to the probability of P, and the phase coding is selected according to the probability of 1-P. The value of P is set according to the actual application requirement, and P is more than 0 and less than 1.
Preferably, the transmitting end is further provided with a reflected light intensity modulator arranged on the reflected light path for implementing decoy.
Preferably, the reflected light intensity modulator is located between the 90-degree rotating faraday mirror and the encoding module. I.e. spoofing is performed using a reflected light intensity modulator prior to encoding.
The reflected light intensity modulator realizes sending of 'decoy state' optical signals by changing light intensity, and can ensure that the efficiency of single photon components is not modified by an attacker, and the single photon components are effective components which can extract a security key in a quantum key distribution system. Accordingly, the light generated by the light source at the receiving end is weak coherent light. The reflected light intensity modulator modulates the intensity of the optical signal reflected back by the 90-degree rotating Faraday reflector to change the proportion of single-photon components to multi-photon components in the output optical pulse.
The reflected light path starts from a 90-degree rotating faraday mirror, the reflected light intensity modulator may act on the optical signal before encoding or after encoding, and the phase modulator and the intensity modulator may also be arranged in different orders on the reflected light path.
Preferably, the reflected light intensity modulator, the phase modulator and the intensity modulator are arranged in sequence on the reflected light path.
The invention also provides a receiving end of the plug-and-play quantum key distribution system based on time-phase coding, which comprises a light source for sending optical signals to a sending end and a decoding module for receiving and detecting the coded optical signals from the sending end, wherein the decoding module comprises a Z-basis vector time detection module and a phase detection module for correspondingly decoding the optical signals, and the phase detection module is an X-basis vector phase detection module.
The Z-basis vector time detection module comprises:
the beam splitting module is used for splitting the optical signal coded by the sending end; and
the single photon detector is connected with the first output end of the beam splitting module to detect Z basis vector time;
and the phase detection module is connected with the second output end of the beam splitting module.
Preferably, the beam splitting module is a beam splitter.
The splitting ratio of the beam splitter can be arbitrarily changed as required. According to the characteristic of the indivisible property of single photon pulses, actually, an optical signal coded by a sending end is output from only one output port (including a first output port and a second output port) after passing through a beam splitting module, namely, the optical signal is output from either the first output port or the second output port, and due to different beam splitting ratios, the probability of output from different output ports is different.
The optical signals coded by the sending end pass through the beam splitting module and then are output from different output ports and enter corresponding detectors, and the corresponding detection basis vectors are selected when the optical signals enter the different detectors. For example: and the single photon detector which performs Z-base vector time detection after passing through the first output port is used for selecting the Z-base vector to perform detection, and the single photon detector which reaches the phase detection module after passing through the second output port is used for selecting the X-base vector.
In the present invention, the beam splitter is understood to be a device capable of splitting or combining beams according to different optical signal transmission directions, and similarly, the first output end and the second output end refer to an optical signal transmission direction from the transmitting end, and if the optical signal transmission direction is from the receiving end to the transmitting end, the second output end can also be regarded as an input end.
The beam splitter is an optical element with a fixed splitting ratio, electric signal control is not needed, the code rate can be further improved, and when the splitting ratio needs to be changed, the beam splitters with different splitting ratios can be directly replaced.
Preferably, the light energy output by the beam splitter to the single-photon detector is not equal to the light energy output by the phase detection module, for example, the light energy output by the beam splitter to the single-photon detector is greater than the light energy output by the phase detection module. Thus, the unbalanced basis vector decoding is realized, and the resultant code rate is favorably improved.
The phase detection module comprises an unequal arm interference module arranged between the beam splitting module and the light source and a detector module used for detecting an optical signal output by the unequal arm interference module; the light path selection is carried out among the light source, the unequal arm interference module and the detector module through the circulator.
The unequal arm interference module receives the other path output of the beam splitting module, the circulator is mainly used for light path selection, the light source transmits light signals to the transmitting end through one path (the light circulator performs path selection to enable the signal light emitted by the light source to be transmitted to the transmitting end after passing through the unequal arm interferometer), and the light source receives and detects coded light signals transmitted from the transmitting end through the other path (the light circulator performs path selection to enable the signal light returned by the receiving end to be transmitted to the detector module after passing through the unequal arm interferometer).
The unequal arm interference module is an unequal arm Mach-Zehnder interferometer or an unequal arm interferometer based on the Michelson principle.
Alternatively, the number of the single photon detectors in the detector module is two, and the two single photon detectors respectively detect two different interference results from the unequal arm interference module.
Preferably, the number of the single-photon detectors in the detector module is one, and two different interference results from the unequal arm interference module are multiplexed to the same single-photon detector through corresponding optical path elements.
As the decoding module, the optical signal output by the unequal arm interference module is understood to be the optical signal output to the single photon detector after receiving the encoded optical signal emitted from the sending end without special description.
Preferably, the number of the single-photon detectors in the decoding module is one, and the output of the first output end of the beam splitting module and two different interference results output by the unequal-arm interference module are both coupled to the same single-photon detector through corresponding optical path elements. The whole decoding module only has one single-photon detector, so that the hardware overhead is further reduced.
When the unequal arm interference module adopts an unequal arm Mach-Zehnder interferometer:
as an alternative, in the unequal arm interference module, two ends of a long arm and a short arm are respectively coupled through a beam splitter on one side close to the circulator and a polarization beam splitter on one side far away from the circulator, and a 90-degree polarization rotator and a delay line are arranged on the long arm.
As another alternative, in the unequal arm interference module, two ends of the long arm and the short arm are respectively coupled through corresponding beam splitters, and only a delay line is arranged on the long arm. I.e. the 90-degree polarization rotator is omitted.
Two single-photon detectors in the detector module are respectively a first single-photon detector and a second single-photon detector, one output end of the beam splitter close to one side of the circulator is connected with the first single-photon detector through a first detection branch, the other output end of the beam splitter is connected with the circulator and is connected with the second single-photon detector through a second detection branch, and the other interface of the circulator is connected with a light source.
When the detector module adopts a single-photon detector, the outputs of the first detection branch and the second detection branch are coupled and then enter the same single-photon detector.
For example, the first detection branch is coupled into the second detection branch through a polarization-maintaining polarization beam splitter (or a beam splitter) after passing through a delay line, and the first detection branch and the second detection branch are coupled and then enter the same single photon detector. Similarly, the delay line may be disposed in the second detection branch, or both detection branches may be disposed, as long as the time to reach the detector can be resolved.
When the whole decoding module adopts a single-photon detector, the first output end of the beam splitting module is connected with a third detection branch, the first detection branch is coupled with the second detection branch and then coupled with the third detection branch through a beam splitter, and the three detection branches enter the same single-photon detector.
The unequal-arm interferometer based on the Michelson principle comprises a first 90-degree rotating Faraday reflector, a second 90-degree rotating Faraday reflector, a polarization beam splitter, a delay line and a half-wave plate; the direction of the fast axis or the slow axis of the half-wave plate forms an angle of 22.5 degrees or-22.5 degrees with any one of the two polarization bases of the polarization beam splitter;
in a reflection light path from the transmitting end, one path of light is transmitted to a first 90-degree rotating Faraday reflector through a polarization beam splitter, the other path of light is reflected to a second 90-degree rotating Faraday reflector through a delay line through the polarization beam splitter, the reflection light from the first 90-degree rotating Faraday reflector and the second 90-degree rotating Faraday reflector is coupled through the polarization beam splitter and then enters a detector module through a circulator through a half-wave plate, and the other interface of the circulator is connected with a light source.
In the scheme, the detector module comprises a polarization beam splitter and two single-photon detectors, and the output of the half-wave plate is divided into two paths after passing through the polarization beam splitter and respectively enters the corresponding single-photon detectors.
The invention also provides a plug-and-play quantum key distribution system based on time-phase coding, which comprises a sending end and a receiving end which are mutually connected in an optical way, wherein a coding module in the sending end comprises a Z basis vector time coding module and a phase coding module which correspondingly code optical signals, and the phase coding module is an X basis vector phase coding module; the decoding module in the receiving end is adapted to the encoding module.
In the plug-and-play quantum key distribution system, the further improvement of the sending end and the receiving end can be both provided by the sending end and the receiving end.
The invention also provides a plug-and-play quantum key distribution method based on time-phase coding, which comprises the steps of carrying out Z basis vector time coding or phase coding after a sending end receives and reflects an optical signal from a receiving end, and then sending the optical signal to the receiving end for decoding and detection, wherein the phase coding is X basis vector phase coding; the decoding mode of the receiving end is adaptive to the coding mode of the transmitting end.
The plug-and-play quantum key distribution system adopts an unequal arm interferometer and a 90-degree rotating Faraday reflector to form a forward loop and a backward loop, and the interference principle of the forward loop and the backward loop is similar to that of a Sagnac loop. The arm length difference phase stability of the unequal-arm interferometer can be ensured for a long time. Two light pulses are transmitted back and forth between a transmitting side and a receiving side, and each light pulse passes through all light paths once to achieve the self-compensation effect. Therefore, the system structure is simpler, and the resultant code rate is higher.
In the existing phase coding system, a phase modulator needs to be added to a measurement part at a receiving end to select an X or Y measurement basis vector, and the added phase modulator brings extra insertion loss and reduces the bit rate. The invention does not need to add a phase modulator at the receiving end. The adopted time-phase encoding takes a Z basis vector and an X basis vector as measurement basis vectors, the Z basis vector time encoding is mainly realized by randomly modulating the intensities of two light pulses through an intensity modulator (the light intensity of one light pulse is modulated to be close to a vacuum state of 0, so that only a first light pulse or only a second light pulse is randomly generated, and the Z basis vector time encoding is realized through the phase modulator).
The existing phase coding system needs to use a signal generator to generate a high-speed random electric signal to actively control the selection of unbalanced basis vectors, the cost is higher, and the accurate modulation of the system phase is limited due to the performance of a device, so that the code rate is reduced. The invention can change the beam splitting ratio of the reflected light entering the Z-basis vector time detection module according to the requirement, and can realize the high-efficiency QKD scheme of the passive unbalanced basis vector at the measuring end without active modulation.
Aiming at the problem that the interference contrast is not high enough, the method selects the Z-basis vector time coding to realize the coding and decoding with ultrahigh contrast, thereby improving the code rate.
Drawings
FIG. 1 is a schematic diagram of a prior art plug-and-play quantum key distribution system;
fig. 2 is a schematic diagram of a plug-and-play quantum key distribution system of embodiment 1;
FIG. 3 is a schematic view of embodiment 2;
FIG. 4 is a schematic view of embodiment 3;
FIG. 5 is a schematic view of embodiment 4;
fig. 6 is a schematic view of embodiment 5.
Detailed Description
Referring to fig. 2, the plug-and-play quantum key distribution system of the present invention includes an Alice end and a Bob end optically connected, where a quantum channel is formed between the Alice end and the Bob end, and the transmission form of the quantum channel may be an optical fiber, a free space, a planar optical waveguide, etc.
The Alice terminal comprises: the 90-degree rotating Faraday reflector 9 and the encoding module are sequentially provided with a phase modulator 8 serving as an X-basis vector phase encoding module and an intensity modulator 7 serving as a Z-basis vector time encoding module along the direction of reflected light, and X-basis vector phase encoding or Z-basis vector time encoding is adopted randomly during encoding.
The Bob end comprises a light source module and a decoding module, wherein the decoding module comprises a Z basis vector time detection module and an X basis vector detection module.
The light source module of the present embodiment adopts a pulse light source 1, which is an ideal single photon source or a weak coherent light source combined with a decoy state, and the Z-basis vector time detection module includes a beam splitting module and a single photon detector 12.
In this embodiment, the beam splitting module may adopt a device for implementing a beam splitting/combining function, preferably adopts the beam splitter 6, the beam splitting ratio thereof can be arbitrarily selected according to the requirement, and in order to increase the code rate, the light energy entering the single photon detector 12 should be greater than or less than the light energy entering the X-base vector detection module.
The beam splitter 6 is an optical element with a fixed splitting ratio, electric signal control is not needed, the code rate can be further improved, and when the splitting ratio needs to be changed, the beam splitters with different splitting ratios can be directly replaced.
The X-base vector detection module comprises an unequal arm interference module, a circulator 2, a single-photon detector 10 (a second single-photon detector) and a single-photon detector 11 (a first single-photon detector). The unequal arm interference module can be an unequal arm Mach-Zehnder interferometer or a Michelson interferometer. In the embodiment, an unequal arm mach-zehnder interferometer is adopted, and the unequal arm mach-zehnder interferometer mainly comprises a beam splitter 3, a 90-degree polarization rotator 4, a delay line and a polarization beam splitter 5.
The polarization beam splitter 5 can be replaced by a beam splitter, but in this case, the 90-degree polarization rotator 4 needs to be removed, and the interference efficiency will be reduced by half after the replacement.
In the operation of the quantum key distribution system of the present embodiment, the pulse light source 1 transmits a linearly polarized light pulse, and the linearly polarized light pulse is input to the beam splitter 3 through the circulator 2.
One path of light pulse output by the beam splitter 3 passes through the long arm of the unequal arm interference module, the polarization state of the light pulse is rotated by 90 degrees by the 90-degree polarization rotator 4 and then is input into the polarization beam splitter 5, and only vertical polarization light pulse (namely | V > light) is output after being reflected by the polarization beam splitter 5. The other path of light pulse output by the beam splitter 3 directly enters the polarization beam splitter 5 through the short arm of the unequal arm interference module, and only horizontal polarized light pulse (i.e., | H > light) is output after being transmitted by the polarization beam splitter 5.
Two beams of light pulses are respectively reflected and transmitted by the polarization beam splitter 5 to be synthesized into a path of light pulse with mutually vertical polarization, then sequentially pass through the beam splitter 6 and the quantum channel 13 to reach an Alice end, and sequentially pass through the intensity modulator 7 and the phase modulator 8 to reach the 90-degree rotating Faraday mirror 9, wherein the intensity modulator 7 and the phase modulator 8 do not work. After the action of the 90-degree rotating Faraday reflector 9, the original horizontal polarized light pulse (| H >) becomes vertical polarized light pulse (| V >), and the original vertical polarized light pulse (| V >) becomes horizontal polarized light (| H >). And returning the original path to an encoding module for encoding, randomly selecting a basis vector during encoding, selecting z basis vector time encoding according to the probability of P, and selecting x basis vector phase encoding according to the probability of 1-P.
When the Z-basis vector time coding is selected, the intensity modulator 7 randomly modulates the intensity of two light pulses, and modulates the light intensity of one light pulse to be close to a zero vacuum state, so that the Z-basis vector time coding is realized.
When X-base vector phase encoding is selected, the phase modulator 8 performs phase modulation on one of the two optical pulses to enable the effective phase difference between the two optical pulses to be 0 or pi, namely, X-base vector phase encoding is performed, and the encoded light returns to the Bob end through the quantum channel 13.
After the optical pulse passes through the beam splitter 6, the first output end of the beam splitter 6 is connected with the single photon detector 12, and the second output end of the beam splitter 6 is connected with the polarization beam splitter 5.
When Z-basis vector detection is selected, detection is carried out through the single-photon detector 12 connected with the first output end.
When X-base vector detection is selected, two beams of light pulses with mutually vertical polarization are reflected and transmitted by the polarization beam splitter 5 to respectively travel through the long arm and the short arm of the unequal arm interference module, wherein:
the vertically polarized light pulse (| V >) passes through the 90-degree polarization rotator 4 to become a horizontally polarized light pulse (| H >) and is input to the beam splitter 3, and the horizontally polarized light pulse (| H >) is directly input to the beam splitter 3.
The two light pulses interfere at the beam splitter 3, if the phase difference before interference is 0, the light passes through the circulator 2 to the single-photon detector 10 (the second single-photon detector), and if the phase difference before interference is pi, the light reaches the single-photon detector 11 (the first single-photon detector), so that the detection under X-base vectors is realized.
The detection under the Z basis vector is to convert the arrival time of the received optical pulse into a bit value, the detection under the X basis vector is to convert the bit value into a bit value according to the phase difference without reaching different detectors, and after the bit value is generated, the quantum key is finally generated through the post-processing processes of basis vector comparison, error correction, privacy amplification and the like through a classical channel.
Example 2
In this embodiment, the difference from embodiment 1 is that the polarization beam splitter 5 is replaced by a beam splitter 5a, a 90-degree polarization rotator is omitted, the original delay line 4a of the long arm part is retained, and the modified part is shown in fig. 3.
Example 3
In this embodiment, the difference from embodiment 1 lies in the unequal arm interference module and the related single photon detector, and in this embodiment, the unequal arm MZ interferometer is implemented by an unequal arm interferometer based on the michelson principle, and the single photon detector is correspondingly improved in adaptability.
Referring to fig. 4, the linearly polarized light pulse emitted from the pulse light source 1 passes through the circulator 2 and reaches the polarization beam splitter 15, which transmits the horizontally polarized light pulse (| H >) and reflects the vertically polarized light pulse (| V >).
The horizontally polarized light pulse (| H >) travels along the long arm, is delayed by the delay line 4a, and then is reflected by the 90-degree rotating faraday mirror 16 (the second 90-degree rotating faraday mirror), and the horizontally polarized light pulse (| H >) becomes the vertically polarized light pulse (| V >) and is reflected and output by the polarization beam splitter 15. After reaching the Alice end and passing through the 90-degree rotating faraday mirror, the vertically polarized light pulse (| V >) becomes the horizontally polarized light pulse (| H >) and then passes through the polarization beam splitter 15 again to be transmitted away from the short arm and reach the 90-degree rotating faraday mirror 17 (the first 90-degree rotating faraday mirror), and after reflection, the polarization state becomes the vertically polarized light pulse (| V >) and is reflected by the polarization beam splitter 15 and output to the circulator 2.
Similarly, the vertical polarized light pulse (| V >) is reflected by the short arm through the 90-degree rotating faraday mirror 17, the polarization state is changed into the horizontal polarized light pulse (| H >) and transmitted through the polarization beam splitter 15 to reach the Alice end, the horizontal polarized light pulse (H >) is changed into the vertical polarized light pulse (| V >) through the 90-degree rotating faraday mirror, the vertical polarized light pulse is reflected by the polarization beam splitter 15 and transmitted through the long arm to the 90-degree rotating faraday mirror 16, and the horizontal polarized light pulse (| H >) is changed into the horizontal polarized light pulse through the reflection polarization state and transmitted through the polarization beam splitter 15 to be output to the circulator 2.
Two beams of light pulses with mutually perpendicular polarization are combined into one path to reach the half-wave plate 18 through the circulator, the direction of the fast axis or the slow axis of the half-wave plate forms an angle of 22.5 degrees or-22.5 degrees with any one of two polarization bases of the polarization beam splitter 15, and equivalently, the projection measurement of the polarization beam splitter 19 under an H/V basis vector is changed into the projection measurement under a +45 degrees/45 degrees basis vector. The light pulse reaches the polarization beam splitter 19 to generate interference, and different polarization states reach different single photon detectors according to the phase difference of the phase modulated by the phase modulator. If the phase difference is 0, the horizontal polarized light pulse (| H >) reaches the single photon detector 20, the phase difference is pi, and the vertical polarized light pulse (| V >) reaches the single photon detector 21.
Example 4
Referring to fig. 5, the difference between this embodiment and embodiment 1 is that the X-base vector detection module is modified, and two single-photon detectors in the X-base vector detection module are replaced by the same single-photon detector, which reduces the cost.
At the Bob end, when two reflected light pulses with the same polarization interfere on the beam splitter, the effective phase difference of the two reflected light pulses is 0 or pi because the Alice-end phase modulator has completed X-basis vector phase encoding.
If the phase difference is 0, the short arm passes through the circulator 2 to reach the polarization-maintaining polarization beam splitter 14 (or adopts a beam splitter), and the transmission is output to the single photon detector 22.
If the phase difference is pi, the traveling arm reaches the polarization-maintaining polarization beam splitter 14 (or a beam splitter is adopted) after being delayed by a delay line 4b, and is output to the single photon detector 22 through reflection.
The present embodiment modification uses two moments to distinguish the arrival of different phase differences at the single photon detector 22. And adding a synchronous time signal in front of the single-photon detector 22 to detect the arrival time, wherein if no delay exists, the phase difference reaches the single-photon detector 22 as 0, and if the delay exists, the phase difference reaches the single-photon detector 22 as pi. This embodiment modification can also be combined with embodiments 2 and 3, in which the polarization-maintaining polarization beam splitter 14 is replaced with a beam splitter when combined with embodiment 2. In which the polarization-maintaining polarizing beam splitter 14 is replaced by a polarizing beam splitter when combined with embodiment 3.
Example 5
Referring to fig. 6, the present embodiment is different from embodiment 4 in that a decoding module is modified, and all single photon detectors are replaced by the same single photon detector, which further reduces the cost.
In the decoding module at the end of Bob, the light pulse from the end of Alice, which is subjected to z-basis vector time coding, is reflected by the beam splitter 6, travels through the long arm, is delayed by the delay line 4c, then reaches the beam splitter 23, and is output to the single photon detector 22 after being reflected.
Two light pulses which are from an Alice end and are subjected to X-base vector phase coding pass through an unequal-arm interferometer, the two light pulses with the same polarization state interfere with each other, if the phase difference is 0, a short arm passes through a circulator 2 to reach a polarization-maintaining polarization beam splitter 14 (or a beam splitter is adopted), and the light pulses are transmitted and output to a beam splitter 23 to reach a single-photon detector 22.
If the phase difference is pi, the traveling arm reaches the polarization-maintaining polarization beam splitter 14 (or adopts a beam splitter) after being delayed by the delay line 4b, and is output to the beam splitter 23 through reflection to reach the single photon detector 22.
The improved scheme of this embodiment uses four time points to distinguish (time-coded signal light arrives before and after two time points for distinguishing, and phase-coded signal light arrives before and after two time points for distinguishing) detection under different basis vectors, and the improved scheme of this embodiment can also be combined with embodiments 2 and 3, in which the polarization-preserving polarization beam splitter 14 is replaced by a beam splitter when being combined with embodiment 2. In which the polarization-maintaining polarizing beam splitter 14 is replaced by a polarizing beam splitter when combined with embodiment 3.
Example 6
In this embodiment, the difference from embodiment 1 is that, in addition to the intensity modulator 7, another reflected light intensity modulator for realizing "decoy state" light source output is additionally provided between the 90-degree rotating faraday mirror 9 and the phase modulator 8 at the Alice end, and the reflected light intensity modulator can randomly perform intensity modulation on the optical signal reflected by the 90-degree rotating faraday mirror 9 to randomly change the ratio of the single photon component to the multi-photon component in the output optical signal.
The idea of the decoy state protocol is based on the consideration that the passing efficiency of single photon and multiphoton is different by an attacker in the PNS attack. In order to find the attack of an attacker, the Alice end randomly sends light sources with different light intensities, wherein the proportion of multi-photon components and single-photon components is different, and the attacker cannot adaptively adjust the passing efficiency of the multi-photon because the attacker cannot distinguish which light source is adopted by the Alice side, so that the statistical results of different light sources reaching the Bob end cannot be kept unchanged at the same time. In contrast, an attacker can only guarantee a single intensity source to reach Bob's statistics. Therefore, the method of providing a 'decoy state' light source can ensure that the efficiency of the single photon component is not modified by an attacker, and the single photon component is an effective component which can extract a security key in a quantum key distribution system.
The modified scheme of this embodiment can also be combined with embodiments 2 to 5, and the light emitted by the corresponding pulse light source (laser) is weak coherent light.
Example 7
In this embodiment, the difference from embodiment 1 is that optical fiber communication is adopted between the Alice end and the Bob end, at this time, the 90-degree polarization rotator 4 may be omitted, and the beam splitter 3 and the polarization beam splitter 5 in embodiment 1 are respectively replaced by a polarization maintaining beam splitter and a polarization maintaining polarization beam splitter, where the tail fibers used by the polarization maintaining beam splitter and the polarization maintaining polarization beam splitter are polarization maintaining optical fibers. The corresponding circulator 2 is a polarization maintaining circulator and its pigtail is a polarization maintaining fiber. Preferably, the pulse light source of the present embodiment transmits a horizontally polarized light pulse.
This embodiment modification can also be combined with embodiments 4 and 5.
Claims (7)
1. A receiving end of a plug-and-play quantum key distribution system based on time-phase coding comprises a light source used for sending optical signals to a sending end and a decoding module used for receiving and detecting the coded optical signals from the sending end, wherein the decoding module comprises a Z basis vector time detection module and a phase detection module which perform corresponding decoding processing on the optical signals, and the plug-and-play quantum key distribution system is characterized in that the phase detection module is an X basis vector phase detection module, and the Z basis vector time detection module comprises:
the beam splitting module is used for splitting the optical signal coded by the sending end; and
the single-photon detector is connected with the first output end of the beam splitting module to detect the Z-basis vector time;
the phase detection module is connected with the second output end of the beam splitting module;
the beam splitting module is a beam splitter, and the light energy output to the single-photon detector by the beam splitter is not equal to the light energy output to the phase detection module;
the phase detection module comprises an unequal arm interference module arranged between the beam splitting module and the light source and a detector module used for detecting an optical signal output by the unequal arm interference module; the light path selection is carried out among the light source, the unequal arm interference module and the detector module through the circulator.
2. The receiving end of a time-phase coding based plug-and-play quantum key distribution system of claim 1, wherein the unequal arm interference module is an unequal arm mach-zehnder interferometer or an unequal arm interferometer based on michelson's principle.
3. The receiving end of the plug-and-play quantum key distribution system based on time-phase encoding as claimed in claim 1, wherein two single photon detectors are disposed in the detector module to detect two different interference results from the unequal arm interference module, respectively.
4. The receiving end of the plug-and-play quantum key distribution system based on time-phase encoding as claimed in claim 1, wherein the number of the single photon detectors in the detector module is one, and two different interference results from the unequal arm interference module are multiplexed to the same single photon detector through corresponding optical path elements.
5. The receiving end of the time-phase coding based plug-and-play quantum key distribution system of claim 1, wherein the decoding module comprises one single photon detector, and the output of the first output end of the beam splitting module and the two different interference results output by the unequal-arm interference module are both coupled to the same single photon detector through corresponding optical path elements.
6. A plug-and-play quantum key distribution system based on time-phase coding comprises a sending end and a receiving end which are mutually connected in an optical way, wherein a coding module in the sending end comprises a Z-base vector time coding module and a phase coding module which correspondingly code optical signals, and the plug-and-play quantum key distribution system is characterized in that the phase coding module is an X-base vector phase coding module; a decoding module in the receiving end is adapted to the encoding module, and the receiving end adopts the receiving end of the plug-and-play quantum key distribution system based on time-phase encoding according to any one of claims 1 to 5.
7. A plug-and-play quantum key distribution method based on time-phase coding comprises the steps of carrying out Z-basis vector time coding and phase coding after a sending end receives and reflects an optical signal from a receiving end, and then sending the optical signal to the receiving end for decoding and detecting, wherein the phase coding is X-basis vector phase coding; the decoding mode of the receiving end is adapted to the coding mode of the transmitting end, and the receiving end adopts the receiving end of the plug-and-play quantum key distribution system based on the time-phase coding according to any one of claims 1 to 5.
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