CN110620652A - Quantum key distribution system and communication method thereof - Google Patents

Quantum key distribution system and communication method thereof Download PDF

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Publication number
CN110620652A
CN110620652A CN201810797022.3A CN201810797022A CN110620652A CN 110620652 A CN110620652 A CN 110620652A CN 201810797022 A CN201810797022 A CN 201810797022A CN 110620652 A CN110620652 A CN 110620652A
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polarization
port
phase
light pulse
optical
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CN110620652B (en
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李东东
刘建宏
汤艳琳
许穆岚
刘仁德
陶俊
李韬
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Anhui Quantum Communication Technology Co Ltd
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Anhui Quantum Communication Technology Co Ltd
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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
    • 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
    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
    • Y02D30/00—Reducing energy consumption in communication networks
    • Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks

Abstract

The invention discloses a quantum key distribution system and a communication method thereof.A light pulse is generated at a transmitting end, the light pulse can be subjected to polarization and phase combined modulation, the light pulse is subjected to phase decoding and polarization decoding in sequence at a receiving end, after the polarization decoding is completed, the disturbance of a fiber channel on the light pulse after the combined modulation is supplemented, and finally the polarization state of the light pulse is detected, wherein in the transmitting end, the light pulse comprises four polarization states of { | P >, | N >, | R >, | L >; detecting each polarization state at the receiving end respectively; wherein | P > and | N > are eigenstates of X basis vector; and | R > and | L > are eigenstates of the Y basis vector. The method and the device realize the simultaneous encoding of multiple degrees of freedom of the single photon, and have simple system structure.

Description

Quantum key distribution system and communication method thereof
Technical Field
The invention relates to the technical field of quantum secret communication, in particular to a quantum key distribution system and a communication method thereof.
Background
Quantum Key Distribution (QKD) technology is of great interest because it enables the generation of perfectly consistent unconditionally secure keys between two communicating parties. Since the proposal of BB84 in 1984, various theoretical schemes are perfected day by day, and the technology is gradually mature and goes to practical application. Quantum Key Distribution (QKD) is fundamentally different from the classical key system in that a single photon or an entangled photon pair is adopted as a carrier of a key, and the three basic principles of Quantum mechanics (heisenberg inaccuracy principle, measurement collapse theory and Quantum unclonable law) ensure the non-eavesdropping and indestructibility of the process, so that a more secure key system is provided.
However, in quantum key distribution, the carrier of the key is a single photon, and under the condition of long-distance transmission, the final key rate is limited by huge channel attenuation. Many theoretical schemes have been proposed for how to improve the quantum key distribution coding rate. One of the schemes that is receiving much attention is the single-photon multi-bit technology, that is, multiple degrees of freedom of a single photon are used for simultaneous encoding, so that one photon carries information of multiple bits, and further, the channel capacity and the final secure key coding rate are effectively improved.
How to provide a quantum key distribution system with a simple structure to realize simultaneous encoding of multiple degrees of freedom of a single photon is a problem to be solved urgently in the technical field of quantum secret communication.
Disclosure of Invention
In order to solve the above problems, the technical solution of the present invention provides a quantum key distribution system and a communication method thereof, which can realize that the quantum key distribution system simultaneously encodes a single photon with multiple degrees of freedom, and the system has a simple structure.
In order to achieve the above purpose, the invention provides the following technical scheme:
a quantum key distribution system, the quantum key distribution system comprising:
the transmitting end is used for generating optical pulses and carrying out polarization and phase joint modulation on the optical pulses;
the receiving end is connected with the transmitting end through an optical fiber channel and is used for sequentially carrying out phase decoding and polarization decoding on the optical pulse, compensating the disturbance of the optical fiber channel on the optical pulse after the combined modulation after the polarization decoding is finished, and finally detecting the polarization state of the optical pulse;
in the transmitting end, the light pulse comprises four polarization states of { | P >, | N >, | R >, | L > };
detecting each polarization state at the receiving end respectively;
wherein | P > and | N > are eigenstates of the X basis vector; and | R > and | L > are eigenstates of the Y basis vector.
Preferably, in the quantum key distribution system, the transmitting end and the receiving end are both controlled by random numbers to implement an unbalanced basis vector scheme by using an unbalanced BB84 protocol.
Preferably, in the quantum key distribution system, the transmitting end includes:
a laser, which is a light source, for generating the light pulse;
the polarization encoding module is used for carrying out polarization encoding on the light pulse emitted by the laser, and randomly selecting one of the four polarization states of | R >, | L >, | P > and | N > of the light pulse for carrying out polarization encoding;
the phase coding module is used for carrying out phase coding on the optical pulses after polarization coding, and randomly selecting a phase difference from {0, pi/2, pi, 3 pi/2 } to carry out phase coding;
and the variable optical attenuator is used for attenuating the optical pulse emitted by the phase coding module to a single photon magnitude.
Preferably, in the quantum key distribution system, the polarization state of the optical pulse emitted by the laser is | P >;
the polarization encoding module comprises a Sagnac interference ring, and the Sagnac interference ring is used for converting the polarization state of the light pulse emitted by the laser into four polarization states of { | P >, | N >, | R >, | L > }.
Preferably, in the quantum key distribution system, the polarization encoding module includes: a first polarization beam splitter, a first phase modulator, and a first random number device;
the first polarization beam splitter has a first port, a second port, a third port, and a fourth port; the first polarization beam splitter acquires the light pulse emitted by the laser through a first port thereof, divides the light pulse into two paths, one path of the light pulse is emitted through a third port thereof, enters a fourth port thereof after passing through the first phase modulator, the other path of the light pulse is emitted through the fourth port thereof, enters a third port thereof after passing through the first phase modulator, and the first phase modulator performs phase modulation on the two paths of the incident light pulse, so that a phase difference is generated between the two paths of the light pulse; the two paths of optical pulses return to the first polarization beam splitter to be converged, form polarization-encoded optical pulses through a second port of the first polarization beam splitter, and send the polarization-encoded optical pulses to the phase encoding module;
wherein the first random number means is configured to perform random number control on a phase change amount of the first phase modulator.
Preferably, in the quantum key distribution system, the phase encoding module includes: the device comprises a first beam splitter, a first Faraday rotator mirror, a second phase modulator and a second random number device;
the first splitter has a first port, a second port, a third port, and a fourth port; the first beam splitter obtains the optical pulse after polarization coding through a first port thereof, divides the optical pulse into two paths, one path of the optical pulse is emitted through a third port thereof, enters the first Faraday rotator mirror after passing through the second phase modulator, returns to the third port thereof through the original path after being reflected by the first Faraday rotator mirror, and the other path of the optical pulse is emitted through a fourth port thereof, returns to the fourth port thereof after being reflected by the second Faraday rotator mirror, and transmits the optical pulse after phase coding to the adjustable optical attenuator through the second port thereof;
wherein the second random number means is for performing random number control on the amount of phase change of the second phase modulator.
Preferably, in the quantum key distribution system, the receiving end includes:
a phase decoding module, configured to perform phase decoding on the jointly modulated optical pulses;
the polarization decoding module is used for carrying out phase decoding on the optical pulse emitted by the phase decoding module and compensating the disturbance of the optical fiber channel on the optical pulse;
and the detector module is used for respectively detecting each polarization state of the light pulse emitted by the polarization decoding module.
Preferably, in the quantum key distribution system, the phase decoding module includes: the device comprises a circulator, a delayer, a third random number device, a second beam splitter, a third phase modulator, a third Faraday rotation mirror and a fourth Faraday rotation mirror;
the circulator has a first port, a second port and a third port; the circulator obtains the light pulse emitted by the optical fiber channel through a first port of the circulator;
the second splitter has a first port, a second port, a third port, and a fourth port; the second beam splitter obtains the light pulse emitted from the second port of the circulator through the first port of the second beam splitter, the light pulse is divided into two paths, one path of light pulse is emitted through the third port of the second beam splitter, the light pulse is emitted into the third Farad rotating mirror after passing through the third phase modulator, the light pulse is reflected through the third Farad rotating mirror and then returns to the third port of the third beam splitter, the other path of light pulse is emitted through the fourth port of the fourth Farad rotating mirror and then returns to the fourth port of the fourth Faraday rotating mirror, the reflected light pulse is simultaneously obtained from the third port and the fourth port of the second beam splitter, the reflected light pulse is emitted from the first port and the second port of the second beam splitter after interference in the second beam splitter, the light pulse emitted from the second port of the second beam splitter is sent to the polarization decoding module, the light pulse emitted from the first port of the second beam splitter is sent to the second port of the circulator and then is sent to the time delay unit through the third port of the circulator, sending the data to the polarization decoding module through the time delay device;
wherein the third random number means is for performing random number control on a phase change amount of the third phase modulator.
Preferably, in the quantum key distribution system, the polarization decoding module includes: the polarization beam splitter comprises a third beam splitter, a first polarization controller, a second polarization beam splitter and a third polarization beam splitter;
the third splitter has a first port, a second port, a third port, and a fourth port; the third beam splitter acquires the light pulse emitted from the second port of the second beam splitter through the first port of the third beam splitter, acquires the light pulse emitted from the time delay unit through the second port of the third beam splitter, performs polarization decoding on the acquired light pulse by the third beam splitter, and then divides the light pulse into two paths, wherein one path of the light pulse is emitted from the third port of the third beam splitter, enters the second polarization beam splitter through the first polarization controller, and the other path of the light pulse is emitted from the fourth port of the third beam splitter, and enters the third polarization beam splitter through the second polarization controller;
the second polarization beam splitter divides the incident light pulse into two paths and sends the two paths of light pulses to the detector module;
the third polarization beam splitter divides the incident light pulse into two paths and sends the two paths of light pulses to the detector module;
wherein the first polarization controller and the second polarization controller are used for compensating the disturbance of the optical fiber channel to the optical pulse.
Preferably, in the quantum key distribution system, the detector module includes four single-photon detectors, two of the single-photon detectors are configured to detect two optical pulses emitted from the second polarization beam splitter respectively, and the other two single-photon detectors are configured to detect two optical pulses emitted from the third polarization beam splitter respectively.
The invention also provides a communication method of the quantum key distribution system, wherein the quantum key distribution system is provided with a transmitting end and a receiving end connected with the transmitting end through an optical fiber channel, and the communication method comprises the following steps:
generating optical pulses at the transmitting end, and carrying out polarization and phase joint modulation on the optical pulses;
transmitting the light pulse subjected to polarization and phase joint modulation to the receiving end through the optical fiber channel;
at the receiving end, phase decoding and polarization decoding are sequentially carried out on the optical pulse, after the polarization decoding is finished, the disturbance of the optical fiber channel on the optical pulse after the combined modulation is compensated, and finally the polarization state of the optical pulse is detected;
the light pulse generated by the transmitting end comprises four polarization states of { | P >, | N >, | R >, | L > };
detecting each polarization state at the receiving end respectively;
| P > and | N > are eigenstates of the X basis vector; and | R > and | L > are eigenstates of the Y basis vector.
Preferably, in the above communication method, the transmitting end and the receiving end are controlled by random numbers to implement an unbalanced basis vector scheme by using an unbalanced BB84 protocol.
Preferably, in the above communication method, the transmitting end includes: the device comprises a laser, a polarization encoding module, a phase encoding module and an adjustable optical attenuator;
the generating of the optical pulses at the transmitting end, the polarization and phase joint modulation of the optical pulses comprising:
generating the light pulse by the laser;
carrying out polarization encoding on the light pulse emitted by the laser through the polarization encoding module, and randomly selecting one of the four polarization states of | R >, | L >, | P > and | N > of the light pulse to carry out polarization encoding;
the polarization-encoded light pulse is subjected to phase encoding through the phase encoding module, and a phase difference is randomly selected from {0, pi/2, pi, 3 pi/2 } to be subjected to phase encoding;
and attenuating the light pulse emitted by the phase coding module to a single photon magnitude by the variable optical attenuator.
Preferably, in the above communication method, the receiving end includes: the device comprises a phase decoding module, a polarization decoding module and a detector module;
the phase decoding and the polarization decoding are sequentially performed on the optical pulse at the receiving end, after the polarization decoding is completed, the disturbance of the optical fiber channel to the optical pulse after the combined modulation is compensated, and finally, the detecting the polarization state of the optical pulse comprises the following steps:
performing phase decoding on the optical pulse subjected to the joint modulation through the phase decoding module;
the polarization decoding module is used for carrying out phase decoding on the optical pulse emitted by the phase decoding module, and the disturbance of the optical fiber channel to the optical pulse is compensated;
and detecting each polarization state of the light pulse emitted by the polarization decoding module through the detector module respectively.
Preferably, in the above communication method, the polarization state | P > of the light pulse emitted from the laser is converted into four polarization states { | P >, | N >, | R >, | L > } at the emitting end through a sagnac interferometric loop.
As can be seen from the above description, in the quantum key distribution system and the communication method thereof provided in the technical scheme of the present invention, at a transmitting end, optical pulses are generated, polarization and phase joint modulation can be performed on the optical pulses, at a receiving end, phase decoding and polarization decoding are performed on the optical pulses in sequence, after the polarization decoding is completed, the optical pulses are supplemented with the perturbation of the optical fiber channel on the optical pulses after the joint modulation, and finally the polarization states of the optical pulses are detected, and at the transmitting end, the optical pulses include four polarization states { | P >, | N >, | R >, | L > }; detecting each polarization state at the receiving end respectively; wherein | P > and | N > are eigenstates of the X basis vector; and | R > and | L > are eigenstates of the Y basis vector. The method and the device realize the simultaneous encoding of multiple degrees of freedom of the single photon, and have simple system structure.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.
Fig. 1 is a schematic structural diagram of a quantum key distribution system according to an embodiment of the present invention;
fig. 2 is a schematic flowchart of a communication method of a quantum key distribution system according to an embodiment of the present invention;
fig. 3 is a schematic flowchart of a polarization and phase joint modulation method according to an embodiment of the present invention;
fig. 4 is a flowchart illustrating a decoding method according to an embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In order to enable the quantum key distribution system to realize simultaneous encoding of multiple degrees of freedom of a single photon:
the first way is to generate photons and then perform active polarization modulation, and combine the BB84 protocol and the DPS protocol to increase the key generation efficiency of a single photon to 7/6, such as an efficient quantum communication protocol proposed by wanggong et al, in which a single photon is encoded by arbitrarily selecting one of 4 polarizations, such as 0 °, 90 °, 45 °, and the like, then divided into three pulses and delayed, and a phase difference of 0 or pi is selected between two adjacent pulses to perform differential phase encoding. At the receiving end, the polarization decoding is firstly carried out, each path after the polarization decoding is carried out is then carried out with the phase decoding, and at the moment, the total number of interferometers required by the phase decoding is 4.
At present, in a free space optical path, active polarization modulation is realized by adopting a rotating wave plate or utilizing a photoelectric modulation crystal. The principle of realizing active polarization modulation by rotating the wave plate is that the optical axis of the wave plate is rotated by an electric rotating platform, the angle of the wave plate is changed, and the photon polarization conversion is realized. The principle of realizing active polarization modulation by the photoelectric modulation crystal is that voltage signals are added at two ends of the crystal to change the relative refractive indexes of o light and e light in the crystal and further change the polarization state of output laser, so that although the modulation rate is high, usually in the order of MHz, and far higher than that of a rotating wave plate, the modulation rate can be used in low-speed quantum communication, the voltage signals required by the photoelectric crystal are usually in the order of hundreds of volts, and the rapid modulation technology of high-voltage signals is difficult, so that the photoelectric modulation crystal is not suitable for the field of high-speed quantum communication. In addition, a plurality of interferometers are also adopted at the receiving end, so that the complexity of the system is increased.
The second method is an improvement on the first method, wherein the polarization dimension coding and the phase dimension coding both use BB84 protocol, 4 lasers are used to generate 4 polarization quantum states required by the polarization coding, and the polarization beam splitter and the polarization independent beam splitter are used to combine beams. The combined photons enter an unequal-arm Mach-Zehnder interferometer to be divided into two beams of front and rear light pulses staggered in time for phase coding. At the decoding end, photons first enter an unequal arm Mach-Zehnder interferometer (Mach-Zehnder interferometer) for phase decoding, and then undergo polarization decoding by a beam splitter and a polarization beam splitter, as suggested by Zhao, etc.
In the second mode, photons with different polarizations are combined and then enter an unequal-arm Mach-Zehnder interferometer for phase encoding. This solution is therefore only suitable for free-space optical paths and is difficult to implement in fiber optic systems. In the optical fiber path, in order to improve the interference contrast, an unequal-arm Mach-Zehnder interferometer is generally constructed by using a polarization maintaining optical fiber. While the polarization maintaining fiber cannot transmit 4 polarization states in the BB84 protocol and remains unchanged.
The third mode is a high-efficiency and stable differential phase and polarization coding composite quantum key distribution system, combines differential phase coding and polarization state coding, and realizes higher system efficiency and better stability by introducing a Faraday mirror structure and improving the traditional system structure. In the method, photons are subjected to active polarization modulation, and then enter an interferometer formed by a Faraday rotator mirror to be divided into three beams of optical pulses for differential phase encoding. At the decoding end, the polarization decoding is performed first followed by the phase decoding, resulting in the need for multiple unequal-arm interferometers for phase decoding. And the polarization decoding firstly adopts an active basis vector selection method to decode the polarization dimension, and then skillfully designs an interference path, so that the phase decoding probability in the differential phase protocol reaches 100%. The key generation efficiency of single photon of the protocol is as high as 3/2, which is the highest level at present. However, in this method, at the encoding end, photons need to be actively polarized and modulated, and the rate is usually low (lower than 1MHz), which is far from meeting the requirement of high-speed quantum communication; at a decoding end, active basis vector selection is required, a rotating wave plate or an electro-optical crystal is generally used for modulation, the rate is low (lower than 1MHz), and the requirement of high-speed quantum communication cannot be met.
In order to solve the above problem, an embodiment of the present invention provides a quantum key distribution system and a communication method thereof, where the quantum key distribution system is a high-speed quantum key distribution system based on joint modulation of polarization phases under unbalanced basis vectors. When the quantum key distribution system carries out polarization coding, the polarization modulation is realized by adopting a skillfully designed scheme of a two-port Sagnac interference ring (Sagnac interference ring) and utilizing a high-speed phase modulator, so that the problem that the active polarization modulation rate is low and is not suitable for high-speed quantum communication is solved; when phase encoding is carried out, a Faraday-Michelson interferometer (Faraday-Michelson interferometer) with automatic polarization compensation is adopted for phase modulation, so that the problem that an unequal arm Mach-Zehnder interferometer is not matched with 4 polarization states is solved; when decoding is carried out, the Faraday-Michelson interferometer is adopted to carry out phase dimension decoding firstly and then carry out polarization dimension decoding, so that the complexity of the system is reduced, and the resource consumption of the system is reduced. In the quantum key distribution system, for polarization encoding and phase encoding, a bias basis vector BB84 protocol is adopted, and encoding efficiency approaching 2 bit/photon can be obtained.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
Referring to fig. 1, fig. 1 is a schematic structural diagram of a quantum key distribution system according to an embodiment of the present invention, where the quantum key distribution system shown in fig. 1 includes: the transmitting terminal 11 is configured to generate an optical pulse, and perform polarization and phase joint modulation on the optical pulse; and the receiving end 12 is connected with the transmitting end 11 through an optical fiber channel 13, and the receiving end 12 is configured to perform phase decoding and polarization decoding on the optical pulse in sequence, compensate for disturbance of the optical fiber channel 13 on the optical pulse after joint modulation after the polarization decoding is completed, and finally detect the polarization state of the optical pulse.
In the quantum key distribution system according to the embodiment of the present invention, in the transmitting end 11, the optical pulse includes { | P>，|N>，|R>，|L>The four polarization states; at the receiving end 12, each polarization state is detected separately. Wherein, | P>And | N>Is eigenstate of X basis vector; | R>And | L>Is eigenstate of the Y basis vector.|H>And | V>Is the eigenstate of the Z basis vector.
Optionally, both the transmitting end 11 and the receiving end 12 are controlled by random numbers to adopt an unbalanced BB84 protocol to implement an unbalanced basis vector scheme.
As shown in fig. 1, the transmitting end 11 includes: a laser LD, which is a light source, for generating the light pulse; the polarization encoding module 111 is configured to perform polarization encoding on the light pulse emitted by the laser LD, and randomly select one of the four polarization states | R >, | L >, | P >, and | N > of the light pulse to perform polarization encoding; a phase encoding module 112, where the phase encoding module 112 is configured to perform phase encoding on the polarization-encoded optical pulses, and randomly select a phase difference from {0, pi/2, pi, 3 pi/2 } to perform phase encoding; and the variable optical attenuator VOA is used for attenuating the light pulse emitted by the phase coding module to a single photon magnitude.
The polarization state of the light pulse emitted by the laser is | P >; the polarization encoding module 111 includes a sagnac interferometric ring, which is used to convert the polarization state of the light pulse emitted by the laser into four polarization states { | P >, | N >, | R >, | L > }.
As shown in fig. 1, the polarization encoding module 111 includes: a first polarizing beam splitter PBS1, a first phase modulator PM1 and a first random number device K1. The first polarizing beam splitter PBS1 and the first phase modulator PM1 form a Sagnac interferometric ring structure, and the first phase modulator PM1 is random number controlled by a first random number device K1.
The first polarizing beam splitter PBS1 has a first port a11, a second port a12, a third port a13, and a fourth port a 14; the first polarization beam splitter PBS1 obtains the optical pulse emitted by the laser LD through a first port a11 thereof, divides the optical pulse into two paths, one path of the optical pulse is emitted through a third port a13 thereof, and enters a fourth port a14 thereof after passing through the first phase modulator PM1, the other path of the optical pulse is emitted through a fourth port a14 thereof, and enters a third port a13 thereof after passing through the first phase modulator PM1, and the first phase modulator PM1 performs phase modulation on the two incident paths of optical pulses, so that a phase difference is generated between the two paths of optical pulses; the two optical pulses are returned to the first polarization beam splitter PBS1 to be merged and sent to the phase encoding module 112 through the second port a12 thereof.
The first random number device K1 is configured to perform random number control on the phase change amount of the first phase modulator PM1, so that the polarization encoding module 111 can randomly select one of the four polarization states | R >, | L >, | P >, and | N > of the optical pulse for polarization encoding.
It should be noted that a circulator or an optical isolator may be added between the laser LD and the a11 port of the first polarization beam splitter PBS1 to reduce the light intensity input to the laser LD through the a11 port after polarization encoding, so as to stabilize the operating state of the laser LD, protect the laser LD, and improve the device lifetime. Optical isolators are generally integrated in commercial lasers LD, and no extra protection measures need to be added; this protection is preferred if the laser LD used does not integrate an optical isolator.
As shown in fig. 1, the phase encoding module 112 includes: a first beam splitter BS1, a first faraday rotator FM1, a second faraday rotator FM2, a second phase modulator PM2 and a second random number device K2.
The first splitter BS1 has a first port a21, a second port a22, a third port a23, and a fourth port a 24; the first beam splitter BS1 obtains the polarization-encoded optical pulse through the first port a21 thereof, and divides the optical pulse into two paths, one path of the optical pulse exits through the third port a23 thereof, enters the first faraday rotator FM1 after passing through the second phase modulator PM2, returns to the third port a23 thereof after being reflected by the first faraday rotator FM1, and exits through the fourth port a24 thereof, returns to the fourth port a24 thereof after being reflected by the second faraday rotator FM2, and sends the phase-encoded optical pulse to the variable optical attenuator VOA through the second port a22 thereof;
the second random number device K2 is used to perform random number control on the phase change amount of the second phase modulator, so that the phase encoding module 112 can randomly select a phase difference from {0, pi/2, pi, 3 pi/2 } to perform phase encoding.
As shown in fig. 1, the receiving end 12 includes: a phase decoding module 121, where the phase decoding module 121 is configured to perform phase decoding on the optical pulses after being jointly modulated; the polarization decoding module 122 is configured to perform phase decoding on the optical pulse emitted by the phase decoding module 121, and is further configured to compensate for disturbance of the optical fiber channel 13 on the optical pulse; and the detector module 123, where the detector module 123 is configured to detect each polarization state of the light pulse emitted by the polarization decoding module 122. In the embodiment of the invention, the 4 polarization states used in the polarization encoding are different from the 4 polarization states used in the polarization decoding, the compensation is carried out through the polarization controller, and each polarization state is respectively distributed to the corresponding single photon detector, so that the polarization state compensation method is more suitable for an optical fiber system, each polarization state is respectively detected, the detection is easier, the system stability is improved, and the cost is reduced.
As shown in fig. 1, the phase decoding module 121 includes: circulator C1, delayer DL1, third random number device K3, second beam splitter BS2, third phase modulator PM3, third faraday rotator FM3 and fourth faraday rotator FM 4.
The circulator C1 has a first port a31, a second port a32 and a third port a 33; the circulator C1 obtains the light pulse emitted from the optical fiber channel 13 through the first port a31 thereof.
The second beam splitter BS2 has a first port a41, a second port a42, a third port a43 and a fourth port a 44. The second beam splitter BS2 obtains the optical pulse emitted from the second port a32 of the circulator C1 through the first port a41 thereof, divides the optical pulse into two paths, one path is emitted through the third port a43 thereof, enters the third faraday rotator FM3 after passing through the third phase modulator PM3, returns to the third port a43 thereof after being reflected by the third faraday rotator FM3, and the other path is emitted through the fourth port a44 thereof, returns to the fourth port a44 thereof after being reflected by the fourth faraday rotator FM4, and the third port a43 and the fourth port a44 thereof simultaneously obtain the reflected optical pulse, after the reflected optical pulse interferes in the second beam splitter BS2, the optical pulse is emitted from the first port a41 and the second port a42 thereof respectively, the optical pulse emitted from the second port a42 thereof is sent to the polarization decoding module 122, and the optical pulse emitted from the first port a41 a1 a32 thereof is sent to the second port C49328 a32 thereof, sent to the delayer DL1 through the third port a33 of the circulator C1 and sent to the polarization decoding module 122 through the delayer DL 1.
Wherein the third random number device K3 is configured to perform random number control on the phase change amount of the third phase modulator PM3, so as to control the phase decoding module 121 to perform measurement basis vector selection under the random number control, and randomly select the phase difference.
As shown in fig. 1, the polarization decoding module 122 includes: a third beam splitter BS3, a first polarization controller PC1, a second polarization controller PC2, a second polarization beam splitter PBS2, and a third polarization beam splitter PBS 3.
The third splitter BS3 has a first port a51, a second port a52, a third port a53, and a fourth port a 54; the third beam splitter BS3 obtains the optical pulse emitted from the second port a42 of the second beam splitter BS2 through the first port a51 thereof, obtains the optical pulse emitted from the retarder DL1 through the second port a52 thereof, and the third beam splitter BS3 performs polarization decoding on the obtained optical pulse, and then divides the optical pulse into two paths, wherein one path is emitted through the third port a53 thereof, passes through the first polarization controller PC1, and enters the second polarization beam splitter PBS2, and the other path is emitted through the fourth port a54 thereof, and passes through the second polarization controller PC2, and enters the third polarization beam splitter PBS 3. The first polarization controller PC1 and the second polarization controller PC2 are configured to compensate for the disturbance of the optical fiber channel 13 to the optical pulse, and separate four polarization states of the optical pulse, so as to detect the four polarization states respectively.
The second polarization beam splitter PBS2 splits the incident light pulse into two paths and sends the two paths to the detector module 123. The third polarization beam splitter PBS3 splits the incident light pulse into two paths and sends the two paths to the detector module 123.
As shown in fig. 1, the detector module 123 includes four single-photon detectors D1, D2, D3 and D4, two of the single-photon detectors (D1 and D2) are configured to detect two light pulses emitted from the second polarization beam splitter PBS2, and the other two of the single-photon detectors (D3 and D4) are configured to detect two light pulses emitted from the third polarization beam splitter PBS 3. The single-photon detectors D1-D4 respectively detect a polarization state of the optical pulse correspondingly.
In the quantum key distribution system shown in fig. 1, the transmitting end 11 has a laser LD, and an optical pulse (i.e., a photon described below) is generated by the laser LD, and the optical pulse outputs a photon polarization state | P > after being pulse-modulated. The single photon is polarized and encoded by a polarization encoding module 111(Sagnac interference ring) at the emitting end 11, and one of the four polarization states | R >, | L >, | P > and | N > is randomly selected to perform polarization encoding in the encoding process.
After polarization coding, the photons carry 1bit polarization information, then the photons carry phase coding through a phase coding module 112(Faraday-Michelson interferometer), a phase difference is randomly selected from {0, pi/2, pi, 3 pi/2 } for phase coding, the photons carry 1bit polarization information and 1bit phase information after phase coding, and the total information amount is 2 bits.
The polarization-encoded and phase-encoded photons are transmitted to the receiving end 12 through the optical fiber channel 13. The photons are then decoded by a phase decoding module 121(Faraday-Michelson interferometer). And then polarization decoding is performed by the polarization decoding module 122. The phase decoding module 121 has two output ports. If the two output ports are respectively subjected to polarization decoding, two polarization decoding modules are needed, 8 single photon detectors are needed in total, the number of system devices is large, and the resource consumption is large. In the embodiment of the present invention, the phase decoding module 121 is provided with a delay device DL1, and after one path of optical signal is delayed, the two paths of optical signals are input into the same polarization decoding module 122 for polarization decoding, so that the number of single photon detectors in the detector module 123 is reduced, and the system structure and the cost are simplified.
After polarization decoding, phase bit information and polarization bit information can be obtained simultaneously according to the detection result of the detector module 123. Therefore, two bits of information can be obtained at the same time by only transmitting one photon, and the channel capacity and the quantum communication key generation rate (namely the code rate) are improved.
In the prior art, a standard BB84 protocol is adopted, the probability of selecting different basis vectors is the same when preparing and measuring different quantum states, basis vector comparison is required, that is, the detection count of the basis vector selection error in general measurement is abandoned, and thus, a lot of resources are wasted.
In the embodiment of the invention, the unbalanced BB84 protocol is used, so that the detection count of the detector discarded in the base vector comparison process can be reduced, and the generation rate of the quantum key is improved. The implementation principle of the unbalanced BB84 protocol adopted in the embodiment of the invention is as follows: the polarization encoding module 111, the phase encoding module 112, and the phase decoding module 121 are respectively provided with a phase modulator controlled by a random number, so that the quantum key distribution system can adopt an unbalanced BB84 protocol, and the probability of selecting different basis vectors is different when different quantum states are prepared and measured, and thus the quantum key distribution system provided by the embodiment of the invention is a high-speed quantum key distribution system of polarization phase combined modulation under unbalanced basis vectors. .
Specifically, when performing polarization encoding, if the standard BB84 protocol is used, the probability of modulating the optical pulse into four states of | R >, | L >, | P >, and | N > is the same, and the emergence probability of each state is 25%, that is, the prior art is a balanced basis vector scheme. The unbalanced BB84 protocol is adopted in the embodiment of the present invention, when performing polarization encoding, the emergence probability of each state can be set according to requirements, for example, the probability of selecting | P > or | N > is 45% (or the probability of selecting X basis vector is 90%, because | P > and | N > are eigen states of X basis vector), the probability of selecting | R > and | L > is 5% (or the probability of selecting Y basis vector is 10%, and | R > and | L > are eigen states of Y basis vector), the probability can be set according to requirements, and which state is controlled by a random number is specifically selected each time, that is, the embodiment of the present invention is an unbalanced basis vector scheme. For example, the emitting state is | R > when the random number is 00, the emitting state is | L > when the random number is 01, the emitting state is | P > when the random number is 10, the emitting state is | N > when the random number is 11, the probability of the occurrence of the random number 00 is 5%, the probability of the occurrence of the random number 01 is 5%, the states corresponding to the two random numbers are only Y-basis vector eigenstates, the probability of the sum is 10%, the probability of the occurrence of the random number 10 is 45%, the probability of the occurrence of the random number 11 is 45%, the states corresponding to the two random numbers are X-basis vector eigenstates, and the probability of the sum is 90%.
It should be noted that, in the embodiment of the present invention, the selection of each polarization state may be set according to a requirement, that is, the probability of the random number is not limited to the manner described in the embodiment of the present invention.
When performing polarization decoding, the probabilities of polarization decoding and polarization encoding selection in the unbalanced basis vector scheme of the present invention may be the same or different. And when the two selection probabilities are the same, the resultant code rate is the highest. For example, when the above probabilities of 90% and 10% are selected during encoding, the two probabilities of 85% and 15% can be arbitrarily selected during decoding, and when the two probabilities of 90% and 10% are selected during decoding, the system performance is best and the coding rate is highest.
In phase encoding, the random number controls the second phase modulator PM2 to select different phase differences. In the prior art, the probabilities of selecting {0, pi/4, pi/2, 3 pi/4 } are the same by adopting a balanced basis vector scheme; in the embodiment of the present invention, the probabilities in the non-equilibrium basis vector scheme are different, for example, the probability of selecting 0 and pi/2 is 45% (or the probability of selecting X basis vector is 90%), and the probability of pi/4 and 3 pi/4 is 5% (or the probability of selecting Y basis vector is 10%). The selection probability setting may be set as desired.
In the embodiment of the present invention, during phase encoding, the phase of the optical pulse is changed by the second phase modulator PM2, and the implementation principle is as follows: the phase modulator changes the phase of the optical pulse under the control of the pulse voltage, the pulse amplitudes of different pulse voltages correspond to different phase change amounts, if the optical pulse passes through the phase modulator, the voltage of the phase modulator is in a high-voltage state (different amplitudes correspond to different phase change amounts), the phase of the optical pulse changes, and if the voltage of the phase modulator is in a low-voltage state (also can be said to be not added with voltage), the phase of the optical pulse does not change. In the embodiment of the invention, the phase difference corresponding to the quantum state needing to be selected is {0, pi/2, pi, 3 pi/2 }, and modulation is realized through a second phase modulator PM 2.
One way is to apply a control pulse voltage of relatively long duration to the second phase modulator PM 2. In this case, the optical pulse needs to pass through the second phase modulator twice in the emitting and reflecting processes, because the duration of the high voltage is long, the second phase modulator is in a high-voltage state in the two processes, and the phase of the optical pulse is changed twice in total, so the phase difference of the second phase modulator only needs to be set to be half of the phase difference {0, pi/2, pi, 3 pi/2 } corresponding to the quantum state needing to be selected, that is, the phase change amount of the second phase modulator PM2 is set to be {0, pi/4, pi/2, 3 pi/4 }. In another embodiment, a pulse voltage with a relatively short duration is applied to the second phase modulator PM2, so that when the optical pulse passes through the second phase modulator PM2 for the first time, the second phase modulator PM2 is in a high-voltage state, and after reflection, the second phase modulator PM2 passes through the second phase modulator PM2 for the second time, and the phase can be changed only once, so that the phase difference set by the second phase modulator PM2 needs to be the same as the phase difference {0, pi/2, pi, 3 pi/2 } corresponding to the quantum state to be selected, and in this way, the second phase modulator PM2 needs to have a relatively high phase difference, so that a relatively high control voltage is needed, which results in relatively high power consumption, relatively high implementation cost, and difficult implementation, and therefore the first way is preferably used.
In phase decoding, the random number controls the third phase modulator PM3 to select a different measured basis vector. In the balanced basis vector scheme, the probabilities of selecting 0 and π/4 are the same; the non-equilibrium basis vector scheme is different, e.g., the probability of selecting 0 is 90% and the probability of selecting π/4 is 10%.
In the quantum key distribution system according to the embodiment of the present invention, when performing polarization encoding, 0 ° (i.e., | H) is usually selected>) 90 ° (i.e. | V)>) 45 ° (i.e. | P)>) And-45 ° (i.e. | N)>) These four polarization states are encoded. In an optical fiber system, if the four polarization states are required to be actively modulated, the four polarization states are difficult to modulate, the modulation rate is low, and the optical fiber system is difficult to be applied to high-speed quantum communication occasions. In the embodiment of the invention, two eigenstates of the Y basis vector are consideredAndthe BB84 protocol coding can be completed by selecting the X basis vector and the Y basis vector as the coding basis vector, and the same safety is achieved as the selection of the X basis vector and the Z basis vector. The X basis vector, the Y basis vector and the Z basis vector are defined in quantum mechanics, each basis vector corresponds to a matrix with 2 rows and 2 columns, and according to linear algebraic knowledge, each basis vector corresponds to two special states, which are called eigenstates.
Wherein the content of the first and second substances,
in the polarization encoding process, a high-speed phase modulator (namely PM1) is utilized to realize polarization modulation through a skillfully designed Sagnac interference ring structure, and eigenstates corresponding to polarization X-basis vectors and polarization Y-basis vectors are selected as polarization encoding states. The laser LD emits a high-speed light pulse with a polarization of +45 °, i.e.The incident light pulse is split into two light pulses, a transmission path and a reflection path, by the PBS 1. The two light pulses have different polarizations, namely 0-degree polarization (H) and 90-degree polarization (V), and the relative phase difference between the H and the V is modulated by PM 1. Assume that the phase change amount of PM1 is θ1Then the output polarization state isWherein the phase change amount of PM1 is theta1By random number control, it is randomly selected between 0, pi/4, pi/2, 3 pi/4. If 0 is selected, the output polarization state is P; if pi/4 is selected, polarization is outputState is | R>(ii) a If pi/2 is selected, the output polarization state is | N>(ii) a If 3 π/4 is chosen, the output polarization state is | L>。
Due to the Sagnac interference loop structure in the polarization encoding module 111, the paths of the optical pulses corresponding to | H > and | V > are consistent and opposite, and the influence of the fiber channel change on the polarization state is automatically compensated.
After the polarization encoding is completed, the light pulse is input to the phase encoding module 112 for phase encoding, and the light may have one of four polarization states (| R >, | L >, | P >, and N). In order to improve the interference contrast, a Mach-Zehnder interferometer in the prior art is generally built by using polarization maintaining fibers and cannot be adapted to 4 kinds of polarization. In the embodiment of the invention, the Faraday-Michelson interferometer (in the phase encoding module 112, the BS1, FM1, FM2 and PM2 form an unequal arm Faraday-Michelson interferometer) is adopted to solve the problem. The input optical pulse is firstly split into two optical pulses by the BS1, and the two optical pulses respectively enter the two arms of the interferometer, and are then reflected back by the FM1 and the FM2, respectively, and are output to the variable optical attenuator VOA.
In the phase encoding module 112, the lengths of the two arms of the Faraday-Michelson interferometer are not the same, so that the arrival times of the returned optical pulses are different, and two adjacent optical pulses are output from the BS1, one optical pulse is at the front | t |0>Another light pulse after | t1>And the time delay between the two light pulses can be reasonably adjusted by controlling the length difference of the two arms. The phase of the interferometer is changed by PM2 provided on one arm and by PM2 provided therein2Output phase state thereofThe phase change amount of PM2 is θ2By random number control, it is randomly selected between 0, pi/4, pi/2, 3 pi/4. If 0 is selected, the phase state is outputIf pi/4 is selected, the output phase state isIf pi/2 is selected, the output phase state isIf 3 pi/4 is selected, the output phase state is
In the embodiment of the invention, the Faraday-Michelson interferometer is adopted to realize phase encoding, can be built by adopting a single-mode optical fiber and is easy to adapt to different input polarization states; meanwhile, a Faraday rotating mirror (Faraday rotating mirror) structure is adopted, so that polarization change caused by birefringence in an optical fiber channel can be automatically compensated, and the polarization state of the optical pulse is kept stable.
During phase decoding, the phase decoding module 121 also has a Faraday-Michelson interferometer (in the phase decoding module 121, the BS2, FM3, FM4, and PM3 also form an unequal-arm Faraday-Michelson interferometer) for phase dimension decoding. The optical pulse is split into two beams by the BS2, and the two beams enter the two arms of the interferometer respectively and are reflected back by FM3 and FM4 respectively. PM3 is arranged on one arm of the interferometer, and one of {0, pi/4 } is randomly selected as a decoding basis vector. Because two optical pulses are output after phase encoding, in the invention, the same arm length difference is set between the Faraday-Michelson interferometer in the phase decoding module 121 and the Faraday-Michelson interferometer in the phase encoding module 122, so that the former optical pulse and the latter optical pulse reach BS2 for interference measurement after passing through the long arm of the interferometer. The optical pulse generated by the former optical pulse passing through the short arm of the interferometer and the optical pulse generated by the latter optical pulse passing through the long arm of the interferometer are unwanted signals, and the unwanted signals are set outside the operating time of the detection module 123 by setting the operating time window of the detection module 123 and are not detected by the detection module 123.
Since the interference optical pulse after phase decoding may exit from the first port a41 of the BS2 and also exit from the second port a42 thereof, the optical pulse output from the two ports needs to be polarization decoded, so that the conventional design needs two polarization decoding modules to count up to 8 single photon detectors, which is resource-consuming and costly. The invention designs a time multiplexing optical path, and designs that the optical pulse emitted from a first port a41 of a BS2 passes through C1 and then is delayed through DL1, the optical pulse and the optical pulse emitted from a second port a42 of the BS2 form delay of preset time in terms of time, and then the two optical pulses are input into the same polarization decoding module 122 for polarization decoding. In this way, the single photon detectors (D1-D4) in the detector module 123 can distinguish from the time of arrival of the light pulse whether the light pulse came from the first port a41 or the second port a42 of the BS2 to determine the phase information of the light pulse. The phase decoding module 121 and the polarization decoding module 122 share one detector module 123.
After polarization conversion by the PBS1, the active polarization decoding method or the passive polarization decoding method can be selected for polarization decoding in accordance with the conventional polarization encoding QKD. The active polarization decoding mode is to actively select a measurement basis vector through an adjusting device, and the passive polarization decoding mode is to divide light into two paths through a beam splitter, wherein each path represents different measurement basis vectors. The embodiment of the invention can realize a passive decoding mode, and the beam splitter BS3 is used for dividing the incident light pulse into two paths during polarization decoding, and the two paths are respectively used for polarization measurement through the PBS2 and the PBS 3. The polarization decoding can also be realized by an active polarization decoding mode, the polarization measurement basis vector is actively selected by methods such as an optical switch or a photoelectric modulator, and then the polarization measurement is carried out by a polarization beam splitter.
The embodiment of the invention can improve the key generation rate by one time by simultaneously using polarization modulation (comprising polarization encoding and polarization decoding) and phase modulation (comprising phase encoding and phase decoding) for single photons.
The unbalanced BB84 protocol is employed for both polarization modulation and phase modulation to further increase the security key rate. In the standard BB84 protocol, during encoding, one of two encoding basis vectors is selected with the same probability, and half of the basis vectors are not selected to be consistent through basis vector comparison, so that the encoding rate of single photons is 1/2, and the encoding efficiency is low. In embodiments of the invention, encoding is performed for either the polarization dimension or the phase dimensionThe selection is made among the X basis vector and the Y basis vector using the probability of imbalance. The probabilities of selecting X-base vector and Y-base vector are respectively p and 1-p during coding, and inconsistent case numbers of base vector selection are discarded after base vector comparison, so that the coding efficiency of a single photon is p2+(1-p)2＝1+2p2-2 p. If one of the basis vectors is chosen with a probability higher than 50%, the coding rate of a single photon will be greater than 1/2, higher than the standard BB84 protocol. If one of the basis vectors is selected with a probability approaching 100%, the single photon encoding efficiency will approach 1 and the key rate will be greatly increased.
In the embodiment of the invention, the realization principle of the non-equilibrium basis vector selection is as follows: in the phase modulation process, the probability of selecting {0, π/2} is p1The probability of selecting { pi/4, 3 pi/4 } is 1-p1The probability of choosing {0, π/2} during polarization encoding is p2The probability of selecting { pi/4, 3 pi/4 } is 1-p2. Correspondingly, in the phase decoding process, the probability of selecting 0 is p1Selecting a probability of pi/4 of 1-p1The probability of choosing the HV basis vector (i.e., the Z basis vector) during polarization decoding is p2The probability of choosing the PN basis vector is 1-p2. (the selection probability can be controlled by controlling the random number if active polarization decoding is used; the selection probability can be controlled by selecting the BS splitting ratio if passive polarization decoding is used). When p is10.5 or p20.5 is consistent with the usual QKD scheme; when p is1Not equal to 0.5 or p2Higher communication rate and coding efficiency can be obtained when the rate is not equal to 0.5; when p is1→ 0 or 1 or p2→ 0 or 1 can obtain coding efficiency close to 100% and communication rate improvement by multiples. If p is1→ 0 or 1 while p2→ 0 or 1, a coding efficiency of 2 bits and a 4-fold increase in communication rate of approximately 1 photon can be obtained.
As can be seen from the above description, in the embodiment of the present invention, the non-equilibrium basis vector scheme is adopted, the quantum communication security key rate is greatly increased, and under the same parameter condition, compared with the standard BB84 protocol of single-dimensional encoding, the key rate is increased by about 4 times.
If the technical scheme of the invention is realized on the basis of the weak coherent light source, the safety of the system can be improved by combining a decoy state method. If the method is a simple superimposed spoofing mode method based on the invention, the method also belongs to the protection scope of the embodiment of the invention.
In the embodiment of the invention, the laser LD is a semiconductor laser and can emit light pulses under the control of an external power-on signal. The PBS1 has four ports, and the optical pulse is input from a11, split into two beams, and output from a13 and a14 respectively, and the polarization states of the optical pulses output from a13 and a14 are | H > and | V > respectively, regardless of the polarization state of the input optical pulse, for example, it can be assumed that the polarization state of the output of a13 is | H > and the polarization state of the output of a14 is | V >.
In the quantum key distribution system of the embodiment of the invention, 3 phase modulators PM1-PM3 are adopted, and each phase modulator adjusts the phase of an optical pulse under the control of an external voltage signal; three polarization beam splitters PBS1-PBS3 are adopted in total, and the polarization beam splitter is provided with two input and output ports; the total use of 4 Faraday rotators FM1-FM4, light can be reflected vertically after being input into the Faraday rotators, and the polarization state changes 90 degrees, such as incidence | H >, emergent | V > after reflection, incidence | V >, and emergent | H > after reflection; adopting an adjustable optical attenuator VOA for attenuating the light pulse intensity to a single photon magnitude; one circulator C1 is adopted, 3 ports a31-a33 are shared, light is input from a31 and output from a32, and light is input from a32 and output from a 33; the method adopts three beam splitters BS1-BS3 in total, the beam splitters are provided with four ports, light is input from one port, and is averagely divided into two paths finally and output from the other two ports; a delay device DL1 is used for adjusting the arrival time of the light pulse; two polarization controllers PC1-PC2 are used for adjusting the polarization state of the light pulse; a total of 4 single photon detectors D1-D4 are used to generate an electrical pulse output when an optical pulse is input.
The working process of the quantum key distribution system of the embodiment of the invention is as follows:
the laser LD generates light pulses with polarization state | P>. The optical pulse is input into the polarization encoding module 111 to be modulated into different polarization states for output. Polarization encodingModule 111 is designed as a fiber Sagnac ring structure, and the light pulse input from a11 of PBS1 is split into two beams, output from a13 and a14 respectively, and has polarization states of H and | V respectively>. The light pulse output from a13 passes through PM1 and then returns to PBS1 through a14, and the light pulse output from a14 passes through PM1 and then returns to PBS1 through a 13. Since the two light pulses travel the same path and in different directions, they reach the PBS1 at the same time, and interfere with each other in the PBS1, and are output from a 12. The polarization state of the output is related to the phase of the PM1 change. Assume that the phase change amount of PM1 is θ1The output polarization state of which isThe amount of phase change of PM1 is controlled by a true random number, chosen randomly between {0, π/4, π/2, 3 π/4 }. If 0 is chosen, the output polarization state is | P>(ii) a If pi/4 is selected, the output polarization state is | R>(ii) a If pi/2 is selected, the output polarization state is | N>(ii) a If 3 π/4 is chosen, the output polarization state is | L>. The optical pulse emitted from the polarization encoding module 111 is input from a21 of the BS1, split into two beams, and output from a23 and a24, respectively. The optical pulse output by a23 is subjected to phase modulation by PM2, reflected by FM1, returned to the original path, and reaches BS1 through a 23. The light pulse output by a24 is directly reflected by FM2 and returns back to BS1 through a 24. The lengths of the optical fibers corresponding to a23 and a24 are different, and the length difference is set according to requirements. Thus, two optical pulses can be output after passing through the phase encoding module 112, one optical pulse preceding | t0>Another light pulse after | t1>There is a difference in phase difference between the two, which is related to the phase difference modulated by PM 2. Assume that the phase change amount of PM2 is θ2Output phase state thereofThe phase change amount of PM2 is θ2By random number control, it is randomly selected between 0, pi/4, pi/2, 3 pi/4. If 0 is selected, the phase state is outputSuch as selectionIs pi/4, the output phase state isIf pi/2 is selected, the output phase state isIf 3 pi/4 is selected, the output phase state is
After combined polarization and phase modulation, the quantum state in the optical pulse isWith both polarization information and phase information.
The optical pulse output from the phase encoding module 112 passes through an adjustable optical attenuator VOA to attenuate the optical pulse intensity to a single photon level.
The optical pulse output from the optical fiber channel 13 enters the phase decoding module 121 for phase decoding. The input is from a31 of circulator C1, the output is from a32 into a41 of BS 2. The first optical pulse of the phase decoding module 121 passes through the long arm of the interferometer, i.e. is output from a43, passes through PM3, enters FM3, is reflected by FM3, and returns to a 43; the second pulse passes through its interferometer short arm, i.e. output from a44, and is reflected by FM4 and returned to a 44. Since the arm length difference of the interferometer in the phase decoding module 121 is equal to the arm length difference of the interferometer in the phase encoding module 112, two optical pulses arrive at the BS2 at the same time and interfere with each other, and are output from a41 and a42 of the BS2, respectively. The PM3 performs measurement basis vector selection under the control of random numbers, and randomly selects phase difference as {0, pi/4 }. The optical pulse output through a42 directly enters the polarization decoding module 122 for polarization decoding, the optical pulse output through a41 is input from a32 of C1, is output from a33 of C1, and enters the polarization decoding module 122 for polarization decoding through DL 1. DL1 functions to time-shift the arrival of two light pulses by a predetermined length to ensure that the same detector module 123 can correctly detect and resolve the two light pulses. DL1 can be implemented by providing a suitable length of optical fiber or quantum storage. By the time multiplexing mode, the number of the polarization decoding modules 122 and the number of the single photon detectors can be effectively reduced, the circuit structure of the polarization decoding modules 122 is simplified, the resource consumption is reduced, and the system cost is saved. It should be noted that the time window of the detector needs to be set properly, and only the interference light pulse is measured. In the embodiment of the present invention, the polarization decoding module 122 and the phase decoding module 121 share the same detector module 123.
After the phase decoding is completed, the optical pulse is input to the polarization decoding module 122 for polarization decoding. The optical pulse is firstly divided into two optical pulses by the BS3 and is respectively output by a53 and a54, the optical pulse output by a53 firstly passes through the PC1 and then the PBS2 to detect two polarization states, and the optical pulse output by a54 firstly passes through the PC2 and then the PBS3 to detect the other two polarization states. The detector module 123 detects four polarization states of the light pulse through the single photon detectors D1-D4, respectively.
As can be seen from the above description, in the quantum key distribution system according to the embodiment of the present invention, on the basis of the unbalanced BB84 protocol, polarization modulation and phase modulation are used simultaneously, so that the coding efficiency of 2 bits per 1 photon is obtained, and the quantum communication rate is increased by four times; the polarization modulation state is not completely consistent with the state at the time of the last polarization measurement; high-speed polarization modulation is realized by utilizing high-speed phase modulation through designing a Sagnac interference ring.
In the embodiment of the invention, the polarization encoding is realized by the phase modulator. One conventional polarization encoding scheme implements polarization encoding by two polarization beam splitters and one phase modulator, essentially a Mach-zedner interferometer. The embodiment of the invention is realized by adopting a polarization beam splitter and a phase modulator, which are Sagnac interference rings essentially, on one hand, the technical scheme of the embodiment of the invention reduces the device requirement and reduces the cost; on the other hand, due to the perfect symmetrical structure of the Sagnac interference ring, paths which the optical pulses pass through are completely the same, the influences of environmental factors such as temperature and the like on polarization coding can be automatically compensated, the stability is good, and meanwhile, the processing technology requirement is greatly reduced. In the traditional polarization encoding scheme, the stability of the interferometer needs to be maintained by adopting technologies such as active temperature control and the like, the resource consumption and the cost are higher, and the application range is smaller; meanwhile, in the processing process, the arm lengths of two arms of the interferometer are required to be completely equal, so that the quality of output light pulses can be ensured, and the process requirement is very strict. In addition, in the technical scheme of the embodiment of the invention, the requirement on the phase modulator is lower, and the modulated voltage and phase difference are only half of those of the traditional polarization coding scheme, which is beneficial to reducing the power consumption of the system and improving the working frequency and stability of the system.
Another conventional polarization encoding scheme adopts a single-channel polarization beam splitter to form an interferometer, and an output port and an input port of the interferometer are the same, so that a circulator is required to separate input light from output light; in the technical scheme of the invention, the dual-channel polarization beam splitter is adopted for polarization coding, and the input port and the output port of the polarization coding module 111 are different ports of the polarization beam splitter, so that on one hand, the number of devices is reduced, and the cost is reduced, and on the other hand, the transmission efficiency and the system extinction ratio can be reduced by using a circulator in the traditional polarization coding.
According to the technical scheme, a single laser is used as a quantum light source, and a traditional QKD system needs 4 lasers as light sources, so that on one hand, the technical scheme simplifies a light path structure and saves cost, on the other hand, compared with a scheme with multiple lasers, the technical scheme can obtain consistent wavelength characteristics, spectrum characteristics and the like, possible attack methods of an eavesdropper on the light source system are reduced, and the safety is higher.
The technical scheme of the invention adopts an active polarization modulation technology, selects the coding basis vector and the Sagnac interference ring, uses the high-speed phase modulator to complete polarization modulation, is completely different from the traditional passive polarization modulation scheme, can obtain higher modulation rate, and can obtain longer working distance and higher safe key rate.
The Faraday-Michelson interferometer designed in the technical scheme of the invention has higher interference contrast and wider application range, and can be simultaneously realized in a free space optical path and an optical fiber optical path. The conventional technical scheme adopts a Mach-Zedner interferometer, so that the Mach-Zedner interferometer can only be built by using a free space optical path, and the interference contrast is low and cannot be used in an optical fiber optical path.
In the technical scheme of the invention, the polarization states are coded by using { | P >, | N >, | R >, | L > } and the four polarization states are respectively detected after polarization decoding, which has significant difference from the prior technical scheme that { | H >, | V >, | P >, | N > } is used for polarization decoding. The technical scheme of the invention has the advantages that the optical fiber provides various high-speed modulation and simple detection at the same time.
In the technical scheme of the invention, the unbalanced BB84 protocol is adopted, compared with the technical scheme adopting the standard BB84 protocol, the key code rate is higher, theoretically, the single-photon coding efficiency in the technical scheme of the invention is close to 2, and is about one time higher than that in the traditional scheme.
In the technical scheme of the invention, only 4 single-photon detectors are needed, and in the traditional technical scheme, 8 single-photon detectors are generally needed. The technical scheme of the invention simplifies the optical path of the system, reduces the number of devices, reduces the cost and improves the stability.
In the technical scheme of the invention, the BB84 scheme is adopted for phase modulation, the safety of the BB84 scheme is strictly proved in theory at present, and the safety of the DPS scheme is only partially proved, so that the safety is higher compared with the realization mode of the DPS scheme.
In the phase modulation stage, the technical scheme of the invention adopts a Faraday-Michelson interferometer, compared with the traditional technical scheme adopting a Mach-Zedner interferometer. The technical scheme of the invention has the polarization automatic compensation function and better interference performance and stability.
In the technical scheme of the invention, only one interferometer is needed when phase encoding is carried out, the optical path structure is simple, and the system performance and stability are better. The polarization modulation function is realized through the phase modulator, and the phase modulator has good stability. When the phase is decoded, only one interferometer is needed for phase decoding, and four interferometers are needed for phase decoding in the traditional technical scheme, so that the technical scheme of the invention simplifies the optical path structure and improves the system performance and stability.
In the technical scheme of the invention, phase decoding is realized by the Faraday-Michelson interferometer, and compared with the traditional technical scheme of the Mach-Zedner interferometer, the interferometer in the technical scheme of the invention has the polarization automatic compensation function and better interference performance and stability.
Based on the foregoing quantum key distribution system embodiment, another embodiment of the present invention further provides a communication method of a quantum key distribution system, which is used in the quantum key distribution system described in the foregoing embodiment, where the communication method is shown in fig. 2, and fig. 2 is a schematic flow diagram of a communication method of a quantum key distribution system provided in an embodiment of the present invention, where the communication method includes:
step S11: and generating optical pulses at the transmitting end, and carrying out polarization and phase joint modulation on the optical pulses.
Step S12: and transmitting the optical pulse subjected to polarization and phase joint modulation to the receiving end through the optical fiber channel.
Step S13: and at the receiving end, sequentially carrying out phase decoding and polarization decoding on the optical pulse, compensating the disturbance of the optical fiber channel on the optical pulse after the combined modulation after the polarization decoding is finished, and finally detecting the polarization state of the optical pulse.
As described in the above embodiment, the transmitting end includes: the device comprises a laser, a polarization encoding module, a phase encoding module and an adjustable optical attenuator.
In the communication method, the light pulse generated by the transmitting terminal includes four polarization states { | P >, | N >, | R >, | L > }. And detecting each polarization state at the receiving end respectively.
As described in the foregoing embodiment, in the communication method according to the embodiment of the present invention, the transmitting end and the receiving end may be controlled by random numbers to implement an unbalanced basis vector scheme by using an unbalanced BB84 protocol.
Optionally, the generating of the optical pulse at the transmitting end, and performing polarization and phase joint modulation on the optical pulse are shown in fig. 3, where fig. 3 is a schematic flow diagram of a polarization and phase joint modulation method provided in an embodiment of the present invention, and the method includes:
step S21: generating the light pulse by the laser.
Step S22: the polarization encoding module is used for carrying out polarization encoding on the light pulse emitted by the laser, and one of the four polarization states of | R >, | L >, | P > and | N > of the light pulse is randomly selected for carrying out polarization encoding.
Step S23: and carrying out phase encoding on the polarization-encoded optical pulses through the phase encoding module, and randomly selecting a phase difference from {0, pi/2, pi, 3 pi/2 } to carry out phase encoding.
Step S24: and attenuating the light pulse emitted by the phase coding module to a single photon magnitude by the variable optical attenuator.
As described in the above embodiment, the receiving end includes: the device comprises a phase decoding module, a polarization decoding module and a detector module.
Optionally, the receiving end sequentially performs phase decoding and polarization decoding on the optical pulse, after the polarization decoding is completed, the disturbance of the optical fiber channel to the optical pulse after the joint modulation is compensated, and finally the detection of the polarization state of the optical pulse is shown in fig. 4, where fig. 4 is a schematic flow diagram of a decoding method provided in an embodiment of the present invention, and the method includes:
step S31: and carrying out phase decoding on the optical pulse subjected to the joint modulation through the phase decoding module.
Step S32: and the polarization decoding module is used for carrying out phase decoding on the optical pulse emitted by the phase decoding module, and the disturbance of the optical fiber channel to the optical pulse is compensated.
Step S33: and detecting each polarization state of the light pulse emitted by the polarization decoding module through the detector module respectively.
Finally, step S34: and carrying out data subsequent processing under the assistance of a classical channel to generate safe quantum keys at a transmitting end and a receiving end.
As described in the foregoing QKD embodiment, the communication method according to the embodiment of the present invention can convert the polarization state | P > of the optical pulse emitted from the laser into four polarization states { | P >, | N >, | R >, | L > } at the emitting end through the sagnac interferometric loop.
The communication method of the embodiment of the invention is used for the quantum key distribution system of the embodiment, is simple in implementation mode, and can double the key generation rate by using polarization modulation (including polarization encoding and polarization decoding) and phase modulation (including phase encoding and phase decoding).
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. As for the communication method disclosed in the embodiment, since it corresponds to the quantum key distribution system disclosed in the embodiment, the description is relatively simple, and the relevant points can be referred to the partial description of the quantum key distribution system.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (15)

1. A quantum key distribution system, comprising:
the transmitting end is used for generating optical pulses and carrying out polarization and phase joint modulation on the optical pulses;
the receiving end is connected with the transmitting end through an optical fiber channel and is used for sequentially carrying out phase decoding and polarization decoding on the optical pulse, compensating the disturbance of the optical fiber channel on the optical pulse after the combined modulation after the polarization decoding is finished, and finally detecting the polarization state of the optical pulse;
in the transmitting end, the light pulse comprises four polarization states of { | P >, | N >, | R >, | L > };
detecting each polarization state at the receiving end respectively;
wherein | P > and | N > are eigenstates of the X basis vector; and | R > and | L > are eigenstates of the Y basis vector.
2. The quantum key distribution system of claim 1, wherein the transmitting end and the receiving end are controlled by random numbers to implement an unbalanced basis vector scheme using an unbalanced BB84 protocol.
3. The quantum key distribution system of claim 1 or 2, wherein the transmitting end comprises:
a laser, which is a light source, for generating the light pulse;
the polarization encoding module is used for carrying out polarization encoding on the light pulse emitted by the laser, and randomly selecting one of the four polarization states of | R >, | L >, | P > and | N > of the light pulse for carrying out polarization encoding;
the phase coding module is used for carrying out phase coding on the optical pulses after polarization coding, and randomly selecting a phase difference from {0, pi/2, pi, 3 pi/2 } to carry out phase coding;
and the variable optical attenuator is used for attenuating the optical pulse emitted by the phase coding module to a single photon magnitude.
4. A quantum key distribution system according to claim 3, wherein the polarization state of the light pulses emitted by the laser is | P >;
the polarization encoding module comprises a Sagnac interference ring, and the Sagnac interference ring is used for converting the polarization state of the light pulse emitted by the laser into four polarization states of { | P >, | N >, | R >, | L > }.
5. The quantum key distribution system of claim 3, wherein the polarization encoding module comprises: a first polarization beam splitter, a first phase modulator, and a first random number device;
the first polarization beam splitter has a first port, a second port, a third port, and a fourth port; the first polarization beam splitter acquires the light pulse emitted by the laser through a first port thereof, divides the light pulse into two paths, one path of the light pulse is emitted through a third port thereof, enters a fourth port thereof after passing through the first phase modulator, the other path of the light pulse is emitted through the fourth port thereof, enters a third port thereof after passing through the first phase modulator, and the first phase modulator performs phase modulation on the two paths of the incident light pulse, so that a phase difference is generated between the two paths of the light pulse; the two paths of optical pulses return to the first polarization beam splitter to be converged, form polarization-encoded optical pulses through a second port of the first polarization beam splitter, and send the polarization-encoded optical pulses to the phase encoding module;
wherein the first random number means is configured to perform random number control on a phase change amount of the first phase modulator.
6. The quantum key distribution system of claim 3, wherein the phase encoding module comprises: the device comprises a first beam splitter, a first Faraday rotator mirror, a second phase modulator and a second random number device;
the first splitter has a first port, a second port, a third port, and a fourth port; the first beam splitter obtains the optical pulse after polarization coding through a first port thereof, divides the optical pulse into two paths, one path of the optical pulse is emitted through a third port thereof, enters the first Faraday rotator mirror after passing through the second phase modulator, returns to the third port thereof through the original path after being reflected by the first Faraday rotator mirror, and the other path of the optical pulse is emitted through a fourth port thereof, returns to the fourth port thereof after being reflected by the second Faraday rotator mirror, and transmits the optical pulse after phase coding to the adjustable optical attenuator through the second port thereof;
wherein the second random number means is for performing random number control on the amount of phase change of the second phase modulator.
7. The quantum key distribution system according to claim 1 or 2, wherein the receiving end comprises:
a phase decoding module, configured to perform phase decoding on the jointly modulated optical pulses;
the polarization decoding module is used for carrying out phase decoding on the optical pulse emitted by the phase decoding module and compensating the disturbance of the optical fiber channel on the optical pulse;
and the detector module is used for respectively detecting each polarization state of the light pulse emitted by the polarization decoding module.
8. The quantum key distribution system of claim 7, wherein the phase decoding module comprises: the device comprises a circulator, a delayer, a third random number device, a second beam splitter, a third phase modulator, a third Faraday rotation mirror and a fourth Faraday rotation mirror;
the circulator has a first port, a second port and a third port; the circulator obtains the light pulse emitted by the optical fiber channel through a first port of the circulator;
the second splitter has a first port, a second port, a third port, and a fourth port; the second beam splitter obtains the light pulse emitted from the second port of the circulator through the first port of the second beam splitter, the light pulse is divided into two paths, one path of light pulse is emitted through the third port of the second beam splitter, the light pulse is emitted into the third Farad rotating mirror after passing through the third phase modulator, the light pulse is reflected through the third Farad rotating mirror and then returns to the third port of the third beam splitter, the other path of light pulse is emitted through the fourth port of the fourth Farad rotating mirror and then returns to the fourth port of the fourth Faraday rotating mirror, the reflected light pulse is simultaneously obtained from the third port and the fourth port of the second beam splitter, the reflected light pulse is emitted from the first port and the second port of the second beam splitter after interference in the second beam splitter, the light pulse emitted from the second port of the second beam splitter is sent to the polarization decoding module, the light pulse emitted from the first port of the second beam splitter is sent to the second port of the circulator and then is sent to the time delay unit through the third port of the circulator, sending the data to the polarization decoding module through the time delay device;
wherein the third random number means is for performing random number control on a phase change amount of the third phase modulator.
9. The quantum key distribution system of claim 8, wherein the polarization decoding module comprises: the polarization beam splitter comprises a third beam splitter, a first polarization controller, a second polarization beam splitter and a third polarization beam splitter;
the third splitter has a first port, a second port, a third port, and a fourth port; the third beam splitter acquires the light pulse emitted from the second port of the second beam splitter through the first port of the third beam splitter, acquires the light pulse emitted from the time delay unit through the second port of the third beam splitter, performs polarization decoding on the acquired light pulse by the third beam splitter, and then divides the light pulse into two paths, wherein one path of the light pulse is emitted from the third port of the third beam splitter, enters the second polarization beam splitter through the first polarization controller, and the other path of the light pulse is emitted from the fourth port of the third beam splitter, and enters the third polarization beam splitter through the second polarization controller;
the second polarization beam splitter divides the incident light pulse into two paths and sends the two paths of light pulses to the detector module;
the third polarization beam splitter divides the incident light pulse into two paths and sends the two paths of light pulses to the detector module;
wherein the first polarization controller and the second polarization controller are used for compensating the disturbance of the optical fiber channel to the optical pulse.
10. The quantum key distribution system of claim 9, wherein the detector module comprises four single photon detectors, two of the single photon detectors are configured to detect two optical pulses emitted from the second polarization beam splitter, and two of the single photon detectors are configured to detect two optical pulses emitted from the third polarization beam splitter.
11. A communication method of a quantum key distribution system having a transmitting end and a receiving end connected to the transmitting end via a fiber channel, the communication method comprising:
generating optical pulses at the transmitting end, and carrying out polarization and phase joint modulation on the optical pulses;
transmitting the light pulse subjected to polarization and phase joint modulation to the receiving end through the optical fiber channel;
at the receiving end, phase decoding and polarization decoding are sequentially carried out on the optical pulse, after the polarization decoding is finished, the disturbance of the optical fiber channel on the optical pulse after the combined modulation is compensated, and finally the polarization state of the optical pulse is detected;
the light pulse generated by the transmitting end comprises four polarization states of { | P >, | N >, | R >, | L > };
detecting each polarization state at the receiving end respectively;
| P > and | N > are eigenstates of the X basis vector; and | R > and | L > are eigenstates of the Y basis vector.
12. The communication method according to claim 11, wherein the transmitting end and the receiving end are controlled by random numbers to implement an unbalanced basis vector scheme using an unbalanced BB84 protocol.
13. The communication method according to claim 11 or 12, wherein the transmitting end comprises: the device comprises a laser, a polarization encoding module, a phase encoding module and an adjustable optical attenuator;
the generating of the optical pulses at the transmitting end, the polarization and phase joint modulation of the optical pulses comprising:
generating the light pulse by the laser;
carrying out polarization encoding on the light pulse emitted by the laser through the polarization encoding module, and randomly selecting one of the four polarization states of | R >, | L >, | P > and | N > of the light pulse to carry out polarization encoding;
the polarization-encoded light pulse is subjected to phase encoding through the phase encoding module, and a phase difference is randomly selected from {0, pi/2, pi, 3 pi/2 } to be subjected to phase encoding;
and attenuating the light pulse emitted by the phase coding module to a single photon magnitude by the variable optical attenuator.
14. The communication method according to claim 11 or 12, wherein the receiving end comprises: the device comprises a phase decoding module, a polarization decoding module and a detector module;
the phase decoding and the polarization decoding are sequentially performed on the optical pulse at the receiving end, after the polarization decoding is completed, the disturbance of the optical fiber channel to the optical pulse after the combined modulation is compensated, and finally, the detecting the polarization state of the optical pulse comprises the following steps:
performing phase decoding on the optical pulse subjected to the joint modulation through the phase decoding module;
the polarization decoding module is used for carrying out phase decoding on the optical pulse emitted by the phase decoding module, and the disturbance of the optical fiber channel to the optical pulse is compensated;
and detecting each polarization state of the light pulse emitted by the polarization decoding module through the detector module respectively.
15. The communication method according to claim 13, wherein the polarization state | P > of the light pulse emitted from the laser is converted into four polarization states { | P >, | N >, | R >, | L > } at the emitting end through a sagnac interferometric loop.
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