CN220554019U - Quantum key distribution system based on polarization coding - Google Patents
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Abstract
The utility model discloses a quantum key distribution system based on polarization coding, which comprises a signal generation unit, a signal modulation unit and a signal demodulation unit; the signal generating unit generates two paths of signals of signal photons and idle photons and inputs the two paths of signals to the signal modulating unit and the signal demodulating unit respectively; the signal modulating unit carries out polarization modulation on the signal photons and then sends the signal photons to the signal demodulating unit for detection; the signal demodulation unit sequentially carries out polarization modulation and detection on idle photons and then compares the idle photons with a coding base and a measuring base of the signal modulation unit; if the count results are consistent, the count results of the coincidence counter are reserved; if the result is inconsistent, the counting result of the coincidence counter is abandoned. The utility model adopts the polarization entangled state as the carrier of the information coding and adopts the method conforming to the counting to decode, so that the system has stronger capability of resisting environmental change in the key distribution process and realizes stable and high-code rate quantum key distribution.
Description
Technical Field
The utility model relates to the technical field of quantum cryptography and optical communication, in particular to a quantum key distribution system based on polarization coding.
Background
Quantum key distribution (Quantum Key Distribution, QKD) is a new cryptographic encoding and distribution technology that arose in the eighties of the twentieth century, and unlike traditional mathematical complexity-based cryptographic techniques, quantum key distribution is based on physical principles with absolute security. Current quantum key distribution technology evolves in two ways, fiber QKD on the one hand and free space QKD on the other. In QKD techniques, information is typically loaded using properties of polarization, phase, orbital angular momentum, etc. of photons. In the optical fiber QKD, due to the instability of photon polarization state in transmission process caused by bending and compression of optical fiber and temperature factors, etc., the error rate is high in long distance communication process, so that a polarization coding scheme is not generally adopted. To solve the above problem, it has been found that the phase information of photons is relatively stable in optical fiber transmission, and a phase encoding scheme is started. The use of phase encoding generally requires the use of a Mach-Zehnder interferometer, where two coherent light pulses are used to interfere with each other, and finally the encoded information is decoded from the interference results. The phase encoding well overcomes the defect that quantum information is easy to be interfered by environment in optical fiber transmission, but the coherence is limited by devices in the encoding and decoding of the information, and extremely high interference contrast is required to achieve lower error rate, so that the scheme is also limited to a certain extent. Later studies found that photons can carry orbital angular momentum information in addition to polarization and phase information of the photons. The light beam carrying the orbital angular momentum has spiral wave front and optical singular point, and has unique quantum topological structure characteristic of the orbital angular momentum, different orbital angular momentums have orthogonal characteristic, and the potential of high-dimensional quantum bit coding is provided, so that the light beam has great research value in quantum communication. However, the modulation efficiency of the spatial light modulator used in the current experiment of orbital angular momentum is not high, and the q-plate device used in some schemes is also in the research and development stage, so that the function of the q-plate device is not ideal.
In free space QKD, the distance limitation is largely overcome, but the phase information of photons is phase shifted and severe in free space transmission, while the polarization state of photons is hardly affected, so polarization encoding is generally employed in free space QKD. In some existing polarization coding systems, a plurality of light sources are often adopted, and side channel attacks are difficult to resist due to the non-ideality of devices. Quantum entanglement is a quantum mechanical phenomenon, which describes a special quantum state in a composite system by definition, the quantum state cannot be decomposed into tensor products of the quantum states of a member system, and refers to non-local correlation among a plurality of separable subsystem quantum systems, and a measurement result of one subsystem cannot be independent of measurement parameters of other subsystems, namely, measurement of one subsystem can influence the other subsystem, so that the efficiency of quantum key distribution is low and unstable.
Accordingly, there is a need to improve upon the deficiencies of the prior art by proposing a quantum key distribution system based on polarization encoding.
Disclosure of Invention
The utility model aims to overcome the defects of the prior art, and provides a quantum key distribution system based on polarization coding by utilizing the entanglement characteristic of photons and adopting a polarization coding mode.
The utility model is realized by the following technical scheme:
the quantum key distribution system based on polarization coding comprises a user side Alice and a user side Bob, wherein the user side Alice is provided with a signal generation unit and a signal modulation unit, and the user side Bob is provided with a signal demodulation unit;
the output port of the signal generating unit is respectively connected with the input ports of the signal modulating unit and the signal demodulating unit, and the output port of the signal modulating unit is connected with the input port of the signal demodulating unit;
the signal generating unit is used for generating two paths of signals of signal photons and idle photons and inputting the two paths of signals into the signal modulating unit and the signal demodulating unit respectively;
the signal modulation unit is used for carrying out polarization modulation on the signal photons input by the signal generation unit and then sending the signal photons to the signal demodulation unit for detection;
the signal demodulation unit is used for comparing the code base with the measuring base after sequentially carrying out polarization modulation and detection on idle photons input by the signal generation unit;
the signal demodulation unit comprises a second polarization modulator, a second computer, a second reflecting mirror, a third reflecting mirror, a first single photon detector, a coincidence counter, a second single photon detector and a fourth reflecting mirror;
the output port of the second polarization modulator is sequentially connected with the second reflecting mirror, the third reflecting mirror and the first single photon according to the detector, the output port of the first single photon detector is connected with the input port of the coincidence counter, the input port of the coincidence counter is also connected with the output port of the second single photon detection, and the input port of the second single photon detection is connected with the fourth reflecting mirror; the input port of the second computer is respectively connected with the output ports of the second polarization modulator and the coincidence counter.
Preferably, the signal generating unit comprises a laser, a focusing lens, a BBO crystal and a beam splitter, which are connected in sequence.
Preferably, the laser generates Gaussian light and sends the Gaussian light to a focusing lens; the focusing lens focuses the Gaussian light to form a beam of optical signals and inputs the beam of optical signals to the BBO crystal; the BBO crystal carries out parametric conversion on the optical signal to obtain a pair of entangled photons and sends the entangled photons to the beam splitter; the beam splitter splits the received pair of entangled photons into signal photons and idle photons, which are respectively input to the signal modulation unit and the signal demodulation unit.
Preferably, the polarization entanglement states of the signal photons and the idle photons are:
|φ>＝∑ l C l |H> a |V> b ；
where a and b represent signal photons and idle photons, respectively, and l represents the number of basis vectors.
Preferably, the signal modulation unit includes a first polarization modulator, a first mirror, and a first computer;
the output port of the first polarization modulator is respectively connected with the input ports of the first reflecting mirror and the first computer, and the output ports of the first reflecting mirror and the first computer are respectively connected with the signal demodulation unit.
Preferably, the first computer controls the first polarization modulator to modulate the polarization state of the signal photons, and outputs photons with polarization states to enter the first reflecting mirror; the first mirror reflects the photons having the polarization state to the signal demodulation unit through free space.
Preferably, the first polarization modulator performs polarization state modulation to randomly select four polarization states to perform polarization state modulation on the signal photons;
the four polarization states include |ψ 11 >＝|H>+|V>、|ψ 12 >＝|H>-|V>And |psi 21 >＝|H>+i|V>、|ψ 22 >＝|H>-i|V>；
Wherein |ψ 11 >And |psi 12 >Mutually orthogonalizing to form a first pair of orthonormal basis vectors;
|ψ 21 >and |psi 22 >Mutually orthogonal to form a second pair of orthogonal normalized basis vectors.
Preferably, the photons with polarization states are transmitted to a fourth reflecting mirror after being reflected to the signal demodulation unit through free space, and are transmitted to a second single photon detector through the fourth reflecting mirror to be detected, and the second single photon detector sends detection results to a coincidence counter to be counted.
Preferably, after the idle photon generated by the signal generating unit is input to the signal demodulating unit, the idle photon enters the second polarization modulator first; the second computer carries out polarization state modulation on the input idle photons by controlling the second polarization modulator and then outputs photons with polarization states, and the photons with the polarization states enter the first single photon detector for detection after being reflected by the second reflecting mirror and the third reflecting mirror in sequence; and finally, the first single photon detector sends the detection result to the coincidence counter for counting.
Preferably, the second polarization modulator randomly selects the following four polarization states to perform polarization state modulation on the signal photons;
the four polarization states include:
|ψ 11 >＝|H>+|V>、|ψ 12 >＝|H>-|V)、|ψ 21 >＝|H>+i|V>and |psi 22 >＝|H>-i|V>；
Wherein |ψ 11 >And |psi 12 >Mutually orthogonalizing to form a first pair of orthonormal basis vectors;
|ψ 21 >and |psi 22 >Mutually orthogonal to form a second pair of orthogonal normalized basis vectors.
Preferably, when the signal photons modulate the polarization state to |ψ m，i >And the idle photon modulation polarization state is |psi n，j >In the time-course of which the first and second contact surfaces,
the coincidence counting function is: c (m, i, n, j) =<φ|.|ψ m，i >|ψ n，j >The method comprises the steps of carrying out a first treatment on the surface of the Wherein: m, i, n, j are positive numbers.
Preferably, after the second computer in the signal demodulation unit acquires the value of the coincidence counter, the second computer compares the code base with the measurement base through a common channel with the first computer of the signal modulation unit to determine whether the code base is consistent with the measurement base;
the second computer judges whether the coding base and the measuring base of the signal photon and the idle photon are consistent according to the coincidence counter, and if so, the counting result of the coincidence counter is reserved; if the result is inconsistent, the counting result of the coincidence counter is abandoned.
Preferably, the principle and procedure for determining whether the coding basis and the measurement basis of the signal photon and the idle photon are identical are as follows:
if the coincidence counting result is 0, the polarization states of the signal photons and the idle photons are mutually orthogonal, and the coding basis and the measuring basis of the signal photons and the idle photons are consistent;
if the coincidence counting result is 0.5, the polarization state of the signal photon is the same as that of the idle photon, and the coding base and the measuring base of the signal photon and the idle photon are consistent;
if the coincidence counting result is '0.25', the coding basis and the measuring basis of the signal photon and the idle photon are inconsistent, and the polarization state of the signal photon cannot be obtained.
The beneficial effects of the utility model are as follows:
1. the utility model adopts the polarization entangled state as the carrier of the information coding and adopts the method conforming to the counting to decode, so that the system has stronger capability of resisting environmental change in the key distribution process and realizes stable and high-code rate quantum key distribution.
2. The quantum coding, measuring and common channel communication process of the utility model has high integration of functions, can conveniently transmit and receive information in real time, and has strong operability.
3. The utility model can determine eavesdropping behaviors of eavesdroppers in real time by utilizing violation verification of Bell-CHSH inequality, and provides a highly-safe quantum key distribution system and method based on polarization coding.
Drawings
FIG. 1 is a diagram of the overall structure of the present utility model;
fig. 2 is a schematic diagram of the system of the present utility model.
The numbers corresponding to the component names in the drawings are as follows:
a signal generation unit: a laser-1, a focusing lens-2, BBO crystals-3, and a beam splitter-4;
a signal modulation unit: a first polarization modulator-5, a first mirror-6, a first computer-7;
a signal demodulation unit: the system comprises a second polarization modulator-8, a second computer-9, a second reflecting mirror-10, a third reflecting mirror-11, a first single photon detector-12, a coincidence counter-13, a second single photon detector-14 and a fourth reflecting mirror-15.
Detailed Description
The present utility model will be further described in detail with reference to the following examples, for the purpose of making the objects, technical solutions and advantages of the present utility model more apparent, but the scope of the present utility model is not limited to the following specific examples.
As shown in fig. 1, the quantum key distribution system based on polarization encoding comprises a user terminal Alice and a user terminal Bob, wherein the user terminal Alice is provided with a signal generation unit and a signal modulation unit, and the user terminal Bob is provided with a signal demodulation unit;
the output port of the signal generating unit is respectively connected with the input ports of the signal modulating unit and the signal demodulating unit, and the output port of the signal modulating unit is connected with the input port of the signal demodulating unit;
the signal generating unit is used for generating two paths of signals of signal photons and idle photons and inputting the two paths of signals into the signal modulating unit and the signal demodulating unit respectively;
the signal modulation unit is used for carrying out polarization modulation on the signal photons input by the signal generation unit and then sending the signal photons to the signal demodulation unit for detection;
the signal demodulation unit is used for comparing the code base with the measuring base after sequentially carrying out polarization modulation and detection on idle photons input by the signal generation unit.
As shown in fig. 2, the signal generating unit includes a laser 1, a focusing lens 2, a BBO crystal 3, and a beam splitter 4, which are sequentially connected.
As shown in fig. 2, the signal modulation unit includes a first polarization modulator 5, a first mirror 6, and a first computer 7;
the output port of the first polarization modulator 5 is connected with the input ports of the first reflecting mirror 6 and the first computer 7, respectively, and the output ports of the first reflecting mirror 6 and the first computer 7 are connected with the signal demodulation unit, respectively.
As shown in fig. 2, the signal demodulation unit includes a second polarization modulator 8, a second computer 9, a second mirror 10, a third mirror 11, a first single photon detector 12, a coincidence counter 13, a second single photon detector 14, and a fourth mirror 15;
the output port of the second polarization modulator 8 is sequentially connected with the second reflecting mirror 10, the third reflecting mirror 11 and the first single photon detector 12, the output port of the first single photon detector 12 is connected with the input port of the coincidence counter 13, the input port of the coincidence counter 13 is also connected with the output port of the second single photon detector 14, and the input port of the second single photon detector 14 is connected with the fourth reflecting mirror 15; the input port of the second computer 9 is connected to the output ports of the second polarization modulator 8 and the coincidence counter 15, respectively.
The principle and process of the quantum key distribution system based on polarization encoding in this embodiment are as follows:
a laser 1 as a signal generating unit in a user side Alice of a transmitting side generates a gaussian light and transmits the gaussian light to a focusing lens 2; the focusing lens 2 focuses the Gaussian light to form a beam of optical signals and inputs the beam of optical signals to the BBO crystal 3; the focusing lens 2 functions to focus the light beam and reduce the dissipation of energy.
The BBO crystal 3 converts parameters of the optical signal to obtain a pair of entangled photons and sends the entangled photons to the beam splitter 4; the beam splitter 4 splits the pair of entangled photons into signal photons and idle photons, and the signal photons and the idle photons are respectively input into a signal modulation unit and a signal demodulation unit; the polarization entanglement states of the signal photons and the idle photons are as follows:
where a and b represent signal photons and idle photons, respectively, and l represents the number of basis vectors.
The first polarization modulator 5 performs polarization state modulation; the first polarization modulator 5 is controlled by the first computer 7, and the first polarization modulator 5 outputs photons with polarization states to enter the first reflecting mirror 6 after carrying out polarization state modulation on signal photons input by the signal generating unit; the first reflecting mirror 6 reflects the photons with polarization state to enter the signal demodulation unit through free space;
wherein the first polarization modulator 5 randomly selects the following four polarization states to perform polarization state modulation on the signal photons,
the four polarization states include:
|ψ 11 >＝|H>+|V>、|ψ 12 >＝|H>-|V>、|ψ 21 >＝|H>+i|V>and |psi 22 >＝|H>-i|V>；
Wherein |ψ 11 >And |psi 12 >Mutually orthogonalizing to form a first pair of orthonormal basis vectors;
|ψ 21 >and |psi 22 >Mutually orthogonal to form a second pair of orthogonal normalized basis vectors.
The relation between the two groups of orthonormal basic vectors shows that, between the two communication parties, if the information sender and the receiver use the same basic vector to encode and decode, the obtained result is definite, and the receiver can infer the encoding information of the sender; and once the information receiver performs measurement with the wrong basis vector, the obtained measurement result will be random. Specifically, in the present embodiment, |ψ 11 >And |psi 21 >The code is set to 0, |ψ 12 >And |psi 22 >The code is set to 1.
After the photons with polarization state reflected by the first reflecting mirror 6 enter the signal demodulation unit through free space, the photons are firstly transmitted to the fourth reflecting mirror 15, the photons with polarization state are transmitted to the second single photon detector 14 by the fourth reflecting mirror 15 to be detected, and then the detection result is sent to the coincidence counter 13 by the second single photon detector 14 to be counted.
After the idle photons generated by the signal generating unit are input to the signal demodulating unit, the idle photons enter the second polarization modulator 8; then the second computer 9 carries out polarization state modulation on the input idle photons by controlling the second polarization modulator 8 and outputs photons with polarization states, and the photons with the polarization states enter the first single photon detector 12 for detection after being reflected by the second reflecting mirror 10 and the third reflecting mirror 11 in sequence; finally, the first single photon detection 12 sends the detection result to the coincidence counter 13 for counting.
The second polarization modulator 8 randomly selects the following four polarization states for polarization state modulation of the signal photons,
the four polarization states include:
|ψ 11 >＝|H>+|V>、|ψ 12 >＝|H>-|V>、|ψ 21 >＝|H>+i|V>、|ψ 22 >＝|H>-i|V>；
wherein |ψ 11 >And |psi 12 >Mutually orthogonalizing to form a first pair of orthonormal basis vectors;
|ψ 21 >and |psi 22 >Mutually orthogonal to form a second pair of orthogonal normalized basis vectors.
When the signal photon modulates the polarization state to be |psi m，i >And the idle photon modulation polarization state is |psi n，j >In the time-course of which the first and second contact surfaces,
the coincidence counting function is:wherein: m, i, n, j are positive numbers.
Further, after the second computer 9 in the signal demodulation unit obtains the value of the coincidence counter 13, the value is compared with the first computer 7 in the signal modulation unit to judge whether the code base and the measurement base are consistent through a common channel;
the second computer 9 judges whether the coding base and the measuring base of the signal photon and the idle photon are consistent according to the coincidence counter, and if so, the counting result of the coincidence counter is reserved; if the count results are inconsistent, discarding the count results of the coincidence counter;
the principle and the process for judging whether the coding base and the measuring base of the signal photon and the idle photon are consistent are as follows:
if the coincidence counting result is 0, the polarization states of the signal photons and the idle photons are mutually orthogonal, and the coding basis and the measuring basis of the signal photons and the idle photons are consistent;
if the coincidence counting result is 0.5, the polarization state of the signal photon is the same as that of the idle photon, and the coding base and the measuring base of the signal photon and the idle photon are consistent;
if the coincidence counting result is '0.25', the coding basis and the measuring basis of the signal photon and the idle photon are inconsistent, and the polarization state of the signal photon cannot be obtained.
Therefore, when the coding base and the measuring base of Alice and Bob are selected to be consistent, if the coincidence counting result is 0, bob can infer that the polarization state coded by Alice is orthogonal to the quantum state modulated by itself;
if the coincidence count is "0.5", bob can infer that Alice encodes the same polarization as his own modulated quantum. Therefore, when the coding base and the measuring base are the same, bob can obtain the coding polarization state information of Alice and further decode the coding bit of Alice according to the polarization state;
when Alice and Bob code bases and measurement bases are inconsistent in selection, the coincidence count values are all 0.25, and Bob cannot judge the code quantum state information of Alice.
The system adopts the polarization entangled state as the carrier of information coding and adopts a method conforming to counting to decode, so that the system realizes stable and high-code rate quantum key distribution.
The method for quantum key distribution by the quantum key distribution system comprises the following steps:
step 1: preparing a polarization entangled state: the signal generation unit is used as a communication sender user terminal Alice, generates two paths of signals of signal photons and idle photons, and inputs the two paths of signals into the signal modulation unit of the user terminal Alice and the signal demodulation unit of the user terminal Bob serving as a receiver;
step 2: polarization coded modulation of signal photons: the signal modulation unit carries out polarization modulation on the signal photons input by the signal generation unit and then sends the signal photons to a second single photon detector in the signal demodulation unit for detection, and the second single photon detector sends a detection result to the coincidence counter for counting;
step 3: polarization measurement modulation of idle photons: the signal demodulation unit is used as a communication party Bob to carry out polarization modulation on idle photons input by the signal generation unit and then send the idle photons to the first single photon detector for detection; the first single photon detection sends the detection result to a coincidence counter for counting;
step 4: coincidence count measurement of signal and idle photons: after the second computer in the signal demodulation unit acquires the value conforming to the counter, comparing the value with the first computer of the signal modulation unit to judge whether the code base and the measurement base are consistent or not through a common channel;
step 5: alignment of coding and measurement bases: the second computer judges whether the coding base and the measuring base of the signal photon and the idle photon are consistent according to the coincidence counter;
if the data are consistent, the counting result of the coincidence counter and the data of the same measuring base are reserved;
if the data are inconsistent, discarding the counting result of the coincidence counter and the data of the same measurement base;
step 6: violation verification of Bell-CHSH inequality: the two parties of communication Alice and Bob take out a part of data to calculate a Bell-CHSH inequality, and judge whether eavesdropping exists according to the violation condition of the Bell-CHSH inequality;
if so, terminating the communication process;
if not, jumping to the step 7;
step 7: obtaining a key by error correction and privacy enhancement: both parties Alice and Bob obtain the final secret key by correcting and confidentiality enhancement to the data of the same measurement base.
The utility model adopts the polarization entangled state as the carrier of the information coding and the method conforming to the counting to decode, so that the system has stronger capability of resisting environmental change in the key distribution process, and realizes stable and high-code rate quantum key distribution.
The quantum coding, measuring and common channel communication process of the utility model has high integration of functions, can conveniently transmit and receive information in real time, and has strong operability.
The utility model utilizes the violation verification of Bell-CHSH inequality to judge the eavesdropping behavior of an eavesdropper in real time, and provides a highly-safe quantum key distribution system based on polarization coding.
Variations and modifications to the above would be obvious to persons skilled in the art to which the utility model pertains from the foregoing description and teachings. Therefore, the utility model is not limited to the specific embodiments disclosed and described above, but some modifications and changes of the utility model should be also included in the scope of the claims of the utility model. In addition, although specific terms are used in the present specification, these terms are for convenience of description only and do not constitute any limitation on the utility model.
Claims (10)
1. The quantum key distribution system based on polarization coding is characterized by comprising a user side Alice and a user side Bob, wherein the user side Alice is provided with a signal generation unit and a signal modulation unit, and the user side Bob is provided with a signal demodulation unit;
the output port of the signal generating unit is respectively connected with the input ports of the signal modulating unit and the signal demodulating unit, and the output port of the signal modulating unit is connected with the input port of the signal demodulating unit;
the signal generating unit is used for generating two paths of signals of signal photons and idle photons and inputting the two paths of signals into the signal modulating unit and the signal demodulating unit respectively;
the signal modulation unit is used for carrying out polarization modulation on the signal photons input by the signal generation unit and then sending the signal photons to the signal demodulation unit for detection;
the signal demodulation unit is used for comparing the code base with the measuring base after sequentially carrying out polarization modulation and detection on idle photons input by the signal generation unit.
2. The polarization encoding based quantum key distribution system of claim 1, wherein the signal demodulation unit comprises a second polarization modulator, a second computer, a second mirror, a third mirror, a first single photon detector, a coincidence counter, a second single photon detector, and a fourth mirror;
the output port of the second polarization modulator is sequentially connected with the second reflecting mirror, the third reflecting mirror and the first single photon according to the detector, the output port of the first single photon detector is connected with the input port of the coincidence counter, the input port of the coincidence counter is also connected with the output port of the second single photon detection, and the input port of the second single photon detection is connected with the fourth reflecting mirror; the input port of the second computer is respectively connected with the output ports of the second polarization modulator and the coincidence counter.
3. A polarization encoding based quantum key distribution system as claimed in claim 2, wherein the signal generating unit comprises a laser, a focusing lens, a BBO crystal and a beam splitter connected in sequence.
4. A polarization encoding based quantum key distribution system as claimed in claim 3 wherein the laser produces gaussian light for transmission to a focusing lens; the focusing lens focuses the Gaussian light to form a beam of optical signals and inputs the beam of optical signals to the BBO crystal; the BBO crystal carries out parametric conversion on the optical signal to obtain a pair of entangled photons and sends the entangled photons to the beam splitter; the beam splitter splits the received pair of entangled photons into signal photons and idle photons, which are respectively input to the signal modulation unit and the signal demodulation unit.
5. The polarization-encoding-based quantum key distribution system of claim 4, wherein the polarization entanglement states of the signal photons and the idle photons are:
where a and b represent signal photons and idle photons, respectively, and l represents the number of basis vectors.
6. A polarization encoding based quantum key distribution system as claimed in claim 1, wherein the signal modulation unit comprises a first polarization modulator, a first mirror and a first computer;
the output port of the first polarization modulator is respectively connected with the input ports of the first reflecting mirror and the first computer, and the output ports of the first reflecting mirror and the first computer are respectively connected with the signal demodulation unit.
7. A polarization encoding based quantum key distribution system as claimed in claim 6 wherein the first computer controls the first polarization modulator to modulate the polarization state of the signal photons, outputting photons having a polarization state into the first mirror; the first mirror reflects the photons having the polarization state to the signal demodulation unit through free space.
8. A polarization encoding based quantum key distribution system as claimed in claim 7 wherein the first polarization modulator performs polarization modulation by randomly selecting four polarization states for polarization modulation of signal photons;
the four polarization states include |ψ 11 >＝|H>+|V>、|ψ 12 ＞＝|H>-|V>And |psi 21 >＝
H>+i|V>、|ψ 22 >＝|H>-i|V>；
Wherein |ψ 11 >And |psi 12 >Mutually orthogonalizing to form a first pair of orthonormal basis vectors;
|ψ 21 >and |psi 22 >Mutually orthogonal to form a second pair of orthogonal normalized basis vectors.
9. The quantum key distribution system based on polarization encoding according to claim 8, wherein the photons with polarization states are transmitted to a fourth mirror after being reflected to the signal demodulation unit through free space, and are transmitted to a second single photon detector through the fourth mirror for detection, and the second single photon detector sends the detection result to a coincidence counter for counting.
10. The quantum key distribution system based on polarization encoding as claimed in claim 9, wherein after the idle photons generated by the signal generating unit are input to the signal demodulating unit, the idle photons first enter the second polarization modulator; the second computer carries out polarization state modulation on the input idle photons by controlling the second polarization modulator and then outputs photons with polarization states, and the photons with the polarization states enter the first single photon detector for detection after being reflected by the second reflecting mirror and the third reflecting mirror in sequence; and finally, the first single photon detector sends the detection result to the coincidence counter for counting.
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