A quantum key distribution method and distribution system based on high-dimensional entanglement

Abstract

The present invention discloses a quantum key distribution method based on high-dimensional entanglement and its distribution system. The method is: preparing a timestamp polarization super-entangled quantum state and encoding information in a high-dimensional space composed of timestamp and polarization degrees of freedom, the information is the quantum state of polarization and timestamp degrees of freedom of photons; using polarization degrees of freedom to divide subspaces, randomly detecting the Z basis and X basis of timestamp degrees of freedom in the subspaces, that is, using timestamp degrees of freedom to establish keys. In the present invention, there is no need to use an active modulator, and only optical devices can be used to implement a protocol based on simultaneous subspace encoding. In addition, the present invention uses polarization and time degrees of freedom encoding of photons, which can be stably transmitted in a single-mode optical fiber channel, and has great application potential in a system where quantum key distribution and classical communication networks coexist.

Description

Technical Field

[0001] The present invention relates to the field of quantum communication technology, and in particular to a quantum key distribution method based on high-dimensional entanglement and a distribution system thereof. Background Art

[0002] With the development of social informatization, people have higher demands for information security. At present, the security of the classical cryptographic system depends on the complexity of calculations. However, with the development of quantum computing, this classical encryption method will no longer be safe. Quantum key distribution (QKD) technology provides a method to establish unconditional security keys between communicators. Combined with "one-time one-pad", it can theoretically achieve unconditional security of information. After more than 30 years of development, quantum key distribution has begun to move towards practical application, but there are still many challenges, such as the inability to code in high-noise environments and high construction costs. In actual application scenarios, environmental noise is often more complex than in laboratories. At present, it is usually dealt with by more stringent filtering to reduce noise or executing QKD protocols that can tolerate high noise.

[0003] In order to reduce the construction cost of QKD system and accelerate the process of commercialization and industrialization, researchers have proposed a solution to integrate QKD system with existing optical fiber infrastructure. However, this solution is also limited by the high-intensity noise in the channel. Therefore, it is very important to improve the anti-noise ability of the system for the practical application of QKD system. To this end, some researchers have theoretically proposed a QKD protocol that uses high-dimensional entangled subspace for encoding.

[0004] There are currently two solutions to implement this protocol, one is to use high-dimensional path entanglement, and the other is to use energy-time entanglement. However, both solutions are difficult to implement. The former uses the path freedom of photons. Although this solution can have a higher decoding efficiency, it is difficult to perform stable transmission over long distances in both fiber channels and free space channels. The latter uses the time freedom of photons and discretizes time through post-processing. This solution can easily realize the preparation of high-dimensional quantum states, but it is easily affected by fiber channel disturbances and has high requirements for the time jitter of the detector. Summary of the invention

[0005] In order to solve the technical problem that the existing QKD system has weak anti-noise ability, the present invention provides a quantum key distribution method based on high-dimensional entanglement and a distribution system thereof.

[0006] The present invention is implemented by the following technical solution: a quantum key distribution method based on high-dimensional entanglement, which comprises the following steps:

[0007] A timestamp polarization super-entangled quantum state is prepared and information is encoded in a high-dimensional space composed of timestamp and polarization degrees of freedom, wherein the information is the quantum state of the polarization and timestamp degrees of freedom of photons; the polarization degrees of freedom are used to divide the subspace, and the Z basis and X basis of the timestamp degrees of freedom are randomly detected in the subspace, that is, the timestamp degrees of freedom are used to establish a key.

[0008] As a further improvement of the above scheme, a polarization beam splitter is used to divide the high-dimensional space into subspaces according to the polarization degree of freedom, and a beam splitter is used to randomly select a basis in the subspace, and the timestamp of the photon is directly detected as the Z basis, and the timestamp information is erased by using an unequal-arm interferometer to detect as the X basis. As a further improvement of the above scheme, when encoding information, the pulse laser is divided into coherent front and back pulse lasers with two front and back timestamps: a front pulse laser with a timestamp of t1 is generated through a short arm of an unequal-arm interferometer 1, and a rear pulse laser with a timestamp of t2 is generated through a long arm of the unequal-arm interferometer 1; the front and back pulses generate polarization entangled photon pairs, and entanglement is also generated in the timestamp degree of freedom, and finally a polarization timestamp super-entangled photon pair is output.

[0009] Furthermore, the forward and backward pulsed lasers generate entangled photon pairs with polarization freedom. At the same time, entanglement is also generated in the timestamp degree of freedom Finally, the photon pair is output Where H represents horizontal polarization and V represents vertical polarization.

[0010] The present invention also provides a quantum key distribution system based on high-dimensional entanglement, which adopts any of the above-mentioned quantum key distribution methods based on high-dimensional entanglement, and the quantum key distribution system comprises:

[0011] A transmitting end, which is used to prepare a timestamp polarization super-entangled quantum state and encode information in a high-dimensional space consisting of timestamp and polarization degrees of freedom, wherein the information is the quantum state of the polarization and timestamp degrees of freedom of photons;

[0012] Two receiving ends, each receiving end is used to divide the subspace by using the polarization degree of freedom, and randomly detect the Z basis and X basis of the timestamp degree of freedom in the subspace, that is, to establish a key by using the timestamp degree of freedom.

[0013] As a further improvement of the above solution, the sending end includes:

[0014] A pulse laser, which is used to generate pulsed laser;

[0015] Unequal-arm interferometer 1, used for dividing the pulse laser into two coherent front and rear pulse lasers with front and rear time stamps: the front pulse laser with time stamp t1 is generated by the short arm of the unequal-arm interferometer 1, and the rear pulse laser with time stamp t2 is generated by the long arm of the unequal-arm interferometer 1;

[0016] The polarization entanglement module is used to generate polarization entangled photon pairs from the front and rear pulses, and also generates entanglement in the timestamp degree of freedom, and finally outputs polarization timestamp super-entangled photon pairs.

[0017] Furthermore, the transmitting end further includes:

[0018] An optical amplifier, which is used to amplify the light intensity of the front and rear pulse lasers;

[0019] And / or, the transmitting end further includes:

[0020] A polarization controller, which is used to adjust the polarization direction of the pulsed laser;

[0021] And / or, the transmitting end further includes:

[0022] A frequency doubling module, which is used for frequency doubling pulsed lasers;

[0023] And / or, the transmitting end further includes: an optical isolator, which is used to optically isolate the pulse laser;

[0024] And / or, the transmitting end further includes an optical filter, which is used to filter the photon pairs and then send them to each receiving end.

[0025] As a further improvement of the above solution, the unequal-arm interferometer is a Faraday-Michelson interferometer or a Mach-Zehnder interferometer.

[0026] As a further improvement of the above solution, the two receiving ends are respectively Alice receiving end and Bob receiving end;

[0027] The Alice receiving end includes a polarization beam splitter and two measurement groups; the polarization beam splitter is used to distinguish different polarizations of the photon pairs, and plays a role in dividing the subspace; each measurement group includes three single-photon detectors, an optical beam splitter, and two unequal-arm interferometers; wherein the optical beam splitter is connected to the polarization beam splitter, and is used to select a basis in the subspace of the corresponding receiving end: one end of the optical beam splitter output end is directly connected to one of the single-photon detectors, and is used to measure the Z basis in the subspace; the optical circulator has three ends A port, wherein one port and the second port form a first channel, and the second port and the third port form a second channel; the third port of the optical circulator communicates with another detector; the other end of the optical beam splitter output end communicates with the first port of the optical circulator; the second port of the optical circulator communicates with the first port of the unequal-arm interferometer for measuring the X basis; the second port of the unequal-arm interferometer communicates with the third detector; the arm length difference of the unequal-arm interferometer 2 is completely consistent with the arm length difference of the unequal-arm interferometer 1;

[0028] The structure of the Bob receiving end is the same as that of the Alice receiving end, and the Alice receiving end is further provided with an optical fiber phase shifter at the long arm of the corresponding unequal-arm interferometer 2.

[0029] Furthermore, the Alice receiving end further includes:

[0030] The optical fiber polarization controller is used to offset the polarization rotation of the photon pair caused by the quantum channel before the photon pair enters the polarization beam splitter, so that the polarization reference system of the receiving end is aligned with the transmitting end.

[0031] Compared with existing technologies:

[0032] 1. The encoding method of the present invention uses a high-dimensional space composed of polarization and timestamp degrees of freedom for encoding, then uses polarization to divide the subspace, and uses timestamps to encode within the subspace. The light source end uses an unequal-arm interferometer combined with a polarization entangled source optical path to prepare polarization-timestamped super-entangled photon pairs. The receiving end uses a polarization beam splitter to divide the subspace, and uses an optical beam splitter to select a basis within the subspace. In the present invention, there is no need to use an active modulator, and only optical devices can be used to implement a protocol based on simultaneous subspace encoding.

[0033] 2. The quantum key distribution method of the present invention improves the robustness to channel disturbances by encoding information in the timestamp and polarization degrees of freedom, and can be compatible with existing classical communication networks. That is, the present invention utilizes the polarization and time degrees of freedom of photons for encoding, can be stably transmitted in a single-mode optical fiber channel, and has great application potential in systems where quantum key distribution and classical communication networks coexist.

[0034] 3. The quantum key distribution system of the present invention is an implementation scheme based on the high-dimensional entangled subspace coding protocol, so it can tolerate higher noise. Secondly, compared with the existing scheme based on energy-time entanglement, the scheme provided by the present invention reduces the requirements for the accuracy of time synchronization between the detector and the communicating parties. Compared with the existing scheme based on path entanglement, the present invention can be integrated with the optical fiber infrastructure to distribute keys in high-noise long-distance scenarios. The quantum key distribution system provided by the present invention can tolerate higher noise and can be integrated with the existing optical fiber infrastructure, which has important scientific significance and practical value for promoting the practical application of QKD systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 A flow chart of a quantum key distribution method based on high-dimensional entanglement provided by the present invention.

[0036] Figure 2 To achieve Figure 1 Functional module diagram of a quantum key distribution system in a quantum key distribution method.

[0037] Figure 3 To achieve Figure 2 A schematic diagram of a quantum key distribution system in Example 1 of the quantum key distribution system.

[0038] Figure 4 To achieve Figure 2 A schematic diagram of a quantum key distribution system in Example 2 of the quantum key distribution method. DETAILED DESCRIPTION

[0039] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] It should be noted that when a component is referred to as being "mounted on" another component, it may be directly on the other component or there may be a central component. When a component is considered to be "set on" another component, it may be directly set on the other component or there may be a central component at the same time. When a component is considered to be "fixed to" another component, it may be directly fixed on the other component or there may be a central component at the same time.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "or / and" used herein includes any and all combinations of one or more of the related listed items.

[0042] See also Figure 1 , which is a flow chart of the quantum key distribution method based on high-dimensional entanglement provided by the present invention. The quantum key distribution method can be divided into two stages: encoding and decoding. Encoding: Encode the information and send it. In this embodiment, the information is the quantum state of the polarization and timestamp freedom of the photon. Decoding: Receive the encoded information and decode it.

[0043] During encoding, a timestamp polarization super-entangled quantum state is prepared and information is encoded in a high-dimensional space consisting of the timestamp and polarization degrees of freedom, wherein the information is the quantum state of the polarization and timestamp degrees of freedom of the photon.

[0044] In this embodiment, the encoding method can be specifically as follows: the pulse laser is divided into two coherent front and rear pulse lasers with front and rear timestamps: the front pulse laser with timestamp t1 is generated through the short arm of the unequal arm interferometer, and the rear pulse laser with timestamp t2 is generated through the long arm of the unequal arm interferometer; the front and rear pulses generate polarization entangled photon pairs, and entanglement is also generated in the timestamp freedom degree, and finally a polarization timestamp super-entangled photon pair is output. The specific form of the polarization timestamp super-entangled state is not limited here, and it can be any one of the sixteen super-entangled states of polarization and timestamp freedom degrees.

[0045] The forward and backward pulsed lasers are passed through the polarization entanglement module to generate polarization degree of freedom entanglement At the same time, entanglement is also generated in the timestamp degree of freedom Finally, the photon pair is output In the formula, H represents horizontal polarization and V represents vertical polarization. Specifically, the forward and backward pulsed lasers are passed through the polarization entanglement module to generate the entanglement of polarization freedom. At the same time, entanglement is also generated in the timestamp degree of freedom Finally, the photon pair is output In the formula, H represents horizontal polarization and V represents vertical polarization. The entangled state of the polarization degree of freedom can also be The timestamp degree of freedom can also be In the present invention, there is no limitation.

[0046] During decoding, the polarization degree of freedom is used to divide the subspace, and the Z basis and X basis of the timestamp degree of freedom are randomly detected in the subspace, that is, the timestamp degree of freedom is used to establish the key. The high-dimensional space can be divided into subspaces according to the polarization degree of freedom using a polarization beam splitter, and a basis is randomly selected in the subspace using an optical beam splitter, and the timestamp of the photon is directly detected as the Z basis, and the timestamp information is erased using an unequal-arm interferometer and then detected as the X basis. In other words, the high-dimensional space is divided into subspaces according to the polarization of the photon using a polarization beam splitter or other device that can distinguish polarization, and the Z basis or X basis for detecting the timestamp degree of freedom is randomly selected in the subspace, and the timestamp of the photon is directly detected as the Z basis, and the timestamp information is erased using an unequal-arm interferometer and then detected as the X basis.

[0047] In order to implement the quantum key distribution method of this embodiment, a specific quantum key distribution system based on high-dimensional entanglement is provided for detailed introduction. The quantum key distribution system is one embodiment of the quantum key distribution method of the present invention.

[0048] See also Figure 2 , which is to achieve Figure 1 A functional module diagram of a quantum key distribution system in a quantum key distribution method. The quantum key distribution system includes a transmitting end 1 (also known as a light source end) and two receiving ends (which can be: Alice receiving end 2 and Bob receiving end 3).

[0049] The transmitting end 1 is used to encode and transmit the pulse laser of the communication band. The transmitting end 1 includes a pulse laser 11, an unequal-arm interferometer 12, and a polarization entanglement module 15.

[0050] The pulse laser 11 generates a pulse laser in the communication band, which is divided into two front and rear pulse lasers after passing through the unequal-arm interferometer 12, that is, two front and rear timestamps. The short arm of the unequal-arm interferometer 12 generates a timestamp t1, and the long arm of the unequal-arm interferometer 12 generates a timestamp t2, which is subsequently used to generate timestamp entanglement.

[0051] The forward and backward pulsed lasers are used to pump the polarization entanglement module, namely the polarization entanglement module 15, to generate polarization entangled photon pairs in the communication band. At the same time, since the two forward and backward coherent pump pulses are generated by the unequal-arm interferometer 12, time stamp entanglement is also generated after passing through the polarization entanglement module 15, and finally a polarization time stamp super entangled photon pair is formed. The generated entangled photon pairs are distributed to the communicating parties, namely Alice receiving end 2 and Bob receiving end 3, through the quantum channel.

[0052] The unequal-arm interferometer 12 is not limited in type, and a Faraday-Michelson interferometer, a Mach-Zehnder interferometer, etc. can be selected. The polarization entanglement module 15 can select a sagnac ring structure, a double crystal structure, etc.

[0053] Alice receiving end 2 comprises: a polarization beam splitter 21 and two measurement groups. Each measurement group comprises a beam splitter 22, a second unequal-arm interferometer 23 and three detectors 24.

[0054] After reaching Alice's receiving end 2, the photon passes through the polarization beam splitter 21, which is used to distinguish different polarizations and divide the subspace. When the polarization state is horizontal |H>, it represents subspace 1 (denoted as m=1), and when the polarization state is vertical |V>, it represents subspace 2 (denoted as m=2). The beam splitter 22 is used to select the basis in the subspace. One end of the output end of the beam splitter 22 is directly connected to one of the detectors 24 to realize the measurement of the Z basis in the subspace, and the other end is connected to the unequal arm interferometer 23 to realize the measurement of the X basis. The arm length difference between the two unequal arm interferometers 23 at Alice's receiving end 2 is exactly the same as the arm length difference of the unequal arm interferometer 12 at the transmitting end 1, which is used to erase the timestamp information. After the timestamps t1 and t2 pass through the long arm and short arm of the unequal arm interferometer 23 respectively, they cannot be distinguished in time, and can be regarded as the projection of the equal superposition of the timestamps t1 and t2.

[0055] The structure of Bob's receiving end 3 is basically the same as that of Alice's receiving end 2, except that Alice's receiving end 2 also adds a fiber phase shifter (not shown) at the long arm of the corresponding unequal-arm interferometer 23. The phase shifter here does not necessarily need to be at Alice's receiving end, but can also be at Bob or the light source end, or each unequal-arm interferometer has a phase shifter. No further details will be given here.

[0056] The method for generating polarization time stamp super entanglement here is an existing technology. In this embodiment, the method is used to generate polarization time stamp super entanglement state The quantum state sent to Alice is then encoded as |0>=|Ht 1 >,|1>=|Ht 2 >,|2>=|Vt 1 >,|3>=|Vt 2 >, the quantum state sent to Bob is encoded as follows, |0>=|Vt 1 >,|1>=|Vt 2 >,|2>=|Ht 1 >,|3>=|Ht 2 >. The subspace 1 composed of |0> and |1> is denoted as m=1, and the subspace 2 composed of |2> and |3> is denoted as m=2. In this way, the division of the subspace can be achieved according to the polarization degree of freedom, and the encoding in the subspace uses the timestamp degree of freedom.

[0057] The present invention includes at least the following beneficial effects: First, the quantum key distribution system provided by the present invention is an implementation scheme based on the high-dimensional entangled subspace coding protocol, and therefore can tolerate higher noise. Secondly, compared with the existing scheme based on energy-time entanglement, the scheme provided by the present invention reduces the requirements for the accuracy of time synchronization between the detector and the communicating parties. Compared with the existing scheme based on path entanglement, the present invention can be integrated with the optical fiber infrastructure to distribute keys in high-noise long-distance scenarios. The quantum key distribution system provided by the present invention can tolerate higher noise and can be integrated with the existing optical fiber infrastructure, which has important scientific significance and practical value for promoting the practical application of QKD systems.

[0058] Example 1

[0059] See also Figure 3 , which is to achieve Figure 2 A schematic diagram of a quantum key distribution system in Embodiment 1 of the quantum key distribution system. The quantum key distribution system also includes: a light source end, an Alice receiving end, and a Bob receiving end.

[0060] The light source end includes not only the pulse laser 11, unequal-arm interferometer 12, optical amplifier 13, frequency doubling module 14, and polarization entanglement module 15 of Example 1, but also includes: optical isolator 16, polarization controller 17. Unequal-arm interferometer 12 adopts Faraday-Michelson interferometer.

[0061] The pulse laser 11 generates a pulse laser in the communication band, and then the light pulse passes through the optical isolator 16 and enters the Faraday Michelson interferometer. The optical isolator 16 is used to block the light returning from the interferometer to the pulse laser 11. The pulse laser is divided into two timestamps after passing through the interferometer, and the timestamp t1 is generated after passing through the short arm, and the timestamp t2 is generated after passing through the long arm, which is subsequently used to generate timestamp entanglement.

[0062] The Faraday-Michelson interferometer includes two Faraday reflectors FM and a beam splitter BS. One of the Faraday reflectors FM is connected to the beam splitter BS through a short optical fiber to form the short arm of the interferometer, and the other Faraday reflector FM is connected to the beam splitter BS through a long optical fiber to form the long arm of the interferometer. The optical amplifier 13 is located after the interferometer, and is used to amplify the light intensity of the pulsed light to ensure that the subsequent polarization entanglement module 15 can obtain sufficient pump power. The polarization controller 17 is located after the optical amplifier 13 and before the frequency doubling module 14, and is used to adjust the polarization direction to meet the incident requirements of the frequency doubling module 14 so that the frequency doubling efficiency is the highest. The frequency doubling module 14 is used to double the frequency of the amplified light, so that the polarization entanglement module 14 can be pumped. The two pulse lasers enter the optical amplifier 13 for power amplification to ensure that the subsequent polarization entanglement module 15 can obtain a sufficiently high pump power. The pulsed laser after optical amplification enters the frequency doubling module 14, and the frequency of the pulsed laser in the communication band is doubled.

[0063] In this embodiment, the polarization entanglement module 15 adopts a Sagnac ring structure, including: a half-wave plate 151, a quarter-wave plate 152, a dichroic mirror 153, a dual-wavelength polarization beam splitter 154, a dual-wavelength half-wave plate 156, a nonlinear crystal 155, and two filters 157. After the pump pulse enters the polarization entanglement module 15, it is adjusted to a suitable polarization state by the half-wave plate 151 and the quarter-wave plate 152, and then reflected by the dichroic mirror 153 to the dual-wavelength polarization beam splitter 154. The horizontal polarization component passes through the nonlinear crystal 155 in the Sagnac ring counterclockwise to generate associated photon pairs, and the vertical polarization component passes through the dual-wavelength half-wave plate 156 and the nonlinear crystal 155 clockwise to generate associated photon pairs. Finally, the photon pairs generated in the clockwise and counterclockwise directions are emitted from the dual-wavelength polarization beam splitter 154. The polarization entanglement module 15 is used to generate entanglement of polarization degrees of freedom. At the same time, the coherent front and rear pulses also become entangled in the timestamp degree of freedom after passing through this module. The final light source can output polarization-time-stamped super-entangled photon pairs The photon pair is filtered by two filters 157 and then sent to Alice's receiving end and Bob's receiving end through the channel.

[0064] The present invention uses the space composed of the timestamp degree of freedom and the polarization degree of freedom to perform high-dimensional encoding. For the photon sent to Alice's receiving end, |0>=|Ht 1 >,|1>=|Ht 2 >,|2>=|Vt 1 >,|3>=|Vt 2 >, for the photon sent to Bob's receiving end, |0>=|Vt 1 >,|1>=|Vt 2 >,|2>=|Ht1 >,|3>=|Ht 2 >, then the super-entangled state generated by the light source can be written as This four-dimensional space is divided into two two-dimensional subspaces. Subspace 1 composed of |0> and |1> is denoted as m=1, and subspace 2 composed of |2> and |3> is denoted as m=2. In this way, the division of subspaces can be achieved based on polarization degrees of freedom, and the encoding in the subspace can be achieved using timestamp degrees of freedom.

[0065] In addition to the polarization beam splitter 21 of Example 1, two measurement groups (two beam splitters 22, unequal-arm interferometer 2 23, and three detectors 24 in each measurement group), the Alice receiving end also includes: a polarization controller 25 and an optical circulator 26. The optical fiber polarization controller 25 is used to offset the polarization rotation caused by the quantum channel so that the polarization reference system of the receiving end is aligned with that of the light source end. The detector 24 adopts a single-photon detector, the polarization controller 25 adopts an optical fiber polarization controller, and the unequal-arm interferometer 2 23 adopts a Faraday-Michelson interferometer.

[0066] The optical fiber polarization beam splitter 25 distinguishes different polarizations and plays a role in dividing the subspace. When the polarization state is horizontal |H>, it indicates m=1, and when the polarization state is vertical |V>, it indicates m=2. Taking the subspace m=1 as an example, the optical beam splitter 22 is connected to the polarization beam splitter 21 for selecting a basis in the subspace. One end of the output end of the optical beam splitter 22 is directly connected to a single-photon detector for measuring the Z basis in the subspace, and the other end is connected to an optical circulator 26, which is connected to a Faraday Michelson interferometer for measuring the X basis. The optical circulator 16 has three ports, one of which forms a first channel with the second port, and the second port forms a second channel with the third port. The first port of the optical circulator 26 is connected to the optical beam splitter, the second port is connected to the first port of the Faraday Michelson interferometer, and the third port is connected to one of the detectors 24. The second port of the Faraday Michelson interferometer is connected to another of the detectors 24. When performing X-based measurement, the photon first enters the Faraday Michelson interferometer through the first channel of the optical circulator 26, and is emitted from the first port or the second port of the interferometer after interference. When emitted from the first port, the photon passes through the second channel of the optical circulator 26 and enters the third single-photon detector (the response of the detector is recorded as 0), and when emitted from the second port, the photon directly enters the single-photon detector (the response of the detector is recorded as 1). Here, the Faraday Michelson interferometer is used to erase the timestamp information. The timestamp t1 after passing through the long arm and the timestamp t2 after passing through the short arm cannot be distinguished in time. This situation can be regarded as a projection on the equal superposition of the timestamps t1 and t2. The detection device in the subspace m=2 is the same as that in m=1, and will not be repeated here.

[0067] The two Faraday Michelson interferometers at Alice's receiving end are basically the same as those at the transmitting end, and also include: a beam splitter BS, two Faraday reflectors FM, but also include a fiber phase shifter PS. One of the Faraday reflectors FM is connected to the beam splitter BS through a short optical fiber to form the short arm of the interferometer, and the other Faraday reflector FM is connected to one end of the fiber phase shifter PS, and the other end of the fiber phase shifter PS is connected to the beam splitter BS to form the long arm of the interferometer. The interferometer at Alice's receiving end has the same arm length difference as the interferometer at the transmitting end, but a fiber phase shifter PS is added to the long arm to adjust the phase difference between the long and short arms. The phase shifter here does not necessarily need to be at Alice's receiving end, but can also be at Bob or the light source end, or there is a phase shifter in each unequal arm interferometer.

[0068] The structure of Bob's receiving end is basically the same as that of Alice's receiving end. The difference between the structure of Bob's receiving end and that of Alice's receiving end is that Alice's receiving end also adds a fiber phase shifter PS at the long arm of the corresponding unequal-arm interferometer 2. The phase shifter here does not necessarily need to be at Alice's receiving end, but can also be at Bob or the light source end, or there is a phase shifter in each unequal-arm interferometer. It will not be described in detail here.

[0069] The Faraday-Michelson interferometers at the transmitting end, Alice's receiving end, and Bob's receiving end are all unequal-arm interferometers and have the same arm length difference. The Faraday-Michelson interferometer described in this embodiment is a prior art. It should be noted that the present invention does not limit the specific form of the unequal-arm interferometer, as long as it is an interferometer that can generate front and back timestamps with equal probability. The polarization entanglement source is also a prior art. The present invention does not limit the specific polarization entanglement implementation method, and it can be any other method that can achieve polarization entanglement. In order to better illustrate this, the present invention provides Example 3, using different unequal-arm interferometers and polarization entanglement modules.

[0070] Example 2

[0071] like Figure 4 As shown, it is realized Figure 2 A schematic diagram of a quantum key distribution system in Example 2 of the quantum key distribution method. The quantum key distribution system also includes: a light source end, an Alice receiving end and a Bob receiving end. In this embodiment, the light source end includes the pulse laser 11, the unequal arm interferometer 12, and the polarization entanglement module 15 of Example 1. The unequal arm interferometer 12 adopts a Mach-Zehnder interferometer.

[0072] The pulse laser 11 directly generates pump pulse light, which then enters the unequal-arm Mach-Zehnder interferometer and is divided into two pulses, namely, two time stamps. The short arm generates time stamp t1, and the long arm generates time stamp t2, which is subsequently used to generate time stamp entanglement. The front and rear pulses then enter the polarization entanglement module 15, which generates polarization entanglement and time stamp entanglement at the same time. The quantum state of the photon pair generated in the end is In this embodiment, the polarization entanglement module 15 adopts a dual crystal structure, including the following components of embodiment 1: a half-wave plate 151, a quarter-wave plate 152, a vertically bonded nonlinear crystal 155, and two filters 157. After the front and rear pump pulses enter the polarization entanglement module 15, the half-wave plate 151 and the quarter-wave plate 152 modulate them to a suitable polarization state, and then pass through the vertically bonded nonlinear crystal 155 to generate an entangled photon pair, which, combined with the coherent front and rear pump pulses, ultimately generates a polarization timestamp super-entangled state. Similarly, the space composed of the timestamp degree of freedom and the polarization degree of freedom is used for high-dimensional encoding. For the photons sent to Alice, let |0>=|Ht 1 >,|1>=|Ht 2 >,|2>=|Vt 1 >,|3>=|Vt 2 >, for the photon sent to Bob, let |0>=|Ht 1 >,|1>=|Ht 2 >,|2>=|Vt 1 >,|3>=|Vt 2 >, then the super-entangled state generated by the light source can be written as This four-dimensional space is divided into two two-dimensional subspaces. Subspace 1 composed of |0> and |1> is denoted as m=1, and subspace 2 composed of |2> and |3> is denoted as m=2. In this way, the division of subspaces can be achieved based on polarization degrees of freedom, and the encoding in the subspace can be achieved using timestamp degrees of freedom.

[0073] Alice receiving end includes the polarization controller 25 of embodiment 2, polarization beam splitter 21, two measurement groups (each measurement group has a beam splitter 22, three detectors 24, and unequal arm interferometer 23). The unequal arm interferometer 23 also uses Mach-Zehnder interferometer, the polarization beam splitter 21 uses a fiber polarization beam splitter, and the detector 24 is a single photon detector.

[0074] The polarization controller 25 offsets the polarization rotation caused by the quantum channel so that the polarization reference system of the receiving end is aligned with the light source end. The optical fiber polarization beam splitter 21 distinguishes different polarizations and plays a role in dividing the subspace. When the polarization state is horizontal |H>, it indicates m=1, and when the polarization state is vertical |V>, it indicates m=2. Taking the subspace m=1 as an example, the optical beam splitter 22 is connected to the polarization beam splitter 21 for selecting a basis in the subspace. One end of the output end of the optical beam splitter 22 is directly connected to a single-photon detector for measuring the Z basis in the subspace, and the other end is connected to a Mach-Zehnder interferometer for measuring the X basis. Here, the Mach-Zehnder interferometer is used to erase the timestamp information of the photon, so that the timestamps t1 and t2 cannot be distinguished in time after passing through the interferometer, thereby realizing the measurement of the X basis.

[0075] It should be noted that the light source end in Example 3 directly causes the pump pulse to pass through the unequal-arm interferometer to generate coherent forward and backward pump pulses, which is equivalent to the light source end in Example 2, and the present invention does not limit the method of generating coherent forward and backward pump pulses. In addition, the unequal-arm interferometer and the polarization entanglement module can be a fiber structure or a free space structure, which is not limited by the present invention.

[0076] In summary, in the quantum key distribution method based on high-dimensional entanglement provided by the present invention, information is encoded in a high-dimensional space composed of timestamps and polarization degrees of freedom, the polarization degrees of freedom are used to divide the subspace, and the timestamp degrees of freedom are used to encode the subspace, which can ensure the stable transmission of the signal in the channel. In the prior art, the quantum key distribution system based on high-dimensional entangled subspace encoding either has high requirements for detectors, or is difficult to distribute over long distances and difficult to be compatible with existing optical fiber communication networks. The scheme for implementing a quantum key distribution protocol based on high-dimensional entangled subspace encoding provided by the present invention not only reduces the requirements for detectors, but is also compatible with existing communication networks, greatly improving the practicality of the protocol, which has important scientific significance and practical value for promoting the practical application of QKD systems.

[0077] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A quantum key distribution method based on high-dimensional entanglement, characterized in that: It includes the following steps: Prepare a timestamp polarization super-entangled quantum state and encode information in a high-dimensional space consisting of timestamp and polarization degrees of freedom, wherein the information is the quantum state of the polarization and timestamp degrees of freedom of the photon; The polarization degree of freedom is used to divide the subspace, and the Z basis and X basis of the timestamp degree of freedom are randomly detected in the subspace, that is, the timestamp degree of freedom is used to establish the key; A polarization beam splitter is used to divide the high-dimensional space into subspaces according to the polarization degree of freedom, and a basis is randomly selected in the subspace using an optical beam splitter. The timestamp of the photon is directly detected as a Z basis, and the timestamp information is erased using an unequal-arm interferometer to detect as an X basis.

2. The quantum key distribution method based on high-dimensional entanglement according to claim 1, characterized in that: When encoding information, the pulse laser is divided into two coherent front and rear pulse lasers with front and rear time stamps: the front pulse laser passes through the short arm of an unequal-arm interferometer to generate a time stamp t1, and the rear pulse laser passes through the long arm of the unequal-arm interferometer to generate a time stamp t2; The forward and backward pulses generate polarization entangled photon pairs, and entanglement is also generated in the timestamp degree of freedom, and finally a polarization-timestamp super-entangled photon pair is output.

3. The quantum key distribution method based on high-dimensional entanglement as claimed in claim 2, characterized in that: The forward and backward pulsed laser generates a photon pair with entangled polarization degree of freedom At the same time, entanglement is also generated in the timestamp degree of freedom Finally, the photon pair is output Where H represents horizontal polarization and V represents vertical polarization.

4. A quantum key distribution system based on high-dimensional entanglement, characterized in that: It adopts the quantum key distribution method based on high-dimensional entanglement as claimed in any one of claims 1 to 3, and the quantum key distribution system includes: A transmitting end, which is used to prepare a timestamp polarization super-entangled quantum state and encode information in a high-dimensional space consisting of timestamp and polarization degrees of freedom, wherein the information is the quantum state of the polarization and timestamp degrees of freedom of photons; Two receiving ends, each receiving end is used to divide the subspace by using the polarization degree of freedom, and randomly detect the Z basis and X basis of the timestamp degree of freedom in the subspace, that is, to establish a key by using the timestamp degree of freedom.

5. The quantum key distribution system based on high-dimensional entanglement as claimed in claim 4, characterized in that: The transmitting end comprises: A pulse laser, which is used to generate pulsed laser; Unequal-arm interferometer 1, used for dividing the pulse laser into two coherent front and rear pulse lasers with front and rear time stamps: the front pulse laser with time stamp t1 is generated by the short arm of the unequal-arm interferometer 1, and the rear pulse laser with time stamp t2 is generated by the long arm of the unequal-arm interferometer 1; The polarization entanglement module is used to generate polarization entangled photon pairs from the front and rear pulses, and also generates entanglement in the timestamp degree of freedom, and finally outputs polarization timestamp super-entangled photon pairs.

6. The quantum key distribution system based on high-dimensional entanglement as claimed in claim 5, characterized in that: The transmitting end also includes: Optical amplifier, which is used to amplify the light intensity of the front and rear pulse lasers.

7. The quantum key distribution system based on high-dimensional entanglement as claimed in claim 5, characterized in that: The transmitting end also includes: A polarization controller is used to adjust the polarization direction of the pulsed laser.

8. The quantum key distribution system based on high-dimensional entanglement as claimed in claim 5, characterized in that: The transmitting end also includes: A frequency doubling module is used for frequency doubling pulsed laser light.

9. The quantum key distribution system based on high-dimensional entanglement as claimed in claim 5, characterized in that: The transmitting end also includes: an optical isolator, which is used to perform optical isolation on the pulse laser.

10. The quantum key distribution system based on high-dimensional entanglement according to claim 5, characterized in that: The transmitting end further comprises an optical filter, which is used for filtering the photon pair and then sending it to each receiving end.

11. The quantum key distribution system based on high-dimensional entanglement according to claim 5, characterized in that: The unequal-arm interferometer is a Faraday-Michelson interferometer or a Mach-Zehnder interferometer.

12. The quantum key distribution system based on high-dimensional entanglement as claimed in claim 4, characterized in that: The two receiving ends are respectively Alice receiving end and Bob receiving end; The Alice receiving end includes a polarization beam splitter, two measurement groups and an optical circulator; the polarization beam splitter is used to distinguish different polarizations of the photon pairs, and plays a role in dividing the subspace; each measurement group includes three single-photon detectors, an optical beam splitter, and two unequal-arm interferometers; wherein the optical beam splitter is connected to the polarization beam splitter, and is used to select a basis in the subspace of the corresponding receiving end: one end of the optical beam splitter output end is directly connected to one of the single-photon detectors, and is used to measure the Z basis in the subspace; the optical circulator has three ports, wherein one port and the second port form a first channel, and the second port and the third port form a second channel; the third port of the optical circulator communicates with another detector; the other end of the optical beam splitter output end communicates with the first port of the optical circulator; the second port of the optical circulator communicates with the first port of the unequal-arm interferometer for measuring the X basis; the second port of the unequal-arm interferometer communicates with the third detector; the arm length difference of the second unequal-arm interferometer is completely consistent with the arm length difference of the first unequal-arm interferometer; The structure of the Bob receiving end is the same as that of the Alice receiving end, and the Alice receiving end is further provided with an optical fiber phase shifter at the long arm of the corresponding unequal-arm interferometer 2.

13. The quantum key distribution system based on high-dimensional entanglement according to claim 12, characterized in that: The Alice receiving end also includes: The optical fiber polarization controller is used to offset the polarization rotation of the photon pair caused by the quantum channel before the photon pair enters the polarization beam splitter, so that the polarization reference system of the receiving end is aligned with the transmitting end.

Images