CN111478767B - Sending end, encoding method and quantum key distribution system for decoy state encoding and polarization encoding - Google Patents
Sending end, encoding method and quantum key distribution system for decoy state encoding and polarization encoding Download PDFInfo
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- CN111478767B CN111478767B CN201910064043.9A CN201910064043A CN111478767B CN 111478767 B CN111478767 B CN 111478767B CN 201910064043 A CN201910064043 A CN 201910064043A CN 111478767 B CN111478767 B CN 111478767B
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
- H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
- H04L9/0852—Quantum cryptography
- H04L9/0858—Details about key distillation or coding, e.g. reconciliation, error correction, privacy amplification, polarisation coding or phase coding
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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
Abstract
The invention relates to a transmitting end, an encoding method and a quantum key distribution system for decoy state and polarization encoding. In the present invention, the transmitting end may include a light source, an encoding module, and a light path returning module, wherein: the encoding module comprises a phase modulation unit and is arranged to perform first-time phase modulation on signal light to be encoded through the phase modulation unit and perform decoy state encoding on the signal light based on the phase modulation; the light path turning back module is used for receiving the signal light coded by the decoy state and turning back the signal light to the coding module; the encoding module is further configured to perform second phase modulation on the folded-back trick-state encoded signal light through the phase modulation unit, and perform polarization encoding on the folded-back trick-state encoded signal light based on the phase modulation. Therefore, the optical path turning-back module is used for multiplexing the coding module, so that the coding module can be arranged to realize the decoy state and the polarization coding at the same time, and the structure of the transmitting end is greatly simplified.
Description
Technical Field
The invention relates to the field of quantum secret communication, in particular to a transmitting end, an encoding method and a Quantum Key Distribution (QKD) system for decoy state encoding and polarization encoding.
Background
Quantum Key Distribution (QKD) is fundamentally different from the classical key system in that it employs a single photon or entangled photon pair as a carrier for the key, and the fundamental principles of quantum mechanics ensure that the process is not eavesdroppable and indecipherable, thereby providing a more secure key system.
The sending end of the QKD system usually uses laser as a light source, but the multiphoton components existing in the laser light source are attacked by photon number separation. In this regard, the light sources of QKD systems often employ decoy-state modulation schemes to solve this problem. The decoy modulation scheme requires random modulation of different light intensities, which is usually implemented by an intensity modulator.
The sending end of the QKD system also needs quantum state BB84 encoding, such as commonly used polarization encoding, for the decoy-state light source. The polarization state coding principle based on phase modulation is as follows:
wherein the light pulse is divided into two light pulse components (| H) with mutually perpendicular polarization directions>And | V>) Readjusting the phase difference between the two optical pulse components
By modulating a particular phase difference between two optical pulse components
It is possible to achieve a specific polarization state on the light pulse formed by combining the two light pulse components, for example when the phase difference is
Taking 0, pi/2, pi and 3 pi/2, respectively, P, R, N and the L polarization state can be obtained by combining the beams correspondingly. In the prior art, a phase modulator is usually used to adjust the phase difference.
In short, the transmitting end of the QKD system needs not only decoy state encoding but also BB84 encoding such as polarization encoding. In the prior art, two coded light paths are mutually independent and are respectively realized by one modulator, and the modulator is high in cost, so that the system is high in cost and low-cost miniaturization is not easy to realize.
Disclosure of Invention
In view of the above problems in the prior art, a first aspect of the present invention provides a transmitting end for spoofing state coding and polarization coding, which includes a light source 1, a coding module 2, and a light path returning module 3. Wherein, the light source 1 is used for providing signal light to be coded; the encoding module 2 comprises phase modulation units 211, 221, and 231, and is configured to perform first phase modulation on the signal light to be encoded through the phase modulation units 211, 221, and 231, and perform decoy state encoding on the signal light to be encoded based on the first phase modulation; the optical path returning module 3 is configured to receive the signal light encoded in a spoofed state and return the signal light to the encoding module 2; and, the encoding module 2 is further configured to perform a second phase modulation on the signal light with the returned spoofed state code by the phase modulation units 211, 221, 231, and perform polarization encoding on the signal light with the returned spoofed state code based on the second phase modulation.
Preferably, the optical path folding module 3 may include a circulator 31 or a reflective structure 32, 33.
Preferably, the encoding module 2 may further include a polarization beam splitting unit 212 and a beam splitting unit 213, and the polarization beam splitting unit 212 and the beam splitting unit 213 are connected through a first optical path and a second optical path having the same optical path to form an equal arm MZ interferometer; the phase modulation unit 211 is provided on the first optical path and/or the second optical path.
Preferably, the polarization beam splitting unit 212 receives the signal light to be encoded and outputs the signal light subjected to spoof state encoding and polarization encoding; also, the beam splitting unit 213 outputs the signal light encoded in a spoofed state and receives the signal light encoded in a wrapped spoofed state.
Preferably, the sending end of the invention may further include a phase detection feedback module, configured to compensate for phase drift in the equal-arm MZ interferometer; and/or, the optical path folding module 3 may be a circulator 31; and/or, the polarization beam splitting unit 212 may be a polarization beam splitter; and/or, the beam splitting unit 213 may be a beam splitter.
Preferably, the encoding module 2 may further include an optical transmission unit 224, a polarization beam splitting unit 222, and an analyzing unit 223, the polarization beam splitting unit 222 being disposed in an optical loop to constitute a sagnac interferometer, and the phase modulation unit 221 being disposed in the optical loop.
Preferably, the encoding module 2 may further include an optical transmission unit 234, a polarization beam splitting unit 232, and a polarization analyzing unit 233, the polarization beam splitting unit 232 is connected to the first faraday rotator 235 and the second faraday rotator 236 through a first optical path and a second optical path having the same optical path, respectively, to form a michelson interferometer, and the phase modulating unit 231 is disposed in the first optical path and/or the second optical path.
Preferably, the optical transmission unit 224,234 may be configured to receive the signal light to be encoded and make it propagate toward the polarization beam splitting unit 222,232, and receive the signal light output by the polarization beam splitting unit 222,232 and output outwards.
Preferably, the analyzing units 223,233 may be disposed between the polarization beam splitting units 222,232 and the optical path folding back module 3.
Preferably, the polarization beam splitting units 222,232 may be polarization beam splitters, and the polarization analyzing units 223,233 may be polarization beam splitters, polarization plates, or a combination of polarization plates and wave plates, and are disposed at an angle of 45 degrees or-45 degrees with respect to each other; and/or the signal light to be coded is one of 45-degree linearly polarized light, left-handed circularly polarized light and right-handed circularly polarized light.
Preferably, the optical transmission unit 224,234 may include a circulator; and/or the optical path folding module 3 may include mirrors or faraday rotators 32, 33.
A second aspect of the present invention relates to a quantum key distribution system including the transmitting end of the present invention.
A third aspect of the present invention relates to an encoding method for simultaneously performing decoy state encoding and polarization encoding, comprising the steps of: a decoy state encoding step of subjecting signal light to be encoded to phase modulation for the first time by a phase modulation unit and intensity-modulating the signal light to be encoded based on the phase modulation for the first time, thereby realizing decoy state encoding; a light path returning step, wherein returning the signal light coded by the decoy state; and a polarization encoding step of passing the signal light of the returned decoy-state-encoded signal light through the phase modulation unit again to perform a second phase modulation thereon, and performing polarization state modulation on the signal light of the returned decoy-state-encoded signal light based on the second phase modulation, thereby realizing polarization encoding.
Preferably, the decoy state encoding step further comprises the steps of: equally dividing the signal light to be encoded into a first signal light component and a second signal light component whose linear polarization directions are perpendicular to each other; performing the first phase modulation on at least one of the first and second signal light components using the phase modulation unit to form a first phase difference θ therebetween1(ii) a And having the first phase difference theta1Is caused to interfere with the first and second signal light components.
Preferably, the polarization encoding step further comprises the steps of: equally dividing the signal light coded by the returned decoy state into a first decoy state coded signal light component and a second decoy state coded signal light component; performing the second phase modulation on at least one of the first and second spoof-state-encoded signal light components with the phase modulation unit to form a second phase difference θ therebetween2(ii) a And having the second phase difference theta2The first and second spoofed state encoded signal light components interfere.
Preferably, in the encoding method of the present invention, an equal-arm MZ interferometer may be further constructed by using a polarization beam splitting unit and a beam splitting unit, and the phase modulation unit is provided in the equal-arm MZ interferometer, wherein the interference in the decoy state encoding step occurs at the beam splitting unit, and the interference in the polarization encoding step occurs at the polarization beam splitting unit; alternatively, a sagnac interferometer may be further configured by using a polarization beam splitting unit and an optical loop, and the phase modulation unit is provided in the sagnac interferometer, wherein the interference in the decoy state encoding step and the interference in the polarization encoding step both occur at the polarization beam splitting unit, and the decoy state encoding step further includes a step of analyzing a result of the interference by using an analyzing unit; alternatively, a michelson interferometer may be constructed by using a polarization beam splitting unit and two faraday rotators, and the phase modulation unit may be provided in the michelson interferometer, wherein the interference in the decoy state encoding step and the interference in the polarization encoding step both occur at the polarization beam splitting unit, and the decoy state encoding step further includes a step of analyzing the result of the interference by using an analyzing unit.
Preferably, in the decoy state encoding step, the intensity of the signal light encoded by the decoy state is (1+ cos θ) of the intensity of the signal light to be encoded1) 2; and/or, in the polarization encoding step, the polarization state of the polarization-encoded signal light is
Preferably, the first phase difference θ1May be set to 180.
Drawings
Fig. 1 illustrates a first exemplary embodiment of a transmitting end and an encoding method for simultaneously implementing spoofed state encoding and polarization encoding according to the present invention;
fig. 2 shows the timing relationship between the signal light pulse and the electric pulse for the phase modulation unit at two phase modulations in the embodiment shown in fig. 1;
FIG. 3 shows a further example of the embodiment shown in FIG. 1;
fig. 4 shows a transmitting end and a coding method for simultaneously implementing spoofed state coding and polarization coding according to a second exemplary embodiment of the present invention;
FIG. 5 shows the timing relationship between the signal light pulse and the electrical pulse for the phase modulation unit at two phase modulations in the embodiment shown in FIG. 4;
fig. 6 shows a third exemplary embodiment of a transmitting end and an encoding method for simultaneously implementing spoofed state encoding and polarization encoding according to the present invention; and
fig. 7 and 8 show two exemplary embodiments of the optical path folding module, respectively.
Detailed Description
Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided by way of illustration in order to fully convey the spirit of the invention to those skilled in the art to which the invention pertains. Accordingly, the present invention is not limited to the embodiments disclosed herein.
In the method of the invention, the decoy state coding and the polarization coding of the signal light are realized by the same phase modulation unit, and the coding process is basically as follows: firstly, signal light passes through a phase modulation unit to carry out primary phase modulation on the signal light, and the intensity modulation of the signal light is realized based on the primary phase modulation, so that decoy state coding is realized; subsequently, the trick-state encoded signal light is turned back to pass through the phase modulation unit again to perform second phase modulation thereon, and polarization state modulation of the signal light is realized based on the second phase modulation, thereby realizing polarization encoding. Namely, the signal light to be coded is made to reciprocate in the same optical path, and the decoy state coding and the polarization coding of the signal light are respectively realized based on the two successive phase modulations of the same phase modulation unit on the signal light.
Specifically, first, signal light to be encoded is equally divided into a first signal light component and a second signal light component whose linear polarization directions are perpendicular to each other; phase-modulating at least one of the first and second signal light components with a phase modulation unit to form a first phase difference θ therebetween1(ii) a And has a first phase difference theta1Interact (e.g., interfere) to form an intensity and first phase difference θ1And (3) related optical signals, namely signal light coded by a decoy state.
As an example, the above process mayThe method is realized by an equal-arm MZ interferometer composed of a polarization beam splitter and a phase modulator arranged in the interferometer; or the device is realized by a Sagnac interferometer composed of a polarization beam splitter and an optical loop, a phase modulator arranged in the interferometer and an analyzer; or by means of a michelson interferometer composed of a polarization beam splitter and two faraday rotators, a phase modulator provided in the interferometer, and an analyzer. For example, the signal light encoded in the decoy state may be made to satisfy (1+ cos θ) in intensity with the signal light to be encoded1) The relationship of/2. As a particular example, when θ1At 180 degrees, the intensity of the decoy-state encoded signal light will be 0, at which time a vacuum-state encoding is implemented on the signal light.
Subsequently, the trap-state-encoded signal light is folded back. As an example, the folding back of the signal light may be achieved by means of a circulator or a reflection unit.
Then, equally dividing the folded signal light coded by the decoy state into a first signal light component coded by the decoy state and a second signal light component coded by the decoy state; phase modulating at least one of the first and second spoof state encoded signal light components with the same phase modulation unit to form a second phase difference θ therebetween2And has a second phase difference theta2Interact to form a polarization direction and a second phase difference theta2The associated optical signal, i.e. the polarization encoded signal light. Here, the polarization encoding process is performed by reversing the signal light through the above-described optical path for decoy state encoding. Therefore, by turning back the signal light, decoy state encoding and polarization encoding of the signal light are realized based on two times of phase modulation by means of the same optical path structure.
It is easily understood by those skilled in the art that, in the present invention, the first phase difference θ1And a second phase difference theta2And may have any suitable value to provide the desired decoy state encoding and polarization state encoding.
To further illustrate the principle of the encoding method of the present invention, fig. 1 shows an exemplary embodiment of the transmitting end of the present invention that implements both spoof state and polarization encoding.
As shown in the figure, the transmitting end of the present invention may include a light source 1, an encoding module 2, and an optical path folding module 3.
In the present invention, the light source 1 is used to output signal light to be encoded, which may be in the form of a laser, for example, to output a laser pulse signal. The encoding module 2 receives the signal light output by the light source 1, performs a first phase modulation on the signal light by using the phase modulation unit 211, and performs an intensity modulation on the signal light based on the first phase modulation, so as to implement a decoy state encoding. The signal light coded by the decoy state is output by the coding module 2 and enters the optical path returning module 3. In the optical path folding module 3, the propagation direction of the signal light is folded back, and the signal light enters the encoding module 2 again. The encoding module 2 receives the decoy-state-encoded signal light folded back by the optical path folding back module 3, performs second phase modulation on the signal light by using the phase modulation unit 211, and modulates the polarization state of the signal light based on the second phase modulation to realize polarization encoding. Finally, the signal light coded by the decoy state and the polarization is output outwards by the coding module 2.
In this embodiment, as shown in fig. 1, the encoding module 2 may include a phase modulation unit 211, a polarization beam splitting unit 212, and a beam splitting unit 213.
The polarization beam splitting unit 212 receives the signal light pulse output from the light source 1 and equally divides it into first and second signal light pulse components of linearly polarized light whose polarization directions are perpendicular to each other. As an example, the polarization beam splitting unit 212 may be in the form of a Polarization Beam Splitter (PBS), and specifically as shown in fig. 1, the signal optical pulse enters via the 4 th port of the polarization beam splitting unit 212, and the split first and second signal optical pulse components are output via the 1 st port and the 2 nd port thereof, respectively.
The first and second signal optical pulse components are output by the polarization beam splitting unit 212 and then propagate along the first optical path and the second optical path, respectively, toward two input ends of the beam splitting unit 213. The first optical path and the second optical path are set to have the same optical path length so that the first and second signal optical pulse components can reach the beam splitting unit 213 at the same time. Therefore, the polarization beam splitting unit 212, the beam splitting unit 213, and the first and second optical paths connecting the two substantially form an equal-arm MZ interferometer structure. Here, the beam splitting unit 213 may be in the form of a Beam Splitter (BS).
The phase modulation unit 211 is disposed on one of the first optical path and the second optical path, and is configured to perform phase modulation on the passed optical pulse component so as to form a preset phase difference between the two optical pulse components. For example, as shown in fig. 1, a phase modulation unit 211 is disposed on the first optical path for performing a phase modulation on the first signal optical pulse component of magnitude θ transmitted along the first optical path1So as to form a phase difference theta between the first and second signal optical pulse components1。
With a phase difference theta1Will arrive at the beam splitting unit 213 simultaneously, so that interference occurs; at this time, the intensity of the interference result (i.e., the first interference light pulse) output by the beam splitting unit 213 will be equal to the first secondary phase modulation amount θ provided by the phase modulation unit 2111In this regard, for example, the intensity of the first interference light pulse may be (1+ cos θ) of the intensity of the signal light pulse1)/2. Obviously, in the encoding block 2, the first-time phase modulation amount θ is controlled1The intensity of the first interference light pulse output by the optical modulator can be adjusted, namely, the modulation of the intensity value of the signal light pulse is realized, so that the decoy state encoding process of the signal light can be realized.
Subsequently, the optical pulse (i.e. the first interference optical pulse) encoded by the decoy state output by the encoding module 2 enters the optical path retracing module 3, and the propagation direction thereof is retraced and returns towards the encoding module 2. In the present invention, the optical path folding module 3 may be implemented in the form of a circulator or in the form of a reflective structure, such as a mirror or a faraday rotator. For example, as shown in fig. 1, the optical path folding back module 3 may include a circulator 31.
When the spoof state encoded light pulse returns to the beam splitting unit 213, it is split into first and second spoof state encoded light pulse components by the beam splitting unit 213 and propagates along first and second optical paths, respectively, toward the polarizing beam splitting unit 212.
With a phase difference theta2The first and second components of the decoy-state encoded light pulse are simultaneously returned to the polarizing beam splitting element 212, thereby causing interference; at this time, as will be readily understood by those skilled in the art, the polarization direction of the interference result (i.e., the second interference light pulse, which is output via the 3 rd port in fig. 1, for example) output by the polarization beam splitting unit 212 will be the same as the second phase modulation amount θ provided by the phase modulation unit 2112With respect to, for example, the polarization state of the second interfering light pulse may be
Obviously, in the encoding block 2, the second-order phase modulation amount θ is controlled2The polarization state of the second interference light pulse output by the optical fiber can be adjusted, namely, the modulation of the polarization state of the signal light pulse is realized, so that the decoy state encoding process of the signal light can be realized. At this time, on the signal light pulse outputted outward by the encoding module 2 (i.e., the polarization splitting unit 212), the decoy state encoding and the polarization encoding have been achieved.
Fig. 2 shows the timing relationship between the signal light pulse and the electric pulse for the phase modulation unit 211 in the transmitting end structure shown in fig. 1 at the time of two phase modulations, so as to further understand the above-mentioned decoy state and polarization encoding process realized based on two successive phase modulations.
In the transmitting end structure shown in fig. 1-2, an equiarm MZ interferometer with a phase modulation function is formed by a polarization beam splitter unit 212 (e.g., a polarization beam splitter), a beam splitter unit 213 (e.g., a beam splitter) and a phase modulation unit 211, and first, with the polarization beam splitter 212 as an input end and the beam splitter 213 as an output end, intensity modulation on a signal optical pulse is realized based on first phase modulation provided by the phase modulation unit 211, and then decoy state encoding is completed; then, the optical path reentry module returns the signal optical pulse subjected to the decoy state encoding to the equal-arm MZ interferometer again, and the beam splitter 213 is used as an input end and the polarization beam splitter 212 is used as an output end, so that the polarization state modulation of the signal optical pulse is realized based on the second phase modulation provided by the phase modulation unit 211, thereby completing the polarization state encoding, and finally completing the decoy state and the polarization state encoding of the signal optical pulse. By means of the special transmitting end structure and the coding method, the multiplexing of the coding module 2 is realized through the optical path retracing module, so that the same optical path structure (even the same phase modulation unit 211) is utilized to realize the decoy state and the polarization coding of the same signal optical pulse. In addition, in such a structure, the requirement for the phase-modulated electrical signal is low, and only two arrival times of the pulse need to be distinguished (i.e., only two times of phase modulation need to be performed) in one cycle.
Further, the present inventors have noticed that, since the sending-end structure of fig. 1 employs the equiarm MZ interferometer structure, a phase shift may occur in a signal optical pulse while propagating therein. Therefore, the phase detection feedback module may also be preferably in the transmit-end architecture of fig. 1.
Fig. 3 shows an example of a phase detection feedback module, which may include a light splitting unit, a phase detection unit, a phase feedback algorithm unit, a phase shifter driving unit, and a phase shifter. The light splitting unit is arranged for splitting a part of the light pulse to the phase detection unit; the phase detection unit is used for detecting the phase drift amount; the phase feedback algorithm unit is used for calculating the phase quantity needing feedback compensation according to the detected phase drift quantity; the phase shifter is arranged in the equal-arm MZ interferometer, and the signal light pulse is correspondingly phase-compensated under the driving signal provided by the phase shifter driving unit. Since those skilled in the art can understand and implement the corresponding phase detection, calculation and feedback processes under the condition that the functions of the units of the phase detection feedback module have been described, details thereof are not repeated herein.
Fig. 4 schematically illustrates another embodiment of the transmitting end structure of the present invention that implements both spoofing state and polarization encoding.
As shown, the transmitting end may include a light source 1, an encoding module 2, and an optical path folding module 3.
In this embodiment, the light source 1 can preferably output 45 ° linearly polarized light (which has
Polarization state of (d) or-45 ° linearly polarized light, or left-circularly polarized light, or right-circularly polarized light, as the signal light.
The encoding module 2 may include an optical transmission unit 224, a phase modulation unit 221, a polarization beam splitting unit 222, and an analyzing unit 223.
The optical transmission unit 224 is configured to receive the signal light pulse output by the light source 1 for subsequent encoding process, and receive and output the signal light pulse encoded by the decoy state and polarization. In this embodiment, the optical transmission unit 224 may include three ports 1-3 and is configured to: light entering from the first port 1 may exit from the second port 2 and light entering from the second port 2 may exit from the third port 3. For example, as shown in fig. 4, the signal light pulse output by the light source 1 propagates toward the polarization beam splitting unit 222 via the first port 1 and the second port 2 of the optical transmission unit 224, and the decoy-state and polarization-encoded signal light pulse output by the polarization beam splitting unit 222 will be output outwards via the second port 2 and the third port of the optical transmission unit 224. As an example, the optical transmission unit 224 may be in the form of a circulator.
The polarization beam splitting unit 222 is disposed in the optical loop to constitute a sagnac interferometer structure, which receives the signal light pulse and equally divides it into first and second signal light pulse components of linearly polarized light whose polarization directions are perpendicular to each other. As an example, the polarization beam splitting unit 222 may be in the form of a Polarization Beam Splitter (PBS). As shown in fig. 4, the signal optical pulse may be input through the 4 th port of the polarization beam splitting unit 222, and the split first and second signal optical pulse components may be output through the 1 st port and the 2 nd port thereof, respectively.
The first and second signal light pulse components enter the optical loop, respectively, and propagate along the loop in opposite directions. The phase modulation unit 221 is disposed on the loop, and is configured to perform phase modulation (i.e., first phase modulation) on the passed optical pulse component, so as to form a preset phase difference between the two optical pulse components. Preferably, the phase modulator 221 may be configured to make the arrival times of the two signal optical pulse components counter-propagating in the loop at the phase modulation unit 221 inconsistent. For example, as shown in fig. 4, the phase modulation unit 221 may perform the first signal optical pulse component transmitted in the clockwise direction with a magnitude of θ1So as to form a phase difference theta between the first and second signal light pulse components1。
With a phase difference theta1Will return simultaneously to the polarization beam splitting unit 222, thereby causing interference; at this time, the polarization direction of the interference result (i.e., the first interference light pulse, which is output via the 3 rd port in fig. 4, for example) output by the polarization beam splitting unit 222 will be equal to the first secondary phase modulation amount θ provided by the phase modulation unit 2211With respect to, for example, the polarization state of the first interfering light pulse may be
The analyzer 223 receives a first interference light pulse, which is set to make the intensity of the light pulse outputted by the analyzer be (1+ cos θ) of the intensity of the signal light pulse1)/2. Obviously, in the encoding module 2, the first-time phase modulation amount θ can be controlled1And adjusting the intensity value of the light pulse, thereby realizing a decoy state encoding process of the signal light. As an example, as shown in fig. 4, the analyzing unit 223 may be a polarization beam splitter, and when the polarization beam splitter unit 222 is a polarization beam splitter, the polarization beam splitter 223 may be set at an angle of 45 degrees (or-45 degrees) with respect to the polarization beam splitter 222 so that the intensity of the light pulse output by the analyzing is the signal intensityIntensity of pulse of signal light (1+ cos theta)1)/2. Alternatively, the analyzing unit 223 may be a polarization plate, or a combination of a polarization plate and a wave plate, instead of the polarization beam splitter.
Subsequently, the optical pulse encoded by the decoy state output by the encoding module 2 enters the optical path retracing module 3, and the propagation direction is retraced under the action of the optical pulse, and the optical pulse returns to the encoding module 2. Likewise, the optical path folding module 3 may be implemented in the form of a circulator or a reflective structure (e.g., a mirror or a faraday rotator). For example, as shown in fig. 4, the optical path folding module 3 may include a mirror 32.
When returning to the encoding module 2 again, the spoof state encoded light pulse is returned to the polarization beam splitting unit 222 via the polarization analyzing unit 223, is split into the first and second spoof state encoded light pulse components, and propagates backward along the loop.
The phase modulation unit 221 in turn phase modulates (i.e., second phase modulates) the decoy state encoded optical pulse component to form a phase difference θ between the two components2. For example, as shown in fig. 4, the phase modulation unit 221 further performs a first component propagating clockwise with a magnitude θ2To form a phase difference θ between the first and second components of the decoy state encoded optical pulse2。
With a phase difference theta2The first and second components of the decoy-state encoded light pulse are simultaneously returned to the polarizing beam splitting unit 222, thereby causing interference; similarly, the polarization direction of the interference result (i.e. the second interference light pulse, which is output via the 4 th port in fig. 4 for example) output by the polarization beam splitting unit 222 will be the same as the second secondary phase modulation amount θ provided by the phase modulation unit 2212With respect to, for example, the polarization state of the second interfering light pulse may be
Obviously, in the encoding block 2, the second-order phase modulation amount θ is controlled2The polarization state of the second interference light pulse output by the optical fiber can be adjusted, namely, the modulation of the polarization state of the signal light pulse is realized, so that the decoy state encoding process of the signal light can be realized. At this time, the process of the present invention,on the signal light pulse output outward by the polarization beam splitting unit 222, decoy state encoding and polarization encoding have been realized. The second interference light pulse output via the polarization beam splitting unit 222 is output to the outside via the second and third ports of the optical transmission unit 224, finally providing a signal light pulse encoded by the decoy state and polarization.
Fig. 5 shows the timing relationship between the signal light pulse and the electric pulse for the phase modulation unit 221 in the transmitting end structure shown in fig. 4 at the time of two phase modulations, so as to further understand the above-mentioned decoy state and polarization encoding process realized based on two successive phase modulations.
In the transmitting-end structure and the encoding method shown in fig. 4 to 5, since the sagnac interferometer having the self-stabilization function is used, in which there is no phase drift, high encoding efficiency can be achieved even if the phase detection feedback module is omitted.
Fig. 6 schematically illustrates another embodiment of the present invention, which is a structure of a transmitting end for simultaneously implementing a spoofing state and a polarization encoding, where the transmitting end also includes a light source 1, an encoding module 2, and a light path returning module 3, and the light source 1 and the light path returning module 3 are similar to the embodiment illustrated in fig. 4, and therefore are not described again.
Compared to the embodiment shown in fig. 4, the encoding module 2 shown in fig. 6 also includes an optical transmission unit 234, a phase modulation unit 321, a polarization beam splitting unit 232, and an analyzing unit 233, but is different in that it further includes a first faraday rotator mirror 235 and a second faraday rotator mirror 236, where: the first and second faraday rotators 235, 236 are connected to the 1 st and 2 nd ports of the polarization beam splitting unit 232 through first and second optical paths having the same optical path, respectively, to constitute a michelson interferometer structure, not a sagnac interferometer. In addition, the optical transmission unit 234, the phase modulation unit 321, the polarization beam splitting unit 232, and the polarization analyzing unit 233 are similar to the embodiment shown in fig. 4, and therefore, are not described in detail.
As shown in fig. 6, the signal light pulse output by the light source 1 propagates toward the polarization beam splitting unit 232 via the first port 1 and the second port 2 of the optical transmission unit 224; the polarization beam splitting unit 232 receives the signal light pulse via its 4 th port and equally splits the signal light pulse into first and second signal light pulse components whose linear polarization directions are perpendicular to each other to be output to the first and second optical paths via its 1 st and 2 nd ports, respectively.
The phase modulation unit 231 is disposed on the first optical path to perform a first phase modulation on the first signal optical pulse component to form a phase difference θ between the two signal optical pulse components1. With a phase difference theta1Respectively, are reflected by the first and second faraday rotation mirrors 235, 236 and then simultaneously returned to the polarization beam splitting unit 232 along the original path, thereby causing interference to output a first interference optical pulse having a first optical pulse at its end 3 port
The polarization state of (c). The analyzer 233 receives the first interference light pulse, and the intensity of the light pulse outputted by the analyzer is (1+ cos θ) of the intensity of the signal light pulse1) 2, so as to modulate theta by means of the first phase1The intensity modulation of signal light pulse is realized, and the decoy state coding is realized. Similarly, as shown in fig. 6, the analyzing unit 233 may be a polarization beam splitter, and when the polarization beam splitter unit 232 is a polarization beam splitter, the polarization beam splitter 233 may be disposed at an angle of 45 degrees (or-45 degrees) with respect to the polarization beam splitter 232, so that the intensity of the optical pulse output by analyzing is (1+ cos θ) of the intensity of the optical pulse of the signal1)/2. Alternatively, the analyzing unit 223 may be a polarization plate, or a combination of a polarization plate and a wave plate, instead of the polarization beam splitter.
Subsequently, the optical pulse encoded by the decoy state output by the encoding module 2 enters the optical path retracing module 3, and the propagation direction is retraced under the action of the optical pulse, and the optical pulse returns to the encoding module 2.
When returning to the encoding module 2 again, the spoof state encoded light pulse is returned to the polarization beam splitting unit 232 via the polarization analyzing unit 233, is split into the first and second spoof state encoded light pulse components, and is output to the first and second optical paths via the 1 st and 2 nd ports thereof.
In the transmitting end structure and the encoding method shown in fig. 6, the advantage is that the michelson interferometer can be built by using a single-mode fiber device, so that a polarization-maintaining optical device is not needed.
In addition, fig. 7 and 8 also respectively show two embodiments of the optical path folding module 3, which are used to further illustrate the working principle of the optical path folding module 3.
The above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and the above alternatives may be used in combination with each other without contradiction. Those skilled in the art will also appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.
Claims (18)
1. A transmitting end for decoy state coding and polarization coding comprises a light source (1), a coding module (2) and a light path returning module (3), wherein,
the light source (1) is used for providing signal light to be coded;
the encoding module (2) comprises a phase modulation unit (211, 221, 231) and is arranged to perform first phase modulation on the signal light to be encoded through the phase modulation unit (211, 221, 231) and perform decoy state encoding on the signal light to be encoded based on the first phase modulation;
the light path turning-back module (3) is used for receiving the signal light coded by the decoy state and turning back the signal light to the coding module (2); and the number of the first and second electrodes,
the encoding module (2) is further configured to perform a second phase modulation on the folded-back trick-state encoded signal light by the phase modulation unit (211, 221, 231), and perform polarization encoding on the folded-back trick-state encoded signal light based on the second phase modulation.
2. The transmitting end according to claim 1, wherein the optical path folding module (3) comprises a circulator (31) or a reflective structure (32, 33).
3. The transmitting end according to claim 1, wherein the encoding module (2) further comprises a polarization beam splitting unit (212) and a beam splitting unit (213), and the polarization beam splitting unit (212) and the beam splitting unit (213) are connected through a first optical path and a second optical path having the same optical path to form an equal-arm MZ interferometer; the phase modulation means (211) is provided on the first optical path and/or the second optical path.
4. The transmitting end according to claim 3, wherein the polarization beam splitting unit (212) receives the signal light to be encoded and outputs the signal light encoded by a decoy state and a polarization; and, the beam splitting unit (213) outputs the signal light encoded by a spoof state and receives the signal light encoded by a spoof state which is folded back.
5. The transmitting end of claim 3, further comprising a phase detection feedback module for compensating for phase drift in the equal-arm MZ interferometer; and/or the light path folding module (3) comprises a circulator (31); and/or the polarizing beam splitting unit (212) comprises a polarizing beam splitter; and/or the beam splitting unit (213) comprises a beam splitter.
6. The transmitting end according to claim 1, wherein the encoding module (2) further comprises an optical transmission unit (224), a polarization beam splitting unit (222) and an analyzing unit (223), the polarization beam splitting unit (222) being arranged in an optical loop to constitute a sagnac interferometer, the phase modulating unit (221) being arranged in the optical loop.
7. The transmitting end according to claim 1, wherein the encoding module (2) further comprises an optical transmission unit (234), a polarization beam splitting unit (232) and a polarization analyzing unit (233), the polarization beam splitting unit (232) connects a first faraday rotator mirror (235) and a second faraday rotator mirror (236) through a first optical path and a second optical path having the same optical path, respectively, to constitute a michelson interferometer, and the phase modulating unit (231) is disposed in the first optical path and/or the second optical path.
8. The transmitting end according to claim 6 or 7, wherein the optical transmission unit (224, 234) is configured to receive the signal light to be encoded and propagate it towards the polarization beam splitting unit (222, 232), and to receive the signal light output by the polarization beam splitting unit (222, 232) and output it outwards.
9. The transmission end according to claim 6 or 7, wherein the analyzing unit (223, 233) is disposed between the polarization beam splitting unit (222, 232) and the optical path folding back module (3).
10. The transmission end according to claim 9, wherein the polarization beam splitting unit (222, 232) is a polarization beam splitter, the polarization analyzing unit (223, 233) is a polarization beam splitter, a polarization plate, or a combination of a polarization plate and a wave plate, and are arranged at an angle of 45 degrees or-45 degrees to each other; and/or the signal light to be coded is one of 45-degree linearly polarized light, left-handed circularly polarized light and right-handed circularly polarized light.
11. The transmitting end according to claim 6 or 7, wherein the optical transmission unit (224, 234) comprises a circulator; and/or the optical path folding module (3) comprises a reflecting mirror or a Faraday rotation mirror (32, 33).
12. A quantum key distribution system comprising the transmitting end of any one of claims 1-11.
13. An encoding method for simultaneously performing decoy state encoding and polarization encoding, comprising,
a decoy state encoding step: enabling signal light to be coded to pass through a phase modulation unit to carry out first-time phase modulation on the signal light, and carrying out intensity modulation on the signal light to be coded based on the first-time phase modulation, so that decoy state coding is realized;
a light path folding step: enabling the signal light coded by the decoy state to generate returning; and
a polarization encoding step: and enabling the folded signal light encoded by the spoof state to pass through the phase modulation unit again to perform second phase modulation on the signal light, and performing polarization state modulation on the folded signal light encoded by the spoof state based on the second phase modulation, thereby realizing polarization encoding.
14. The encoding method of claim 13, wherein the decoy state encoding step further comprises the steps of: equally dividing the signal light to be encoded into a first signal light component and a second signal light component whose linear polarization directions are perpendicular to each other; performing the first phase modulation on at least one of the first and second signal light components using the phase modulation unit to form a first phase difference θ therebetween1(ii) a And make it haveFirst phase difference theta1Is caused to interfere with the first and second signal light components.
15. The encoding method of claim 14, wherein the polarization encoding step further comprises the steps of: equally dividing the signal light coded by the returned decoy state into a first decoy state coded signal light component and a second decoy state coded signal light component; performing the second phase modulation on at least one of the first and second spoof-state-encoded signal light components with the phase modulation unit to form a second phase difference θ therebetween2(ii) a And having the second phase difference theta2The first and second spoofed state encoded signal light components interfere.
16. The encoding method of claim 15, wherein:
constructing an equal-arm MZ interferometer by using a polarization beam splitting unit and a beam splitting unit, and arranging a phase modulation unit in the equal-arm MZ interferometer, wherein the interference in the decoy state encoding step occurs at the beam splitting unit, and the interference in the polarization encoding step occurs at the polarization beam splitting unit;
or, a sagnac interferometer is constructed by using a polarization beam splitting unit and an optical loop, and the phase modulation unit is arranged in the sagnac interferometer, wherein the interference in the decoy state encoding step and the interference in the polarization encoding step both occur at the polarization beam splitting unit, and the decoy state encoding step further comprises a step of analyzing the result of the interference by using an analyzing unit;
or, a michelson interferometer is constructed by using a polarization beam splitting unit and two faraday rotating mirrors, and the phase modulation unit is provided in the michelson interferometer, wherein the interference in the decoy state encoding step and the interference in the polarization encoding step both occur at the polarization beam splitting unit, and the decoy state encoding step further includes a step of analyzing the result of the interference by using an analyzing unit.
17. The encoding method of claim 15, wherein in the decoy state encoding step, the intensity of the signal light encoded by the decoy state is (1+ cos θ) of the intensity of the signal light to be encoded1) 2; and/or, in the polarization encoding step, the polarization state of the polarization-encoded signal light is
18. The encoding method of claim 17, wherein the first phase difference θ1＝180°。
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