CN103546280B - Encoder for quantum cryptography communication - Google Patents
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Abstract
The present invention proposes the encoder/decoder for quantum cryptography communication, it is possible to achieve the coding and decoding function of BB84 agreement.According to the present invention, using optical switch device to make light pulse at random by multiple interferometers at transmitting terminal, the phase contrast between the length arm of interferometer differs k 2 π+pi/2 successively, thus realizing the encoding function needed for BB84 agreement；Randomly choosing measurement base at receiving terminal passively by optical beam-splitter, the interferometer being differed k 2 π+pi/2 by the phase contrast between two length arms completes decoding function.By using multiple interferometer, without using phase modulator at a high speed in interferometer, solve the two-way time of the restriction to speed, and phase modulation drive circuit can use low speed design and device to realize, greatly reduce the difficulty that high speed quantum cipher communication system realizes, be highly suitable for realizing high speed quantum cryptography communication.If using Michelson's interferometer (F-M) structure adopting faraday's reflecting mirror, then can have further the adaptive equalization ability of polarization scrambling in light path.
Description
Technical Field
The invention belongs to the technical field of quantum cryptography communication, and particularly relates to an encoder and a decoder for quantum cryptography communication.
Background
Quantum cryptography combines quantum physics principles with modern communication technologies. The quantum cryptography communication guarantees the security of the key negotiation process and the result in different places by virtue of a physical principle, and can realize secret communication independent of algorithm complexity by combining with a one-time pad encryption technology.
At present, quantum cryptography mainly uses optical quantum as a carrier for realization, and the optical quantum is distributed through free space or an optical fiber channel. The classical random bit is loaded on the physical quantity such as polarization, phase and the like of the optical quantum by means of polarization coding, phase coding and the like. Nowadays, optical fiber communication has become the basic architecture and development trend of modern information transmission, and quantum cryptography communication in optical fiber channel has very important meaning and application prospect. When transmitting a light quantum signal in an optical fiber channel, the polarization state of the light quantum signal is affected by the intrinsic birefringence characteristics of the optical fiber, and the birefringence characteristics in the optical path and the channel are changed by the external environment, so that a polarization feedback device is required to be used in order to ensure the stability of the polarization encoding quantum cryptography system. When the interference on the system changes rapidly, the feedback process becomes more time-consuming, and the difficulty in maintaining the stable operation of the system is greatly increased.
The key generation rate is one of the core indicators of quantum cryptography communication systems. Therefore, the quantum cryptography system with high working frequency is an important direction for the development of quantum cryptography communication technology. As the operating frequency of the system increases, the system needs to use a high-bandwidth phase modulator, which causes several difficulties in system implementation:
(1) photons with mutually vertical polarization states can be transmitted in the phase modulator with low loss, while the phase modulator used in the application of conventional optical communication and the like mostly only allows a single polarization state to pass through, and the transmission of the polarization state vertical to the phase modulator in the phase modulator has high loss, so that special requirements are provided for the design process and the implementation technology of the phase modulator;
(2) the high-bandwidth electro-optic phase modulation device generally needs to be matched with a low-impedance resistor, so that a driving circuit is needed to provide a large driving current, and heat is possibly generated after long-time operation, so that the stability of a system is influenced;
(3) in order to modulate the photons to obtain a stable and precise phase, the modulation driving voltage needs to be kept stable at the arrival time of the photons, and the round-trip structure prolongs the time, so that the increase of the system working speed can be limited;
(4) in order to obtain a phase modulator driving signal which is high in bandwidth, stable, accurate and adjustable and has large driving capacity, higher requirements are put forward on a driving circuit of a system;
(5) both the transmitting and receiving require active phase control, so both the transmitting and receiving require true random number driving source.
Disclosure of Invention
The invention provides an encoder and a decoder for quantum cryptography communication, which can realize the encoding and decoding functions of BB84 protocol. The invention is characterized in that: an optical switch device is used at a transmitting end to enable light pulses to randomly pass through a plurality of interferometers, and the phase difference between the long arm and the short arm of each interferometer is k & lt 2& gt pi + pi/2 in sequence, so that the coding function required by a BB84 protocol can be completed; and a measuring base is passively and randomly selected at a receiving end through an optical beam splitter, and a decoding function is completed through an interferometer with the phase difference k & lt 2& gt pi + pi/2 between the two long and short arms. Through using a plurality of interferometers, can need not to use high-speed phase modulation device in the interferometer to solved the restriction of round trip time to speed, and the phase modulation drive circuit can use low-speed design scheme and device to realize, greatly reduced the degree of difficulty that high-speed quantum cryptography communication system realized, consequently especially adapted is used for realizing high-speed quantum cryptography communication. If a Michelson interferometer (F-M) structure using Faraday mirrors is used, it is possible to have the capability of adaptive compensation of polarization perturbations in the optical path.
According to an aspect of the present invention, there is provided an encoder for quantum cryptography communication, comprising: a first branch consisting of a first optical switching element and a first encoding interferometer; a second branch consisting of a second optical switching element and a second encoding interferometer; a third branch consisting of a third optical switching element and a third encoding interferometer; a fourth branch consisting of a fourth optical switching element and a fourth encoding interferometer; a four-splitting optical splitter, connected to the input end, the first branch, the second branch, the third branch and the fourth branch, for splitting an input optical pulse into four optical pulses, which are input to the first branch, the second branch, the third branch and the fourth branch, respectively; a four-in-one light combiner connected to the first branch, the second branch, the third branch, the fourth branch and the output end for combining the coded light pulses output by the first branch, the second branch, the third branch and the fourth branch into one path and outputting the path from the output end, wherein the first branch is connected to the second branch, the third branch and the fourth branchThe phase difference between the front and back wave packets generated after the light pulse is interfered by the encoding interferometer is a first predetermined phaseThe phase difference between the front and back wave packets generated after the second coding interferometer interferes the optical pulse is the first preset phase + k.2 pi + pi/2The phase difference between the front and back wave packets generated after the third coding interferometer interferes the optical pulse is the first predetermined phase + k.2 pi + piThe phase difference between the front and back wave packets generated after the fourth coding interferometer interferes the optical pulse is the first preset phase + k.2 pi +3 pi/2Where k is an integer.
Preferably, the encoder may further include: an optical switching element driving circuit for randomly turning on one of the first optical switching element, the second optical switching element, the third optical switching element, and the fourth optical switching element in accordance with a random signal.
Preferably, the encoder may further include: and the random light intensity modulator is connected between the input end and the one-to-four optical splitter and is used for performing random light intensity modulation on the input light pulse. More preferably, the random light intensity modulator may be a waveguide type electro-optical intensity modulator, and a polarization maintaining type single mode fiber may be used between the input end and the one-to-four optical splitter.
Preferably, the encoder may further include: and the attenuator is connected between the four-in-one light combiner and the output end and is used for controlling the light intensity of the output light pulse so that the light pulse reaches the single photon magnitude.
Preferably, the encoder may further include: and the random light intensity modulator is connected between the four-in-one light combiner and the output end and is used for carrying out random light intensity modulation on the output light pulse to realize a decoy state protocol.
Preferably, the encoder may further include: and the attenuator is connected between the four-in-one light combiner and the random light intensity modulator or between the random light intensity modulator and the output end and is used for controlling the light intensity of the output light pulse so that the light pulse reaches the single photon magnitude.
Preferably, the divide-by-four optical splitter equally divides an input light pulse into four light pulses, which are input to the first branch, the second branch, the third branch, and the fourth branch, respectively. More preferably, the input of the one-to-four optical splitter is a polarization maintaining single mode fiber.
Preferably, the first optical switching element, the second optical switching element, the third optical switching element and the fourth optical switching element are electro-optical intensity modulators, micro-opto-electro-mechanical system optical switches, all-optical switches based on nonlinear effects (NOLM and SOA), or mechanical type optical switching elements. More preferably, the first optical switching element, the second optical switching element, the third optical switching element and the fourth optical switching element are waveguide-type electro-optical intensity modulators.
Preferably, the first, second, third and fourth encoding interferometers are michelson interferometers or Mach-Zehnder type interferometers using faraday mirrors.
Preferably, the first optical switch element is connected between the four-splitting optical splitter and the first coding interferometer, or connected between the first coding interferometer and the four-combining optical device; the second optical switch element is connected between the four-splitting optical splitter and the second coding interferometer or between the second coding interferometer and the four-combining optical device; the third optical switch element is connected between the four-in-one optical splitter and the third coding interferometer or between the third coding interferometer and the four-in-one optical combiner; the fourth optical switch element is connected between the one-to-four optical splitter and the fourth coding interferometer, or between the fourth coding interferometer and the four-in-one optical combiner.
According to another aspect of the present invention, there is provided a decoder for quantum cryptography communication, comprising: the first branch consists of a first decoding interferometer, a first single-photon detector and a second single-photon detector; the second branch consists of a second decoding interferometer, a third single-photon detector and a fourth single-photon detector; and the two-splitting optical splitter is connected with the input end, the first branch and the second branch and is used for splitting an input optical pulse into two optical pulses which are respectively input into the first branch and the second branch, wherein the phase difference between two wave packets output by the first decoding interferometer and the second decoding interferometer is pi, a random one of the first decoding interferometer and the second decoding interferometer corresponds to a group of bases of {0, pi }, and the other one of the first decoding interferometer and the second decoding interferometer corresponds to a group of bases of { pi/2, 3 pi/2 }.
Preferably, the one-to-two splitter is a wavelength insensitive device, i.e. the splitting ratio is substantially uniform for input light of different wavelengths.
Drawings
The above and other objects, features and advantages of the present invention will become more apparent by describing a preferred embodiment thereof with reference to the accompanying drawings, in which:
FIG. 1 is a schematic block diagram of an optical encoder 1000 according to the present invention;
FIG. 2 is a schematic block diagram of an optical encoder 1100 according to an embodiment of the present invention;
FIG. 3 is a schematic block diagram of a coding interferometer 1140-1 in accordance with one embodiment of the present invention;
FIG. 4 is a schematic block diagram of a coding interferometer 1140-2 in accordance with another embodiment of the present invention;
FIG. 5 is a schematic block diagram of an optical decoder 2000 in accordance with the present invention; and
FIG. 6 is a schematic block diagram of an optical encoder 2100 in accordance with an embodiment of the present invention.
Detailed Description
In the following detailed description of the preferred embodiments of the present invention, reference is made to the accompanying drawings, in which details and functions that are not necessary for the invention are omitted so as not to obscure the understanding of the present invention.
First, an optical encoder 1000 according to the present invention will be described in detail with reference to fig. 1, and fig. 1 is a schematic block diagram of the optical encoder 1000 according to the present invention.
As shown in fig. 1, the optical encoder 1000 includes: by a first optical switching element 10301And a first encoding interferometer 10401A first branch of; by a second optical switching element 10302And a second encoding interferometer 10402A second branch of; by a third optical switching element 10303And a third encoding interferometer 10403A third branch of; by a fourth optical switching element 10304And a fourth encoding interferometer 10404A fourth branch of; a four-splitting optical splitter 1010 connected to the input terminal IN and the first to fourth branches, for splitting the input optical pulse into four optical pulses (which may be equal or unequal) and inputting the four optical pulses to the first to fourth branches, respectively; a four-in-one combiner 1020 connected to the first to fourth branches and the output terminal OUT for combining the first to fourth branchesThe coded light pulses output by the first branch, the second branch and the fourth branch are combined into a path and output from an output end OUT.
First optical switching element 10301Can be connected to a four-splitting optical splitter 1010 and a first encoding interferometer 10401Between, or connected to, the first encoding interferometer 10401And a four-in-one light combiner 1020; second optical switching element 10302Can be connected to a divide-by-four optical splitter 1010 and a second encoding interferometer 10402Or connected to the second encoding interferometer 10402And a four-in-one light combiner 1020; third optical switching element 10303Can be connected with a four-splitting optical splitter 1010 and a third coding interferometer 10403Or connected to a third encoding interferometer 10403And a four-in-one light combiner 1020; fourth optical switching element 10304Can be connected to a divide-by-four beam splitter 1010 and a fourth encoding interferometer 10404Or connected to a fourth encoding interferometer 10404And a four-in-one light combiner 1020.
The optical encoder 1000 may further include: an optical switching element drive circuit (not shown) for randomly turning on the first to fourth optical switching elements 1030 according to a random signal1～10304One of them. Thus, the first to fourth encoding interferometers 1040 can be selected1～10404One of the branches modulates the input optical pulse, i.e. only one branch acts as modulation code.
A four-splitting optical splitter 1010 may equally split an input optical pulse into four optical pulses, which are input to the first branch, the second branch, the third branch, and the fourth branch, respectively.
First to fourth code interferometers 10401～10404It may be a michelson interferometer using a faraday mirror or a Mach-Zehnder type interferometer. After passing through the interferometer, the light pulse is divided into a front wave packet and a rear wave packet, and the phase difference between the front wave packet and the rear wave packetPhase modulated by long and short arm optical path length difference △ L of interferometer and phase modulation device thereinAnd (6) determining. For example, phase differenceThe following formula can be satisfied:
for a system implementing the BB84 protocol, the first to fourth encoding interferometers 1040 are required1～10404The relative phase difference of the phase difference between the long and short arms of (2) is k · 2 pi + pi/2 (k is an integer). Disregarding k.2 pi components not actually affecting modulation and demodulation, and subjecting the first encoding interferometer 10401The generated phase differenceAs a reference origin of the phase, the remaining three interferometers (second to fourth encoding interferometers 1040)2～10404) The modulation phase difference of (a) satisfies the following relationship:
first to fourth optical switching elements 10301～10304Can be an electro-optical intensity modulator, a micro-optical-electro-mechanical system lightSwitches, all-optical switches based on nonlinear effects (NOLM and SOA), or mechanical-type optical switching elements. To meet the need to tell quantum key distribution, more preferably, the first to fourth optical switching elements 10301～10304A waveguide type electro-optic intensity modulator may be employed.
In the prior phase-coded high-speed quantum cryptography communication system, a waveguide type electro-optical phase modulation device with high modulation bandwidth is generally required. When the invention is used in a high-speed quantum cryptography communication system, the invention passes through the first to fourth optical switch elements 10301～10304Fast photon random phase selection is achieved and therefore slow phase modulation devices, such as piezoelectric ceramic phase modulation devices, can be used, which greatly reduces the requirements of the system for electronic control. The phase modulation devices selected by the present invention are not limited to high bandwidth devices, and therefore, polarization insensitive phase modulation devices (first to fourth encoding interferometers 1040) with very low insertion loss can be selected1～10404)。
FIGS. 2-4 specifically show schematic block diagrams of an optical encoder 1100 and an encoding interferometer 1140 therein according to embodiments of the present invention.
Referring to FIG. 2, there is shown a schematic block diagram of an optical encoder 1100 in accordance with an embodiment of the present invention. The same or similar elements as in fig. 1 are denoted by similar reference numerals, and a detailed description thereof will be omitted where appropriate.
The optical encoder 1100 includes: by the first optical switching element 11301And a first encoding interferometer 11401A first branch of; by the second optical switching element 11302And a second encoding interferometer 11402A second branch of; by the third optical switching element 11303And a third encoding interferometer 11403A third branch of; by the fourth optical switching element 11304And a fourth encoding interferometer 11404The fourth branch of the structure. A four-to-one splitter 1110 connected to the first to fourth branches for splitting the input light pulseThe impulse is divided into four light pulses (which can be equal in proportion or unequal in proportion) and respectively input into the first branch, the second branch and the fourth branch; and a four-in-one light combiner 1120, connected to the first to fourth branches, for combining the coded light pulses output by the first to fourth branches into one path for output.
The optical encoder 1100 further includes: a random light intensity modulator 1105 connected between the input terminal IN and the one-to-four optical splitter 1110 for performing random light intensity modulation on the input light pulse, thereby realizing the random light intensity modulation function required by the decoy state protocol; and the attenuator 1125 is connected between the four-in-one optical combiner 1120 and the output end OUT, and is configured to perform light intensity modulation on the output optical pulse, so as to realize overall intensity control of the optical pulse, and enable the optical pulse entering the quantum channel to reach a single photon level.
The arrangement of the random intensity modulator (1105) and the attenuator (1125) is not limited thereto. For example, as another example (not shown), the random light intensity modulator (1105) may be connected between the four-in-one combiner (1120) and the output terminal (OUT) for performing random light intensity modulation on the output light pulse, thereby implementing the random light intensity modulation function required by the spoof state protocol; the attenuator (1125) can be connected between the four-in-one light combiner (1120) and the random light intensity modulator (1105), or can be connected between the random light intensity modulator (1105) and the output end (OUT), and is used for controlling the light intensity of the output light pulse, so that the light pulse reaches the single photon magnitude.
The random light intensity modulator can be a waveguide type electro-optical intensity modulator.
A polarization maintaining single mode fiber may be used between the input terminal IN and the one-to-four splitter 1110.
In the present embodiment, the first to fourth optical switching elements 10301～10304Is a waveguide type high-speed light intensity modulator.
Similarly, the optical encoder 1100 may further include: an optical switching element driving circuit (not shown) for randomly turning on the first to fourth optical switching elements 1130 according to a random signal1～11304One of them. Thus, the first to fourth encoding interferometers 1140 can be selected1～11404One of the branches modulates the input optical pulse, i.e. only one branch acts as modulation code.
Most of the light sources used in the current quantum cryptography system are coherent lasers with strong attenuation. Such sources present a proportion (typically less than 1%) of multiphoton pulses. The long quantum channels (typically above about 30 Km) make the system less than ideal unconditional security. Such attacks are referred to as Photon Number Separation (PNS) attacks. The decoy state technology estimates the information quantity obtained by an eavesdropper through randomly modulating the light intensity of each pulse (completed by a random light intensity modulator 1105) and through simultaneous counting rate and bit error rate equations of a signal state and a decoy state, and calculates the final safe code rate, thereby achieving the purpose of resisting PNS attack. The signal state, the trick-state light intensity for the trick-state implementation of the BB84 protocol are typically set to an average of 0.6 photons/pulse and 0.2 photons/pulse, respectively, with the vacuum state as the second trick state. In this embodiment, the speed of the decoy state intensity modulation should be the same as the speed of the optical pulse transmission, so in a high speed system, a single high speed waveguide optical intensity modulator 1105 is typically used for the decoy state intensity modulation. Since the first to fourth optical switching elements 10301～10304The insertion loss of the high-speed waveguide electro-optic intensity modulator is generally changed with voltage, and the first to fourth optical switching elements 10301～10304Applying different switching voltages across the respective optical switching elements 1030iThe attenuation of the light intensity of (a) varies randomly according to the requirements of the spoof state and the random light intensity modulator 1105 may be omitted.
FIGS. 3 and 4 show encoding interferometers (first through fourth encoding interferometers 1140), respectively, that may be used in embodiments of the invention1～11404Any one or more of).
Figure 3 is a schematic block diagram of a michelson interferometer (F-M)1140-1 using a faraday mirror, according to one embodiment of the invention.
As shown in FIG. 3, F-M interferometer 1140-1 comprises: a 2 × 2 optical splitter 1141, faraday mirrors 1145 and 1147, and a controlled phase modulation device 1143. Two ends of the same side of the 2 × 2 optical splitter 1141 are respectively used as an input end and an output end of the F-M interferometer 1140-1, and two ends of the other side of the 2 × 2 optical splitter 1141 are respectively connected with faraday reflectors 1145 and 1147 to form two interference arms. A controlled phase modulation device 1143 is connected to one of the two interferometric arms.
The optical pulse entering the F-M interferometer 1140-1 is first split into two equal wave packets by the 2 × 2 beam splitter 1141 (50: 50 beam splitter) and enters the long arm and the short arm of the unequal arm interferometer 1140-1, because of the Faraday mirror, the polarization state of the input arbitrary polarization light P is changed into the orthogonal state P⊥Then, returning from the original path, and setting the jones matrix of the interferometer single-mode fiber as L, the process can be described as follows:
wherein,represents a matrix conjugate transpose operation, and FM represents the Jones matrix of the Faraday mirror (·)*Representing a matrix conjugate operation and det (-) representing a matrix determinant operation. It can be seen that the light pulse returning through the faraday mirror reaches 50 again: the 50 beamsplitter polarization state is always orthogonal to the polarization state at its input. Because the speed of the 50: the polarization states of two light pulse wave packets entering the long arm and the short arm of the 50 beam splitter are the same, so the polarization states of two reflected wave packets are also the same. For the channel disturbance, because the time interval of the two wave packets with the same polarization is short, the two wave packets can be regarded as the same polarization state change in the channel, and if the same structure is adopted at the receiving end, the interference can be guaranteedThe polarization states of the two optical pulse wave packets are always the same. Therefore, the whole interference system can be compensated in a self-adaptive mode no matter how the polarization states of the optical fibers of the interferometer and the channel change, and therefore polarization robustness of the whole system can be guaranteed.
Fig. 4 is a schematic block diagram of a Mach-Zehnder type interferometer (M-Z)1140-2 in accordance with another embodiment of the present invention.
As shown in FIG. 4, M-Z interferometer 1140-2 comprises: a 1 × 2 beam splitter 1142 (50: 50 beam splitter), a controlled phase modulation device 1144, and a 2 × 1 light combiner 1146 (50: 50 light combiner). As shown in fig. 4, a port on the left side of the 1 × 2 optical splitter 1142 is used as an input port, and an optical pulse enters the 1 × 2 optical splitter 1142 and is divided into two optical pulses, which pass through an arm including the controlled phase modulation device 1144 and an arm not including the controlled phase modulation device 1144, respectively, and then interfere with each other at the 2 × 1 optical combiner 1146, and is output from a port on the right side of the 2 × 1 optical combiner 1146. The controlled phase modulation device 1144 may be constituted by elements similar to those of the controlled phase modulation device 1143. In the configuration shown in fig. 4, the light pulse passes only unidirectionally from the interferometer. The structure can not complete the self-adaptive compensation of the polarization state of the optical path, and an additional polarization recovery module is required to be added.
Next, the optical decoder 2000 according to the present invention will be described in detail with reference to fig. 5, and fig. 5 is a schematic block diagram of the optical decoder 2000 according to the present invention.
As shown in fig. 5, the decoder 2000 includes: by a first decoding interferometer 20201And a first single photon detector 20301Second single photon detector 20302A first branch of; by a second decoding interferometer 20202And a third single photon detector 20303Fourth single photon detector 20304A second branch of; a two-splitting optical splitter 2010 connected to the input terminal IN, the first branch and the second branch for splitting the input optical pulse into two optical pulses (which may be equal or unequal) and respectively input to the first branch and the second branch, wherein a first decoding interferometer 20201And the second solutionCode interferometer 20202The phase difference between the two wave packets is pi, and the first decoding interferometer 20201Set of basis, second decoding interferometers 2020 corresponding to 0, pi2A set of bases corresponding to { pi/2, 3 pi/2 }. The one-to-two splitter 2010 can be used as a quantum random number generator to realize the passive random selection of two groups of bases of the BB84 protocol. First decoding interferometer 20201And a second decoding interferometer 20202Are two separate decoding interferometers respectively corresponding randomly to the two groups of bases 0, pi/2, 3 pi/2 of the BB84 protocol, and a first decoding interferometer 20201And a second decoding interferometer 20202The base phase of (2) differs by pi/2. The phase difference between the optical pulse wave packets output by the two arms of the interferometer is determined to be equal to pi by the interference characteristics of the interferometer, so that the four quantum states of the BB84 protocol can be decoded.
Here, for system security, the modulation mode may be generated by a random generator at the decoding end without being published to the outside by adjusting the phase modulation units of the first decoder and the second decoder such that their corresponding relationship with the basis vector group is changed randomly.
The one-to-two splitter 2010 may be a wavelength insensitive device, i.e., the splitting ratio is substantially the same for input light of different wavelengths.
First decoding interferometer 20201And a second decoding interferometer 20202A Mach-Zehnder type interferometer (M-Z) may be used, and a michelson interferometer (F-M) using a faraday mirror may also be used.
Hereinafter, a detailed description will be given of specific embodiments of the present invention, taking an F-M interferometer as an example. Here, the advantage of using an F-M interferometer is that robustness to changes in the polarization state of the light path can be achieved.
FIG. 6 is a schematic block diagram of an optical encoder 2100 in accordance with an embodiment of the present invention. The same or similar elements as in fig. 5 are denoted by similar reference numerals, and a detailed description thereof will be omitted, where appropriate.
As shown in fig. 6, the optical encoder 2100 includes: by a first circulator 21251First decoding interferometer 21201And a first single photon detector 21301Second single photon detector 21302A first branch of; by a second circulator 21252A second decoding interferometer 21202And a third single photon detector 21303And a fourth single photon detector 21304A second branch of; a two-splitting optical splitter 2110 connected to the input terminal IN, the first branch and the second branch for splitting the input optical pulse into two optical pulses (which may be equal or unequal) to be input to the first branch and the second branch, respectively, wherein the first decoding interferometer 21201And a second decoding interferometer 21202The phase difference between the two wave packets is pi, and the first decoding interferometer 21201A set of basis, second decoding interferometers 2120 corresponding to {0, π }2A set of bases corresponding to { pi/2, 3 pi/2 }.
The optical pulse packets split by the one-to-two splitter 2110 pass through the first circulator 2125 respectively1And a second circulator 212521 → 2 port into the first decoding interferometer 21201And a second decoding interferometer 21202(Michelson interferometer), the two arm outputs of the interferometer directly enter into the single photon detector (the second single photon detector 2130)2And a fourth single photon detector 21304) And through a first circulator 21251And a second circulator 212522 → 3 port into single photon detector (first single photon detector 2130)1And a third single photon detector 21303). Decoding of the photon quantum states can thereby be accomplished. First decoding interferometer 21201And a second decoding interferometer 21202The phase modulation device used in (1) may also be a low-speed modulation phase modulation device.
First decoding interferometer 21201And a second decoding interferometer 21202May have a configuration as shown in FIG. 3, including mounting on an unequal arm interferometerA controlled phase modulation device on the long or short arm. The detailed description may refer to the related description of fig. 3, and will not be repeated herein.
Aiming at the decoy state technology, random modulation of three states of light pulse with two intensities and a vacuum state can be realized, wherein when the vacuum state is realized, the laser does not generate light pulse (does not emit light), namely, a vacuum pulse (vacuum state) with the photon number of 0 is generated. The light intensity of the laser, the modulation voltage of the random light intensity modulator 1105, or the first to fourth optical switching elements 1030 may be controlled1～10304The modulation voltage of the (waveguide type high speed optical intensity modulator) is modulated or controlled to produce the appropriate optical intensity and/or extinction ratio.
The optical switching element driving circuit may be implemented by an FPGA, and the random signal may be generated by an external true random number or pseudo random number generator, or may use a random sequence generated internally by the FPGA. A two-bit high-speed serial signal may be used to implement to control the four optical switching elements separately. For example, random numbers 00, 01, 10, 11 and four optical switching elements 10301～10304(11301～11304) The control relationship (on/off) between them can be realized as shown in the following table.
The invention has thus been described with reference to the preferred embodiments. It should be understood by those skilled in the art that various other changes, substitutions, and additions may be made without departing from the spirit and scope of the invention. The scope of the invention is therefore not limited to the particular embodiments described above, but rather should be determined by the claims that follow.
Claims (11)
1. An encoder for quantum cryptography communication, comprising:
a first branch consisting of a first optical switching element and a first encoding interferometer;
a second branch consisting of a second optical switching element and a second encoding interferometer;
a third branch consisting of a third optical switching element and a third encoding interferometer;
a fourth branch consisting of a fourth optical switching element and a fourth encoding interferometer;
a four-splitting optical splitter, connected to the input end, the first branch, the second branch, the third branch and the fourth branch, for splitting an input optical pulse into four optical pulses, which are input to the first branch, the second branch, the third branch and the fourth branch, respectively;
a four-in-one light combiner connected to the first branch, the second branch, the third branch, the fourth branch and the output end for combining the encoded light pulses output by the first branch, the second branch, the third branch and the fourth branch into one path and outputting the path from the output end,
wherein, the phase difference between the front and the back wave packets generated after the first coding interferometer interferes the light pulse is a first preset phase, the phase difference between the front and the back wave packets generated after the second coding interferometer interferes the light pulse is the first preset phase + k.2 pi + pi/2, the phase difference between the front and the back wave packets generated after the third coding interferometer interferes the light pulse is the first preset phase + k.2 pi + pi, the phase difference between the front and the back wave packets generated after the fourth coding interferometer interferes the light pulse is the first preset phase + k.2 pi +3 pi/2, wherein k is an integer,
wherein the first optical switching element, the second optical switching element, the third optical switching element, and the fourth optical switching element are electro-optical intensity modulators, micro-opto-electro-mechanical systems optical switches, all-optical switches based on nonlinear effects, or mechanical type optical switching elements,
wherein the first, second, third and fourth encoding interferometers are michelson interferometers or Mach-Zehnder type interferometers using faraday reflectors.
2. The encoder of claim 1, further comprising:
an optical switching element driving circuit for randomly turning on one of the first optical switching element, the second optical switching element, the third optical switching element, and the fourth optical switching element in accordance with a random signal.
3. The encoder of claim 1, further comprising:
and the random light intensity modulator is connected between the input end and the one-to-four optical splitter and is used for performing random light intensity modulation on the input light pulse.
4. The encoder of claim 1, further comprising:
and the attenuator is connected between the four-in-one light combiner and the output end and is used for controlling the light intensity of the output light pulse so that the light pulse reaches the single photon magnitude.
5. The encoder of claim 1, further comprising:
and the random light intensity modulator is connected between the four-in-one light combiner and the output end and is used for performing random light intensity modulation on the output light pulse.
6. The encoder of claim 5, further comprising:
and the attenuator is connected between the four-in-one light combiner and the random light intensity modulator or between the random light intensity modulator and the output end and is used for controlling the light intensity of the output light pulse so that the light pulse reaches the single photon magnitude.
7. The encoder according to claim 1, wherein the divide-by-four optical splitter equally divides an input light pulse into four light pulses, which are input to the first branch, the second branch, the third branch, and the fourth branch, respectively.
8. The encoder of claim 1, wherein the first, second, third and fourth optical switching elements are waveguide-type electro-optic intensity modulators.
9. The encoder of claim 1, wherein,
the first optical switch element is connected between the four-splitting optical splitter and the first coding interferometer or between the first coding interferometer and the four-combining optical combiner;
the second optical switch element is connected between the four-splitting optical splitter and the second coding interferometer or between the second coding interferometer and the four-combining optical device;
the third optical switch element is connected between the four-in-one optical splitter and the third coding interferometer or between the third coding interferometer and the four-in-one optical combiner;
the fourth optical switch element is connected between the one-to-four optical splitter and the fourth coding interferometer, or between the fourth coding interferometer and the four-in-one optical combiner.
10. A decoder for quantum cryptography communication, comprising:
the first branch consists of a first decoding interferometer, a first single-photon detector and a second single-photon detector;
the second branch consists of a second decoding interferometer, a third single-photon detector and a fourth single-photon detector;
a two-splitting optical splitter connected to the input terminal, the first branch and the second branch for splitting an input optical pulse into two optical pulses, which are input to the first branch and the second branch, respectively,
wherein the phase difference between the two wave packets output by the first decoding interferometer and the second decoding interferometer is pi, the random one of the first decoding interferometer and the second decoding interferometer corresponds to a group of bases of {0, pi }, and the other one of the first decoding interferometer and the second decoding interferometer corresponds to a group of bases of { pi/2, 3 pi/2 },
wherein the first decoding interferometer and the second decoding interferometer are Michelson interferometers or Mach-Zehnder type interferometers using Faraday mirrors.
11. The decoder according to claim 10, wherein the one-to-two splitter is a wavelength insensitive device, i.e., the splitting ratio is substantially uniform for different wavelengths of input light.
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