The polarization compensation implementation method of continuous variable quantum key distribution system
Technical field
The present invention relates to the technology in a kind of quantum cryptography communication field, specifically, is a kind of polarization compensation implementation method of continuous variable quantum key distribution system.
Background technology
Along with the progress of expanding economy and science, information-based, digitlization process is constantly accelerated, and information transmission and exchange are more and more frequent, therefore passes on the safety and reliability of process that more and more higher requirement is arranged to information.How safely transmission of information has become the major issue in modern communications field.Applied cryptography is learned and is arisen at the historic moment thus, and especially being widely used in World War II embodied cryptographic importance more.Up to the present, cryptography all plays an important role in the various aspects of the social life such as political, military, economic, cultural.
Classical cryptographic communication can be divided into two large classes: asymmetric cryptosystem and symmetric cryptosystem.Asymmetric cryptosystem is called again open key system, such as the RSA Algorithm that widely adopts now.The recipient selects first one group of private key that only has him to know, calculates successively corresponding public-key cryptography, is used for enciphered message so public-key cryptography is given the people again.The fail safe of asymmetric cryptosystem depends on the complexity of calculating.In case the quantum computer development is born, the speed of calculating will greatly promote, and the difficulty that cracks asymmetric cryptosystem will significantly reduce; Symmetric cryptosystem is called again the private key system, and as generally acknowledging the one-time pad bag (One-Time-Pad) that is perfectly safe, transmit leg has identical key with the recipient.Key is only used once, and key length is not less than message length.Fail safe just is guaranteed like this, and cost is the service efficiency that has reduced password.
Quantum cryptography communication is different from classical cryptographic communication, and its fail safe is based on the fundamental characteristics of quantum physics but not the complexity of calculating.It is not to utilize merely computational complexity that the listener-in can not be broken a code in finite time, but utilize quantum-mechanical basic principle and characteristic to find whether to have the listener-in to exist, establish the Information Superiority that contrasts between legitimate correspondence person and the listener-in with this, thereby guarantee the safety of communicating by letter.Therefore, in today of quantum calculation development, research also realizes that quanta cryptology technique is very important.
Through this development in twenties years, quantum cryptography communication progressively moves towards practical application from theoretical research.According to the difference of implementation, mainly be divided into discrete variable quantum key distribution system and continuous variable quantum key distribution system.Wherein the discrete variable quantum key distribution realizes by the mode of single photon.Owing to now also can't use desirable single-photon source, mainly be by coherent light is decayed, so that the average photon number of each pulse is less than 1 therefore.The number of photons of lasing light emitter is obeyed Poisson distribution, also contains the multi-photon pulse except containing single photon pulses.These multi-photon pulses are ravesdropping can not discovered afterwards, thereby affects the fail safe of cryptographic system.Use and inveigle attitude (Decoy State) to address this problem to a great extent, but increased the complexity of real system.
Continuous variable quantum cryptography starting will be a little later than discrete variable, until Ralph in 1999 at first proposes to utilize continuous variable to carry out the concept of quantum key distribution.Different from discrete variable, continuous variable is information coding (such as regular position X and canonical momentum P) on continuous canonical component, can be than the discrete variable more information of encoding on each bit.The light source that adopts can be coherent states, squeezed state or Entangled State.Consider convenience experimentally, generally all adopt coherent laser as light source.The continuous variable quantum key distribution adopts the Homodyne detector as quantum detector, compares with discrete variable, has the advantage on the cost.Therefore the continuous variable quantum key distribution has good practical development prospect.
At present, two unequal arm interference structures of all adopting of continuous variable quantum key distribution system.This kind structure is easy to be subject to the impact of various factors.These factors include but not limited to:
1. variations in temperature: the electrooptic modulator of lithium niobate material itself is relatively more responsive for the variation of temperature, and unsettled ambient temperature can produce larger deviation when modulation.
2. the suffered mechanical oscillation of system, the mechanical oscillation meeting produces stress in optical fiber, cause refractive index to change, so that the polarization state that transmits in optical fiber produces drift.
3. the impact of Air Flow.Air Flow can produce disturbance to the optical fiber that does not compact fixing, causes polarization state to be drifted about.
4. the impact of electromagnetic radiation field: electromagnetic radiation (such as the electromagnetic wave of mobile phone signal, alternating current generation) can produce apparent in view interference to the circuit part of system, and the interference of receiving is the most obvious to be exactly the quantum detector part.
In order to address these problems, allow the continuous variable quantum key distribution system can have higher stability, we need to monitor the impact of environment, and calibrate by the mode of feedback, so that system has resistivity for environmental interference, and can long time continuous working.
Summary of the invention
For defective of the prior art, the object of the invention is to the blank for continuous variable quantum key distribution system polarization control scheme, a kind of brand-new polarization control scheme has been proposed, advance the practical of continuous variable quantum cryptography, effectively suppressed simultaneously the environmental interference that the polarization of continuous variable quantum signal in the quantum communications process is subject to.
According to an aspect of the present invention, provide a kind of polarization compensation implementation method of continuous variable quantum key distribution system, be specially:
At transmitting terminal: the pulse laser that laser is produced is divided into flashlight and local oscillator light two bundles by polarization-maintaining beam splitter after the attenuator decay; After making flashlight through amplitude modulation(PAM) and phase-modulation, decay to Quantum Level by attenuator, then enter by polarizing coupler with local oscillator light and transfer to receiving terminal in the optical fiber;
At receiving terminal: make transmission comes in the optical fiber signal behind dynamic polarization controller, be divided into flashlight and local oscillator light two bundles by polarization beam apparatus; Make flashlight directly enter polarization-maintaining coupler; Make local oscillator light be divided into two bundles by polarization-maintaining beam splitter first, wherein, the polarization control unit receives and a branch of to dynamic polarization controller output polarization feedback control signal according to wherein, another bundle is finished phase compensation by phase-modulator, enters quantum detector with flashlight by polarizing coupler and does relevant the detection.
Preferably, at receiving terminal: the polarization control unit comprises photodiode, amplifier and the RMS-DC converter circuit that connects successively, wherein, by photodiode local oscillator light a branch ofly is converted into electric pulse by light pulse by what polarization-maintaining beam splitter was told, then amplify electric pulse by amplifier, the electric pulse after order is amplified again is converted into the DC feedback Voltage-output to dynamic polarization controller through the RMS-DC converter circuit.
Preferably, dynamic polarization controller is controlled polarization state by the voltage on four optical fiber squeezers that change its inside, makes DC feedback voltage reach maximum, specifically comprises the steps:
Step (1): dynamic polarization controller gathers DC feedback voltage;
Step (2): the control voltage of selecting first optical fiber squeezer;
Step (3): suitably increase control voltage, gather the DC feedback voltage that feed back this moment;
Step (4): if DC feedback voltage increases, then return step (3), otherwise enter step (5);
Step (5): suitably reduce to control voltage, gather the DC feedback voltage of this moment;
Step (6): if DC feedback voltage increases, then return step (5), otherwise enter step (7);
Step (7): select the control voltage of next optical fiber squeezer, enter step (3).
Preferably, the RMS-DC converter circuit adopts effective value direct current transducer (RMS-DC) to calculate the real effective of pulse signal, and the effective value direct current transducer is measured in real time, finishes pulse voltage to the real-time conversion of direct voltage.
Preferably, at transmitting terminal, flashlight and local oscillator light are transmitted in same optical fiber by the mode of palarization multiplexing; At receiving terminal, by polarization beam apparatus the local oscillator light of palarization multiplexing and flashlight are divided into two-way after, local oscillator light is carried out light splitting.
More specifically, for achieving the above object, the technical scheme that a preferred embodiment of the present invention adopts is as follows:
Continuous variable quantum key distribution system with polarization feedback function of the present invention comprises transmitting terminal and receiving terminal two parts, and transmitting terminal and receiving terminal form by light path part and circuit part separately.Wherein said light path part mainly is comprised of laser, attenuator, polarization-maintaining beam splitter, polarization-maintaining coupler, faraday mirror, adjustable delay line, polarizing coupler, polarization beam apparatus, dynamic polarization controller, amplitude modulator and phase-modulator; Described circuit part is comprised of control circuit, quantum detector and the polarization control unit of truly random key generator, transmitting terminal and receiving terminal.
Transmitting terminal: laser produces pulse laser, is divided into two bundles by the 10:90 polarization-maintaining beam splitter after the attenuator decay, 10% a branch of as flashlight, 90% a branch of as local oscillator light.Behind flashlight process amplitude modulation(PAM) and the phase-modulation, decay to Quantum Level by attenuator, then enter in the optical fiber by polarizing coupler with local oscillator light and transmit.
Receiving terminal: the signal in the optical fiber is polarized beam splitter and is divided into flashlight and local oscillator light two bundles by behind the dynamic polarization controller.Flashlight directly enters polarization-maintaining coupler, and local oscillator light is divided into two bundles by the polarization-maintaining beam splitter of 10:90 first, 10% a branch of being used for as the polarization control signal, and all the other finish phase compensation by phase-modulator, do relevant the detection with flashlight.By regulating the adjustable delay line so that two unequal arm interference structure reaches balance.
The polarization control implementation method that the present invention proposes is divided into two stages, light splitting detection-phase and polarization recovery stage.Scheme is as follows:
The light splitting detection-phase: the receiving terminal of continuous variable quantum key distribution carries out polarization demultiplexing with the signal of receiving with polarizing coupler, afterwards the local oscillator light that obtains is told a part, by pulse conversion circuit pulsed optical signals is converted to d. c. voltage signal.
Pulse conversion circuit adopts photodiode that light pulse is converted into electric pulse.Because it is very little to be used for the light intensity of light splitting detection, electric impulse signal is very faint, therefore needs to use amplifier to amplify.Electric pulse after the amplification is processed with effective value converter (RMS-DC), is translated into direct voltage.
The polarization recovery stage: the dynamic polarization controller that the direct voltage of pulse conversion circuit output is fed back to receiving terminal, dynamic polarization controller is sampled to DC feedback voltage, and continue to regulate dynamic polarization controller by following polarization control algorithm, so that DC feedback voltage keeps maximum.
Dynamic polarization controller is controlled polarization state by the voltage on four optical fiber squeezers that change its inside.The key step of polarization feedback algorithm is as follows:
Step (1): dynamic polarization controller gathers DC feedback voltage;
Step (2): the control voltage of selecting first optical fiber squeezer;
Step (3): suitably increase voltage, gather the DC feedback voltage that feed back this moment;
Step (4): if DC feedback voltage increases, then return step (3), otherwise enter step (5);
Step (5): suitably reduce voltage, gather the DC feedback voltage of this moment;
Step (6): if DC feedback voltage increases, then return step (5), otherwise enter step (7);
Step (7): select the control voltage of next optical fiber squeezer, enter step (3).
By above process, DC feedback voltage can be remained on the maximum state always.This moment, the local oscillator light intensity of correspondence was the strongest, had namely realized the function of polarization control.
Compared with prior art, major advantage of the present invention is as follows:
1, fail safe aspect: because in the present invention, polarization control adopts the method for telling part local oscillator light, do not carry out light splitting and measurement for flashlight, according to the analysis theories of quantum key distribution, the present invention can not have any impact to key safety.
2, therefore the quantum signal of continuous variable quantum key distribution is subject to environmental interference especially easily owing to reached the quantum rank in the transmission course of fiber channel, causes the polarization drift, and then causes the increase of the error rate of system.Receiving terminal among the present invention separates local oscillator light and flashlight by polarization beam apparatus; Afterwards a part of local oscillator light is detected, to learn the degree of polarization drift; This part local oscillator luminous intensity is less, illustrates that the degree of drift is more serious; When this part local oscillator luminous intensity reaches maximum, be the state that polarization is compensated.Thereby can the environmental interference of establishment in the quantum key distribution process, so that polarization continues to be in stable state, reduced the error rate, strengthened Systems balanth.
Description of drawings
By reading the detailed description of non-limiting example being done with reference to the following drawings, it is more obvious that other features, objects and advantages of the present invention will become:
Fig. 1 is the block diagram according to the polarization compensation implementation method of continuous variable quantum key distribution system provided by the invention.
Among the figure: 100 is fiber channel, and 901 is the adjustable attenuator of transmitting terminal, and 902 is the faraday mirror of transmitting terminal, and 801 is the adjustable attenuator of receiving terminal, and 802 is faraday mirror, and 803 is true effective value converting circuit, and 804 is amplifier, and 805 is photodiode.
Embodiment
The present invention is described in detail below in conjunction with specific embodiment.Following examples will help those skilled in the art further to understand the present invention, but not limit in any form the present invention.Should be pointed out that to those skilled in the art, without departing from the inventive concept of the premise, can also make some distortion and improvement.These all belong to protection scope of the present invention.
In the present embodiment, described continuous variable quantum key distribution system polarization compensation implementation method, concrete steps are as follows:
(1) light splitting detection-phase: the receiving terminal of continuous variable quantum key distribution system with quantum signal by polarization beam apparatus, according to the character of polarisation of light theory and polarizing coupler, if polarization without drift, then local oscillator light will separate fully with flashlight.In the situation that has the polarization drift, local oscillator light can be revealed a part to the flashlight end, so that local oscillator light self-energy reduces.By a part of local oscillator light of telling is detected, can learn the drift degree of polarization.In the situation of polarized matching, it is maximum that the local oscillator light intensity reaches.Be best polarization state this moment.
It is very little to be used for the employed local oscillator luminous intensity of continuous variable quantum key distribution system, and each pulse has 10 approximately
8Individual photon is in the situation of 1MHz at the clock trigger rate, and power only has-48.9dBm.The light of telling accounts for 10% of local oscillator light energy, i.e. each pulse has an appointment 10
7Photon, power is-58.9dBm under the clock frequency of 1MHz.Through after the photodiode converts, the electric current that obtains very faint (being about 1uA) therefore needs amplifier that it is amplified.Amplifier is amplified to the pulse voltage signal that peak value is 500mV with weak current.
Pulse voltage signal after the amplification is converted to DC feedback voltage by pulse conversion circuit.The present invention adopts effective value direct current transducer (RMS-DC) to calculate the real effective of pulse voltage, output dc voltage.The effective value direct current transducer can be measured in very wide incoming frequency scope in real time, finishes pulse signal to the conversion of direct current signal.
(2) the polarization recovery stage: the direct voltage of pulse conversion circuit output feeds back to recipient's dynamic polarization controller, dynamic polarization controller is sampled to feedback voltage, and continue to regulate dynamic polarization controller by following polarization control algorithm, so that DC feedback voltage keeps maximum.
Dynamic polarization controller is controlled polarization state by voltage V1, V2, V3, the V4 on four optical fiber squeezers that change its inside, and the voltage-regulation scope is-and 12V is between+the 12V.The polarisation of light attitude is described with the point on the poincare sphere, and stokes parameter S1, S2, S3 correspondence the reference axis of poincare sphere.If increase voltage V1 or V3, then polarization state can turn clockwise around the S1 axle; On the contrary, if reduce V1 or V3, then polarization state can be rotated counterclockwise around the S1 axle.On the other hand, if increase voltage V2 or V4, then polarization state can turn clockwise around the S2 axle, if reduce V2 or V4, polarization state then can be rotated counterclockwise around the S2 axle.Thus, as long as the direction of input polarisation of light attitude and S1 and S2 out of plumb is all inputted so the polarisation of light attitude and can be changed to any one polarization state by 2 voltages of minimal manipulation.
Thus, the idiographic flow of a preferred polarization feedback algorithm is as follows:
Step (1): the single-chip microcomputer of dynamic polarization controller is to DC feedback voltage sample and record data;
Step (2): the control voltage of four optical fiber squeezers all is set to 0V, selects first optical fiber squeezer control voltage;
Step (3): suitably increase control voltage, polarization state can be on poincare sphere be done around the vector axle of correspondence and was clockwise rotated this moment, gathered the DC feedback voltage of this moment, if voltage has reached+12V, then skipped to step (7);
Step (4): if DC feedback voltage increases, then return step 3, otherwise enter step (5);
Step (5): suitably reduce to control voltage, polarization state can be on poincare sphere be done around the vector axle of correspondence and was rotated counterclockwise this moment, gathered the DC feedback voltage of this moment, if voltage has reached-12V, then skipped to step (7);
Step (6): if DC feedback voltage increases, then return step (5), otherwise enter step (7);
Step (7): select the control voltage of next optical fiber squeezer, enter step (3).
By above process, DC feedback voltage can be remained on the maximum state always.According to top analysis, at this moment corresponding local oscillator luminous intensity is the strongest, and namely local oscillator light does not leak, and polarization reaches optimum state.
The fail safe aspect: because in the present invention, polarization control adopts the method for telling part local oscillator light, does not carry out light splitting and measurement for flashlight, and according to the analysis theories of quantum key distribution, the present invention can not have any impact to key safety.
Therefore the quantum signal of continuous variable quantum key distribution is subject to environmental interference especially easily owing to reached the quantum rank in the transmission course of fiber channel, cause the polarization drift, and then causes the increase of the error rate of system.The present invention adopts technique scheme, establishment the environmental interference in the quantum key distribution process so that polarization continues to be in stable state, reduced the error rate, strengthened Systems balanth.
More than specific embodiments of the invention are described.It will be appreciated that the present invention is not limited to above-mentioned particular implementation, those skilled in the art can make various distortion or modification within the scope of the claims, and this does not affect flesh and blood of the present invention.