The polarization compensation implementation method of continuous variable quantum key distribution system
Technical field
The present invention relates to a kind of technology 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, information transmission and exchange more and more frequent, therefore has more and more higher requirement to the safety and reliability of communicating information process.How safely transmission of information has become the major issue in modern communications field.Applied cryptography arises at the historic moment thus, widely using especially in World War II, has more embodied cryptographic importance.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 also called open key system, as the RSA Algorithm widely adopted now.Recipient first selects one group of private key only having him to know, calculates corresponding public-key cryptography successively, then is given by public-key cryptography so people is used for enciphered message.The fail safe of asymmetric cryptosystem depends on the complexity of calculating.Once quantum computer development is born, the speed of calculating will significant increase, and the difficulty cracking asymmetric cryptosystem will significantly reduce; Symmetric cryptosystem is also called private key system, and as generally acknowledged the one-time pad bag (One-Time-Pad) be perfectly safe, transmit leg and recipient have identical key.Key is only used once, and key length is not less than message length.Such fail safe is just guaranteed, and cost is reduction of the service efficiency of password.
Quantum cryptography communication is different from classical cryptographic communication, and its fail safe is the complexity of non-computational based on the fundamental characteristics of quantum physics.It is not utilize merely computational complexity that listener-in can not be broken a code in finite time, but utilize quantum-mechanical general principle and characteristic to find whether have listener-in to exist, establish with this Information Superiority contrasted between legitimate correspondence and listener-in, thus guarantee the safety of communication.Therefore, in today of quantum calculation development, study and realize quanta cryptology technique and be 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, be mainly divided into discrete variable quantum key distribution system and continuous variable quantum key distribution system.Wherein discrete variable quantum key distribution is realized by the mode of single photon.Due to also desirable single-photon source cannot be used now, therefore mainly by being decayed by coherent light, the average photon number of each pulse is made to be less than 1.The number of photons of lasing light emitter obeys Poisson distribution, except also containing multi-photon pulses containing except single photon pulses.Can not be discovered after these multi-photon pulses are ravesdropping, thus affected the fail safe of cryptographic system.Use and inveigle state (Decoy State) to address this problem to a great extent, but be the increase in the complexity of real system.
The starting of continuous variable quantum cryptography will a little later than discrete variable, until first Ralph in 1999 proposes to utilize continuous variable to carry out the concept of quantum key distribution.Different from discrete variable, continuous variable, by information coding (as regular position X and canonical momentum P) on continuous print canonical component, each bit can be encoded more information than discrete variable.The light source adopted can be coherent states, squeezed state or Entangled State.Consider convenience experimentally, generally all adopt coherent laser as light source.Continuous variable quantum key distribution adopts Homodyne detector as quantum detector, compared with discrete variable, has the advantage on cost.Therefore 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 of structure is easy to the impact being subject to various factors.These factors include but not limited to:
1. variations in temperature: the electrooptic modulator of lithium niobate material itself is more responsive for the change of temperature, unstable ambient temperature can produce larger deviation when modulating.
2. the mechanical oscillation suffered by system, mechanical oscillation can produce stress in a fiber, cause refractive index to change, and make the polarization state transmitted in a fiber produce drift.
3. the impact of air flowing.Air flowing can produce disturbance to the optical fiber not compacting fixing, causes polarization state to be drifted about.
4. the impact of electromagnetic radiation field: electromagnetic radiation (electromagnetic wave that such as mobile phone signal, alternating current produce) can produce obvious interference to the circuit part of system, and the interference received is exactly the most obviously quantum detector part.
In order to address these problems, allow continuous variable quantum key distribution system can have higher stability, we need to monitor the impact of environment, and are calibrated by the mode of feedback, system is made to have resistivity for environmental interference, and can long time continuous working.
Summary of the invention
For defect of the prior art, the object of the invention is to the blank for continuous variable quantum key distribution system polarization control scheme, propose a kind of brand-new polarization control scheme, advance the practical of continuous variable quantum cryptography, the environmental interference that the polarization that simultaneously restrained effectively continuous variable quantum signal in quantum communications process is subject to.
According to an aspect of the present invention, a kind of polarization compensation implementation method of continuous variable quantum key distribution system is provided, is specially:
At transmitting terminal: the pulse laser that laser produces is divided into flashlight and local oscillator light two bundle by polarization-maintaining beam splitter after attenuator attenuates; Make flashlight after amplitude modulation(PAM) and phase-modulation, by attenuator attenuates to Quantum Level, then entered in optical fiber by polarizing coupler together with local oscillator light and transfer to receiving terminal;
At receiving terminal: make in optical fiber the signal transmitting and after dynamic polarization controller, be divided into flashlight and local oscillator light two bundle by polarization beam apparatus; Flashlight is made directly to enter polarization-maintaining coupler; Local oscillator light is made first to be divided into two bundles by polarization-maintaining beam splitter, wherein, polarization control unit receives and exports polarization control signal according to wherein a branch of to dynamic polarization controller, another bundle completes phase compensation by phase-modulator, enters quantum detector do relevant detection together with flashlight by polarizing coupler.
Preferably, at receiving terminal: polarization control unit comprises the photodiode, amplifier and the RMS-DC converter circuit that connect successively, wherein, what separated by polarization-maintaining beam splitter by local oscillator light by photodiode is a branch ofly converted into electric pulse by light pulse, then amplify electric pulse by amplifier, then order amplify after electric pulse be converted into DC feedback voltage through RMS-DC converter circuit and output to dynamic polarization controller.
Preferably, dynamic polarization controller is controlled polarization state by the voltage changed on four optical fiber squeezers of 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 selecting first optical fiber squeezer;
Step (3): suitably increase control voltage, gathers the DC feedback voltage now fed back;
Step (4): if DC feedback voltage increases, then return step (3), otherwise enter step (5);
Step (5): suitably reduce control voltage, gathers DC feedback voltage now;
Step (6): if DC feedback voltage increases, then return step (5), otherwise enter step (7);
Step (7): the control voltage selecting next optical fiber squeezer, enters step (3).
Preferably, RMS-DC converter circuit adopts effective value direct current transducer (RMS-DC) to calculate the real effective of pulse signal, and effective value direct current transducer is measured in real time, completes the real-time conversion of pulse voltage to 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, after the local oscillator light of palarization multiplexing and flashlight being divided into two-way by polarization beam apparatus, light splitting is carried out to local oscillator light.
More specifically, for achieving the above object, the technical scheme that adopts of a preferred embodiment of the present invention is as follows:
The 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 are by respective light path part and circuit part composition.Wherein said light path part forms primarily 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 made up of the control circuit of truly random key generator, transmitting terminal and receiving terminal, quantum detector and polarization control unit.
Transmitting terminal: laser produce pulse laser, after attenuator attenuates, be divided into two bundles by 10:90 polarization-maintaining beam splitter, 10% a branch of as flashlight, 90% a branch of as local oscillator light.Flashlight, after amplitude modulation(PAM) and phase-modulation, by attenuator attenuates to Quantum Level, is then entered in optical fiber by polarizing coupler and transmits together with local oscillator light.
Receiving terminal: the signal in optical fiber, by after dynamic polarization controller, is polarized beam splitter and is divided into flashlight and local oscillator light two bundle.Flashlight directly enters polarization-maintaining coupler, and local oscillator light is first divided into two bundles by the polarization-maintaining beam splitter of 10:90,10% to be a branch ofly used for as polarization control signal, and all the other complete phase compensation by phase-modulator, doing relevant detection together with flashlight.Two unequal arm interference structure is made to reach balance by regulating adjustable delay line.
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:
Light splitting detection-phase: the signal polarizing coupler received is carried out polarization demultiplexing by the receiving terminal of continuous variable quantum key distribution, afterwards the local oscillator light obtained is separated 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 the light intensity detected for light splitting is very little, electric impulse signal is very faint, therefore needs to use amplifier to amplify.Electric pulse effective value converter (RMS-DC) after amplification processes, and is translated into direct voltage.
The polarization recovery stage: the dynamic polarization controller direct voltage that pulse conversion circuit exports being 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, make DC feedback voltage keep maximum.
Dynamic polarization controller is controlled polarization state by the voltage changed on four optical fiber squeezers of 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 selecting first optical fiber squeezer;
Step (3): suitably increase voltage, gathers the DC feedback voltage now fed back;
Step (4): if DC feedback voltage increases, then return step (3), otherwise enter step (5);
Step (5): suitably reduce voltage, gathers DC feedback voltage now;
Step (6): if DC feedback voltage increases, then return step (5), otherwise enter step (7);
Step (7): the control voltage selecting next optical fiber squeezer, enters step (3).
By above process, DC feedback voltage can be remained on maximum state always.The intensity of now corresponding local oscillator light is the strongest, namely achieves the function of polarization control.
Compared with prior art, major advantage of the present invention is as follows:
1, fail safe aspect: due in the present invention, polarization control adopts the method separating 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 the fail safe of key.
2, the quantum signal of continuous variable quantum key distribution is owing to reaching quantum rank, therefore in the transmitting procedure of fiber channel, is especially easily subject to environmental interference, causes polarization to drift about, and then causes the increase of the error rate of system.Local oscillator light and flashlight, by polarization beam apparatus, separate by the receiving terminal in the present invention; Afterwards a part of local oscillator light is detected, to learn the degree that polarization drifts about; 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.Thus effectively can suppress the environmental interference in quantum key distribution process, make polarization continue to be in stable state, reduce the error rate, enhance the stability of system.
Accompanying drawing explanation
By reading the detailed description done non-limiting example with reference to the following drawings, other features, objects and advantages of the present invention will become more obvious:
Fig. 1 is the block diagram of the polarization compensation implementation method according to continuous variable quantum key distribution system provided by the invention.
In figure: 100 is fiber channel, 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
Below in conjunction with specific embodiment, the present invention is described in detail.Following examples will contribute to those skilled in the art and understand the present invention further, but not limit the present invention in any form.It should be pointed out that to those skilled in the art, without departing from the inventive concept of the premise, some distortion and improvement can also be made.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 is by quantum signal by polarization beam apparatus, and the character of and polarizing coupler theoretical according to polarisation of light, if polarization is without drift, then local oscillator light will separate completely with flashlight.When there is polarization drift, local oscillator light can reveal a part to flashlight end, and local oscillator light self-energy is reduced.By detecting a part of local oscillator light separated, the drift degree of polarization can be learnt.When polarized matching, the intensity of local oscillator light reaches maximum.Now be best polarization state.
The local oscillator luminous intensity used for continuous variable quantum key distribution system is very little, and each pulse about has 10
8individual photon, when clock trigger rate is 1MHz, power only has-48.9dBm.The light separated accounts for 10% of local oscillator light energy, and namely each pulse has an appointment 10
7photon, under the clock frequency of 1MHz, power is-58.9dBm.After photodiode converts, the electric current obtained very faint (being about 1uA), therefore needs amplifier to amplify it.Weak current is amplified to the pulse voltage signal that peak value is 500mV by amplifier.
Pulse voltage signal after 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.Effective value direct current transducer can be measured in real time within the scope of very wide incoming frequency, completes the conversion of pulse signal to direct current signal.
(2) the polarization recovery stage: the direct voltage that pulse conversion circuit exports feeds back to the dynamic polarization controller of recipient, dynamic polarization controller is sampled to feedback voltage, and continue to regulate dynamic polarization controller by following polarization control algorithm, make DC feedback voltage keep maximum.
Dynamic polarization controller is controlled polarization state by voltage V1, V2, V3, the V4 changed on four optical fiber squeezers of its inside, and voltage regulation limits is between-12V to+12V.Point in polarisation of light state poincare sphere describes, and stokes parameter S1, S2, S3 correspond to the reference axis of poincare sphere.If increase voltage V1 or V3, then polarization state can turn clockwise around S1 axle; On the contrary, if reduce V1 or V3, then polarization state can be rotated counterclockwise around S1 axle.On the other hand, if increase voltage V2 or V4, then polarization state can turn clockwise around S2 axle, if reduce V2 or V4, polarization state then can be rotated counterclockwise around S2 axle.Thus, as long as direction all out of plumb of input polarisation of light state and S1 and S2, so input polarisation of light state and can change to any one polarization state by minimal manipulation 2 voltages.
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 is all set to 0V, selects first optical fiber squeezer control voltage;
Step (3): suitably increase control voltage, now polarization state can rotate clockwise by the vector axle around correspondence in poincare sphere, gathers DC feedback voltage now, if voltage reaches+12V, then skips to step (7);
Step (4): if DC feedback voltage increases, then return step 3, otherwise enter step (5);
Step (5): suitably reduce control voltage, now polarization state can rotate counterclockwise by the vector axle around correspondence in poincare sphere, gathers DC feedback voltage now, if voltage reaches-12V, then skips to step (7);
Step (6): if DC feedback voltage increases, then return step (5), otherwise enter step (7);
Step (7): the control voltage selecting next optical fiber squeezer, enters step (3).
By above process, DC feedback voltage can be remained on maximum state always.According to analysis above, at this moment corresponding local oscillator luminous intensity is the strongest, and namely local oscillator light does not leak, and polarization reaches optimum state.
Fail safe aspect: due in the present invention, polarization control adopts the method separating part local oscillator light, and 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 the fail safe of key.
The quantum signal of continuous variable quantum key distribution, owing to reaching quantum rank, is therefore especially easily subject to environmental interference, causes polarization to drift about in the transmitting procedure of fiber channel, and then causes the increase of the error rate of system.The present invention adopts technique scheme, effectively inhibits the environmental interference in quantum key distribution process, makes polarization continue to be in stable state, reduces the error rate, enhance the stability of system.
Above specific embodiments of the invention are described.It is to be appreciated that the present invention is not limited to above-mentioned particular implementation, those skilled in the art can make various distortion or amendment within the scope of the claims, and this does not affect flesh and blood of the present invention.