Method for enhancing security of a quantum key distribution communication line using a phase-coherent technique

Abstract

Method for enhancing security of a communication line for quantum key distribution, the method including the following steps: a) an encryption key is transmitted from an encryption device (4) to a quantum key distribution transmitter (6) from which the quantum key is distributed in the form of a photonic radiation to a first end of an optical fiber (10) for quantum key distribution, and at the same time a laser radiation having a different wavelength than the photonic radiation wavelength of the distributed quantum key is delivered from a laser (L) to a first end of an optical fiber (10) for quantum key distribution, b) radiation exiting a second end of the optical fiber (10) for quantum key distribution is split into photonic radiation of the distributed quantum key which is transmitted to quantum key distribution receiver (7), which transmits the respective encryption key to a decryption device (5), and laser radiation, which is reflected by means of a mirror back into the optical fiber (10) for quantum key distribution, wherein the reflected laser radiation is transmitted for evaluation from the first end of the optical fiber (10) for quantum key distribution, c) evaluation of unauthorized manipulation of the optical fiber (10) for quantum key distribution is performed based on evaluation of changes of phase of the reflected laser radiation relative to the phase of the laser radiation fed into the first end of the optical fiber (10) for quantum key distribution during step a).

Description

[0001] METHOD FOR ENHANCING SECURITY OF A QUANTUM KEY DISTRIBUTION

[0002] COMMUNICATION LINE USING A PHASE-COHERENT TECHNIQUE

[0003] Field of Art

[0004] The invention relates to a method for enhancing security of a physical layer of a communication line for distributing a quantum encryption key for symmetrical cryptography using a phase-coherent technique.

[0005] Background Art

[0006] Confidential data which is sent from location A to location B is encrypted at location A using an encryption key and decrypted at location B. The transmission of the encryption key from location A to location B can be performed via a separate optical fiber using a device for quantum key distribution that encodes the key into a quantum states of photons which are distributed through the quantum channel. Eavesdropping on such channels causes a loss of photons that indicates such eavesdropping. In a case only a small percentage of photons is removed, the photon loss could be evaluated as operational and thus could remain undetected. Neverthless, theoretically, the encryption key could be compromised even from the obtained incomplete information.

[0007] The objective of this invention is to increase the security of quantum encryption key distribution by increasing the ability to detect attempts to intercept quantum encryption key distribution on the communication line.

[0008] Summary of the Invention

[0009] The above specified aim is achieved by a method for enhancing security of a communication line for quantum key distribution, the method including the following steps: a) an encryption key is transmitted from an encryption device to a quantum key distribution transmitter, from which the quantum key is distributed in the form of a photonic radiation to a first end of an optical fiber for quantum key distribution, and at the same time a laser radiation having a different wavelength than the photonic radiation wavelength of the distributed quantum key is fed from a laser to a first end of an optical fiber for quantum key distribution, b) radiation exiting a second end of the optical fiber for quantum key distribution is split into photonic radiation of the distributed quantum key, which is transmitted to a quantum key distribution receiver, which transmits the respective encryption key to a decryption device, and laser radiation, which is in turn reflected by means of a mirror back into the optical fiber for quantum key distribution, wherein the reflected laser radiation is exiting the first end of the optical fiber for quantum key distribution for the sake of the evaluation, c) evaluation of unauthorized manipulation of the optical fiber for quantum key distribution is carried out on the basis of the evaluation of phase changes of the reflected laser radiation compared to the phase of the laser radiation delivered in step a) to the first end of the optical fiber for quantum key distribution.

[0010] Preferably, in step a), the laser radiation and the quantum key in the form of photonic radiation are coupled into the optical fiber for the quantum key distribution via a first optical add-drop multiplexer, and / or, in step b), the radiation exiting the second end of the optical fiber for quantum key distribution is split using a second add-drop multiplexer. Advantageously, in step a), the laser radiation from the laser is first fed to the input of the optical splitter, from which a first part of the laser radiation is fed to the first end of the optical fiber for quantum key distribution and a second part is fed to a first Faraday mirror from which it is reflected and brought through the optical splitter to a photodetector, wherein the laser radiation reflected in step b) and exits at the first end of the optical fiber for quantum key distribution is also brought to the mentioned photodetector, wherein the evaluation of phase changes of the reflected laser radiation passing through the optical fiber is performed by evaluating a beat-note frequency changes detected by the photodetector.

[0011] Also preferably, an encryption device is connected to the first communication system and a decryption device is connected to the second communication system, wherein the encryption device encrypts data using the encryption key that it distributes to the distribution transmitter in step a), and the encrypted data are transmitted via a communication line for transmitting encrypted data from the first communication system to the second communication system. Preferably, the encryption device changes the encryption key at least once every 24 hours, preferably at least once per hour, preferably at least once every 5 minutes, preferably at least once per minute, preferably once per second, wherein after each change of the encryption key, the newly changed encryption key is distributed in step a) and b) from the encryption device to the decryption device.

[0012] Preferably, a maximum permissible phase change of the reflected laser radiation compared to the laser radiation fed in step a) to the first end of the optical fiber for quantum key distribution is determined, and when a phase change larger than the specified maximum permissible phase change is detected in step c), the distribution of the encrypted data from the first communication system to the second communication system is stopped.

[0013] Advantageously, the wavelength of the laser radiation fed in step a) to the first end of the optical fiber for quantum key distribution differs from the wavelength of the photonic radiation of the distributed quantum key by at least 15 nm, preferably by at least 20 nm, and / or the wavelength of the photonic radiation of the distributed quantum key is in the range of 1500 to 1560 nm, preferably 1530 nm.

[0014] Advantageously, in step b), before the photonic radiation of the distributed quantum key is delivered to the distribution receiver, it is filtered by a bandpass to filter out any residual laser radiation.

[0015] The above-specified aim is also achieved by a device for quantum key distribution that incorporates the following: an encryption device, a quantum key distribution transmitter having its input connected to the output of the encryption device and being adapted to convert the encryption key to photonic radiation, first optical coupling element having its first input connected to the output of the quantum key distribution transmitter, optical fiber for quantum key distribution having its first end connected to the optical coupling element, second optical coupling element having its input connected to the second end of the optical fiber for quantum key distribution, quantum key distribution receiver having its input connected to the first output of the second optical coupling element, decryption device having its input connected to the output of the quantum key distribution receiver, a laser, an optical splitter and mirrors, wherein the laser has its output optically connected to an input of the optical splitter, which has a first output optically connected to a second input of the first optical coupling element and a second output is directed onto a first mirror, wherein a second output of the second optical coupling element is directed onto a second mirror, and a photodetector which is optically connected to the optical splitter for receiving radiation provided by reflection from the first mirror and by reflection from the second mirror.

[0016] Advantageously, the device includes an evaluation unit which is interconnected with the photodetector and adapted to evaluate phase changes of the laser radiation reflected from the second mirror compared to the phase of the laser radiation reflected from the first mirror. Preferably, the photodetector is adapted to detect a beat-note frequency between the laser radiation reflected from the second mirror and the laser radiation reflected from the first mirror.

[0017] Preferably, the device includes a frequency mixer which has its input connected to the photodetector and is adapted to demodulate beat-note frequencies from the photodetector to a useful signal.

[0018] It is also advantageous when the device includes a proportional-integral-derivative controller having its input connected to the frequency mixer, and a voltage-controlled oscillator having its input connected to the proportional- integral-derivative controller, and an acousto-optic modulator arranged for modulating the laser radiation transmitted from the optical splitter into the first optical coupling element.

[0019] Preferably, the evaluating unit is interconnected to the photodetector via the frequency mixer or via the frequency mixer and the proportional-integral-derivative controller.

[0020] In the device according to the invention, the acousto-optic modulator arranged for modulating the laser radiation transmitted from the optical splitter into the first optical coupling element shifts the frequency of the transmitted laser radiation only by a constant frequency offset and the compensation of phase fluctuations in the optical fiber for quantum key distribution is provided by a fiber coil (not shown in the drawings), the fiber coil having a variable length of optical path, wherein the length of the optical path is controlled by an output signal of the proportional-integral-derivative controller. Controlling the length of the optical path of the (not shown) fiber coil may be carried out preferably by a piezo-electric actuator, by an induction coil, motor driving unit, heating body, Peltier device.

[0021] The invention is based on the finding that due to external influences, such as mechanical disturbance and movement of the optical fiber, a phase change of the currently distributed laser radiation occurs in the optical fiber for photonic, i.e. quantum, encryption key distribution due to the Doppler effect. When the optical fiber is tampered with, the manipulation can be detected almost instantly from these phase changes and the distribution of the data encrypted with the given encryption key can be stopped.

[0022] Some changes may also occur as a result of weather conditions or as a result of shocks or vibrations caused by regular operation in the area through which the respective optical fiber passes. However, the impact of these common changes can be largely eliminated.

[0023] Brief Description of the Drawings

[0024] The invention is further described with reference to drawings, wherein FIG. 1 illustrates a diagram of an assembly for separate transmission of encrypted data and of the encryption key, utilizing which a method according to the invention is carried out, and FIG. 2 is a diagram of an exemplary embodiment of an electronic unit for compensation of delay fluctuation on a photonic connection according to this technical solution.

[0025] Exemplifying Embodiment of the Invention

[0026] The assembly shown in the drawing includes a first communication system 1 located at location A and a second communication system 2 located at location B, wherein these communication systems 1, 2 are mutually interconnected via a communication line 3 for transmitting encrypted data, e.g. via an optical fiber.

[0027] At location A, an encryption device 4 is connected to the communication system 1, and at location B, a decryption device 5 is connected to the communication system 2.

[0028] At location A, the encrypting device 4 is connected to an input of a quantum key distribution transmitter 6, the output of which is connected to a first optical add-drop multiplexer 8, to the output of which the optical fiber 10 for quantum key distribution is connected with its first end. At location B, a second end of the optical fiber 10 for quantum key distribution is connected through a second optical add-drop multiplexer 9 to a quantum key distribution receiver 7, the output of which is interconnected with the decryption device 5.

[0029] Thus, an interconnection is established between the first communication system 1 at location A and the second communication system 2 at location B, firstly by an independent communication line 3 for an encrypted data transmission, and secondly by an independent communication line for the distribution of the quantum key, which is intended for encryption and decryption of said data. The distance between location A and location B can be from tens of meters to a hundred kilometres. In an advantageous non-illustrated embodiment, there can be a third optical add-drop multiplexer arranged between the second optical add-drop multiplexer and the quantum key distribution receiver 7, which filters out any residual radiation from the laser L, which could otherwise infiltrate the quantum key distribution receiver 7. More add-drop multiplexers or optical filters can be advantageously arranged for an enhancement of the third optical adddrop multiplexer function and thus an even better elimination of the residual laser L radiation, which could otherwise infiltrate the quantum key distribution receiver 7.

[0030] At location A, the shown assembly further includes laser L providing a source of stable optical frequency, the wave of which, having an optical frequency vo, is transmitted via an optical fiber line, optical splitter OC for beam splitting arranged on the side of transmitting the signal, which is by its input connected to the output of the laser L, acousto-optic modulator AOM of the light allowing for the modification of the optical frequency of the transmitted signal “against” the influence of the Doppler effect, for example, with the same magnitude and opposite polarity as the fluctuation of the optical frequency created by the influence of the Doppler effect, first F araday mirror FM 1 , optical isolator I preventing back-reflections into the laser L, wherein the optical splitter OC is optically connected by its first output to the optical input of the above-mentioned acousto-optic modulator AOM and by its second output to the first Faraday mirror FM1. The acousto-optic modulator AOM is connected by its output to the second input of the optical add-drop multiplexer 8 mentioned above.

[0031] The second output of the second optical add-drop multiplexer 9 is optically focused on a second Faraday mirror FM2.

[0032] The assembly further includes: photodetector FD being connected by its input to the above-mentioned optical splitter OC and adapted for beat-note frequency detection between a wave reflected by the first Faraday mirror FM1 and a wave propagating through the optical fiber 10 for quantum key distribution and being reflected by the second Faraday mirror FM2, frequency mixer MIX that is connected by its input to the photodetector FD and adapted for demodulation of the beat-note frequency from the photodetector FD into a useful signal, proportional-integral-derivative controller PID being connected by its input to the frequency mixer MIX, voltage-controlled oscillator VCO being connected by its input to the controller PID and by its output to the acousto-optic modulator AOM and setting the frequency for the excitation of the acousto-optic modulator AOM so that, based on regulatory action, it generates, at a given moment, at its output an optical frequency adjusted (reduced or increased) by a value that corresponds to the immediate change in the optical frequency of the wave due to the Doppler effect, which propagates through the optical fiber 10 for quantum key distribution.

[0033] The assembly works as follows:

[0034] An encryption key is generated in the encryption device 4 and then used for the encryption of data being sent from the first communication system 1 to the second communication system 2 via the communication line 3.

[0035] At the same time, the encryption key is forwarded to the quantum key distribution transmitter 6 which converts it into a photonic radiation and transmits it to the first optical add-drop multiplexer 8, from which it is sent from location A by the optical fiber 10 for quantum key distribution to the second optical add-drop multiplexer 9 at location B. This photonic radiation is transmitted from the second optical add-drop multiplexer 9 to the quantum key distribution receiver 7, from which the encryption key is transmitted to the decryption device 5 that uses it to decrypt data transmitted by the communication line 3 to the second communication system 2.

[0036] Laser radiation from the laser L is coupled to the optical splitter OC where it is split into a first part which is transmitted through the acousto-optic modulator AOM to the first end of the optical fiber 10 for quantum key distribution, and a second part which is reflected back by the first Faraday mirror FM1 through the optical splitter OC onto the photodetector FD.

[0037] A signal is delivered from the photodetector FD to the frequency mixer MIX that is adapted to demodulate the beat-note frequency from the photodetector FD into a useful signal.

[0038] From the frequency mixer MIX, the signal is brought to the proportional-integral-derivative controller PID from which the signal is brought to the evaluation unit 11 for evaluating the differences in the phases of the laser radiation reflected from the first Faraday mirror FM1 and the second Faraday mirror FM2, and also to the voltage-controlled oscillator VCO, which has its output connected to the acousto-optic modulator AOM and which, based on regulatory action, is setting the frequency for the excitation of the acousto-optic modulator AOM so that at a given moment it generates at its output an optical frequency an adjusted (reduced or increased) by a value that corresponds to the immediate change in the optical frequency of the wave due to the Doppler effect, which propagates through the optical fiber 10 for quantum key distribution.

[0039] In an alternative non-illustrated embodiment, the signal is not delivered to the evaluation unit 11 from the proportional-integral-derivative controller PID but from the frequency mixer MIX.

[0040] The evaluation unit 11 performs an evaluation of an unauthorized manipulation with the optical fiber 10 for quantum key distribution. A test or initial operation reveals the nominal signal amplitude of the above-mentioned phase changes or the beat-note frequency, and a maximum permissible amplitude threshold of this signal is determined based on these findings. During routine operation, if the signal amplitude exceeds the determined maximum permissible amplitude, the encrypted data transmission between location A and B, i.e. between the first communication system 1 and the second communication system 2, is stopped. Consequently, the integrity of the optical fiber 10 for quantum key distribution must be checked, i.e. whether, for example, an eavesdropping device was installed or not.

[0041] During the operation, a compensation of Doppler shift is preferably performed, as the Doppler shift may be induced in the optical fiber 10 for quantum key distribution, which is caused by normal operating conditions. The compensation is performed by an instantaneous regulatory action using the acousto-optic modulator AOM, by which the value of the optical frequency of the coherent wave emitted from location A, specifically the wave fed to the first optical add-drop multiplexer 8, is regulated.

[0042] The acousto-optic modulator AOM shifts the optical frequency of the propagating coherent wave having the optical frequency vo by the value of the modulation frequency / AOM according to the formula:

[0043] V / = v0+ fA0M(1)

[0044] Due to the Doppler effect, the optical frequency VA of the emitted coherent wave from location A, or rather from the acousto-optic modulator AOM, which is propagated through the optical fiber 10 for quantum key distribution to location B is shifted to a value of wherein the value vs is an instantaneous shift of the wave caused by the optical fiber 10 for quantum key distribution. When the wave is reflected by the second Faraday mirror FM2 back to location A, there is additional frequency shift by the value vs of the original wave due to the Doppler effect in the optical fiber 10 for quantum key distribution, and after another pass through the acousto-optic modulator AOM, a coherent wave with a frequency VBA is gained, which is defined by a formula:

[0045] VBA =B+vs+ fA0M(3) which - by substituting the previous formulas - may be converted to the following form:

[0046] VBA = (Vo + / / 10M + ) +vs + fAOM =V0 + 2( / ^0M+Vs) (4)

[0047] If the reflected wave is mixed with the original coherent wave of the frequency vo, we obtain a beat-note radio frequency / beat, the value of which can be defined according to a formula:

[0048] Based on the knowledge of the beat-frequency frequency beat, it is possible to adjust the frequency of the transmitted laser radiation even before its emission so that the original wave frequency shift vs caused by the Doppler effect in the optical fiber 10 for quantum key distribution is fully compensated. The Doppler shift compensation is therefore possible via a quick change of the modulation frequency / AOM of the acousto-optic modulator AOM in a control loop by a value of a modulation frequency / s which is defined by a formula:

[0049] A = -vs= - fbeat ~ 2fAOM2 (6)

[0050] However, introducing the regulation offset / s into the acousto-optic modulator AOM must respect the length of the optical fiber 10 for quantum key distribution and thus also the transport delay introduced into the detected beat-note frequency / beat information. Thus, the entire control system operates in the mode of phase lock of the reflected wave from location B to location A to the wave of a stable laser normal, and the acousto-optic modulator AOM is connected here as an actuator of the control loop. The result of this compensation is the transport of the coherent wave to location B with an optical frequency VBcomp given by a formula: whereby the user acquires the original coherent wave with a highly suppressed frequency shift of the optical fiber 10, at location B. Systems based on this technique currently achieve a level of compensation of the Doppler shift induced in the fiber, which allows the transmission of a stable optical frequency with the relative stability of up to 10'17for an integration time of 100 s over distances of up to hundreds of kilometres.

[0051] In the case of use for very long optical fibers 10 for quantum key distribution, the change in the optical frequency of the wave propagating through this optical fiber 10 for quantum key distribution and being influenced by external influences is already frequency- shifted by the Doppler shift to such a degree that a quick action by the controller PID and debugging the input optical wave using the acousto-optic modulator AOM results in a phase slip. Only after the short-term external disturbance has subsided the phase lock will be re-locked because the wave frequency changes by the Doppler effect are already slower than the transport delay of the optical fiber 10. However, in case of a phase lock failure, the transmitted optical wave is not compensated correctly for the environmental influences for a certain period of time, and while measuring its frequency at location B, a short-term change in its frequency can be observed. Thus, a sufficiently frequency-stable transmission of this wave is not ensured. In such a case, using an electronic unit for delay compensation may be advantageous. A block scheme of such an electronic unit ensuring the desired function of delay compensation is shown in FIG. 2. In the illustrated embodiment, the electronic unit includes the first bandpass BP1 adapted to connect with the photodetector FD in order to pass only a high-frequency beat-note signal corresponding to the difference between a wave reflected by the first Faraday mirror FM1 and a wave propagating through the optical fiber 10 for quantum key distribution and being reflected by the second Faraday mirror FM2.

[0052] The electronic unit further includes the first amplifier Al having an input connected to the first bandpass BP1 and an output connected to the first input of the mixer MIX.

[0053] The above-mentioned electronic unit further includes a direct digital synthesis circuit DDS for generating reference signals LO1 and LO2. The first output of the direct digital synthesis circuit DDS is connected to the second input of the mixer MIX in order to feed the first reference signal LO1.

[0054] The output of the mixer MIX is connected to a second band pass BP2 having an output connected to a second amplifier A2, the output of which is connected to a programmable divider DDS:N.

[0055] Another component of the electronic unit is a phase detector PD having a first input connected to the programmable divider DDS:N and a second input being connected to the direct digital synthesis circuit DDS in order to feed the second reference signal LO2.

[0056] An output of the phase detector PD is connected to the input of the first analogue proportional-integral controller PID1 (i.e. the first proportional-integral-derivative controller) having an output connected to a voltage-controlled oscillator VCO, an output of which is connected to a first input of a voltage-controlled gain amplifier VGA.

[0057] An output of the voltage-controlled gain amplifier VGA is connected to an input of a third amplifier A3 having an output adapted for connecting with the acousto-optic modulator AOM (not shown in FIG. 2).

[0058] The shown electronic unit further includes a second proportional-integral controller PID2 (i.e. a second proportional-integral-derivative controller) with an output being connected to a second input of the voltage-controlled gain amplifier VGA, which is an input for gain control. An electrical signal from the photodetector FD corresponding to the detected intensity of both electromagnetic waves (reflected from the Faraday mirrors FM1, FM2) is fed to an input of said controller PID2 that keeps the intensity at a constant predetermined level in a closed control loop via changing the voltage at the voltage-controlled gain amplifier VGA, resulting in a change in the amplitude of the modulating signal for the acousto-optic modulator AOM (not shown in FIG. 2).

[0059] The illustrated embodiment of the electronic unit for compensating delay fluctuation on the photonic link works as follows:

[0060] Through a “RF IN” input, a high-frequency beat-note signal between the transmitted laser radiation (location A) and the laser radiation reflected back from the opposite end of the trajectory (location B) is fed from the photodetector FD. It is usually a beat-note frequency of 240 or 160 MHz. The bandpass BP1 selects a useful frequency spectrum from the input signal so that other spectral components of the signal from the photodetector FD do not cause error values in the detection chain. The first amplifier Al amplifies the useful signal to a level required for a proper functioning of the mixer MIX. Mixing with the reference signal LO1 translates the useful signal to a higher intermediate frequency (usually 400 MHz), thereby creating a signal IF1 required for a proper functioning of the programmable splitter DDS:N of the frequency. To improve the signal / noise ratio, this signal is filtered by the second bandpass BP2 before being amplified by the second amplifier A2.

[0061] For optical trajectories several hundreds of km long, the bandwidth of the beat-note signal IF1 is in the order of tens of kHz. However, due to transport delays in the order of milliseconds, the maximum bandwidth of the regulation of optical frequency shifts (drifts) on such a long fiber route is limited to tens to hundreds of Hz. Therefore, the signal IF1 must be adapted to the regulated transmission assembly. By dividing the frequency of the signal IF1 by the programmable splitter DDS:N with a division rate of 1 :N, an IF2 signal with an N-times smaller bandwidth with an N-times smaller center frequency is obtained. The programmable splitter DDS:N of the frequency is used to limit the bandwidth of the useful signal without losing information regarding its phase. Typically, having a signal IF1 with a frequency of 400 MHz and a bandwidth of approx. 30 kHz and dividing the frequency in a ratio of, e.g. 1 : 1600, a signal IF2 is obtained having a frequency of 250 kHz and a bandwidth of approx. 20 Hz, which is suitable for regulating the optical trajectory with a delay of 3 ms.

[0062] The phase detector PD compares the phase of the signal IF2 with the phase of the reference signal LO2. Assuming zero shifts in the optical frequency of the laser L when passing through the optical fiber 10 for quantum key distribution, the phase difference of these signals is constant; the phase lock loop principle can thus be used for compensating shifts in the transmitted optical frequency. The output of the phase detector PD is the regulation error signal VeiTwhich is further processed by an analogue low-noise controller, namely the proportional-integral controller PID1. The output of the first proportional-integral controller PID1 is a voltage signal controlling the voltage-controlled oscillator VCO. Then, in a closed control loop, where the acousto-optic modulator AOM is excited by the amplified signal from the voltage-controlled oscillator VCO, the frequency deviation of the voltage-controlled oscillator VCO precisely compensates for the shift in optical frequency caused by the optical path, i.e. the passage of the signal through the optical fiber 10.

[0063] When transmitting the stable optical frequency along the optical fiber 10, in addition to fluctuations in wavelength (i.e. optical frequency), it is also necessary to compensate for intensity fluctuation of transmitted coherent radiation on the transmitting side. The electrical signal from the photodetector FD corresponding to the detected intensity is fed to the input of the second proportional-integral controller PID2, which maintains the intensity at a constant predetermined level in a closed control loop. An output signal Vieveiof the second proportional-integral controller PID2 controls the gain of the variable-gain amplifier VGA which is connected to the output of the voltage-controlled oscillator VCO. The acousto-optic modulator AOM then compensates for the intensity shift of the coherent radiation caused by the optical path, i.e. the passage of the signal through the optical fiber 10 for quantum key distribution.

[0064] The design of the proportional-integral controllers PID1 and PID2 may be based on analogue circuits, and their parameters can be set by suitable values of electronic components that influence the value of the proportional, integral, and, possibly, even derivative terms of the control algorithm.

[0065] An alternative design of the proportional-integral controllers PID1 and PID2 is based on analogue-to-digital and digital-to-analogue converters; the control algorithm as such is implemented using a digital signal processor, and the values of the proportional, integral, and, possibly, even derivative terms of the control algorithm can be set by suitable values of the processor’s software variables via communication bus CAN from the master control system. As follows from the above, the method can be implemented using various specific procedures. In one embodiment, a stabilization of the optical frequency of the laser L is carried out using the acousto-optic modulator, the proportional-integral-derivative controller PID is turned on, and, using the output and control of the voltage-controlled oscillator VCO, it controls the acousto-optic modulator AOM and therefore compensates for the phase changes caused by external influences. The signal that is generated by the proportional-integral-derivative controller PID at its output is used to evaluate an attack on the optical fiber 10 for quantum key distribution.

[0066] In an alternative embodiment, the proportional-integral-derivative controller PID is not used, and the voltage-controlled oscillator VCO generates a stable signal and the acousto-optic modulator AOM shifts the frequency of the propagated optical wave by a constant frequency offset. In such a case, the signal vs from the frequency mixer MIX is used for evaluating the attack on the optical fiber 10 for quantum key distribution.

[0067] And in yet another embodiment, the delay in the optical fiber 10 is being compensated for again, but the acousto-optic modulator AOM is being excited by a harmonic oscillator (not shown) at a predetermined constant frequency to provide a frequency shift of the wave that goes from the laser L to the optical fiber 10 for quantum key distribution and then after being reflected from the second Faraday mirror FM2 back to the optical splitter OC and to the photodetector FD. Simultaneously, a fiber coil (not shown) with a possibility of length modulation, for example, using a piezo-electric transducer or temperature control, is advantageously placed after the first optical add-drop multiplexer 8 and before the input into the optical fiber 10 or directly in the optical fiber 10 for quantum key distribution. Such a fiber coil (with a length in the order of several tens of meters) has almost no attenuation and, therefore almost does not limit the communication over the optical fiber 10. Then, the piezoelectric element controlling the length of the fiber coil joins the phase compensation control of the optical fiber 10 at the input of the proportional-integral-derivative controller PID. The proportional-integral-derivative controller PID then generates a voltage signal that is amplified by the amplifier and fed to the piezo-electric transducer. Therefore, when the optical fiber 10 is extended due to mechanical stress, the control mentioned above ensures shortening of the fiber coil using the piezo-electric transducer; when the optical fiber 10 for quantum key distribution is shortened, the fiber coil is actively extended. Thus, the regular phase delay changes on the optical fiber 10 for quantum key distribution are compensated. At the same time, signals from the proportional-integral-derivative controller PID are evaluated to detect irregular changes that indicate fiber attack.

[0068] This solution advantageously has a highly positive influence on the quantum key distribution reliability, as in this case, the length of the optical trajectory in the optical fiber 10 is stabilized, and the transmitted quantum key information is not burdened by random fluctuations of changes in the length of this fiber thanks to the above-mentioned active phase stabilization. It can be expected that in such a case, a smaller error rate of the quantum key distribution can be achieved, or extending the distance (optical fiber 10 length), which is advantageous when connecting large distances between locations A and B with such a secure communication system.

[0069] Laser L is preferably a low-noise fiber laser or semiconductor laser operating in a telecommunication bandwidth (i.e. 1300 - 1700 nm) with its laser radiation having a spectral line with a width in the order of tenths of Hz to tens of kHz. In the case of a bi-directional communication requirement, the assembly is duplicated.

Claims

CLAIMS1. Method for enhancing security of a communication line for quantum key distribution; the method including the steps: a) an encryption key is transmitted from an encryption device (4) to a quantum key distribution transmitter (6), from which the quantum key is distributed in the form of a photonic radiation to a first end of an optical fiber (10) for quantum key distribution, and at the same time, laser radiation having a different wavelength than the photonic radiation wavelength of the distributed quantum key is fed from a laser (L) to a first end of an optical fiber (10) for quantum key distribution, b) radiation exiting a second end of the optical fiber (10) for quantum key distribution is split into photonic radiation of the distributed quantum key, which is transmitted to a quantum key distribution receiver (7), which transmits the respective encryption key to a decryption device (5), and laser radiation, which is reflected by means of a mirror back into the optical fiber (10) for quantum key distribution, wherein the reflected laser radiation is transmitted, for evaluation, from the first end of the optical fiber (10) for quantum key distribution, c) evaluation of unauthorized manipulation of the optical fiber (10) for quantum key distribution is performed based on evaluation of changes of the phase of the reflected laser radiation relative to the phase of the laser radiation fed into the first end of the optical fiber (10) for quantum key distribution during step a).

2. Method according to claim 1, characterized in that in step a), the laser radiation and the quantum key in the form of photonic radiation are fed into the optical fiber (10) for the quantum key distribution via a first optical add-drop multiplexer (8) and / or, in step b), the radiation exiting the second end of the optical fiber (10) for quantum key distribution is split using a second add-drop multiplexer (9).

3. Method according to claim 1 or 2, characterized in that, in step a), the laser radiation from laser (L) is brought first into an optical splitter (OC) from which a first part of thelaser radiation is coupled to the first end of the optical fiber (10) for quantum key distribution and a second part is coupled to a first Faraday mirror (FM1) from which it is reflected and passes through the optical splitter (OC) to a photodetector (FD), wherein the laser radiation reflected in step b) and exiting at the first end of the optical fiber (10) for quantum key distribution is also being brought to the said photodetector (FD), wherein the evaluation of phase changes of the reflected laser radiation passing through the optical fiber (10) is performed by evaluating changes in the beat-note frequency detected by the photodetector (FD).

4. Method according to any one of claims 1 to 3, characterized in that an encryption device (4) is connected to the first communication system (1) and a decryption device (5) is connected to the second communication system (2), wherein the encryption device (4) encrypts data using the encryption key that it distributes to the distribution transmitter (6) in step a), and the encrypted data is transmitted via a communication line (3) for transmitting encrypted data from the first communication system (1) to the second communication system (2).

5. Method according to any of claims 1 to 4, characterized in that the encryption device (4) changes the encryption key at least once every 24 hours, preferably at least once per hour, preferably at least once every 5 minutes, preferably at least once per minute, preferably once per second, wherein after each change of the encryption key, the newly changed encryption key is distributed in step a) and b) from the encryption device (4) to the decryption device (5).

6. Method according to claim 4 or 5, characterized in that a maximum permissible phase change of the reflected laser radiation compared to the laser radiation fed in step a) to the first end of the optical fiber (10) for quantum key distribution is determined, and when a phase change larger than the specified maximum permissible phase change is detected in step c), the distribution of the encrypted data from the first communication system (1) to the second communication system (2) is stopped.

7. Method according to any one of the preceding claims, characterized in that the wavelength of the laser radiation fed in step a) to the first end of the optical fiber (10) forquantum key distribution differs from the wavelength of the photonic radiation of the distributed quantum key by at least 15 nm, preferably by at least 20 nm, and / or the wavelength of the photonic radiation of the distributed quantum key is in the range of 1500 to 1560 nm, preferably 1530 nm.

8. Method according to any of the preceding claims, characterized in that, in step b), before being delivered into the distribution receiver (7), the photonic radiation of the distributed quantum key is filtered by a bandpass to filter out any residual laser radiation.

9. Device for quantum key distribution including: an encryption device (4), a quantum key distribution transmitter (6) having its input connected to the output of the encryption device and being adapted to convert the encryption key to photonic radiation, first optical coupling element having its first input connected to the output of the quantum key distribution transmitter (6), an optical fiber (10) for quantum key distribution having its first end connected to the optical coupling element, second optical coupling element having its input connected to the second end of the optical fiber (10) for quantum key distribution, quantum key distribution receiver (7), having its input connected to the first output of the second optical coupling element, decryption device (5), having its input connected to the output of the quantum key distribution receiver (7), a laser (L), an optical splitter (OC) and mirrors, wherein the laser (L) has its output optically connected to an input of the optical splitter (OC), which has a first output optically connected to a second input of the first optical coupling element and a second output is directed onto a first mirror, wherein a second output of the second optical coupling element is directed onto a second mirror, and a photodetector (FD) which is optically connected to the optical splitter (OC) for receiving radiation provided by reflection from the first mirror and by reflection from the second mirror.

10. Device according to claim 9, characterized in that the device includes an evaluation unit (11) which receives the signal from the photodetector (FD) and is adapted to evaluate phase changes of the laser radiation reflected from the second mirror compared to the phase of the laser radiation reflected from the first mirror.

11. Device according to claim 9 or 10, characterized in that the photodetector (FD) is adapted to detect a beat-note frequency between the laser radiation reflected from the second mirror and the laser radiation reflected from the first mirror.

12. Device according to claim 11, characterized in that it includes a frequency mixer (MIX) which has its input connected to the photodetector (FD) and is adapted to demodulate beat-note frequencies from the photodetector (FD) into a useful signal.

13. Device according to claim 12, characterized in that it includes a proportional-integral-derivative controller (PID) having its input connected to the frequency mixer (MIX), and a voltage-controlled oscillator (VCO) having its input connected to the proportional-integral-derivative controller (PID), and an acousto-optic modulator (AOM) arranged for modulating the laser radiation transmitted from the optical splitter into the first optical coupling element.

14. Device according to claim 12 or 13, characterized in that the evaluating unit (11) is signal connected to the photodetector (FD) via the frequency mixer (MIX) or via the frequency mixer (MIX) and the proportional-integral-derivative controller (PID).

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