RU2840296C1 - System and method for compensating for time shift of optical pulses in quantum channels of quantum key distribution device with untrusted central node - Google Patents

Abstract

FIELD: quantum distribution of keys.

SUBSTANCE: invention relates to quantum key distribution with an untrusted central node (QKD UCN). Technical result is achieved by implementing a system for compensating for the time shift of optical pulses in quantum channels of a quantum key distribution device (QKD) with an untrusted central node (UCN), in which UCN contains a controlled optical delay line (ODL) placed in one of the input arms of the beam splitter, on which interference of input optical pulses occurs, wherein the controlled ODL is included in a feedback loop with single photon detectors by means of a control unit (CU) and is configured to control overlapping of laser pulses coming from transmitting units (TU) to the UCN with accuracy of up to a unit of picoseconds.

EFFECT: improved visibility of interference in real operating conditions of the device, thereby increasing the maximum distance between TU and increasing the speed of generating a secret key.

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Description

AREA OF TECHNOLOGY

[0001] This technical solution relates to the field of quantum key distribution with an untrusted central node.

PRIOR ART

[0002] Quantum key distribution with an untrusted central node (QKU NTs) [1] is a promising technology of quantum communications, which allows to get rid of all information leaks associated with the measuring device (single photon detectors - DOF) [2-4]. Of great importance in the QKU NTs protocol is the visibility of the interference of weak coherent states on the beam splitter of the NTs [5, 6]. To achieve high visibility, they strive to create indistinguishable quantum states: to get rid of the distinguishability of spectra, to reduce jitter and polarization mismatch of interfering pulses. Practical implementation of the QKU NTs system faces a number of additional difficulties associated with the remoteness of the transmitting units (TUs) from the NTs, which can be connected by quantum channels tens of kilometers long. In this case, it is necessary not only to take measures to ensure synchronization of the TUs and NTs, but also to take into account possible temperature fluctuations leading to a change in the lengths of the quantum channels ΔL. Fluctuations in ΔL, in turn, lead to fluctuations in the arrival time of optical pulses at the beam splitter.

[0003] This solution describes a system for compensating for the time shift of optical pulses in quantum channels of a quantum key distribution device with an untrusted central node. The proposed system is based on the use of an adjustable optical delay line in one of the NCU inputs, which is included in the feedback loop with the DOF and allows for the overlap of laser pulses coming from the PB to the NCU beam splitter to be controlled with an accuracy of up to a few picoseconds. This solution should significantly improve the visibility of interference in real operating conditions of the device, and therefore increase the maximum distance between the PB and increase the speed of secret key generation.

ESSENCE OF THE INVENTION

[0004] The claimed invention is aimed at solving a technical problem in terms of implementing an optical quantum key distribution scheme with an untrusted central node, which makes it possible to adjust the time shift of optical pulses arriving at the NCU.

[0005] The technical result consists in increasing the visibility of interference in real operating conditions of the device, due to which the maximum distance between the PBs increases and the speed of generating the secret key increases.

[0006] The claimed technical result is achieved by implementing the NCU KRK system, consisting of at least two transmitting units and a central node, while

Each PB is connected to the NCU via a fiber-optic quantum channel; the NCU contains:

a beam splitter that receives optical pulses from two PBs at the input and ensures their interference with subsequent entanglement of the quantum states of the pulses, and the beam splitter is connected to two single photon detectors (SPDs) that ensure measurement of the obtained quantum states in the Bell basis;

an adjustable optical delay line located in one of the input arms of the beam splitter.

[0007] The claimed technical result is also achieved by implementing a method for compensating for the time shift of optical pulses in the quantum channels of the QRC NCU device, while, when implementing the method

each transmitting unit forms between the trains of information signals calibration trains of optical signals in the form of weakened laser pulses with a given intensity, which are transmitted to the NCU;

The NCU receives calibration signals and, by changing the strobing time of the DOF and accumulating at each step clicks corresponding to a given number of calibration trains, restores the time profiles of the pulses that the PB prepares;

The control unit (CU) of the NCU processes the measured profiles of optical pulses and determines the time difference Δt of their arrival;

The BU transmits the measured value of Δt to the optical delay line, which shifts the optical path length of the signal of one of the transmitting units, thus minimizing the overlap inaccuracy of their time profiles.

BRIEF DESCRIPTION OF DRAWINGS

[0008] The accompanying drawings, which are included in this description to provide further understanding of the essence of the claimed solution and form part of it, illustrate embodiments and, together with the description, serve to explain the principles of implementation and operation of the claimed solution.

On the drawings;

Digital designations: 1 - untrusted central node (UCN), 2, 3 - transmitting units (TU1 and TU2), 4 - quantum channel, 5 - optical delay line, 6 - 50:50 beam splitter, 7, 8 - single photon detectors (SPD), 9 - control unit (CU), 10 - train of information pulses, 11 - train of calibration pulses.

[0009] Fig. 1 shows a diagram of a quantum key distribution device with an untrusted central node, which contains a system for compensating for the time shift of optical pulses in quantum channels.

[0010] Fig. 2 schematically shows trains of information and calibration pulses.

[0011] Fig. 3 schematically shows the profiles of optical pulses arriving from the transmitting units to the untrusted central node and measured using single photon detectors.

IMPLEMENTATION OF THE INVENTION

[0012] The circuit diagram of the QRC NCU device, which contains a system for compensating the time shift of optical pulses in quantum channels, is shown in Fig. 1. Transmitting units PB1 and PB2 (2, 3) prepare weak laser pulses and send them through quantum channels (4) to the NCU (1). The optical pulses meet at the beam splitter (6), where they interfere. After the beam splitter, the resulting quantum state is detected in the Bell basis using the DOF (7, 8). The signals from the DOF are processed in the control unit (9). The measurement results are then communicated to the transmitting units via classical authenticated channels; a secret key is generated based on these results.

[0013] In real conditions, it is necessary to take into account the uncontrolled time shift between the optical pulses arriving from PB1 and PB2 to the beam splitter of the NCU. This time shift is caused by the effect of thermal expansion of the optical fiber: when the temperature changes, the quantum channel can become slightly longer or shorter, which leads to a change in the pulse propagation time from the PB to the NCU. The degree of pulse overlap is thus a function of time and requires continuous adjustment. One possible way to implement such an adjustment is to use an adjustable optical delay line (OL3-5 in Fig. 1) in one of the input arms of the beam splitter.

[0014] To adjust the pulse arrival time on the fly, i.e. during the key distribution session, one can use time multiplexing, as shown in Fig. 2. Calibration pulse trains are placed between the information pulse trains. Each calibration train is a sequence of identical quantum states, i.e. a periodic sequence of identical laser pulses. First, calibration trains are sent by PB1. By changing the DOF gating time and accumulating clicks at each step corresponding to a given number of calibration trains, one can reconstruct the time profile of the pulse prepared by PB1. Then, the described procedure should be repeated with PB2. As a result, we obtain the time profiles of the pulses (see Fig. 3) prepared by the transmitting units, and we can determine the difference in their arrival times at the NCU, Δt.

[0015] The measured value of Δt is transmitted to the OLZ, which shifts the optical path length of the signal of one of the transmitter units, thus minimizing the inaccuracy of the overlap of their time profiles. If Δt exceeds the maximum value of the delay time that can be introduced using the OLZ, then a phase-locked loop on one of the TUs or another available phase shift mechanism on the TU can be additionally used.

Sources of information

[1] N.-K. Lo, M. Curty, and W. Qi, "Measurement-Device-Independent Quantum Key Distribution," Phys. Rev. Lett., vol. 108, pp.130503-130501-130503-130505, 2012.

[2] C. Wiechers, L. Lydersen, C. Wittmann, D. Elser, J. Skaar, C. Marquardt, V. Makarov, and G. Leuchs, “After-gate attack on a quantum cryptosystem,” New Journal of Physics, vol. 13, p. 013043,2011.

[3] L. Lydersen, N. Jain, C. Wittmann, 0. Maray, J. Skaar, C. Marquardt, V. Makarov, and G. Leuchs, “Superlinear threshold detectors in quantum cryptography,” Phys. Rev. A, vol. 84, p.032320, 2011.

[4] H. Weier, H. Krauss, M. Rau, M. Fiirst, S. Nauerth, and H. Weinfurter, “Quantum eavesdropping without interception: an attack exploiting the dead time of single-photon detectors,” New Journal of Physics, vol. 13, p.073024, 2011.

[5] Z. L. Yuan, M. Lucamarini, J. F. Dynes, B. Frohlich, M. B. Ward, and A. J. Shields, “Interference of Short Optical Pulses from Independent Gain-Switched Laser Diodes for Quantum Secure Communications,” Physical Review Applied, vol. 2, p.064006, 2014.

[6] L. C. Comandar, M. Lucamarini, B. Frohlich, J. F. Dynes, Z. L. Yuan, and A. J. Shields, “Near perfect mode overlap between independently seeded, gain-switched lasers,” Opt. Express, vol. 24, pp.17849-17859, 2016.

Claims (9)

1. A system for compensating for the time shift of optical pulses in quantum channels of a quantum key distribution (QKD) device with an untrusted central node (UCN), The NCU contains an adjustable optical delay line (OLD) located in one of the input arms of the beam splitter, on which interference of incoming optical pulses occurs, at the same time The adjustable OLZ is included in the feedback loop with single photon detectors via the control unit (CU) and is designed to control the overlap of laser pulses coming from the transmitting units (TU) to the NCU with an accuracy of up to units of picoseconds. 2. A method for compensating for the time shift of optical pulses in quantum channels of the QRC NCU device, performed using the system according to item 1, wherein when implementing the method: Each PB forms between the trains of information signals calibration trains of optical signals in the form of weakened laser pulses with a given intensity, which are transmitted to the NCU; The NCU receives calibration signals and, by changing the strobing time of the single photon detectors (SPD) and accumulating at each step clicks corresponding to a given number of calibration trains, restores the time profiles of the pulses that prepare the PB; The NCU BU processes the measured profiles of optical pulses and determines the time difference Δ t of their arrival; The BU transmits the measured value of Δ t to the optical delay line, which shifts the optical path length of the signal of one of the transmitting units, thus minimizing the overlap inaccuracy of their time profiles.