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Laser Physics Letters

Security of quantum key distribution with a laser reference coherent state, resistant to loss in the communication channel

S. N. Molotkov, T. A. Potapova

Published Apr 23, 2015• 6 citations
Publisher

Abstract

The problem of quantum key distribution security in channels with large losses is still open. Quasi-single-photon sources of quantum states with losses in the quantum communication channel open up the possibility of attacking with unambiguous state discrimination (USD) measurements, resulting in a loss of privacy. In this letter, the problem is solved by counting the classic reference pulses. Conservation of the number of counts of intense coherent pulses makes it impossible to conduct USD measurements. Moreover, the losses in the communication channel are considered to be unknown in advance and are subject to change throughout the series parcels. Unlike other protocols, differential phase shift (Inoue et al 2002 Phys. Rev. Lett. 89 037902, Inoue et al 2003 Phys. Rev. A 68 022317, Takesue et al 2007 Nat. Photon. 1 343, Wen et al 2009 Phys. Rev. Lett. 103 170503) and coherent one way (Stucki et al 2005 Appl. Phys. Lett. 87 194108, Branciard et al 2005 Appl. Phys. Lett. 87 194108, Branciard et al 2008 New J. Phys. 10 013031, Stucki et al 2008 Opt. Express 17 13326), the simplicity of the protocol makes it possible to carry out a complete analysis of its security.

DOI
10.1088/1612-2011/12/6/065201

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The problem of quantum key distribution security in channels with large losses is still open. Quasi-single-photon sources of quantum states with losses in the quantum communication channel open up the possibility of attacking with unambiguous state discrimination (USD) measurements, resulting in a loss of privacy. In this letter, the problem is solved by counting the classic reference pulses. Conservation of the number of counts of intense coherent pulses makes it impossible to conduct USD measurements. Moreover, the losses in the communication channel are considered to be unknown in advance and are subject to change throughout the series parcels. Unlike other protocols, differential phase shift (Inoue et al 2002 Phys. Rev. Lett. 89 037902, Inoue et al 2003 Phys. Rev. A 68 022317, Takesue et al 2007 Nat. Photon. 1 343, Wen et al 2009 Phys. Rev. Lett. 103 170503) and coherent one way (Stucki et al 2005 Appl. Phys. Lett. 87 194108, Branciard et al 2005 Appl. Phys. Lett. 87 194108, Branciard et al 2008 New J. Phys. 10 013031, Stucki et al 2008 Opt. Express 17 13326), the simplicity of the protocol makes it possible to carry out a complete analysis of its security.
Since a source of quantum states is not strictly single-photon and losses exist in a communication channel, an attack with unambiguous measurements is possible, leading to loss of security. The problem of the stability of quantum key distribution protocols in a channel with large losses is still unsolved. A radical solution of this problem has been proposed by completely excluding unambiguous measurements by an eavesdropper. The problem is solved by means of counting of classical reference pulses. The conservation of the number of classical sync pulses results in the impossibility of unambiguous measurements. In this case, losses in the communication channel are considered as a priori unknown and can vary during a series of messages.
Since a strictly single-photon source is not yet available, in quantum cryptography systems, one uses, as information quantum states, coherent radiation of a laser with an average number of photons of μ ≈ 0.1–0.5 in a pulse, attenuated to the quasi-single-photon level. The linear independence of a set of coherent quasi-single-photon information states leads to the possibility of unambiguous measurements that, in the presence of losses in the line, restrict the transmission range of secret keys. Starting from a certain value of critical loss (the length of the line), the eavesdropper knows the entire key, does not make errors, and is not detected—the distribution of secret keys becomes impossible. This problem is solved by introducing an additional reference state with an average number of photons of μcl ≈ 103–106, depending on the length of the communication line. It is shown that the use of a reference state does not allow the eavesdropper to carry out measurements with conclusive outcome while remaining undetected. A reference state guarantees detecting an eavesdropper in a channel with high losses. In this case, information states may contain a mesoscopic average number of photons in the range of μ q ≈ 0.5–102. The protocol proposed is easy to implement technically, admits flexible adjustment of parameters to the length of the communication line, and is simple and transparent for proving the secrecy of keys.
We now have a safe way to communicate, thanks to quantum key distribution (QKD). In recent years there have been the emergence of brand-new challenges in the realm of QKD. Let us take a look at the analysis of the study on the topic of QKD. This research informs us that protected keys cannot be created when a weaker laser is used as the source of the photons, so let us look at that. The sequential unambiguous state discrimination (USD) attack, a variation of the photon number splitting (PNS) attack, is at fault for this issue. The mechanism used to generate keys must have a higher security level. An experiment based on superconducting single photon detector (SSPD) was reported, in which a differential phase shift of 10 GHz was applied to a clock utilized for QKD. We can produce protected keys using a 10-GHz clock system due to the minimal timing jitter and low 2 dark count rate provided by the SSPD. For us to scatter protected keys against sequential USD attacks across 200 km of fiber with a channel loss of 42.1 dB and general collective attacks for single photons, we needed to use thread. In addition, the protected key rate that we accomplished at 105 km was 17 kilobits per second, far greater than the previous record by two orders of magnitude. If we could do this, it would be a significant step forward in our capacity to understand the interrelationships of cities in real time. Installation of a QKD system and a worldwide QKD system using geostationary satellites. It is essential to consider that the 30-35 dB channel loss is enough for most geo-synchronized satellites.
Abstract Quantum key distribution (QKD) is a quantum technology which creates a common secret key between two distant users without any technological and computational assumptions about the eavesdropper. Nevertheless, not all the QKD protocols have tight upper and lower security bounds, thus the study of eavesdropping strategies and countermeasures is important. Using large number of symmetric coherent states is one of possible technologies in quantum key distribution against unambiguous state discrimination (USD) attack. Here we propose an attack based on heterodyne measurement, which is relatively simple for practical implementation and which becomes more powerful than USD attack when the number of states in the protocol is large enough. We study the performance of this attack for various symmetric coherent states QKD protocol parameters and for various distances between the legitimate users.
Quantum cryptography involves the fundamental question of the existence of secure protocols of quantum key distribution at any length of a communication channel (at any losses in it). A remarkable feature and novelty of the proposed protocol is that it ensures dual control; first, the intensity of a reference (intense) quantum state is controlled classically before the comparison (measurement) of coherent states from different time windows, and, second, the loss of coherence of a state is detected by interference measurements of an information quantum state and the reference quantum state tested against the conservation of the intensity. Thus, this is the only existing protocol stable to any losses in the communication channel. The length of the communication channel is limited only by dark noise in detectors.
A new attack on the quantum key distribution is proposed involving joint collective unambiguous measurements of reflected probe states from an intensity modulator and photon number splitting (PNS) measurements, i.e., nondestructive measurements of the number of information photons in the quantum communication channel. This attack does not change the relative statistics of photocounts of states with different numbers of photons, does not lead to errors on the receiver side, and, thereby, is not detected by any of known methods, including the modified decoy state method. The attack results only in additional losses in the communication channel, which are not “controlled” by the decoy state method. The dependence of introduced losses on the intensity of reflected probe states is estimated. The critical level of losses depends on a particular physical implementation of the quantum cryptography system, which determines the upper bound of the intensity of reflected probe states. The knowledge of this bound is fundamentally necessary to ensure the security of keys. The fact that the attack does not lead to errors on the receiver side and does not change the relative statistics of photocounts but only results in additional losses, which depend on the intensity according to the distinguishability of reflected probe states, does not mean that this attack transforms quantum cryptography systems from the type of cryptographic systems where the security of keys is guaranteed by fundamental quantum mechanical laws to the type of systems where this security is guaranteed by technical restrictions. Even in the presence of side channels of information leakage, the security of keys is still guaranteed by fundamental quantum mechanical restrictions on the distinguishability of states. A low level of distinguishability (“intensity”) of quantum states in side channels is naturally reached by technical tools.