We consider a subcarrier wave quantum key distribution (QKD) system, where quantum encoding is carried out at weak sidebands generated around a coherent optical beam as a result of electro-optical phase modulation. We study security of two protocols, B92 and BB84, against one of the most powerful attacks for this class of systems, the collective beam-splitting attack. Our analysis includes the case of high modulation index, where the sidebands are essentially multimode. We demonstrate numerically and experimentally that a subcarrier wave QKD system with realistic parameters is capable of distributing cryptographic keys over large distances in presence of collective attacks. We also show that BB84 protocol modification with discrimination of only one state in each basis performs not worse than the original BB84 protocol in this class of QKD systems, thus significantly simplifying the development of cryptographic networks using the considered QKD technique.

In this paper we propose feasibility demonstration of twin-field quantum key ditribution system based on multi-mode weak coherent phase-coded states. Their utilization provides indisputable advantages described in the paper. We also provide the detailed description of nontrivial interference scheme for those states and derive detection and quantum bit error rates. Since we propose the feasibility scheme we present in this paper only asymptotic secure key estimation and show that in principle it can beat well-known fundamental limit of repeaterless quantum communications , i.e., the secret key capacity of the lossy communication channel. Also we present here the experimental setup and provide the experimental values of detection rates dependence on the modulation signals phase difference.

We demonstrate qubit transmission via an optical cable in a metropolitan area network using quantum communications via sidebands with record-setting system parameters for such systems in network mode. The sifted-key generation rate was 1.06 Mbits/s with losses of 1.63 dB and a quantum error coefficient of 1%.

In this paper we investigate the advanced unambiguous state discrimination (USD) attack that is crucial for quantum key distribution based on phase-coded coherent states. Performing the attack Eve imposes all discriminated states while she can maintain both detection and error rates at Bob’s side. In this work we give the detailed description of the attack and propose the general approach for countermeasures. As the particular case we introduce the so-called ‘mousetrap’ (uninformative) state which allows to decrease the USD probability and preserve high sifted key rate. We also propose some candidates for ‘mousetrap’ states and provide the estimation on the discrimination probability for each of them.

In this paper, we consider the influence of device imperfections on the performance of the subcarrier wave quantum key distribution system. For instance, we consider the utilization of a non-phase-randomized coherent light source, the possibility of detector blinding, and its consequences. The common opinion is that the presence of a coherent light source in the system provides the possibility for a photon number splitting attack to an eavesdropper. We show that the attack is inapplicable or inefficient in the case of the considered states (without assumption of phase randomization). We also provide the conditions for estimation of a detector blinding attack. Moreover, we demonstrate that not only hardware countermeasures are applicable to revealing the attack, but also utilization of special protocol construction can be implied. Thus, we conclude that the imperfections of the considered devices can easily be leveled due to the features of implemented states and protocol construction.

In this paper, we compare two solutions (well known and newly proposed) for subcarrier wave quantum key distribution systems that decrease the crucial impact of the errorless unambiguous state discrimination attack in order to ensure security in channels with higher losses. One of the well-known methods is based on adding more signal states and allows a decrease in the probability of successful discrimination. However, sifting fractions will decrease too, leading to lower key rates. Thus, we propose a technique that utilizes additional vacuum state (unmodulated carrier and vacuum states at the sidebands) with low a priori sending probability and that provides the same decrease (in the first order) of errorless unambiguous discrimination as for four signal states while maintaining the same sifting fraction as for two states.