Two-dimensional distributed-phase-reference protocol for quantum key distribution

Davide Bacco, Jesper Bjerge Christensen, Mario A. Usuga Castaneda, Yunhong Ding, Søren Forchhammer, Karsten Rottwitt, Leif Katsuo Oxenløwe
2016 Scientific Reports  
Quantum key distribution (QKD) and quantum communication enable the secure exchange of information between remote parties. Currently, the distributed-phase-reference (DPR) protocols, which are based on weak coherent pulses, are among the most practical solutions for long-range QKD. During the last 10 years, long-distance fiber-based DPR systems have been successfully demonstrated, although fundamental obstacles such as intrinsic channel losses limit their performance. Here, we introduce the
more » ... t two-dimensional DPR-QKD protocol in which information is encoded in the time and phase of weak coherent pulses. The ability of extracting two bits of information per detection event, enables a higher secret key rate in specific realistic network scenarios. Moreover, despite the use of more dimensions, the proposed protocol remains simple, practical, and fully integrable. Sharing sensitive information has always been a great challenge within our society. In particular, QKD, first introduced by Bennett and Brassard, provides a unique procedure for exchanging a private key, based on the laws of quantum mechanics 1 . During the last decade, the effort from the scientific community has been focused on an enhancement of the quantum communication performances in terms of key rate, transmission distance and security aspects 2-9 . In later years this technology has matured enormously, but the lack of compact, efficient, inexpensive, and reliable systems, has restricted wide spreading of practical QKD systems. The basic idea behind QKD systems, in the case of "prepare and measure" schemes, is based on quantum states prepared by Alice (the transmitter) and sent through a quantum channel towards Bob (the receiver). Depending on the quantum measurement, Bob can deduce which state was prepared by Alice. This way, after error reconciliation and privacy amplification methods established in a classical channel, the two users share an identical bit sequence. Ideally, QKD systems are secure with no chance for an eavesdropper to extract information on the key. However, in real implementations of the systems, due to the losses and imperfections of devices, the secret key rate defines a bound on how much information can be assumed secure 10-12 . We here propose a new QKD protocol, which we refer to by the name: Differential phase time shifting (DPTS). In its essence, the protocol utilizes two degrees of freedom -time and phase -to encode information in a quaternary alphabet, i.e. {0, 1, 2, 3} 13 . The DPTS belongs to the family of distributed phase-reference (DPR) protocols, which rather than using the principle of random basis-choices between different mutually unbiased bases, encodes information in adjacent weak coherent pulses 6,10,14-18 . We study the performance of the DPTS protocol using infinite-key analysis in the case of collective attacks, and further show that the protocol holds great potential in intracity network scenarios. Results Principle of DPTS. As in most practical implementations of QKD, the DPTS protocol, which is sketched in Fig. 1 , uses a source of weak coherent pulses to establish a key of random numbers between two authenticated parties, Alice and Bob. To initiate the key distribution process, Alice randomly encodes information in the train of pulses in two dimensions, time and phase. The time encoding is performed using an intensity modulator (IM) as in the coherent-one way (COW) protocol 15 . For every pair of pulses (we refer to such a pair as a sub-block), one pulse is transmitted with mean photon number μ < 1 (|α〉 ), and one is blocked completely (|vac〉 ). Hence, within each sub-block, information is carried by the time-of-arrival of a non-empty pulse 15, 19 . The phase encoding
doi:10.1038/srep36756 pmid:28004821 pmcid:PMC5177871 fatcat:rqtjstkg7ngoxhpt4b2ls37lo4