Timing and synchronisation for high‐loss free‐space quantum communication with Hybrid de Bruijn Codes

Abstract

Satellite‐based, long‐distance free‐space quantum key distribution has the potential to realise global quantum secure communication networks. Detecting faint quantum optical pulses sent from space requires highly accurate and robust classical timing systems to pick out signals from the noise and allow for reconciliation of sent and received key bits. For such high‐loss applications, a fault‐tolerant synchronisation signal coding and decoding scheme based on de Bruijn sequences is proposed. A representative synchronisation timing system was tested in laboratory conditions and it demonstrated high fault tolerance for the error‐correction algorithm even under high loss. The performance limitations of this solution are also discussed, and the maximum error tolerance of the scheme and the estimated computational overhead are analysed, allowing for the possibility of implementation on a real‐time system‐on‐chip. This solution not only can be used for synchronisation of high‐loss channels such as channels between satellites and ground stations but can also be extended to applications with low loss, high bit error rate, but require reliable synchronisation such as quantum and non‐quantum communications over terrestrial free space or fibre optic channels.

Real-time gigahertz free-space quantum key distribution within an emulated satellite overpass

“…Alignment of the absolute time has been addressed by many studies, and a number of solutions have been proposed, including using synchronization beacons, global positioning systems, de Bruijn sequences, etc. (37)(38)(39). However, the implementation of this synchronization is outside the scope of this work and will be the focus of future research.…”

Modelling and experimental testing of an optical synchronisation beacon designed for high‐loss satellite quantum communication

“…Agnesi et al used a laser ranging system to launch a 55 ps laser pulse to a satellite at a distance of 7500 km [8] achieving a detection accuracy of 230 ps of the reflected photons, but requires the co-location of the transmitter and receiver and an internal clock network to gate the detection signal as well as a known target distance. Zhang et al proposed an encoding scheme based on a hybrid de Bruijn code which could directly decode the quantum signal index with high-loss tolerance and real-time synchronisation potential [9]. The newest work demonstrated by Berra also represents a synchronisation system using a bright laser, which is locked to the same master clock with a quantum signal [10].…”

Applications of combinatorics in coding theory

“…Recently, a special version of the de Bruijn sequence is used to synchronize the signals in quantum communication [64]. In a freespace quantum key distribution, synchronizing the transmitted and received signals accurately is a challenge.…”

“…Previous works on designing an e cient and reliable timing and synchronization system are in [30,17]. In [64], a new timing and synchronization is introduced using a beacon with the on-o↵ model. In this model, a sequence of beacon pulses is used to represent a binary de Bruijn sequence.…”

“…If one pulse slot represents a binary bit, for example, on is 1 and o↵ is 0, a long run of 0's in the sequence results in a long period of no-pulses. In [64], two pulse slots are used to represent a single bit, on-on is 1 and on-o↵ is 0, so the scheme can avoid two consecutive o↵ pulses. However, the above scheme requires 2N pulse slots to represent a de Bruijn sequence of length N .We observe that it is possible to use less redundant pulse slots to achieve both goals: to synchronize accurately and to avoid a long period of no-pulses.…”