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arXiv:2007.02256 · npj Quantum Information

A universal, plug-and-play synchronisation scheme for practical quantum networks

Virginia D’Auria, Bruno Fedrici, Lutfi Arif Ngah, Florian Kaiser, Laurent Labonté, Olivier Alibart, Sébastien Tanzilli

Published Feb 10, 2020• 22 citations
arXivHTMLPDF

Abstract

Abstract We propose and experimentally demonstrate a plug-and-play, practical, and enabling method allowing to synchronise the building blocks of a quantum network in an all-optical way. Our scheme relies on mature and reliable classical telecommunication and non-linear optical technologies, and can be implemented in a universal way with off-the-shelf components. Compared to already reported solutions, it allows achieving high-quality synchronisation compatible with high network-operation rate and is free from opto-electronic jitters affecting servo-loop-based configurations. We test our scheme with a genuine quantum optical method in terms of the interference between two photons coming from two remotely synchronised sources spaced by distances of up to 100 km. Measured visibilities well above 90% confirm the validity of our approach. Due its simplicity and high-quality performance, our scheme paves the way for the synchronisation of long-distance quantum networks based on fibre, free-space, as well as hybrid solutions.

DOI
10.1038/s41534-020-0245-9

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We present an all-optical method for synchronizing remote quantum network stations dedicated to the generation or the storage of photonic entanglement. We apply our method to the connection of two sources emitting pairs of photons at telecom wavelengths. Our experimental setup is based on a pulsed telecom fiber laser operating at 2.5 GHz that pumps, after frequency doubling, two nonlinear waveguide photon-pair sources. Our synchronization scheme is validated via the implementation of a two-photon interference experiment performed at the relay station, i.e. where the Bell state measurement takes place. We show a raw visibility greater than 90%, obtained in the 4-fold coincidence counts, for a separation distance of 100 km between the photon- pair sources. This result demonstrates the relevance of our approach.
Precise time synchronisation underpins critical infrastructure from telecommunications and financial markets to power grids and scientific metrology. Several families of quantum protocols have been proposed and demonstrated for clock synchronisation and time distribution, exploiting entangled photon pairs, quantum key distribution (QKD) correlations, Hong-Ou-Mandel interference, and entangled clock networks. We critically assess these approaches, reviewing the main quantum time synchronisation (QTS) protocol families, quantifying the gap between theory and experiment, and identifying practical bottlenecks in sources, detectors, and channels. We survey the classical timing landscape from Network Time Protocol (NTP) and GPS to laboratory-grade optical frequency transfer, and compare quantum and classical methods at equivalent maturity. We examine use cases including financial trading, power grids, telecommunications, scientific metrology, and military applications, evaluating whether quantum timing offers a realistic advantage. We show that time transfer, not clock performance, is now the bottleneck for distributed optical timekeeping: the best demonstrated synchronisation uncertainty (2.46~ps) falls two to three orders of magnitude short of what optical clocks with fractional frequency uncertainties of 10−1810^{-18}10−18--10−1910^{-19}10−19 require. Our assessment is that quantum time synchronisation will not replace classical methods for most applications in the near-to-medium future. Its near-term value lies in physical-layer security against timing manipulation and integration with quantum communication networks, while closing the synchronisation gap for scientific metrology remains the most critical open challenge.
We report an all-optical realisation of a quantum relay experiment at a telecom wavelength. The main idea relies on utilizing a picosecond 2.5 GHz repetition rate telecom laser as a master clock to allow, thanks to dedicated distribution channels, the generation of synchronized entangled pairs of photons at remote locations. This is a necessary condition for achieving a reliable quantum relay function over a significant distance. We report a two-photon interference visibility at the relay station greater than 99%, validating our synchronization scheme at optical table scale, and paving the way for an extend of the communication distance.
Quantum networks involve entanglement sharing between multiple users. Ideally, any two users would be able to connect regardless of the type of photon source they employ, provided they fulfill the requirements for two-photon interference. From a theoretical perspective, photons coming from different origins can interfere with a perfect visibility, provided they are made indistinguishable in all degrees of freedom. Previous experimental demonstrations of such a scenario have been limited to photon wavelengths below 900 nm, unsuitable for long distance communication, and suffered from low interference visibility. We report two-photon interference using two disparate heralded single photon sources, which involve different nonlinear effects, operating in the telecom wavelength range. The measured visibility of the two-photon interference is 80 ± 4%, which paves the way to hybrid universal quantum networks.