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arXiv:quant-ph/0311162 · AIP conference proceedings

Clock Synchronization based on Second-Order Quantum Coherence of Entangled Photons

Thomas B. Bahder

Published Jan 1, 2004• 26 citations
arXivHTMLPDF

Abstract

We present an algorithm for synchronizing two clocks based on second‐order quantum interference between entangled photons generated by parametric down‐conversion. The procedure is distinct from the standard Einstein two‐way clock synchronization method in that photon correlations are used to define simultaneous events in the frame of reference of a Hong‐Ou‐Mandel (HOM) interferometer. Once the HOM interferometer is balanced, by use of an adjustable optical delay in one arm, arrival times of simultaneously generated photons are recorded by each clock. Classical information on the arrival times is sent from one clock to the other, and a correlation of arrival times is done to determine the clock offset.

DOI
10.1063/1.1834461

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We present an algorithm for synchronizing two clocks based on second‐order quantum interference between entangled photons generated by parametric down‐conversion. The procedure is distinct from the standard Einstein two‐way clock synchronization method in that photon correlations are used to define simultaneous events in the frame of reference of a Hong‐Ou‐Mandel (HOM) interferometer. Once the HOM interferometer is balanced, by use of an adjustable optical delay in one arm, arrival times of simultaneously generated photons are recorded by each clock. Classical information on the arrival times is sent from one clock to the other, and a correlation of arrival times is done to determine the clock offset.
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Quantum clock synchronization (QCS) aims to establish a shared temporal reference between distant nodes by exploiting uniquely quantum phenomena such as entanglement, single-photon interference, and quantum correlations. In contrast to classical synchronization and time-transfer techniques, which are limited by signal propagation delays, atmospheric disturbances, and oscillator drift, QCS protocols offer the potential to surpass classical precision bounds and enhance resilience against adversarial manipulations. As precise and secure time synchronization underpins distributed quantum networks, navigation systems, and emerging quantum Internet infrastructures, understanding QCS principles, capabilities, and implementation challenges has become increasingly important. This survey provides a unified and critical overview of the rapidly growing QCS research landscape, highlighting fundamentals, protocol types, enabling resources, performance constraints, security considerations, and practical implementations of QCS. We first introduce the theoretical underpinnings of QCS, including entanglement-assisted time transfer, Hong-Ou-Mandel interference-based synchronization, and quantum slow-clock transport. We then categorize the main QCS protocols, ranging from ticking-qubit and entanglement-based schemes to time-of-arrival correlation methods, conveyor-belt synchronization, and quantum-enhanced two-way time transfer. This organization clarifies the relationships between protocol families and their achievable precision advantages over classical methods. Key quantum resources such as spontaneous parametric down-conversion-based entangled photon pairs, Greenberger-Horne-Zeilinger and W multipartite states, squeezed and frequency-entangled light, quantum frequency combs, and quantum memories are reviewed in the context of scalability and robustness.
We present a source of indistinguishable photons at telecom wavelength, synchronized to an external clock, for the use in distributed quantum networks. We characterize the indistinguishability of photons generated in independent parametric down-conversion events using a Hong-Ou-Mandel interferometer, and show non-classical interference with coalescence, C = 0.83(5). We also demonstrate the synchronization to an external clock within sub-picosecond timing jitter, which is significantly shorter than the single-photon wavepacket duration of ≈ 35 ps. Our source enables scalable quantum protocols over multi-node, long-distance optical networks using network-based clock recovery systems.