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Quantum Nanophotonic Materials, Devices, and Systems

Remote clock synchronization with entangled photon pairs (Conference Presentation)

Christian Kurtsiefer, Jianwei Lee

Published Sep 10, 2019• 0 citations

Abstract

Photon pairs generated in spontaneous parametric downconversion exhibit a strong correlation in time, which has been used to synchronize spatially separated clocks in the past in quantum key distribution implementations to a high accuracy. A symmetric version of this scheme can be used to allow for an absolute clock synchronization, and the entanglement properties of photons generated in spontaneous down conversion allows to strengthen the authenticity of the received signals through entanglement witnesses. Aspects of this time synchronization technique with respect to use in optical fibers and remaining vulnerabilities are discussed. [1] C. Ho, A. Lamas-Linares, and C. Kurtsiefer: Clock synchronization by remote detection of correlated photon pairs, New Journal of Physics 11, 045011 (2009). [2] J. Lee, L. Shen, A. Cerè, J. Troupe, A. Lamas-Linares, C. Kurtsiefer: Symmetrical clock synchronization with time-correlated photon pairs, Appl. Phys. Lett. 114, 101102 (2019).

DOI: 10.1117/12.2531739

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Photon pairs generated in spontaneous parametric downconversion exhibit a strong correlation in time, which has been used to synchronize spatially separated clocks in the past in quantum key distribution implementations to a high accuracy. A symmetric version of this scheme can be used to allow for an absolute clock synchronization, and the entanglement properties of photons generated in spontaneous down conversion allows to strengthen the authenticity of the received signals through entanglement witnesses. Aspects of this time synchronization technique with respect to use in optical fibers and remaining vulnerabilities are discussed. [1] C. Ho, A. Lamas-Linares, and C. Kurtsiefer: Clock synchronization by remote detection of correlated photon pairs, New Journal of Physics 11, 045011 (2009). [2] J. Lee, L. Shen, A. Cerè, J. Troupe, A. Lamas-Linares, C. Kurtsiefer: Symmetrical clock synchronization with time-correlated photon pairs, Appl. Phys. Lett. 114, 101102 (2019).

In this study, we present an algorithm to detect the time and frequency differences of independent clocks based on observation of time-correlated photon pairs. This enables remote coincidence identification in entanglement-based quantum key distribution schemes without dedicated coincidence hardware, pulsed sources with a timing structure or very stable reference clocks. We discuss the method for typical operating conditions and show that the requirement for reference clock accuracy can be relaxed by about five orders of magnitude in comparison with previous schemes.

Based on the second-order quantum interference between frequency entangled photons that are generated by parametric down conversion, a quantum strategic algorithm for synchronizing two spatially separated clocks has been recently presented. In the reference frame of a Hong-Ou-Mandel (HOM) interferometer, photon correlations are used to define simultaneous events. Once the HOM interferometer is balanced by use of an adjustable optical delay in one arm, arrival times of simulta- neously generated photons are recorded by each clock. The clock offset is determined by correlation measurement of the recorded arrival times. Utilizing this algorithm, we demonstrate a proof-of-principle experiment for synchronizing two clocks separated by 4 km fiber link. A minimum timing stability of 0.44 ps at averaging time of 16000 s is achieved with an absolute time accuracy of 73.2 ps. The timing stability is verified to be limited by the correlation measurement device and ideally can be better than 10 fs. Such results shine a light to the application of quantum clock synchronization in the real high-accuracy timing system.

We demonstrate a point-to-point clock synchronization protocol based on bidirectionally exchanging photons produced in spontaneous parametric down conversion. The technique exploits tight timing correlations between photon pairs to achieve a precision of 51 ps in 100 s with count rates of order 200 s−1. The protocol is distance independent, is secure against symmetric delay attacks, and provides a natural complement to techniques based on Global Navigation Satellite Systems. The protocol works with mobile parties and can be augmented to provide authentication of the timing signal via a Bell inequality check.

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.