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Synchronized Pump Lasers for Network-Compatible Sources of Indistinguishable Photons

Cory M. Nunn, N. Lal, Ivan A. Burenkov, Ya-Shian Li-Baboud, Paulina S. Kuo, Thomas Gerrits, Sergey V. Polyakov

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We demonstrate picosecond-level synchronization (≈3 ps) of two actively mode-locked Ti:Sapphire lasers via the White Rabbit Precision Time Protocol across 50 km of optical fiber, meeting the requirements of pump lasers for indistinguishable single-photon sources.
DOI
10.1364/cleo_at.2025.jps200_156

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We demonstrate picosecond-level synchronization of two actively mode-locked Ti:Sapphire lasers via the white rabbit precision time protocol (WR-PTP), tested over 120 km of deployed optical fiber. This synchronization capability, in combination with the highly single-mode, transform-limited pulses produced by each laser, enables their use as pump lasers for indistinguishable photon sources at remote locations in a quantum network. Here, the WR-PTP serves as a scalable network synchronization protocol, and its performance is compared to traditional methods of local synchronization. We demonstrate pulse-to-pulse synchronization better than 3 ps and time deviation (TDEV) values below 4 ps for all averaging times up to 10 s. With a designed coherence time of 35 ps for single photon sources utilizing these lasers, the achievable temporal overlap corresponds to 98% Hong-Ou-Mandel (HOM) interference visibility between independent sources.
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On-demand single photon sources with a high purity and indistinguishability are the key component for many quantum applications such as quantum cryptography, quantum computation and quantum metrology [1]. Quantum dots, which are usually pumped with picosecond titanium sapphire (Ti:Sa) lasers, have recently proven to be an excellent source for these single photons [2]. However, the use of Ti:Sa lasers requires an in-depth expertise of laser alignment, bulky cooling devices and a stabilized optical table, restraining the application of the generated single photons.
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.
Abstract Photonic quantum technology requires precise, time-resolved identification of photodetection events. In distributed quantum networks with spatially separated and drifting time references, achieving high precision is particularly challenging. Here we build on recent advances of using single-photons for time transfer and employ and quantify a fast postprocessing scheme designed to pulsed single-photon sources. We achieve an average root mean square synchronization jitter of 3.0 ps. The stability is comparable to systems with Rb vapor cell clocks with 19 ps at 1 s integration time, in terms of Allan time deviation. Remarkably, our stability is even better than classical high-precision time transfer, like the White Rabbit protocol, although we use significantly less signal (single-photon level). Our algorithms allow local processing of the data and do not affect the secure key rate. It compensates substantial clock imperfections from crystal oscillators and we foresee great potential for low signal scenarios. The findings are naturally suited to quantum communication networks and provide simultaneous time transfer without adding hardware or modifying the single-photon sources.