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Optics Express

Synchronized source of indistinguishable photons for quantum networks

Nijil Lal, Ivan A. Burenkov, Ya-Shian Li-Baboud, M. V. Jabir, Paulina S. Kuo, Thomas Gerrits, Oliver Slattery, Sergey V. Polyakov

Published Apr 15, 2024• 12 citations
Publisher

Abstract

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.

DOI
10.1364/oe.521083

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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.
We build a network source of indistinguishable photons, synchronized to an external clock, that could be used as scalable modular unit in an extended quantum network infrastructure. We characterize the indistinguishability and synchronization of this source. The Hong-Ou-Mandel dip shows near unity indistinguishability. Allan deviation analysis shows sub-picosecond jitter when locked to an external clock. The jitter figure is more than 10 times smaller than the pulse duration of 30ps. The source is compatible with many clock recovery systems, including the White Rabbit Precision Time Protocol (WR-PTP). This source enables scalable quantum protocols over multi-node, long-distance optical networks.
Using a network-compatible source of single photons, we study the interference of heralded signal and idler photons that are not part of the same biphoton pair. We characterize the indistinguishability of such arbitrary single photons that are generated in independent parametric down-conversion events using a Hong-Ou-Mandel interferometer, and show non-classical coalescence, C = 0.85(9). Separately, we measure coalescence of multi-mode thermal fields corresponding to the signal and idler photons and characterize their indistinguishability. This source exhibits ubiquitous indistinguishability between any photons that it generates and thus enables the interchangeable use of signal and idler photons in multi-node, photon-interference-based protocols for scalable quantum networks. The indistinguishability between arbitrary photons from such sources enables requirement-driven entanglement to be generated in nodes within a quantum network.
Two-photon interference, or the indistinguishability of single photons, such as those from spontaneous parametric down-conversion, is required for scalable quantum networks. Here, we study the indistinguishability between signal and idler photons that are not members of the same bi-photon pair, thereby enabling the scalable and interchangeable use of any photon from any of the conjugated paths of down-conversion. We build a Fourier transform-limited source of indistinguishable photons in the telecom C-band that generates identical signal and idler photons distinguished only by polarization. Using a Hong-Ou-Mandel interferometer, we characterize the indistinguishability between signal and idler photons from independent down-conversion events and compare that to the indistinguishable biphotons. Achieving successful two photon interference between signal and idler photons from independent trials enables our source as a modular unit for practical quantum networks.
Time synchronization is a fundamental requirement in entanglement-based quantum networks, where the indistinguishability of photons in the time domain is essential for enabling Hong-Ou-Mandel interference and entanglement swapping. In addition to precise temporal alignment, it is equally crucial to ensure the stability of the reference clock over time, as even small fluctuations can degrade the overall performance of the network. In this work, we investigate the stability of clock synchronization for entanglement distribution based on entangled-photon sources operating in the telecommunication C-band. Temporal correlations between photon detection events are analyzed using time-tagged coincidence measurements, enabling the extraction of synchronization peaks and their long-term stability. Experimental results demonstrate that, once locked, the sources exhibit stable temporal correlations over an 8-hour acquisition period, with a maximum observed drift of approximately 120 ps, primarily associated with long fiber links. The width of the correlation peak remains consistent with detector jitter, indicating negligible additional system-induced temporal broadening. A central result of this work is the experimental synchronization between two entanglement-photon sources in a realistic metropolitan deployment, where entanglement distribution is performed over existing telecommunication infrastructure characterized by non-negligible losses and background noise. In this scenario, despite the presence of significant imperfections introduced by the metropolitan-scale fiber network, the observed correlations remain clearly detectable and are consistently well-approximated by Gaussian statistics. This confirms that the two sources can be reliably synchronized not only in controlled laboratory conditions but also under real-world operating constraints.
Advanced quantum communication protocols require high-visibility quantum interference between photons generated at distant nodes, which places stringent demands on optical synchronization. Conventionally, synchronization of optical wave packets relies on pulsed sources and precise optical path stabilization. An alternative approach employs continuous-wave (CW) photon-pair sources, where temporal indistinguishability is enforced by post-selecting detection events within a coincidence window τwτ_wτw​ shorter than the photon coherence time TcT_cTc​. Despite its conceptual simplicity, the quantitative relation between relevant time scales, achievable interference visibility, and usable multi-photon rates has remained unclear. Here, we develop in detail and experimentally validate a theoretical framework that quantitatively describes time-resolved multi-photon interference in the CW regime. We explicitly incorporate detector timing jitter, photon coherence time, and temporal post-selection. The model is verified using four-photon Hong-Ou-Mandel interference measurements. Based on this validated framework, we determine the coincidence window that maximizes usable four-photon rates for a target visibility. Finally, we compare CW and pulsed SPDC sources under equivalent indistinguishability constraints and show that CW operation can achieve comparable rates while relaxing optical synchronization requirements.