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Quantum Computing, Communication, and Simulation

Indistinguishability between arbitrary signal and idler photons for scalable quantum networks

Nijil Lal Cheriya Koyyottummal, Ivan A. Burenkov, Paulina S. Kuo, Oliver Slattery, Sergey V. Polyakov

Published Mar 19, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.3042508

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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.
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.
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 characterize an entangled-photon-pair source that produces signal and idler photons at 1533 nm and 1567 nm using fiber-assisted signal-photon spectroscopy. By erasing the polarization distinguishability, we observe interference between the two down-conversion paths. The observed interference signature is closely related to the spectral correlations between photons in a Hong-Ou-Mandel interferometer. These measurements suggest good indistinguishability between the two down-conversion paths, which is required for high entanglement visibility.
We have developed an entangled photon pair source based on a domain-engineered, type-II periodically poled lithium niobate crystal that produces signal and idler photons at 1533 nm and 1567 nm. We characterized the spectral correlations of the generated entangled photons using fiber-assisted signal-photon spectroscopy. We observed interference between the two down-conversion paths after erasing polarization distinguishability of the down-converted photons. The observed interference signature is closely related to the spectral correlations between photons in a Hong- Ou-Mandel interferometer. These measurements suggest good indistinguishability between the two downconversion paths, which is required for high entanglement visibility.
We implement an ultrafast pulsed type-II parametric down conversion source in a periodically poled KTP waveguide at telecommunication wavelengths with almost identical properties between signal and idler. As such, our source resembles closely a pure, genuine single mode photon pair source with indistinguishable modes. We measure the joint spectral intensity distribution and second order correlation functions of the marginal beams and find with both methods very low effective mode numbers corresponding to a Schmidt number below 1.16. We further demonstrate the indistinguishability as well as the purity of signal and idler photons by Hong-Ou-Mandel interferences between signal and idler and between signal/idler and a coherent field, respectively. Without using narrowband spectral filtering, we achieve a visibility for the interference between signal and idler of 94.8% and determine a purity of more than 80% for the heralded single photon states. Moreover, we measure raw heralding efficiencies of 20.5% and 15.5% for the signal and idler beams corresponding to detector-loss corrected values of 80% and 70%. (C) 2013 Optical Society of America