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arXiv:2403.12866 · Physical Review Letters

Purifying Photon Indistinguishability through Quantum Interference

Carlos F. D. Faurby, Lorenzo Carosini, Huan Cao, Patrik Isene Sund, L. M. Hansen, Francesco Giorgino, Andrew B. Villadsen, S. N. van den Hoven, Peter Lodahl, Stefano Paesani, J. C. Loredo, Philip John Walther

Published Jul 18, 2024• 13 citations
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Abstract

Indistinguishability between photons is a key requirement for scalable photonic quantum technologies. We experimentally demonstrate that partly distinguishable single photons can be purified to reach near-unity indistinguishability by the process of quantum interference with ancillary photons followed by heralded detection of a subset of them. We report on the indistinguishability of the purified photons by interfering two purified photons and show improvements in the photon indistinguishability of 2.774(3)% in the low-noise regime, and as high as 10.2(5)% in the high-noise regime.

DOI
10.1103/physrevlett.133.033604

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Indistinguishability between photons is a key requirement for scalable photonic quantum technologies. We experimentally demonstrate that partly distinguishable single photons can be purified to reach near-unity indistinguishability by the process of quantum interference with ancillary photons followed by heralded detection of a subset of them. We report on the indistinguishability of the purified photons by interfering two purified photons and show improvements in the photon indistinguishability of 2.774(3)% in the low-noise regime, and as high as 10.2(5)% in the high-noise regime.
High-visibility quantum interference (indistinguishability) among single photons is the key to scalable, high-fidelity linear optical quantum gates. Measuring indistinguishability by interference is laborious and time-consuming, though, and thus not scalable. The authors find that the faster, simpler second-order correlation functions provide results equivalent to the indistinguishability, and could be useful in rapid-prototyping source design of large-scale photonic circuits. Also, for mature guided-wave integrated optics such as silicon photonics, the high-visibility bottleneck is due to the physics of a process that tends to produce single photons in multiple spectral modes.
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Photonic quantum technology provides a viable route to quantum communication, quantum simulation, and quantum information processing. Recent progress has seen the realisation of boson sampling using 20 single-photons and quantum key distribution over hundreds of kilometres. Scaling the complexity requires photonic architectures containing a large number of single photons, multiple photon-sources and photon-counters. Semiconductor quantum dots are bright and fast sources of coherent single-photons. For applications, a significant roadblock is the poor quantum coherence upon interfering single photons created by independent quantum dots. Here, we demonstrate two-photon interference with near-unity visibility using photons from remote quantum dots. Exploiting the quantum interference, we demonstrate a photonic controlled-not circuit and a high-fidelity entanglement between photons of different origins. Our results provide a long-awaited solution to the challenge of creating coherent single-photons in a scalable way. In the near future, they point to a demonstration of quantum advantage using quantum-dot single photons and an implementation of device-independent quantum key distribution.
We report on two-photon interference of highly indistinguishable single photons emitted by a quantum dot. Strictly resonant excitation with picosecond laser pulses has been used to prepare coherent states with a significantly increased coherence time $({T}_{2}\ensuremath{\sim}1\text{ns})$ and reduced lifetime $({T}_{1}\ensuremath{\sim}650\text{ps})$, as compared to a nonresonant excitation scheme. Indistinguishable photons, with visibilities greater than 70%, have been observed by measuring the Hong-Ou-Mandel dip without postselection of the interfering photons. Near-unity indistinguishable photons should be achievable by preventing fluctuations in the electrostatic environment in the vicinity of the dots, considered as an important source of decoherence.
In quantum mechanics, particles in identical states are indistinguishable, giving rise to effects with no classical analog. For instance, the bosonic nature of light insures that upon interference two indistinguishable photons will coalesce into a single inseparable state. Through this coalescence, we demonstrate that photons produced from two separate quantum dots are indistinguishable. Further, we show that single photons created in a fundamentally different process - parametric down-conversion in a nonlinear crystal - can be manipulated to be indistinguishable from those from quantum dots. The quantum interference in both experiments occurs with a visibility reduced from unity because the quantum dot photons are not lifetime-limited due to the presence of pure dephasing. The measured visibility closely matches the theoretical visibility predicted for photons with the parameters of those measured here.
Photonic quantum computers use the bosonic statistics of photons to construct, through quantum interference, the large entangled states required for measurement-based quantum computation. Therefore, any which-way information present in the photons will degrade quantum interference and introduce errors. While quantum error correction can address such errors in principle, it is highly resource-intensive and operates with a low error threshold, requiring numerous high-quality optical components. We experimentally demonstrate scalable, optimal photon distillation as a substantially more resource-efficient strategy to reduce indistinguishability errors in a way that is compatible with fault-tolerant operation. Photon distillation is an intrinsically bosonic, coherent error-mitigation technique which exploits quantum interference to project single photons into purified internal states, thereby reducing indistinguishability errors at both a higher efficiency and higher threshold than quantum error correction. We observe unconditional error reduction (i.e., below-threshold behaviour) consistent with theoretical predictions, even when accounting for noise introduced by the distillation gate, thereby achieving actual net-gain error mitigation under conditions relevant for fault-tolerant quantum computing. We anticipate photon distillation will find uses in large-scale quantum computers. We also expect this work to inspire the search for additional intrinsically bosonic error-reduction strategies, even for fault-tolerant architectures.