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arXiv:2505.03960 · Physical Review Letters

Quantum cnot Gate with Actively Synchronized Photon Pairs

Haim Nakav, Tanim Firdoshi, Omri Davidson, Bankim Chandra Das, Ofer Firstenberg

Published Nov 25, 2025• 1 citation
arXivHTMLPDF

Abstract

Controlling the synchronization of photons from probabilistic quantum sources plays a pivotal role in advancing efficient quantum information processing. We report the realization of a probabilistic entangling gate operating on actively synchronized photon pairs, using a quantum memory based on warm atomic vapor. We achieve a truth-table fidelity exceeding 85% and demonstrate Bell-inequality violation for all four Bell states. The reduction in visibility of the Hong-Ou-Mandel interference introduced by the storage process is identified as a primary factor limiting the gate fidelity. We derive an exact quantitative relation between Hong-Ou-Mandel visibility and gate fidelity for pure single photons, which applies to all photonic gates relying on interference, regardless of the physical platform.

DOI
10.1103/c318-t2vn

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Controlling the synchronization of photons from probabilistic quantum sources plays a pivotal role in advancing efficient quantum information processing. We report the realization of a probabilistic entangling gate operating on actively synchronized photon pairs, using a quantum memory based on warm atomic vapor. We achieve a truth-table fidelity exceeding 85% and demonstrate Bell-inequality violation for all four Bell states. The reduction in visibility of the Hong-Ou-Mandel interference introduced by the storage process is identified as a primary factor limiting the gate fidelity. We derive an exact quantitative relation between Hong-Ou-Mandel visibility and gate fidelity for pure single photons, which applies to all photonic gates relying on interference, regardless of the physical platform.
Efficient synchronization of single photons that are compatible with narrow band atomic transitions is an outstanding challenge, which could prove essential for photonic quantum information processing. Here we report on the synchronization of independently generated single photons using a room-temperature atomic quantum memory. The photon source and the memory are interconnected by fibers and employ the same ladder-level atomic scheme. We store and retrieve the heralded single photons with end-to-end efficiency of η_{e2e}=25% and final antibunching of g_{h}^{(2)}=0.023. Our synchronization process results in an over tenfold increase in the photon-pair coincidence rate, reaching a rate of more than 1000 detected synchronized photon pairs per second. The indistinguishability of the synchronized photons is verified by a Hong-Ou-Mandel interference measurement.
Imperfect photon sources degrade the performance of interference-based linear-optical entangling gates. We derive a universal fidelity law for these gates in terms of two standard source metrics: Hong-Ou-Mandel visibility and second-order coherence. We model the multiphoton component as an arbitrary overlap between noise and signal photons. While different values of the signal-noise overlap produce different error structures, associated with different physical platforms, all result in the same fidelity law. We test the model on our actively synchronized CNOT-gate experiment and against published results across different platforms, with no free parameters. This framework turns source characterization into a unified benchmark for photonic quantum computing.
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Time synchronization and phase shaping of single photons both play fundamental roles in quantum information applications that rely on multi-photon quantum interference. Phase shaping typically requires separate modulators with extra insertion losses. Here, we develop an all-optical built-in phase modulator for single photons using a quantum memory. The fast phase modulation of a single photon in both step and linear manner are verified by observing the efficient quantum-memory-assisted Hong–Ou–Mandel interference between two single photons, where the anti-coalescence effect of bosonic photon pairs is demonstrated. The developed phase modulator may push forward the practical quantum information applications.
Heralded entangling quantum gates are an essential element for the implementation of large-scale optical quantum computation. Yet, the experimental demonstration of genuine heralded entangling gates with free-flying output photons in linear optical system, was hindered by the intrinsically probabilistic source and double-pair emission in parametric down-conversion. Here, by using an on-demand single-photon source based on a semiconductor quantum dot embedded in a micropillar cavity, we demonstrate a heralded controlled-NOT (CNOT) operation between two single photons for the first time. To characterize the performance of the CNOT gate, we estimate its average quantum gate fidelity of (87.8±1.2)%. As an application, we generated event-ready Bell states with a fidelity of (83.4±2.4)%. Our results are an important step towards the development of photon-photon quantum logic gates.