Quantum CNOT Gate with Actively Synchronized Photon Pairs

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 (HOM) interference introduced by the storage process is identified as a primary factor limiting the gate fidelity. We derive an exact quantitative relation between HOM visibility and gate fidelity for pure single photons, which applies to all photonic gates relying on interference, regardless of the physical platform.