A pulsed Sagnac source of narrowband polarization-entangled photons

TL;DR

This paper presents a pulsed, narrowband, polarization-entangled photon source based on a Sagnac interferometer, demonstrating high entanglement quality suitable for quantum communication applications.

Contribution

It introduces a pulsed Sagnac source capable of generating high-visibility entangled photons with precise characterization methods, advancing quantum source technology.

Findings

Achieved 99% quantum-interference visibility

Generated entangled pairs at 0.7 per pulse at high rate

Measured high fidelity and CHSH S parameter indicating strong entanglement

Abstract

We demonstrate pulsed operation of a bidirectionally pumped polarization Sagnac interferometric down-conversion source and its generation of narrowband, high-visibility polarization-entangled photons. Driven by a narrowband, mode-locked pump at 390.35 nm, the phase-stable Sagnac source with a type-II phase-matched periodically poled KTiOPO$_4$ crystal is capable of producing 0.01 entangled pair per pulse in a 0.15-nm bandwidth centered at 780.7 nm with 1 mW of average pump power at a repetition rate of 31.1 MHz. We have achieved a mean photon-pair generation rate of as high as 0.7 pair per pulse, at which multi-pair events dominate and significantly reduce the two-photon quantum-interference visibility. For low generation probability $\alpha$, the reduced visibility $V=1-\alpha$ is independent of the throughput efficiency and of the polarization analysis basis, which can be utilized to yield an accurate estimate of the generation rate $\alpha$. At low $\alpha$ we have characterized the source entanglement quality in three different ways: average quantum-interference visibility of 99%, the Clauser-Horne-Shimony-Holt $S$ parameter of $2.739 \pm 0.119$, and quantum state tomography with 98.85% singlet-state fidelity. The narrowband pulsed Sagnac source of entangled photons is suitable for use in quantum information processing applications such as free-space quantum key distribution.