High-fidelity entangled photon pairs from a quantum-dot-based single-photon source

TL;DR

This paper demonstrates a high-fidelity, semiconductor quantum dot-based source of entangled photon pairs suitable for quantum networks, surpassing traditional SPDC sources in fidelity and indistinguishability.

Contribution

The authors develop a novel quantum dot microcavity system that produces entangled photon pairs with over 98% fidelity and high indistinguishability, advancing scalable quantum network technology.

Findings

Achieved 96.1% entanglement fidelity, improved to 98.1% with post-selection.

Generated entangled photon pairs at rates exceeding 0.5 Gpairs/sec.

Produced mutually indistinguishable photons suitable for entanglement swapping.

Abstract

Entangled photon pairs are a ubiquitous resource in quantum technologies, used in quantum key distribution and quantum networking as well as fundamental tests of non-locality. For scalable quantum networks, pairs that are indistinguishable in all unentangled degrees of freedom are essential, as they enable high-fidelity entanglement swapping across network nodes. To date the most-studied sources of "swappable" entangled photon pairs have been based on spontaneous parametric down-conversion (SPDC) in non-linear crystals. However, the probabilistic nature and unavoidable trade-off between brightness and unwanted multi-photon emission limits their performance in lossy channels. Here, we demonstrate a high-fidelity source of "swappable" entangled photon pairs using a semiconductor quantum dot (QD) coupled to a tunable microcavity. By actively modulating the QD emission between orthogonal polarisation states, delaying one path in a low-loss Herriott cell, and recombining the two on a balanced beam splitter, we generate entangled photon pairs with a fidelity of $96.1\pm0.5$ %. We identify and mitigate fidelity-limiting factors, achieving a maximum fidelity of $98.1\pm0.5$ % through time-resolved post-selection. The scheme suppresses residual multi-photon events concentrated near the excitation pulse and has only a modest impact on the rate. Furthermore, the photons are mutually indistinguishable, enabling efficient entanglement swapping. Our results establish semiconductor QDs as a viable platform for quantum network-compatible swappable entangled photon pair generation, with feasible entanglement generation rates exceeding 0.5 Gpairs/s.