Operating Spin Echo in the Quantum Regime for an Atomic-Ensemble Quantum Memory

TL;DR

This paper demonstrates that with optimized pulse arrangements, spin echo can be effectively used in atomic ensemble quantum memories at the single-photon level, significantly extending their coherence time.

Contribution

It is the first experimental demonstration of using spin echo to extend the lifetime of ensemble-based quantum memories at the single-quanta level.

Findings

Quantum memory lifetime extended by 5 times.

Successfully observed nonclassical photon-photon correlations.

Noise due to rephasing pulse imperfections is highly directional.

Abstract

Spin echo is a powerful technique to extend atomic or nuclear coherence time by overcoming the dephasing due to inhomogeneous broadening. However, applying this technique to an ensemble-based quantum memory at single-quanta level remains challenging. In our experimental study we find that noise due to imperfection of the rephasing pulses is highly directional. By properly arranging the beam directions and optimizing the pulse fidelities, we have successfully managed to operate the spin echo technique in the quantum regime and observed nonclassical photon-photon correlations. In comparison to the case without applying the rephasing pulses, quantum memory lifetime is extended by 5 folds. Our work for the first time demonstrates the feasibility of harnessing the spin echo technique to extend lifetime of ensemble-based quantum memories at single-quanta level.