Gradient echo memory in an ultra-high optical depth cold atomic ensemble

TL;DR

This paper demonstrates a gradient echo memory in a cold atomic ensemble with high optical depth, achieving 80% efficiency and extended coherence times, advancing quantum memory technology for quantum communication.

Contribution

Developed a high optical depth cold atomic ensemble for GEM, achieving record efficiency and coherence times surpassing previous warm vapor implementations.

Findings

Memory efficiency of 80% achieved.

Coherence times up to 195 microseconds.

Optical depth of 1000 in the atomic ensemble.

Abstract

Quantum memories are an integral component of quantum repeaters - devices that will allow the extension of quantum key distribution to communication ranges beyond that permissible by passive transmission. A quantum memory for this application needs to be highly efficient and have coherence times approaching a millisecond. Here we report on work towards this goal, with the development of a $^{87}$Rb magneto-optical trap with a peak optical depth of 1000 for the D2 $F=2 \rightarrow F'=3$ transition using spatial and temporal dark spots. With this purpose-built cold atomic ensemble to implement the gradient echo memory (GEM) scheme. Our data shows a memory efficiency of $80\pm 2$% and coherence times up to 195 $\mu$s, which is a factor of four greater than previous GEM experiments implemented in warm vapour cells.