Would one rather store squeezing or entanglement in continuous variable quantum memories?

TL;DR

This paper compares two strategies for storing quantum information in continuous variable memories—storing entangled states directly versus storing squeezed states and combining them later—and determines which yields higher entanglement under realistic noise conditions.

Contribution

It analytically evaluates the optimal storage strategy for continuous variable quantum memories considering noise, providing guidelines for maximizing entanglement and teleportation capability.

Findings

Storing squeezed states and combining them later can outperform direct entanglement storage under certain noise conditions.

The optimal strategy depends on the noise characteristics of the memory, especially which quadrature is more affected.

For nearly ideal memories, protecting the quadrature with the largest noise enhances entanglement.

Abstract

Given two quantum memories for continuous variables (e.g., the collective pseudo-spin of two atomic ensembles) and the possibility to perform passive optical operations (typically beam-splitters) on the optical modes before or after the storage, two possible scenarios arise resulting in generally different degrees of final entanglement. Namely, one could either store an entangled state and retrieve it directly from the memory, or rather store two separate single-mode squeezed states and then combine them with a beam-splitter to generate the final entangled state. In this paper, we address the question of which of these two options yields the higher entanglement. By adopting a well established descrip- tion of QND feedback memories, and a simple but realistic noise model, we analytically determine the optimal choice for several regions of noise parameters and quantify the advantage it entails, not only in terms of final entanglement but also in terms of the capability of the final state to act as a shared resource for quantum teleportation. We will see that, for 'ideal' or 'nearly ideal' memories, the more efficient of the two options is the one that better protects the quadrature subject to the largest noise in the memory (by increasing it and making it more robust).