Dynamical Quantum Memories

TL;DR

This paper introduces a dynamical quantum memory model using an oscillator-cavity system that achieves high fidelity over long storage times by mode-matching and critical coupling, applicable to various linear media.

Contribution

It presents an exact response calculation and mode-matching criteria for optimizing quantum memory performance in a generic linear medium.

Findings

Mode-matching determines optimal input/output pulse shapes.

Critical atom-cavity coupling enables high fidelity with long storage.

Dynamical memory surpasses classical memory bounds.

Abstract

We propose a dynamical approach to quantum memories using an oscillator-cavity model. This overcomes the known difficulties of achieving high quantum input-output fidelity with storage times long compared to the input signal duration. We use a generic model of the memory response, which is applicable to any linear storage medium ranging from a superconducting device to an atomic medium. The temporal switching or gating of the device may either be through a control field changing the coupling, or through a variable detuning approach, as in more recent quantum memory experiments. An exact calculation of the temporal memory response to an external input is carried out. This shows that there is a mode-matching criterion which determines the optimum input and output mode shape. This optimum pulse shape can be modified by changing the gate characteristics. In addition, there is a critical coupling between the atoms and the cavity that allows high fidelity in the presence of long storage times. The quantum fidelity is calculated both for the coherent state protocol, and for a completely arbitrary input state with a bounded total photon number. We show how a dynamical quantum memory can surpass the relevant classical memory bound, while retaining a relatively long storage time.