Unwanted couplings can induce amplification in quantum memories despite negligible apparent noise

TL;DR

This paper reveals that unwanted couplings in quantum memories can cause signal amplification and efficiency exceeding unity, even with negligible apparent noise, highlighting overlooked effects in simplified models.

Contribution

It demonstrates the impact of undesired energy levels on quantum memory performance and proposes strategies to mitigate amplification effects.

Findings

Unwanted couplings can cause signal amplification in quantum memories.

Memory efficiencies can exceed unity due to these couplings.

The effect occurs even with negligible apparent noise.

Abstract

Theoretical quantum memory design often involves selectively focusing on certain energy levels to mimic an ideal $\Lambda$-configuration, a common approach that may unintentionally overlook the impact of neighboring levels or undesired couplings. While this simplification may be justified in certain protocols or platforms, it can significantly distort the achievable memory performance. Through numerical semi-classical analysis, we show that the presence of unwanted energy levels and undesired couplings in an NV-center-based absorptive memory can significantly amplify the signal, resulting in memory efficiencies exceeding unity, a clear indication of unwanted noise at the quantum level. Strikingly, this effect occurs even when the apparent noise i.e., output in the absence of an input field, is negligible. We then generalize our results using semi-analytical estimations to analyze this amplification, and propose a strategy to reduce its effect. Our findings extend to memory platforms beyond NV centers; as an example, we also analyze a cavity-based rubidium memory that experiences the same issue.