On the impossibility of faithfully storing single-photons with the three-pulse photon echo

TL;DR

The paper demonstrates that the three-pulse photon echo technique cannot reliably store single photons due to a low signal-to-noise ratio, limiting its use for quantum memory applications.

Contribution

It provides explicit calculations showing the fundamental limitations of the three-pulse photon echo for single-photon quantum storage.

Findings

Echo signal to noise ratio is less than one for single-photon inputs.

The protocol's fidelity is too low for quantum memory applications.

Three-pulse photon echo is unsuitable for faithful single-photon storage.

Abstract

The three-pulse photon echo is a well-known technique to store intense light pulses in an inhomogeneously broadened atomic ensemble. This protocol is attractive because it is relatively simple and it is well suited for the storage of multiple temporal modes. Furthermore, it offers very long storage times, greater than the phase relaxation time. Here, we consider the three-pulse photon echo in both two- and three-level systems as a potential technique for the storage of light at the single-photon level. By explicit calculations, we show that the ratio between the echo signal corresponding to a single-photon input and the noise is smaller than one. This severely limits the achievable fidelity of the quantum state storage, making the three-pulse photon echo unsuitable for single-photon quantum memory.