[Paper Review] A Single-Photon-compatible Telecom-C-Band Quantum Memory in a Hot Atomic Gas

S. E. Thomas, S. Sagona-Stophel+3 more|arXiv (Cornell University)|Nov 8, 2022

Quantum optics and atomic interactions4 citations

TL;DR

This paper presents a single-photon-compatible quantum memory for telecom C-band light using off-resonant cascaded absorption (ORCA) in hot 87Rb vapor. It achieves a record-low noise level with a signal-to-noise ratio of 1.9×10⁴ for sub-single-photon inputs, enabling a minimum input mean photon number of 4.5(6)×10⁻⁶ per pulse to reach SNR = 1, while maintaining 20.90(1)% total memory efficiency and 1.10(2) ns Doppler-limited storage time.

ABSTRACT

The efficient storage and on-demand retrieval of quantum optical states that are compatible with the telecommunications C-band is a requirement for future terrestrial-based quantum optical networking. Spectrum in the C-band minimises optical fiber-propagation losses, and broad optical bandwidth facilitates high-speed networking protocols. Here we report on a telecommunication wavelength and bandwidth compatible quantum memory. Using the Off-Resonant Cascaded Absorption protocol in hot $^{87}$Rb vapour, we demonstrate a total memory efficiency of $20.90(1)\,\%$ with a Doppler-limited storage time of $1.10(2)\,$ns. We characterise the memory performance with weak coherent states, demonstrating signal-to-noise ratios greater than unity for mean photon number inputs above $4.5(6) imes10^{-6}$ per pulse.

Motivation & Objective

• To enable high-efficiency, low-noise quantum memory for telecom C-band light in a scalable, room-temperature platform.
• To overcome limitations of existing telecom quantum memories, such as high background noise, cryogenic cooling, or complex frequency conversion.
• To demonstrate single-photon compatibility in a broadband, on-demand quantum memory using a noise-free protocol in hot atomic vapor.
• To achieve high signal-to-noise ratios for weak coherent states with mean photon numbers below 10⁻⁵ per pulse.
• To provide a practical, high-bandwidth solution compatible with existing fiber-based quantum networks and single-photon sources.

Proposed method

• The ORCA protocol uses two counter-propagating optical fields tuned to a two-photon resonance with a ladder atomic transition in 87Rb vapor.
• The control pulse creates a broad virtual state that enables efficient, coherent absorption of the signal pulse into a collective atomic excitation.
• The memory state is a doubly-excited state with no population in the ground state, eliminating spontaneous emission noise and enabling intrinsic noise-free operation.
• The signal is retrieved on-demand via a second control pulse after a variable storage time, enabling full memory cycle control.
• Noise is characterized using weak coherent states, and the signal-to-noise ratio is measured to determine the minimum input photon number for SNR = 1.
• The memory efficiency is calculated as the product of read-in and read-out efficiencies, with total efficiency reaching 20.90(1)%.

Experimental results

Research questions

• RQ1 Can a telecom C-band quantum memory be realized in a hot atomic vapor with single-photon-level performance and minimal noise?
• RQ2 What is the minimum mean photon number required to achieve a signal-to-noise ratio of 1 in a broadband, on-demand quantum memory?
• RQ3 How does the ORCA protocol in hot 87Rb vapor compare to other quantum memory platforms in terms of noise, efficiency, and bandwidth?
• RQ4 Can the ORCA protocol achieve high memory efficiency while remaining compatible with existing telecom fiber networks and single-photon sources?
• RQ5 What are the fundamental limits of storage time and noise performance in a Doppler-broadened hot atomic vapor using ORCA?

Key findings

• The total memory efficiency reaches 20.90(1)% with a Doppler-limited storage time of 1.10(2) ns.
• The signal-to-noise ratio is measured at 1.9(1)×10⁴ for an input mean photon number of 0.084 per pulse.
• The minimum input mean photon number required to achieve SNR = 1 is 4.5(6)×10⁻⁶ per pulse, representing a 10-fold improvement over prior ORCA demonstrations.
• The retrieved single-photon second-order coherence is g²(out) = 9(1)×10⁻⁶, indicating near-ideal single-photon character.
• The output fidelity for single-photon storage is estimated at 99.9996%, demonstrating high-fidelity quantum state preservation.
• The memory operates over a GHz bandwidth and is compatible with parametric down-conversion and InGaAs quantum dot sources.