Short answer: Warm rubidium vapor can serve as an optical quantum memory for single photons without cryogenics. A 2022 PRX Quantum paper reports the first single-photon storage and retrieval in a ground-state vapor cell memory, and a 2023 Physical Review Letters paper reports a microfabricated vapor cell memory with an end-to-end efficiency of 3.12%. A third paper, from 2024, reports gradient echo memory work, but only its title could be read. These are laboratory memories for photonic networks, with modest efficiencies.
The 2022 interface
The PRX Quantum abstract says interfacing single-photon sources with photon memories is foundational, but compatibility between heterogeneous devices is a long-standing challenge. The authors built and interfaced a heralded single-photon source, based on cavity-enhanced spontaneous parametric down-conversion in periodically poled potassium titanyl phosphate, with a matched memory based on electromagnetically induced transparency in warm rubidium-87 vapor.
The photons have a bandwidth of 370 MHz, which the authors call technologically relevant while within the memory's acceptance bandwidth. All components operate at or above room temperature. Read-out noise is considerably reduced by using polarization selection rules in the hyperfine structure of spin-polarized atoms. They report demonstrating, for the first time, single-photon storage and retrieval in a ground-state vapor cell memory, with a retrieved second-order correlation of 0.177 (uncertainty 0.023), which they say shows the single-photon character of the light. They call the platform attractive for future room-temperature quantum networks at high bandwidth.
The 2023 microfabricated cell
The Physical Review Letters abstract says scalability is a central challenge that microfabrication can address. The authors demonstrate a high-bandwidth optical memory using a warm alkali atom ensemble in a microfabricated vapor cell compatible with wafer-scale fabrication. They apply an external tesla-order magnetic field and explore a ground-state scheme in the hyperfine Paschen-Back regime, where individual optical transitions can be addressed in a Doppler-broadened medium.
Working on the rubidium-87 D2 line, where deterministic quantum dot single-photon sources are available, they demonstrate bandwidth matching with hundreds of megahertz broad pulses. For an 80 ns storage time they measure an end-to-end efficiency of 3.12% (uncertainty 0.17) and an internal efficiency of 24% (uncertainty 3), with a signal-to-noise ratio of 7.9 (uncertainty 0.8) using coherent pulses at the single-photon level. The signal-to-noise figure comes from coherent pulses, not single photons.
A gradient echo memory paper
An APL Quantum article from 2024 is titled "Highly efficient storage of cavity SPDC single photons in room temperature gradient echo memory". The abstract could not be read, so this post does not describe its results.
Limits
An end-to-end efficiency of a few percent is low for network use, and storage times here are tens of nanoseconds in the 2023 work. The 2022 correlation value of 0.177 is the authors' measure of single-photon behavior in one setup.
FAQ
Why use a warm vapor?
The 2022 authors note that components operate at or above room temperature, which keeps complexity low.
How efficient is the 2023 memory?
3.12% end to end for an 80 ns storage time, per the authors.
What is a microfabricated vapor cell?
A small vapor cell made with wafer-scale techniques, as described in the 2023 abstract.
Was single-photon storage shown?
Yes in 2022. The 2023 test used coherent pulses at the single-photon level.
Is this ready for a quantum network?
The authors describe it as attractive for future networks.
All field notes