Oh, that's very neat, I can imagine that some future front-end to a SDR will have no antenna at all, just a bunch of solid state.

The abstract of the paper:

"""Coupling a Rydberg vapour medium to both microwave and optical fields enables the benefits of all-optical detection, such as minimal disturbance of the measured field and resilience to very strong signals, since no conventional antenna is required. However, peak sensitivity typically relies on adding a microwave local oscillator, which compromises the all-optical nature of the measurement. Here we introduce an alternative, optical-bias detection, that maintains fully optical operation while achieving high sensitivity. To address laser phase noise, which is critical in this approach, we perform a simultaneous measurement of the noise using a nonlinear process and correct it in real time via data processing. This yields a 35 dB improvement in signal-to-noise ratio compared with the basic method. We demonstrate a sensitivity of 176 nV / cm / sqrt(f Hz) , reliable operation up to 3.5 mV/cm at 13.9 GHz, and quadrature-amplitude modulated data transmission, underlining the ability to detect microwave field quadratures while preserving the unique advantages of all-optical detection."""

Emph. mine, at about -36 dBm that's not super sensitive yet though, but that formula suggests that at lower frequencies it should be a lot more sensitive.

The paper is at: https://www.nature.com/articles/s41467-025-63951-9

A company called Infleqtion already has an RF sensing product that uses Rydberg atoms.

https://infleqtion.com/quantum-rf-receiver/

That raised the question, how do you make Rydberg atoms, and the answer is (always!) with lasers: https://en.wikipedia.org/wiki/Rydberg_atom

We demonstrated simultaneous reception of neighboring channels with strong isolation between them." This enabled the researchers to monitor numerous radio channels at once, instead of tuning into them individually.

Can anyone elaborate on this? How does a single receiver produce multiple concurrent outputs, and how are they isolated in this context?

I immediately thought of the old crystal radios and a short search brought up this comparison. While old radios used a natural mineral (galena) to detect radio waves, modern Rydberg radio receivers use a synthetic photonic crystal (often made of silicon or glass) to guide and enhance the radio signal for improved performance.

Interesting, I learnt something new today. My only comment was the noise floor of the simple graph was very high-25dBm which (without having a clue how the physics works and skimming the article) sounds about right for something with no proper RF front-end.

What modulation technique?