[peter_retief]

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.

[jacquesm]

That's not the function of the crystal though, it is not optical but 100% electrical and serves to demodulate the signal, not to receive it.

[peter_retief]

Thanks for that, I was partially aware of that, just made me think of crystal radios and how magical they are.

[jacquesm]

They are magical! A radio powered by the transmitter, it's an incredible thing, really. The trick is that the diode is a very low forward drop one, typically Germanium, which has only 0.2V forward drop. But for more magic, you can also use a razor blade or a little bit of lead and then seek for naturally occurring diodes on the surface of the material. These can be even more sensitive than the Germanium ones.

[peter_retief]

https://www.mpg.de/9142418/magnetism-in-artificial-crystal and this