Low-frequency radio waves (from 30 to 300 kilohertz) carry information through water and earth. But catching them with an ordinary antenna is hard: the wavelength reaches several kilometers, and a small receiver barely feels such a field. Physicists found a way out: they turned a single ion into an antenna.
A calcium-40 ion was suspended in a vacuum using electric fields on a chip surface. Laser beams made the ion oscillate like a tiny pendulum, and these oscillations began to behave like a laser beam — creating a so-called phonon laser (a laser based on sound waves). When an external electric field hits the trap, it slightly changes the ion's oscillation rhythm. By measuring the shift in beat frequency between the laser beam and the ion's motion, one can extract three signal parameters at once: frequency, phase, and amplitude.
The main trick of the method is that it doesn't require the ultra-low cooling of the ion that is usually needed in such experiments. The system itself maintains stable oscillations and handles noise quite well. Such a sensor, the size of a pinhead, opens the way to underground communication, ultra-precise mass measurements, and monitoring of biosignals.
The trapped calcium ion oscillates at about 1 megahertz — thousands of times higher than the note "A" above middle C, but still in a range that electronics can "hear."