Locating radio-frequency waves' angle-of-arrival by imaging interference patterns with Rydberg atoms.

We recently developed a method to image electromagnetic fields in two dimensions via spatially resolved fluorescence readout of electromagnetically induced transparency of Rydberg states of alkali atoms in a vapor cell. Here, we use this technique to determine the angle of arrival of an incident RF wave by imaging the standing-wave patterns created by internal reflections from the partially reflective glass cell walls. By fitting the resulting standing-wave node spacing, we measure in-plane angles of arrival to within one degree over essentially all in-plane angles (θ). We further extend the approach to recover arbitrary three-dimensional angles (θ, φ) with uncertainties of a few degrees.