  In this work, we demonstrate the use of a Rydberg atom-based sensor for
determining the angle-of-arrival of an incident radio-frequency (RF) wave or
signal. The technique uses electromagnetically induced transparency in Rydberg
atomic vapor in conjunction with a heterodyne Rydberg atom-based mixer. The
Rydberg atom mixer measures the phase of the incident RF wave at two different
locations inside an atomic vapor cell. The phase difference at these two
locations is related to the direction of arrival of the incident RF wave. To
demonstrate this approach, we measure phase differences of an incident 19.18
GHz wave at two locations inside a vapor cell filled with cesium atoms for
various incident angles. Comparisons of these measurements to both full-wave
simulation and to a plane-wave theoretical model show that these atom-based
sub-wavelength phase measurements can be used to determine the angle-of-arrival
of an RF field.
