Three-dimensional k-vector measurements with an electrically small Rydberg rf polarimeter

Radio frequency (rf) receivers that use Rydberg atoms and their broadband sensitivity to electric fields offer potential advantages over classical sensors, such as their size, extreme tunability, and lack of field absorption. We present a vector electrometry and k-vector measurement method through an extension of current rf heterodyne techniques. Using three independent local oscillators, one for each cardinal axis, we are able to fully measure the polarization of an incoming field through the phases and amplitudes of the heterodyne beats. The phase sensitive measurements allows us to extract the full polarization ellipse in three dimensions, not just the amplitude of the polarization projections. We also present the ability of this sensor to extract the k-vector in three dimensions for an elliptical input field. Compared to previous techniques, this method is compatible with a sensor much smaller than the rf wavelength.