A Theory of Single-Antenna Atomic Beamforming

Abstract: Leveraging the quantum advantages of highly excited atoms, Rydberg atomic receivers (RAREs) represent a paradigm shift in radio wave detection, offering high sensitivity and broadband reception. However, existing studies largely model RAREs as isotropic point receivers and overlook the spatial variations of atomic quantum states within vapor cells, thus inaccurately characterizing their reception patterns. To address this issue, we present a theoretical analysis of the aforementioned spatial responses of a standard local-oscillator (LO)- dressed RARE. Our results reveal that increasing the vapor-cell length produces a receive beam aligned with the LO field, with a beamwidth inversely proportional to the cell length. This finding enables atomic beamforming to enhance received signal-to-noise ratio using only a single-antenna RARE. Furthermore, we derive the achievable beamforming gain by characterizing and balancing the fundamental tradeoff between the effects of increasing the vapor cell length and the exponential power decay of laser propagating through the cell. To overcome the limitation imposed by exponential decay, we propose a novel RARE architecture termed segmental vapor cell. This architecture consists of vapor-cell segments separated by clear-air gaps, allowing the total cell length (and hence propagation loss) to remain fixed while the effective cell length increases. As a result, this segmented design expands the effective atom-field interaction area without increasing the total vapor cell length, yielding a narrower beamwidth and thus higher beamforming gain as compared with a traditional continuous vapor cell.