Since its theoretical sensitivity is limited by quantum noise, radio wave
sensing based on Rydberg atoms has the potential to replace its traditional
counterparts with higher sensitivity and has developed rapidly in recent years.
However, as the most sensitive atomic radio wave sensor, the atomic
superheterodyne receiver lacks a detailed noise analysis to pave its way to
achieve theoretical sensitivity. In this work, we quantitatively study the
noise power spectrum of the atomic receiver versus the number of atoms, where
the number of atoms is precisely controlled by changing the diameters of
flat-top excitation laser beams. The results show that under the experimental
conditions that the diameters of excitation beams are less than or equal to 2
mm and the read-out frequency is larger than 70 kHz, the sensitivity of the
atomic receiver is limited only by the quantum noise and, in the other
conditions, limited by classical noises. However, the experimental
quantum-projection-noise-limited sensitivity this atomic receiver reaches is
far from the theoretical sensitivity. This is because all atoms involved in
light-atom interaction will contribute to noise, but only a fraction of them
participating in the radio wave transition can provide valuable signals. At the
same time, the calculation of the theoretical sensitivity considers both the
noise and signal are contributed by the same amount of atoms. This work is
essential in making the sensitivity of the atomic receiver reach its ultimate
limit and is significant in quantum precision measurement