  Microwave receivers using electromagnetically-induced transparency (EIT) in
Rydberg atoms have recently demonstrated improved sensitivities. It is not
evident how their state-of-the-art electric field sensitivities compare to
those achieved using standard electronic receivers consisting of low-noise
amplifiers (LNAs) and mixers. In this paper, we show that conventional
room-temperature electronic receivers greatly outperform the best demonstrated
sensitivities of room-temperature Rydberg electrometers in standard free-space
coupled configurations. However, Rydberg-atom receivers can surpass the
sensitivity of conventional receivers if resonant or confining microwave
structures are designed to enhance the electric fields sensed by the atoms. For
a given microwave resonator, the external (coupling) quality factor must be
carefully chosen to minimize their thermal and quantum noise contributions.
Closed-form expressions for these optimal design points are found, and compared
in terms of noise temperature with conventional LNAs reported in the literature
from 600 MHz to 330 GHz.
