The absorption spectrum of a system of $N$ atoms interacting with a single mode of the quantized radiation field is exactly calculated. Such a spectrum shows vacuum-field Rabi splittings, and thus microwave absorption by Rydberg atoms in a cavity should be a useful way to observe these splittings.

• Received 20 June 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.1732