  Vacuum Rabi splitting relies on symmetrical splitting of the common resonance
frequency of atoms and the cavity in which the atoms reside. In this work, we
argue that vacuum Rabi splitting is a manifestation of virtual light-matter
two-mode squeezing. We establish a connection between squeezing parameters of
virtual excitations and frequency shifts of the physical modes. To this end, we
use the mapping between the Dicke model and two interacting harmonic
oscillators, which we analyze in the framework of bare and physical modes.
Finally, we suggest that such virtual squeezing of quantum fields might also
play a role in quantum field theories.
