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The mutual coupling between atoms and the electromagnetic field is typically very weak on the level of single atoms and photons. When confining the photons to a cavity, the strength of their interaction with an atom can be increased many orders of magnitude.
This strong-coupling regime of cavity quantum electrodynamics has been reached for real atoms in optical cavities, and for artificial atoms in circuit QED and quantum-dot systems. A strong-coupling signature is the splitting of the cavity transmission peak into a pair of resolvable peaks when a single resonant atom is placed inside the cavity – an effect known as vacuum Rabi splitting.
The image below shows that the avoided atom-photon crossing "fans out" when probing the transmission beyond linear response. This effect has recently been observed in a circuit QED experiment at Yale, using a transmon qubit as the artificial atom.
Credit to Circuit QED Team, Yale University. Collaborating scientists include L.S. Bishop , J.M. Chow, Jens Koch, A.A. Houck, M.H. Devoret , E. Thuneberg, S.M. Girvin, and R.J. Schoelkopf. This work is sponsored by Academy of Finland, NSA, and NSF.
This image was presented with the abstract "Nonlinear response of the vacuum Rabi resonance" at the APS 2009 March Meeting, author Jens Koch.
Image credit: Circuit QED Team, Yale University