Ultrastrong coupling of atoms to electromagnetic fields

Atomic spontaneous decay, a basic and fundamental phenomenon, arises from the coupling of the atom to the electromagnetic field – a system with an infinite number of degrees of freedom. In conventional atomic physics the coupling is weak such that the atomic emission rate is much smaller than the atomic frequency.

We showed that with superconducting circuits we can explore a new regime, where the coupling is so large that the decay rate becomes comparable to the atomic frequency. This type of system allows for investigations of basic matter-light phenomena in a new regime.

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Spectroscopy of a two level atom (a superconducting flux qubit) coupled to a transmission line. Left and right panels correspond to strong and ultrastrong coupling respectively. The top panels show transmission of microwaves versus frequency and qubit bias flux. The bottom panels are transmission spectra at the qubit symmetry point.

[1] P. Forn-Diaz et al.,Nature Physics13,39–43.