Phys. Rev. X 6, 031004 (2016)

Quantum optics and 'squeezed' light have been leveraged in applications ranging from improving the measurement precision of gravitational wave detectors to bioimaging. Some other directions have remained more theoretical; researchers previously predicted that resonance fluorescence is modified when the light exciting the atomic system is squeezed and this could be used to characterize light–atom interactions. David Toyli and colleagues from the US and Canada have now experimentally demonstrated this phenomenon. The team coupled squeezed microwaves to a superconducting artificial atom and developed techniques to measure the fluorescence in weak and strong regimes. The fluorescence linewidth was reduced by 3.1 dB compared with a conventional vacuum. Moreover, the Mollow triplet spectrum was shown to depend strongly on the phase of the driving and squeezed vacuum fields. Good agreement between the experimentally measured spectra and the almost 30-year-old predictions was found. The approach may create new directions for microwave quantum photonics such as characterizing squeezing and a new tool for studies on multimode strong coupling and ultrastrong coupling.