ALP Seminar: An Accidental Laser and Why We Study It

The advent of the laser revolutionised atomic physics by providing a light source with the spectral brightness to match the intrinsic lineshapes of unperturbed atoms. The strength of light-matter coupling can be further enhanced via confinement within optical resonators. I will describe our experiments with a laser-cooled gas of potassium atoms trapped within a high-finesse ring cavity. As the in-cavity optical depth increases, the regime of collective strong coupling becomes evident as a normal-mode splitting of the cavity transmission spectrum. As the coupling continues to increase, the gas undergoes a transition to lasing driven solely by the cooling and trapping fields. We observe random switching between clockwise and anti-clockwise lasing modes as well as a controllable breaking of symmetry between the two directions. Measurements of photon bunching below the lasing threshold show the coherence time can be extended by two orders of magnitude through a combination of gain and dispersion. Surprisingly (to us), most of these observations will be familiar to professional 'laserists', but in the last part of the talk I will describe our plans to open new horizons for active quantum sensing with cold-atom lasers.