PhD Defense by Sofus Laguna Kristensen

Superradiant lasers harness the collective effects of cold atoms trapped inside an optical cavity. The phase information of the lasing process is stored in the atoms rather than the electric field in the optical cavity, constituting a frequency reference with greatly suppressed cavity noise without needing a separate ultrastable reference cavity.

In this thesis I describe the development and the results of a system designed to produce extended narrow linewidth superradiant pulses for a proof-of-concept measurement. The thesis reviews the relevant theory for sustaining lasing, cooling and trapping atoms, cavity QED, and collective effects in cold atom samples. The experimental apparatus needed to perform experiments with cold atoms in an optical cavity is described, with an emphasis on the transfer of atoms between two magneto-optical trap cooling stages, where the atoms are cooled to 2-3 uK. The collective effects responsible for the superradiant lasing are quantized by measuring the vacuum Rabi splitting, and the system is determined to be in the strong coupling regime where the strength of the atom-cavity interaction exceeds all other relevant decoherence rates. The characteristics of the superradiant pulses produced by the system are discussed and characterised. Crucially, the superradiant pulses are extended by several orders of magnitude to push the Fourier limit of the lasing frequency below the natural linewidth of the transition. This provides the main result of the thesis, a linewidth of just 820 Hz for a superradiant laser working on the 7.5 kHz intercombination line in strontium 88, which is a very promising prospect for future ultra stable lasers. The thesis also presents a superradiant Ramsey readout scheme that provides a fast and non-destructive readout of the atomic state.

https://ucph-ku.zoom.us/j/3674384298?pwd=NjI2bW9nMnVQQXNiTXZQNTFQTFM1Zz09