Group seminar at MPQ and Zoom: Narrow linewidth superradiant lasing using cold 88Sr for frequency metrology and sensing

Sofus Kristensen,  Niels-Bohr Institute, Copenhagen
Group seminar at MPQ lecture hall and Zoom
Thursday, November, 30 at 9:00am (MEZ)

Superradiant lasers harness 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, and a continuous superradiant laser can constitute a frequency reference with greatly suppressed cavity noise without needing a separate ultrastable reference cavity. In this talk I will discuss the development and the results of a system designed to produce extended narrow linewidth superradiant pulses for proof-of-concept measurements. I will describe the experimental apparatus needed to produce lasing, with an emphasis on a two stage magneto-optical trap for Sr88 which can trap over 100e6 atoms at uK level temperatures. We quantize the collective effects responsible for the superradiant 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. I will also discuss the characteristics of the single population inversion superradiant pulses produced by the system. We use optical repumping to extend the superradiant pulses several orders of magnitude compared to single population inversion pulses. This serves to push the Fourier limit of the lasing frequency below the natural linewidth of the transition. We observe a linewidth of just 820 Hz for a superradiant laser working on the 7.5 kHz intercombination line in Sr88, which is a very promising prospect for future ultra stable lasers. Additionally, I will also present a superradiant Ramsey readout scheme that provides a fast and non-destructive readout of the atomic state.