PhD Defense by Julian Robinson-Tait

Title: Cavity Phase Measurements and Continuous Atomic Clocks

The advancement of optical atomic clocks leads to new methods of testing fundamental physics, and discovering impactful applications for society. A technological gap exists for an atomic clock that is better than what can be achieved with vapor cells but less complicated than the best optical atomic clocks. This thesis explores themes of optical atomic clocks based on continuous and passive cavity-enhanced phase measurements of neutral 88Sr. These methods may be a simpler way of realizing state-of-the-art atomic clocks while still maintaining high performance. We study two regimes that differ in the number of atoms: the weak and the strong collective cooperativity regimes. 

The weak regime is where the atom number is low, and their behavior is linear and classical. We build a 88Sr beam machine from scratch to demonstrate continuous cavity-enhanced phase measurements on a travelling atomic beam. We only use a single cooling transition to increase the atomic density and signal size. We use the narrow 1S0-3P1 transition of strontium with a decay rate of 2π 7.5 kHz as our clock transition and employ a cavity-enhanced measurement technique called NICE-OHMS. The resulting signal has a Doppler-free saturation feature that can be used as a narrow error signal to stabilize a laser. The foundations have been set for the first reporting of a NICE-OHMS measurement on a continuous atomic beam, and only a few technical adjustments are required before the machine can be tested as a frequency standard. 

The strong regime is where the atom number is high, and it is qualified by the emergence of collective effects characterized by the theory of cavity quantum electro-dynamics. Our atom-cavity system exhibits a non-linear phenomenon called bi-stability. Two stable solutions can exist for the cavity field for the same input parameters; the chosen solution depends on the history of the atom-cavity system. On a pulsed machine (different from the continuous machine mentioned above), we experimentally explore this regime and employ a simple and versatile measurement scheme inspired by NICE-OHMS that allows cavity phase and transmission information to be retrieved. The phase exhibits a sharp π step linked to the atomic resonance that could be used as an error signal to lock a laser. For a laser stabilized to this error signal, we estimate an achievable short-term stability below 3.7 x 10^-14 for 1s of integration time, assuming a continuous replenishment of atoms into the cavity.

Zoom Link: https://ucph-ku.zoom.us/j/3841208851?pwd=M2xSQjExV2NtQWtvdU9jNDU2dmpGUT09

Reception time: 13:00 (M Building Library)