Low-Noise Lasers for Cavity-Mediated Interactions

Cold atomic ensembles coupled to optical cavities provide a powerful platform for studying many-body physics and enabling entanglement-enhanced atom interferometry. Using ultracold rubidium atoms inside a high-finesse optical cavity, we have demonstrated collective interactions and generated spin-squeezed states for precision measurements beyond the standard quantum limit (SQL). Now we aim to disregard the internal spin degrees of freedom and access beyond–mean-field dynamics among pure momentum states.

Currently, phase noise on our laser inducing the squeezing interactions limits our ability to resolve below the SQL. In particular, our momentum states are separated by 500 kHz, and noise at this offset from carrier drives random Bragg rotations between the states. We estimate that achieving sub-SQL operation requires a dressing laser with phase noise below −110 dBc/Hz (≈5 Hz²/Hz) at this offset.

To mitigate this noise, we are pursuing two different approaches. First, we are constructing a narrow-linewidth optical filter cavity designed to suppress laser phase noise near 500 kHz. Additionally, we are also exploring stimulated Brillouin scattering (SBS)–based low-noise laser sources as an alternative route to achieving the required phase noise performance.