Progress in Continuous-Wave Superradiance and Gap Protection Against Doppler Dephasing in a High-Finesse Optical Ring Cavity*

Superradiant lasers offer narrow linewidth, high precision, and broad bandwidth by shifting phase memory from the optical cavity to an ultra-narrow atomic transition of cold trapped atoms. Our pulsed superradiance demonstration on the mHz transition in 87Sr achieved a fractional Allan deviation of 6.7×10⁻¹⁶ at 1s averaging. To enable continuous-wave superradiance with vastly improved short-term stability, we continuously load 2.1(3)×10^{7 88}Sr atoms/s into a high-finesse ring cavity and transport them via a moving lattice to a low-decoherence region [1]. This high-flux system provides a foundation for continuous superradiant lasers, dead-time-free interferometry, and high-precision atomic clocks. Additionally, we observe continuous lasing from laser-cooled ⁸⁸Sr atoms in the ring cavity, driven by a collective recoil-induced resonance [2]. This lasing process, related to but distinct from self-organization phase transitions, exhibits 120 times less sensitive to cavity frequency fluctuations due to a lasing-induced atomic loss mechanism, providing a novel approach to suppressing low-frequency cavity noise. These results open new possibilities for continuous cavity QED quantum simulations and the realization of continuous superradiant lasers. Additionally, we observe extended coherence of collective dipole moments by suppressing Doppler decoherence through cavity-mediated spin-exchange interactions on the 7.5 kHz ¹S₀–³P₁ transition without magic wavelength trapping [3]. This offers an alternative to Lamb-Dicke confinement for quantum sensing and many-body physics.

[1] J. R. K. Cline, V. M. Schäfer, et al. Physical Review Letters 134 (2025): 013403.

[2] V. M. Schäfer, Z. Niu, et al. Nature Physics in press, arXiv:2405.20952 (2024).

[3] Z. Niu, V. M. Schäfer, et al. arXiv:2409.16265 (2024).