Routes to Continuous-Wave Superradiant Lasing: Atomic Beam and Collective Repumping Schemes

Superradiant lasers are a promising platform to generate ultra-stable, highly coherent light with exciting applications in active atomic clocks. Unlike good-cavity lasers, where coherence is stored in the cavity field, superradiant lasers operate in the bad-cavity regime and store coherence in the atomic ensemble. This makes them remarkably insensitive to cavity-length fluctuations, an advantage that becomes crucial in continuous-wave (CW) operation. Yet, realizing CW superradiant lasers has remained challenging due to heating from spontaneous emission during atomic repumping. In this talk, I will discuss two approaches toward achieving CW superradiant lasing: (i) Atomic beam lasing, where a continuous stream of excited atoms passes through an optical cavity. Because each atom interacts only briefly, heating is minimized, and fresh atoms constantly replenish the system. At sufficiently high flux, collective synchronization overcomes Doppler and transit-time broadening, enabling robust superradiant emission. (ii) Collective repumping, which suppresses incoherent scattering by coupling multi-level atoms collectively on separate pumping and decay transitions. This approach supports narrow-linewidth CW lasing and even allows operation with vanishing sensitivity to cavity-length fluctuations through a lasing-without-inversion mechanism. Together, these strategies outline practical routes toward realizing CW superradiant lasers.