Continuous Superradiant Laser – CONSULA

Quantum technologies have the potential to lead to significant improvements for precision measurements. Among them, superradiant (SR) lasers are composed of an atomic ensemble placed inside a high-finesse Fabry-Perot cavity and rely on quantum correlations to generate an enhanced collective spontaneous emission. They are operated as "active optical clocks", in which the ultra-stable radiation is directly emitted by atoms and only weakly coupled to the cavity, which makes it essentially insensitive to many technical limitations. Their spectral properties could challenge even the best optical clocks, with an optical linewidth that could reach the mHz regime and a remarkable frequency stability at all timescales, so that they may be the optical clocks of the future. Yet, metrological operation is still at its early beginning and requires additional developments. In this project, we address two major questions: the realization of a continuous SR laser, and the impact of decoherence on the SR laser metrological performances.

Our proposal consists in realizing the first continuous superradiant lasers and characterizing their metrological potential. It relies on two complementary architectures: we plan to realize at FEMTO-ST a continuous SR laser with metrological performances that will use ultra-cold ytterbium atoms, and the design of which will rely on preliminary studies performed at LPL on a hot strontium atomic beam. To reach the continuous regime, two strategies will be employed: at FEMTO-ST, we will rely on repumping the atoms and a sequential refilling of the cavity, while at LPL, the cavity will be refilled continuously by the already-excited atomic strontium beam.

We will explore in particular questions related to the competition between the build-up of coherence, that is necessary for superradiant emission, and decoherence induced by repumping or motion of atoms refilling the cavity.
We will also characterize the fractional frequency stability reached by the two set-ups. The LPL set-up should lead to a reduction by a factor 100 of the linewidth from the intercombination line, and reach a short-term emission linewidth on the order of 100 Hz. At FEMTO-ST, we aim at reaching in the frame of this project a linewidth in the order of 1 Hz (the fundamental limit of order 1 mHz will be the subject of future work). Metrological application perspectives for both systems will therefore be evaluated and may guide future works.
The complementarity between the two set-ups gives access to a wide range of parameters, providing a broader scientific picture of the operating regimes of SR lasers. Morevover, our consortium gathers experts in quantum correlations and in metrology. This federation of competences from different scientific fields will be crucial for the study of these dissipative, strongly correlated systems with a high metrological potential.