The aim of this work package is to develop continuous active frequency standards based on ultra-cold atoms with engineered lattice topologies to supersede thermal-noise limited optical cavities.

The state-of-the-art ultra-stable optical atomic clocks rely on the frequency stability of reference lasers [blo14, let13, ush15]. These lasers have been demonstrated with linewidths down to a few tens of mHz [bis13], relying heavily on stabilisation to ultra-stable reference cavities [kes12, mat17]. Therefore, the fractional frequency stability of the reference lasers is currently limited by the thermal fluctuations in the reference cavity mirror coatings, substrates, and spacer. This also limits the stability of the clock: for neutral atom clocks by the Dick effect and for single-ion clocks by the coherence time of the laser that limits the interrogation time and thus the resolved linewidth.

The solution to this problem can be found by replacing the reference cavity with an ensemble of atoms. The proposed approaches include non-linear spectroscopy [wes15, mar11], magnetically induced optical transparency [win17], and direct emission of radiation in superradiant lasers [mei09, che12, nor16]. Interestingly, all the aforementioned methods are cavity assisted. The system, however, operates in the so-called bad cavity regime, where the cavity linewidth is much larger than the natural linewidth of the atoms.

The first task will be dedicated to zero dead-time clock operation with laser frequency stabilisation based on non-linear feature of the coherent atom dipole-cavity interaction and magnetically induced optical transparency in Sr and investigate the prospects of Ca, which has a nearly 20-fold narrower linewidth compared to Sr. In the second task we will develop advanced techniques to tackle current limitation of the superradiant lasers (namely pulsed operation).

Lead participant

UMK (Prof. Michał Zawada)