Lectio Magistralis

Andrea Bertoldi

Researcher @ LCF (Laboratoire Charles Fabry) de l'Institut d'Optique

Cold atoms, weak measurements and feedback for timekeeping

''Measurement devices based on Ramsey interferometry rely on the evolution signal of coherent ensembles of 2-level atoms. Such devices are used to measure frequency, magnetic field, inertial and gravitational forces. Trapped atomic ensembles allow long observation times, hence potential extreme precision, provided that one mitigates the coherence loss due to ambient noise. If disturbances act homogeneously over the ensemble, it becomes possible to measure the collective noise effect without perturbing the atoms by using weak measurements. A feedback correction is then applied to fight the noise induced decoherence.

We demonstrated the feedback control of a cloud of cold 87Rb atoms trapped in an optical cavity and prepared in a coherent superposition of two internal states. This superposition is coherently manipulated with a microwave, and the state read out by a heterodyne non destructive detection. We evaluate the effect of two collective noise models acting on the atoms, and show how to use weak measurements and feedback to significantly reduce the ensemble decoherence. The coherent probing based on weak non-demolition measurements can be exploited in timekeeping to increase the effective interrogation time. We phase-locked a quartz oscillator to the phase of an atomic quantum state, and are now implementing a clock on this concept. This will improve the performance of conventional, frequency-lock based atomic clocks.''


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