Protecting ensemble atomic coherence with weak measurements and feedback

Etienne Cantin

Thomas Vanderbruggen, Ralf Kohlhaas, Andréa D Bertoldi +4 more

Weak measurements provide new ways to observe and control quantum systems. Here we use weak non-destructive measurements in a feedback scheme to protect an atomic ensemble in a coherent superposition against a simple decoherence model. The feedback efficiency, defined as the ability to recover the coherence of the initial state, is studied versus the number of photons in the probe beam. This allows us to precisely characterize the trade-off between information retrieval and destructivity. The lifetime of the coherent spin state is increased by one order of magnitude when the correction cycle is iterated.

David Vitali, P. Tombesi, Mauro Fortunato

Etienne Cantin, Ralf Kohlhaas, Andréa D Bertoldi +4 more

We demonstrate the feedback control of the internal states of atomic ensembles and their protection against collective noise. Weak measurements with negligible back-action and coherent microwave manipulations are used in a feedback controller to protect a superposition state. The efficiency of the feedback is studied for a simple binary noise model and characterized in terms of the trade-off between information retrieval and destructivity from the optical probe. More complex feedback scenarios are implemented which provide a way towards novel atom interferometry schemes using repeated measurements and feedback.

Yu. P. Malakyan, Dmitry Budker, Simon Rochester +3 more

Thomas Vanderbruggen, Ralf Kohlhaas, Andréa D Bertoldi +3 more

We consider the decoherence of a pseudospin ensemble under collective random rotations and study, both theoretically and experimentally, how a nondestructive measurement combined with real-time feedback correction can protect the state against such a decoherence process. We theoretically characterize the feedback efficiency with different parameters---coherence, entropy, fidelity---and show that a maximum efficiency is reached in the weak measurement regime, when the projection of the state induced by the measurement is negligible. This article presents in detail the experimental results published previously [T. Vanderbruggen et al., Phys. Rev. Lett. 110, 210503 (2013)], where the feedback scheme stabilizes coherent spin states of trapped ultracold atoms and nondestructively probes them with dispersive optical detection. In addition, we study the influence of several parameters, such as atom number and rotation angle, on the performance of the method. We analyze the various decoherence sources limiting the feedback efficiency and propose a way to mitigate their effect. The results demonstrate the potential of the method for the real-time coherent control of atom interferometers.