Weak Optical Measurements and Control of Atomic Spin Ensembles

Shimiao Fan, Hongyu Pei, Wei Quan +3 more

L. S. Walker

N. Behbood, Giorgio Colangelo, Ferran Martin Ciurana +3 more

We apply entropy removal by measurement and feedback to a cold atomic spin ensemble. Using quantum nondemolition probing by Faraday rotation measurement, and feedback by weak optical pumping, we drive the initially random collective spin variable F toward the origin F=0. We use input-output relations and ensemble quantum noise models to describe this quantum control process and identify an optimal two-round control procedure. We observe 12 dB of spin noise reduction, or a factor-of-63 reduction in phase-space volume. The method offers a nonthermal route to generation of exotic entangled states in ultracold gases, including macroscopic singlet states and strongly correlated states of quantum lattice gases.

G. Caimi

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.

Jonathan Kohler, Nicolas Spethmann, Sydney Schreppler +1 more

We demonstrate continuous measurement and coherent control of the collective spin of an atomic ensemble undergoing Larmor precession in a high-finesse optical cavity. The coupling of the precessing spin to the cavity field yields phenomena similar to those observed in cavity optomechanics, including cavity amplification, damping, and optical spring shifts. These effects arise from autonomous optical feedback onto the atomic spin dynamics, conditioned by the cavity spectrum. We use this feedback to stabilize the spin in either its high- or low-energy state, where, in equilibrium with measurement backaction heating, it achieves a steady-state temperature, indicated by an asymmetry between the Stokes and the anti-Stokes scattering rates. For sufficiently large Larmor frequency, such feedback stabilizes the spin ensemble in a nearly pure quantum state, in spite of continuous measurement by the cavity field.