Tzahi Grunzweig
Weizmann Institute of Science, Israel
Note different date and time
Friday, April 27, 2007
11:00 am in SPL 52
Perturbation Dependent Dephasing of Internal State Coherence of Cold Atoms
Abstract: The possibility to control and manipulate cold atoms holds the promise of applying cold atoms to quantum information processing, as well as to precision measurements. Once the atoms are cooled and trapped, microwave fields can probe their internal hyperfine split ground states for extended time. However, the trapping potential is dependent on the atomic internal state, i.e. a differential potential (or “perturbation”) to the trap is seen by the atom as it’s internal state changes. This coupling between the atomic internal and external degrees of freedom causes dephasing of the internal state coherence. The dephasing due to inhomogeneous broadening can be reduced by the use of coherence echoes [1] and "compensating" techniques [2]. Residual loss of coherence is related to the trapped atoms dynamics. I will describe our attempts to use this loss of coherence as a probe to study cold atoms dynamics in dark optical dipole traps.
We found that when the perturbation and the trap share dynamical timescale and symmetry, a distinct echo signal revival emerges even for a classically chaotic atom optics billiard. When perturbation strength increases a perturbation independent regime is identified [3]. We also investigated a broader class of "generic" perturbations, while minimizing the trap related dephasing using compensating techniques [2]. For example in Fig. 1 we consider the case of complete spatial symmetry breaking of the perturbation and trap, by adding a weak speckle beam. Unlike the non-generic perturbation of the trap itself, the speckle field causes a monotonically increasing dephasing reflecting the fact that no time scale emerges from the perturbation. When the speckle generic perturbation strength is increased, the dephasing rate increases. No perturbation independent regime was observed here.
We then applied various phase space selection techniques to manipulate dephasing characteristics. We used a mixed phase space wedge trap in which trapped atoms trajectories are either chaotic or stable. We were able to “address” by a spatial perturbation both the chaotic regimes and the island of stability. We also demonstrated that we can selectively populate either phase space regime, by loading the wedge from a Gaussian beam trap whose relative position is appropriately set.
Finally we tracked the evolution of phase space in an open system. To this end we punched a hole in a mixed phase space wedge. As the chaotic atoms leak out the hole over time, details of the stable trajectories manifest themselves in the observed dephasing properties of the trapped ensemble.
Fig. 1 Echo signal for wedge billiard with classically mixed phase space dynamics shows coherence revival at time scale associated with trap parameters (blue circles) , while for a compensated wedge in the presence of perturbing spackle pattern revival is suppressed (red cross).