Michael d'Arcy

Experimental observation of high-order quantum accelerator modes.

Using a freely falling cloud of cold cesium atoms periodically kicked by pulses from a vertical standing wave of laser light, we present the first experimental observation of high-order quantum accelerator modes. This confirms the recent prediction by Fishman, Guarneri, and Rebuzzini [Phys. Rev. Lett. 89, 084101 (2002)]]. We also show how these accelerator modes can be identified with the stable regions of phase space in a classical-like chaotic system, despite their intrinsically quantum origin.

Signatures of quantum stability in a classically chaotic system

We experimentally and numerically investigate the quantum accelerator mode dynamics of an atom optical realization of the quantum delta-kicked accelerator, whose classical dynamics are chaotic. Using a Ramsey-type experiment, we observe interference, demonstrating that quantum accelerator modes are formed coherently. We construct a link between the behavior of the evolutions fidelity and the phase space structure of a recently proposed pseudoclassical map, and thus account for the observed interference visibilities.

Decoherence as a probe of coherent quantum dynamics

The effect of decoherence, induced by spontaneous emission, on the dynamics of cold atoms periodically kicked by an optical lattice is experimentally and theoretically studied. Ideally, the mean energy growth is essentially unaffected by weak decoherence, but the resonant momentum distributions are fundamentally altered. It is shown that experiments are inevitably sensitive to certain nontrivial features of these distributions, in a way that explains the puzzle of the observed enhancement of resonances by decoherence [Phys. Rev. Lett. 87, 074102 (2001)]. This clarifies both the nature of the coherent evolution, and the way in which decoherence disrupts it.

Gravity-Sensitive Quantum Dynamics in Cold Atoms

We subject a falling cloud of cold cesium atoms to periodic kicks from a sinusoidal potential created by a vertical standing wave of laser light. By controllably accelerating the potential, we show quantum accelerator mode dynamics to be highly sensitive to the effective gravitational acceleration when close to specific, resonant values. This quantum sensitivity to a control parameter is reminiscent of that associated with classical chaos and promises techniques for precision measurement.

How dark is a grey state

We present experimental results showing that a dark state with finite lifetime, a grey state, becomes darker the more the atom is exposed to on-resonant light. We discuss this phenomenon theoretically in the context of the complex eigenenergies and show that it can be understood qualitatively in terms of the quantum Zeno effect. The predicted dependence of the lifetime on the composition of the grey state and on the light intensity is experimentally confirmed with caesium atoms. This has implications for velocity-selective coherent population trapping and adiabatic transfer.

Limits of the separated-path Ramsey atom interferometer

We describe in detail our caesium atom interferometer which uses a combination of microwaves and momentum-changing adiabatic transfer pulses. This combination allows us to achieve spatial separation between the arms of the interferometer. We account for the observed visibility of the resulting interference fringes and find that the effects which contribute the most are optical pumping and magnetic fields.