The quantum kicked rotor is studied experimentally in an atom-optics setting, where we observe the center-of-mass motion of cold cesium atoms. Dynamical localization in this system typically suppresses classical diffusive motion, but is susceptible to the addition of various forms of noise. We study in detail the effects of timing noise, where variations are introduced to the times at which the kicks occur. This noise is particularly interesting because it does not directly induce momentum diffusion. However, it is found that the addition of timing noise efficiently destroys both the classical correlations that give rise to fluctuations in the classical diffusion rate as well as quantum coherences that lead to dynamical localization. Ó 2002 Elsevier Science Ltd. All rights reserved.The interface between quantum and classical dynamics serves as a testing ground for decoherence and is a central feature of mesoscopic physics. It is believed that decoherence, the destruction of quantum interferences, is necessary to reconcile the distinctly different behavior in the quantum and classical limits [1]. Experimental progress towards understanding decoherence has been made on a number of different fronts. One system that has the advantage of conceptual simplicity is a perturbed atom interferometer [2,3], where spontaneous scattering introduces dissipation into the system. Decoherence has also been observed with Rydberg atoms coupled to microwave cavities [4] and motional Schr€ o odinger cat states in an ion trap [5].One of the most interesting settings in which to study correspondence is a classically chaotic system, where quantum effects suppress classical diffusive motion [6]. While the majority of research in decohering effects on ''quantum chaos'' has been theoretical in nature [7][8][9][10][11], there has been a steadily growing body of experimental work in recent years. Some of the pioneering experiments in this field have been conducted by studying the internal dynamics of Rydberg atoms [12][13][14][15]. Mesoscopic condensed-matter systems have provided another important testing ground [16]. Recent great advances in atom optics, the manipulation of atomic de Broglie waves, have also made it possible to study quantum dynamics by observing the center-of-mass motion of cold atoms [17,18]. Here, laboratory tests have allowed the direct observation of such phenomena as dynamical localization [19], quantum resonances [20], and chaos-assisted tunneling [21].Of particular interest is dynamical localization [22], a quantum suppression of classically chaotic (diffusive) motion. A prototypical system is the kicked rotor, or standard map, which has been a model system for the study of both classical and quantum chaos. The kicked rotor is a particularly suitable system for experimental study because it can be directly realized by exposing cold atoms to a pulsed standing wave of light. Classical particles in such a chaotic potential tend to exhibit diffusive behavior, where the energy of the system grows linearly as a f...