We present an experimental investigation of the local dynamics and spatial diffusion of atoms in a rubidium optical lattice using polarization-selective intensity correlation spectroscopy in both the time and frequency domains. Autocorrelations of the circularly polarized components of the fluorescence light are shown to give access to the atomic dynamics in a single optical potential well. By contrast, cross correlations between opposite circular components of the scattered light provide information about atomic transport in the lattice through escape and recapture between neighbor potential wells.
We present an experimental investigation of the local dynamics and spatial diffusion of atoms in a rubidium optical lattice using polarization-selective intensity correlation spectroscopy in both the time and frequency domains. Autocorrelations of the circularly polarized components of the fluorescence light are shown to give access to the atomic dynamics in a single optical potential well. By contrast, cross correlations between opposite circular components of the scattered light provide information about atomic transport in the lattice through escape and recapture between neighbor potential wells.
This thesis includes both experimental and theoretical investigations of fluctuation-induced transport phenomena, presented in a series of nine papers, by studies of the dynamics of cold atoms in dissipative optical lattices. With standard laser cooling techniques about 108 cesium atoms are accumulated, cooled to a few μK, and transferred into a dissipative optical lattice. An optical lattice is a periodic light-shift potential, and in dissipative optical lattice the light field is sufficiently close to resonance for incoherent light scattering to be of importance. This provides the system with a diffusive force, but also with a friction through laser cooling mechanisms. In the dissipative optical lattices the friction and the diffusive force will eventually reach a steady state. At steady state, the thermal energy is low enough, compared to the potential depth, for the atoms to be localized close to the potential minima, but high enough for the atoms to occasionally make inter-well flights. This leads to a Brownian motion of the atoms in the optical lattices. In the normal case these random walks average to zero, leading to a symmetric, isotropic diffusion of the atoms. If the optical lattices are tilted, the symmetry is broken and the diffusion will be biased. This leads to a fluctuation-induced drift of the atoms. In this thesis an investigation of such drifts, for an optical lattice tilted by the gravitational force, is presented. We show that even though the tilt over a potential period is small compared to the potential depth, it clearly affect the dynamics of the atoms, and despite the complex details of the system it can, to a good approximation, be described by the Langevin equation formalism for a particle in a periodic potential. The linear drifts give evidence of stop-and-go dynamics where the atoms escape the potential wells and travel over one or more wells before being recaptured. Brownian motors open the possibility of creating fluctuation-induced drifts in the absence of bias forces, if two requirements are fulfilled: the symmetry has to be broken and the system has to be brought out of thermal equilibrium. By utilizing two distinguishable optical lattices, with a relative spatial phase and unequal transfer rates between them, these requirements can be fulfilled. In this thesis, such a Brownian motor is realized, and drifts in arbitrary directions in 3D are demonstrated. We also demonstrate a real-time steering of the transport as well as drifts along pre-designed paths. Moreover, we present measurements and discussions of performance characteristics of the motor, and we show that the required asymmetry can be obtained in multiple ways.
The present work investigates the state-selective transport of single neutral cesium atoms in a one-dimensional optical lattice. It demonstrates experimental applications of this transport, including a single atom interferometer, a quantum walk and controlled two-atom collisions. The atoms are stored one by one in an optical lattice formed by a standing wave dipole trap. Their positions are determined with sub-micrometer precision, while atom pair separations are reliably inferred down to neighboring lattice sites using real-time numerical processing. Using microwave pulses in the presence of a magnetic field gradient, the internal qubit states, encoded in the hyperfine levels of the atoms, can be separately initialized and manipulated. This allows us to perform arbitrary single-qubit operations and prepare arbitrary patterns of atoms in the lattice with single-site precision. Chapter 1 presents the experimental setup for trapping a small number of cesium atoms in a one-dimensional optical lattice. Chapter 2 is devoted to fluorescence imaging of atoms, discussing the imaging setup, numeric methods and their performance in detail. Chapter 3 focuses on engineering of internal states of trapped atoms in the lattice using optical methods and microwave radiation. It provides a detailed investigation of coherence properties of our experimental system. Finally manipulation of individual atoms with almost single-site resolution and preparation of regular strings of atoms with predefined distances are presented. In Chapter 4, basic concepts, the experimental realization and the performance of the state-selective transport of neutral atoms over several lattice sites are presented and discussed in detail. Coherence properties of this transport are investigated in Chapter 5, using various two-arms single atom interferometer sequences in which atomic matter waves are split, delocalized, merged and recombined on the initial lattice site, while the interference contrast and the accumulated phase difference are measured. By delocalizing a single atom over several lattice sites, possible spatial inhomogeneities of fields along the lattice axis in the trapping region are probed. In Chapter 6, experimental realization of a discrete time quantum walk on a line with single optically trapped atoms is presented as an advanced application of multiple path quantum interference in the context of quantum information processing. Using this simple example of a quantum walk, fundamental properties of and differences between the quantum and classical regimes are investigated and discussed in detail. Finally, by combining preparation of atom strings, position-dependent manipulation of qubit states and state-selective transport, in Chapter 7, two atoms are deterministically brought together into contact, forming a starting point for investigating two-atom interactions on the most fundamental level. Future prospects and suggestions are finally presented in Chapter 8.
We present an experimental study of a four-beam optical lattice using the light scattered by the atoms in the lattice. We use both intensity correlations and observations of the transient behaviour of the scattering when the lattice is suddenly switched on. We compare results for three different configurations of the optical lattice. We create situations in which the Lamb–Dicke effect is negligible and show that, in contrast with what has been stated in some of the literature, the damping rate of the ‘coherent’ atomic oscillations can be much longer than the inelastic photon scattering rate.
Summary form only given. The investigation of coherent superposition states for atoms interacting with external potentials allows a completely new class of experiments directed towards coherent trapping and guiding of neutral atoms. In addition, the process of decoherence, i.e., the collapse of superposition states, is one of the basic concepts for our understanding of the connection between classical and quantum physics. We study the creation and decay of superpositions of different components of the center-of-mass wavefunction of rubidium atoms in a one-dimensional near-resonant optical lattice. This makes it possible to gain insight into the dependence of decoherence processes on the coupling of the atoms to the light field. We discuss the dependence of the wave packet oscillations and echoes on experimental parameters (potential depth, light intensity and detuning, time delay, size of the shift, etc.). We compare our experimental results with numerical simulations of the echo effect. The observation of this new effect in the coherent wave packet motion of atoms trapped in optical potentials will allow us to selectively study the decoherence process and measure the decoherence time in regimes where this was previously impossible.