Symmetry breaking and directed transport of cold atoms in optical lattices
The central theme of this thesis is the directed transport of cold atoms in optical lattices. Several methods for producing such an outcome are proposed, experimentally realised and characterised, in both dissipative and non-dissipative regimes. Accordingly, this thesis may be thought to be composed of two parts. The first section reports results of directed transport in non-dissipative optical lattices, where we used the atom optics realisation of the delta-kicked rotor (DKR) as a model system. Initial experiments performed on this system showed evidence of dynamical localisation, the signature of quantum chaos, and we exploited the presence of the momentum boundary (a barrier to diffusion arising from the approximation of delta-kicks with finite width impulses) to produce asymmetric momentum diffusion. Breaking the DKR system symmetries produced directed diffusion due to chaotic dynamics alone. We observed directed transport in a spatially symmetric system whenever temporal symmetry was broken, and also when the spatial and temporal symmetry of the DKR were simultaneously broken. We also report the first experimental evidence for a 'double-DKR', where experiments performed using a kick sequence composed of closely spaced pairs of kicks instead of single kicks caused significant alterations in the observed behaviour of the kicked rotor system. The origin of this new behaviour was explained in the framework of kick-to-kick correlations and our results were found to be in excellent agreement with numerical simulations. The second part of this thesis describes the realisation of a Brownian ratchet in a 3D dissipative lattice, where fluctuations between different potential surfaces acted as the source of noise. These fluctuations were rectified by the application of a periodic, bi-harmonic driving force, which broke the temporal symmetry of the system. Both current reversal and stochastic resonance behaviour characteristic of a Brownian ratchet were observed.