Dynamics stabilization and transport coherency in a rocking ratchet for cold atoms

Cold atoms in optical lattices have emerged as an ideal system to investigate the ratchet effect, as demonstrated by several recent experiments. In this work we analyze theoretically two aspects of ac driven transport in cold atoms ratchets. We first address the issue of whether, and to which extent, an ac driven ratchet for cold atoms can operate as a motor. We thus study theoretically a dissipative motor for cold atoms, as obtained by adding a load to a 1D non-adiabatically driven rocking ratchet. We demonstrate that a current can be generated also in the presence of a load, e.g. the ratchet device can operate as a motor. Correspondingly, we determine the stall force for the motor, which characterizes the range of loads over which the device can operate as a motor, and the differential mobility, which characterizes the response to a change in the magnitude of the load. Second, we compare our results for the transport in an ac driven ratchet device with the transport in a dc driven system. We observe a peculiar phenomenon: the bi-harmonic ac force stabilizes the dynamics, allowing the generation of uniform directed motion over a range of momentum much larger than what is possible with a dc bias. We explain such a stabilization of the dynamics by observing that a non-adiabatic ac drive broadens the effective cooling momentum range, and forces the atom trajectories to cover such a region. Thus the system can dissipate energy and maintain a steady-state energy balance. Our results show that in the case of a finite-range velocity-dependent friction, a ratchet device may offer the possibility of controlling the particle motion over a broader range of momentum with respect to a purely biased system, although this is at the cost of a reduced coherency

Dissipation-induced symmetry breaking in a driven optical lattice

We analyze the atomic dynamics in an ac driven periodic optical potential which is symmetric in both time and space. We experimentally demonstrate that in the presence of dissipation the symmetry is broken, and a current of atoms through the optical lattice is generated as a result

Electromagnetic imaging with atomic magnetometers: applications in security and surveillance

We give an overview of our research programme on the use of atomic magnetometers to detect and image concealed conductive objects via electromagnetic induction. The extreme sensitivity of atomic magnetometers at low frequency, several orders of magnitude higher than a coil-based system of similar size, allows for their operation in such a frequency range, thus permitting deep penetration through different barriers. This overcomes the limitations usually associated with electromagnetic detection. Applications in security and surveillance are discussed

Current reversals in a rocking ratchet: dynamical vs symmetry-breaking mechanisms

Directed transport in ratchets is determined by symmetry-breaking in a system out of equilibrium. A hallmark of rocking ratchets is current reversals: an increase in the rocking force changes the direction of the current. In this work for a bi-harmonically driven spatially symmetric rocking ratchet we show that a class of current reversal is precisely determined by symmetry-breaking, thus creating a link between dynamical and symmetry-breaking mechanisms

Inhibition of electromagnetically induced absorption due to excited state decoherence in Rb vapor

The explanation presented in [Taichenachev et al, Phys. Rev. A {\bf 61}, 011802 (2000)] according to which the electromagnetically induced absorption (EIA) resonances observed in degenerate two level systems are due to coherence transfer from the excited to the ground state is experimentally tested in a Hanle type experiment observing the parametric resonance on the $$ line of $^{87}$Rb. While EIA occurs in the $F=1\to F^{\prime}=2$ transition in a cell containing only $Rb$ vapor, collisions with a buffer gas ($30 torr$ of $Ne$) cause the sign reversal of this resonance as a consequence of collisional decoherence of the excited state. A theoretical model in good qualitative agreement with the experimental results is presented.Comment: 8 pages, 7 figures, submitted to Physical Review