476 research outputs found
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
Rayleigh scattering and atomic dynamics in dissipative optical lattices
We investigate Rayleigh scattering in dissipative optical lattices. In particular, following recent proposals [S. Guibal et al., Phys. Rev. Lett. 78, 4709 (1997); C. Jurczak et al., Phys. Rev. Lett. 77, 1727 (1996)], we study whether the Rayleigh resonance originates from the diffraction on a density grating and is therefore a probe of transport of atoms in optical lattices. It turns out that this is not the case: the Rayleigh line is instead a measure of the cooling rate, while spatial diffusion contributes to the scattering spectrum with a much broader resonance
- …