1,883 research outputs found
Asymptotic theory of quasiperiodically driven quantum systems
The theoretical treatment of quasi-periodically driven quantum systems is
complicated by the inapplicability of the Floquet theorem, which requires
strict periodicity. In this work we consider a quantum system driven by a
bi-harmonic driving and examine its asymptotic long-time limit, the limit in
which features distinguishing systems with periodic and quasi-periodic driving
occur. Also, in the classical case this limit is known to exhibit universal
scaling, independent of the system details, with the system's reponse under
quasi-periodic driving being described in terms of nearby periodically driven
system results. We introduce a theoretical framework appropriate for the
treatment of the quasi-periodically driven quantum system in the long-time
limit, and derive an expression, based on Floquet states for a periodically
driven system approximating the different steps of the time evolution, for the
asymptotic scaling of relevant quantities for the system at hand. These
expressions are tested numerically, finding excellent agreement for the
finite-time average velocity in a prototypical quantum ratchet consisting of a
space-symmetric potential and a time-asymmetric oscillating force
Hidden symmetries, instabilities, and current suppression in Brownian ratchets
The operation of Brownian motors is usually described in terms of
out-of-equilibrium and symmetry-breaking settings, with the relevant
spatiotemporal symmetries identified from the analysis of the equations of
motion for the system at hand. When the appropriate conditions are satisfied,
symmetry-related trajectories with opposite current are thought to balance each
other, yielding suppression of transport. The direction of the current can be
precisely controlled around these symmetry points by finely tuning the driving
parameters. Here we demonstrate, by studying a prototypical Brownian ratchet
system, the existence of {\it hidden} symmetries, which escape the
identification by the standard symmetry analysis, and require different
theoretical tools for their revelation. Furthermore, we show that system
instabilities may lead to spontaneous symmetry breaking with unexpected
generation of directed transport.Comment: To appear in Phys. Rev. Let
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
Sub-picotesla widely tunable atomic magnetometer operating at room-temperature in unshielded environments
We report on a single-channel rubidium radio-frequency atomic magnetometer
operating in un-shielded environments and near room temperature with a measured
sensitivity of 130 fT/\sqrt{Hz}. We demonstrate consistent, narrow-bandwidth
operation across the kHz - MHz band, corresponding to three orders of magnitude
of magnetic field amplitude. A compensation coil system controlled by a
feedback loop actively and automatically stabilizes the magnetic field around
the sensor. We measure a reduction of the 50 Hz noise contribution by an order
of magnitude. The small effective sensor volume, 57 mm^3, increases the spatial
resolution of the measurements. Low temperature operation, without any magnetic
shielding, coupled with the broad tunability, and low beam power, dramatically
extends the range of potential field applications for our device.Comment: Main text: 6 pages, 9 figures. Supplementary material: 3 pages, 3
figures. Published version can be found at
https://aip.scitation.org/doi/full/10.1063/1.5026769 . V2: Added journal
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