77 research outputs found
Spin Wave Diffraction Control and Read-out with a Quantum Memory for Light
A scheme for control and read-out of diffracted spins waves to propagating
light fields is presented. Diffraction is obtained via sinusoidally varying
lights shifts and ideal one-to-one mapping to light is realized using a
gradient echo quantum memory. We also show that dynamical control of the
diffracted spin waves spatial orders can be implemented to realize a quantum
pulse sequencer for temporal modes that have high time-bandwidth products. Full
numerical solutions suggest that both co-propagating and couterpropagating
light shift geometries can be used, making the proposal applicable to hot and
cold atomic vapours as well as solid state systems with two-level atoms.Comment: 5 pages, 3 figure
Fast quantum control in dissipative systems using dissipationless solutions
We report on a systematic geometric procedure, built up on solutions designed
in the absence of dissipation, to mitigate the effects of dissipation in the
control of open quantum systems. Our method addresses a standard class of open
quantum systems modeled by non-Hermitian Hamiltonians. It provides the
analytical expression of the extra magnetic field to be superimposed to the
driving field in order to compensate the geometric distortion induced by
dissipation, and produces an exact geometric optimization of fast population
transfer. Interestingly, it also preserves the robustness properties of
protocols originally optimized against noise. Its extension to two interacting
spins restores a fidelity close to unity for the fast generation of Bell state
in the presence of dissipation
Connection between inverse engineering and optimal control in shortcuts to adiabaticity
We consider fast high-fidelity quantum control by using a shortcut to
adiabaticity (STA) technique and optimal control theory (OCT). Three specific
examples, including expansion of cold atoms from the harmonic trap, atomic
transport by moving harmonic trap, and spin dynamics in the presence of
dissipation, are explicitly detailed. Using OCT as a qualitative guide, we
demonstrate how STA protocols designed from inverse engineering method, can
approach with very high precision optimal solutions built about physical
constraints, by a proper choice of the interpolation function and with a very
reduced number of adjustable parameters.Comment: 23 pages, 16 figure
Shortcuts To Adiabaticity for L\'evy processes in harmonic traps
L\'evy stochastic processes, with noise distributed according to a L\'evy
stable distribution, are ubiquitous in science. Focusing on the case of a
particle trapped in an external harmonic potential, we address the problem of
finding "shortcuts to adiabaticity": after the system is prepared in a given
initial stationary state, we search for time-dependent protocols for the
driving external potential, such that a given final state is reached in a
given, finite time. These techniques, usually used for stochastic processes
with additive Gaussian noise, are typically based on a inverse-engineering
approach. We generalise the approach to the wider class of L\'evy stochastic
processes, both in the overdamped and in the underdamped regime, by finding
exact equations for the relevant characteristic functions in Fourier space.Comment: 15 pages, 7 figure
Chaotic dynamics and fractal structures in experiments with cold atoms
We use tools from nonlinear dynamics to the detailed analysis of cold atom
experiments. A powerful example is provided by the recent concept of basin
entropy which allows to quantify the final state unpredictability that results
from the complexity of the phase space geometry. We show here that this enables
one to reliably infer the presence of fractal structures in phase space from
direct measurements. We illustrate the method with numerical simulations in an
experimental configuration made of two crossing laser guides that can be used
as a matter wave splitter
Optimal work in a harmonic trap with bounded stiffness
We apply Pontryagin's principle to drive rapidly a trapped overdamped
Brownian particle in contact with a thermal bath between two equilibrium states
corresponding to different trap stiffness . We work out the optimal
time dependence by minimising the work performed on the particle
under the non-holonomic constraint , an
experimentally relevant situation. Several important differences arise, as
compared with the case of unbounded stiffness that has been analysed in the
literature. First, two arbitrary equilibrium states may not always be
connected. Second, depending on the operating time and the
desired compression ratio , different
types of solutions emerge. Finally, the differences in the minimum value of the
work brought about by the bounds may become quite large, which may have a
relevant impact on the optimisation of heat engines.Comment: 16 pages, 9 figures; submitted to Physical Review
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