3,323 research outputs found

### Frictionless quantum quenches in ultracold gases: a quantum dynamical microscope

In this article, a method is proposed to spatially scale up a trapped
ultracold gas while conserving the quantum correlations of the initial
many-body state. For systems supporting self-similar dynamics, this is achieved
by implementing a many-body finite-time frictionless quantum quench of the
harmonic trap which acts as a quantum dynamical microscope.Comment: 5 pages, 3 figure

### Non Singular Origin of the Universe and the Cosmological Constant Problem (CCP)

We consider a non singular origin for the Universe starting from an Einstein
static Universe in the framework of a theory which uses two volume elements
$\sqrt{-{g}}d^{4}x$ and $\Phi d^{4}x$, where $\Phi$ is a metric independent
density, also curvature, curvature square terms, first order formalism and for
scale invariance a dilaton field $\phi$ are considered in the action. In the
Einstein frame we also add a cosmological term that parametrizes the zero point
fluctuations. The resulting effective potential for the dilaton contains two
flat regions, for $\phi \rightarrow \infty$ relevant for the non singular
origin of the Universe and $\phi \rightarrow -\infty$, describing our present
Universe. Surprisingly, avoidance of singularities and stability as $\phi
\rightarrow \infty$ imply a positive but small vacuum energy as $\phi
\rightarrow -\infty$. Zero vacuum energy density for the present universe is
the "threshold" for universe creation.Comment: awarded an honorable mention in the Gravity Research Foundation 2011
Awards for Essays in Gravitation for 201

### Controlling quantum critical dynamics of isolated systems

Controlling the non adiabatic dynamics of isolated quantum systems driven
through a critical point is of interest in a variety of fields ranging from
quantum simulation to finite-time thermodynamics. We briefly review the
different methods for designing protocols which minimize excitation (defect)
production in a closed quantum critical system driven out of equilibrium. We
chart out the role of specific driving schemes for this procedure, point out
their experimental relevance, and discuss their implementation in the context
of ultracold atom and spin systems.Comment: Second version of invited review article submitted to EPJ-ST.
References added, typos corrected. 3 figures, 14 p

### Atom laser dynamics in a tight-waveguide

We study the transient dynamics that arise during the formation of an atom
laser beam in a tight waveguide. During the time evolution the density profile
develops a series of wiggles which are related to the diffraction in time
phenomenon. The apodization of matter waves, which relies on the use of smooth
aperture functions, allows to suppress such oscillations in a time interval,
after which there is a revival of the diffraction in time. The revival time
scale is directly related to the inverse of the harmonic trap frequency for the
atom reservoir.Comment: 6 pages, 5 figures, to be published in the Proceedings of the 395th
WE-Heraeus Seminar on "Time Dependent Phenomena in Quantum Mechanics ",
organized by T. Kramer and M. Kleber (Blaubeuren, Germany, September 2007

### Non Singular Origin of the Universe and its Present Vacuum Energy Density

We consider a non singular origin for the Universe starting from an Einstein
static Universe, the so called "emergent universe" scenario, in the framework
of a theory which uses two volume elements $\sqrt{-{g}}d^{4}x$ and $\Phi
d^{4}x$, where $\Phi$ is a metric independent density, used as an additional
measure of integration. Also curvature, curvature square terms and for scale
invariance a dilaton field $\phi$ are considered in the action. The first order
formalism is applied. The integration of the equations of motion associated
with the new measure gives rise to the spontaneous symmetry breaking (S.S.B) of
scale invariance (S.I.). After S.S.B. of S.I., it is found that a non trivial
potential for the dilaton is generated. In the Einstein frame we also add a
cosmological term that parametrizes the zero point fluctuations. The resulting
effective potential for the dilaton contains two flat regions, for $\phi
\rightarrow \infty$ relevant for the non singular origin of the Universe,
followed by an inflationary phase and $\phi \rightarrow -\infty$, describing
our present Universe. The dynamics of the scalar field becomes non linear and
these non linearities are instrumental in the stability of some of the emergent
universe solutions, which exists for a parameter range of values of the vacuum
energy in $\phi \rightarrow -\infty$, which must be positive but not very big,
avoiding the extreme fine tuning required to keep the vacuum energy density of
the present universe small. Zero vacuum energy density for the present universe
defines the threshold for the creation of the universe.Comment: 28 pages, short version of this paper awarded an honorable mention by
the Gravity Research Foundation, 2011, accepted for publication in
International Journal of Modern Physics

### Shortcuts to Adiabaticity Assisted by Counterdiabatic Born-Oppenheimer Dynamics

Shortcuts to adiabaticity (STA) provide control protocols to guide the
dynamics of a quantum system through an adiabatic reference trajectory in an
arbitrary prescheduled time. Designing STA proves challenging in complex
quantum systems when the dynamics of the degrees of freedom span different time
scales. We introduce Counterdiabatic Born-Oppenheimer Dynamics (CBOD) as a
framework to design STA in systems with a large separation of energy scales.
CBOD exploits the Born-Oppenheimer approximation to separate the Hamiltonian
into effective fast and slow degrees of freedom and calculate the corresponding
counterdiabatic drivings for each subsystem. We show the validity of the CBOD
technique via an example of coupled harmonic oscillators, which can be solved
exactly for comparison, and further apply it to a system of two-charged
particles.Comment: 14 pages, 3 figures, published New Journal of Physic

### Scaling-up quantum heat engines efficiently via shortcuts to adiabaticity

The finite-time operation of a quantum heat engine that uses a single
particle as a working medium generally increases the output power at the
expense of inducing friction that lowers the cycle efficiency. We propose to
scale up a quantum heat engine utilizing a many-particle working medium in
combination with the use of shortcuts to adiabaticity to boost the nonadiabatic
performance by eliminating quantum friction and reducing the cycle time. To
this end, we first analyze the finite-time thermodynamics of a quantum Otto
cycle implemented with a quantum fluid confined in a time-dependent harmonic
trap. We show that nonadiabatic effects can be controlled and tailored to match
the adiabatic performance using a variety of shortcuts to adiabaticity. As a
result, the nonadiabatic dynamics of the scaled-up many-particle quantum heat
engine exhibits no friction and the cycle can be run at maximum efficiency with
a tunable output power. We demonstrate our results with a working medium
consisting of particles with inverse-square pairwise interactions, that
includes noninteracting and hard-core bosons as limiting cases.Comment: 15 pages, 3 figures; typo in Eq. (51) fixed. Feature paper in the
Special Issue "Quantum Thermodynamics" edited by Prof. Dr. Ronnie Koslof

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