3,093 research outputs found
Assisted finite-rate adiabatic passage across a quantum critical point: Exact solution for the quantum Ising model
The dynamics of a quantum phase transition is inextricably woven with the
formation of excitations, as a result of the critical slowing down in the
neighborhood of the critical point. We design a transitionless quantum driving
through a quantum critical point that allows one to access the ground state of
the broken-symmetry phase by a finite-rate quench of the control parameter. The
method is illustrated in the one-dimensional quantum Ising model in a
transverse field. Driving through the critical point is assisted by an
auxiliary Hamiltonian, for which the interplay between the range of the
interaction and the modes where excitations are suppressed is elucidated.Comment: 2 figures, 5 page
Suitability of the approximate superposition of squeezed coherent states for various quantum protocols
A state in a d-dimensional Hilbert space can be simulated by a state defined
in a different dimension with high fidelity. We assess how faithfully such the
approximated state can perform quantum protocols, using an example of the
squeezed coherent superposition state which was recently experimentally
generated.Comment: 6 pages, 4 figure
Acceleration of the Universe driven by the Casimir force
We investigate an evolutional scenario of the FRW universe with the Casimir
energy scaling like . The Casimir effect is used to explain the
vacuum energy differences (its value measured from astrophysics is so small
compared to value obtained from quantum field theory calculations). The
dynamics of the FRW model is represented in terms of a two-dimensional
dynamical system to show all evolutional paths of this model in the phase space
for all admissible initial conditions. We find also an exact solution for non
flat evolutional paths of Universe driven by the Casimir effect. The main
difference between the FRW model with the Casimir force and the CDM
model is that their generic solutions are a set of evolutional paths with a
bounce solution and an initial singularity, respectively. The evolutional
scenario are tested by using the SNIa data, FRIIb radiogalaxies, baryon
oscillation peak and CMB observation. We compare the power of explanation of
the model considered and the CDM model using the Bayesian information
criterion and Bayesian factor. Our investigation of the information criteria of
model selection showed the preference of the CDM model over the model
considered. However the presence of negative like the radiation term can remove
a tension between the theoretical and observed primordial He and D
abundance.Comment: RevTeX4, 17 pages, 9 figure
"Background Field Integration-by-Parts" and the Connection Between One-Loop and Two-Loop Heisenberg-Euler Effective Actions
We develop integration-by-parts rules for Feynman diagrams involving massive
scalar propagators in a constant background electromagnetic field, and use
these to show that there is a simple diagrammatic interpretation of mass
renormalization in the two-loop scalar QED Heisenberg-Euler effective action
for a general constant background field. This explains why the square of a
one-loop term appears in the renormalized two-loop Heisenberg-Euler effective
action. No integrals need be evaluated, and the explicit form of the background
field propagators is not needed. This dramatically simplifies the computation
of the renormalized two-loop effective action for scalar QED, and generalizes a
previous result obtained for self-dual background fields.Comment: 13 pages; uses axodraw.st
Do all pure entangled states violate Bell's inequalities for correlation functions?
Any pure entangled state of two particles violates a Bell inequality for
two-particle correlation functions (Gisin's theorem). We show that there exist
pure entangled N>2 qubit states that do not violate any Bell inequality for N
particle correlation functions for experiments involving two dichotomic
observables per local measuring station. We also find that
Mermin-Ardehali-Belinskii-Klyshko inequalities may not always be optimal for
refutation of local realistic description.Comment: 4 pages, journal versio
Cosmological zoo -- accelerating models with dark energy
ecent observations of type Ia supernovae indicate that the Universe is in an
accelerating phase of expansion. The fundamental quest in theoretical cosmology
is to identify the origin of this phenomenon. In principle there are two
possibilities: 1) the presence of matter which violates the strong energy
condition (a substantial form of dark energy), 2) modified Friedmann equations
(Cardassian models -- a non-substantial form of dark matter). We classify all
these models in terms of 2-dimensional dynamical systems of the Newtonian type.
We search for generic properties of the models. It is achieved with the help of
Peixoto's theorem for dynamical system on the Poincar{\'e} sphere. We find that
the notion of structural stability can be useful to distinguish the generic
cases of evolutional paths with acceleration. We find that, while the
CDM models and phantom models are typical accelerating models, the
cosmological models with bouncing phase are non-generic in the space of all
planar dynamical systems. We derive the universal shape of potential function
which gives rise to presently accelerating models. Our results show explicitly
the advantages of using a potential function (instead of the equation of state)
to probe the origin of the present acceleration. We argue that simplicity and
genericity are the best guide in understanding our Universe and its
acceleration.Comment: RevTeX4, 23 pages, 10 figure
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