1,977 research outputs found
Unconditional Pointer States from Conditional Master Equations
When part of the environment responsible for decoherence is used to extract
information about the decohering system, the preferred {\it pointer states}
remain unchanged. This conclusion -- reached for a specific class of models --
is investigated in a general setting of conditional master equations using
suitable generalizations of predictability sieve. We also find indications that
the einselected states are easiest to infer from the measurements carried out
on the environment.Comment: 4 pages, 3 .eps figures; final version to appear in Phys.Rev.Let
Topological Schr\"odinger cats: Non-local quantum superpositions of topological defects
Topological defects (such as monopoles, vortex lines, or domain walls) mark
locations where disparate choices of a broken symmetry vacuum elsewhere in the
system lead to irreconcilable differences. They are energetically costly (the
energy density in their core reaches that of the prior symmetric vacuum) but
topologically stable (the whole manifold would have to be rearranged to get rid
of the defect). We show how, in a paradigmatic model of a quantum phase
transition, a topological defect can be put in a non-local superposition, so
that - in a region large compared to the size of its core - the order parameter
of the system is "undecided" by being in a quantum superposition of conflicting
choices of the broken symmetry. We demonstrate how to exhibit such a
"Schr\"odinger kink" by devising a version of a double-slit experiment suitable
for topological defects. Coherence detectable in such experiments will be
suppressed as a consequence of interaction with the environment. We analyze
environment-induced decoherence and discuss its role in symmetry breaking.Comment: 7 pages, 4 figure
Decoherence, Chaos, and the Second Law
We investigate implications of decoherence for quantum systems which are
classically chaotic. We show that, in open systems, the rate of von Neumann
entropy production quickly reaches an asymptotic value which is: (i)
independent of the system-environment coupling, (ii) dictated by the dynamics
of the system, and (iii) dominated by the largest Lyapunov exponent. These
results shed a new light on the correspondence between quantum and classical
dynamics as well as on the origins of the ``arrow of time.''Comment: 13 Pages, 2 Figures available upon request, Preprint LA-UR-93-, The
new version contains the text, the previous one had only the Macros: sorry
Quantum to classical transition in a system with a mixed classical dynamics
We study how decoherence rules the quantum-classical transition of the Kicked
Harmonic Oscillator (KHO). When the amplitude of the kick is changed the system
presents a classical dynamics that range from regular to a strong chaotic
behavior. We show that for regular and mixed classical dynamics, and in the
presence of noise, the distance between the classical and the quantum phase
space distributions is proportional to a single parameter which relates the effective Planck constant
, the kick amplitude and the diffusion constant . This
is valid when , a case that is always attainable in the semiclassical
regime independently of the value of the strength of noise given by . Our
results extend a recent study performed in the chaotic regime.Comment: 10 pages, 7 figure
Fragility of a class of highly entangled states of many quantum-bits
We consider a Quantum Computer with n quantum-bits (`qubits'), where each
qubit is coupled independently to an environment affecting the state in a
dephasing or depolarizing way. For mixed states we suggest a quantification for
the property of showing {\it quantum} uncertainty on the macroscopic level. We
illustrate in which sense a large parameter can be seen as an indicator for
large entanglement and give hypersurfaces enclosing the set of separable
states. Using methods of the classical theory of maximum likelihood estimation
we prove that this parameter is decreasing with 1/\sqrt{n} for all those states
which have been exposed to the environment.
Furthermore we consider a Quantum Computer with perfect 1-qubit gates and
2-qubit gates with depolarizing error and show that any state which can be
obtained from a separable initial state lies inbetween a family of pairs of
certain hypersurfaces parallel to those enclosing the separable ones.Comment: 9 Pages, RevTe
Closed timelike curves in superfluid He
It is shown that the curved spacetime induced in a thin film of superfluid
He-A by the presence of symmetric vortices with the unbroken symmetry
phase, admits the existence of closed timelike curves through which only
superfluid clusters formed by anti-He atoms can travel and violate
causality.Comment: 7 pages, LaTex, to appear in Phys. Lett.
Finite-Time Disentanglement via Spontaneous Emission
We show that under the influence of pure vacuum noise two entangled qubits
become completely disentangled in a finite time, and in a specific example we
find the time to be given by times the
usual spontaneous lifetime.Comment: revtex, 4 pages, 2 figure
Moduli Stabilization and Supersymmetry Breaking in Deflected Mirage Mediation
We present a model of supersymmetry breaking in which the contributions from
gravity/modulus, anomaly, and gauge mediation are all comparable. We term this
scenario "deflected mirage mediation," which is a generalization of the
KKLT-motivated mirage mediation scenario to include gauge mediated
contributions. These contributions deflect the gaugino mass unification scale
and alter the pattern of soft parameters at low energies. In some cases, this
results in a gluino LSP and light stops; in other regions of parameter space,
the LSP can be a well-tempered neutralino. We demonstrate explicitly that
competitive gauge-mediated terms can naturally appear within phenomenological
models based on the KKLT setup by addressing the stabilization of the gauge
singlet field which is responsible for the masses of the messenger fields. For
viable stabilization mechanisms, the relation between the gauge and anomaly
contributions is identical in most cases to that of deflected anomaly
mediation, despite the presence of the Kahler modulus. Turning to TeV scale
phenomenology, we analyze the renormalization group evolution of the
supersymmetry breaking terms and the resulting low energy mass spectra. The
approach sets the stage for studies of such mixed scenarios of supersymmetry
breaking at the LHC.Comment: 33 pages, 8 figures. Published version in Journal of High Energy
Physic
Dynamics of a Quantum Phase Transition
We present two approaches to the dynamics of a quench-induced phase
transition in quantum Ising model. The first one retraces steps of the standard
approach to thermodynamic second order phase transitions in the quantum
setting. The second one is purely quantum, based on the Landau-Zener formula
for transition probabilities in avoided level crossings. We show that the two
approaches yield compatible results for the scaling of the defect density with
the quench rate. We exhibit similarities between them, and comment on the
insights they give into dynamics of quantum phase transitions.Comment: 4 pages, 3 figures. Replaced by revised versio
- …