5,837 research outputs found
Conditional quantum dynamics with several observers
We consider several observers who monitor different parts of the environment
of a single quantum system and use their data to deduce its state. We derive a
set of conditional stochastic master equations that describe the evolution of
the density matrices each observer ascribes to the system under the Markov
approximation, and show that this problem can be reduced to the case of a
single "super-observer", who has access to all the acquired data. The key
problem - consistency of the sets of data acquired by different observers - is
then reduced to the probability that a given combination of data sets will be
ever detected by the "super-observer". The resulting conditional master
equations are applied to several physical examples: homodyne detection of
phonons in quantum Brownian motion, photo-detection and homodyne detection of
resonance fluorescence from a two-level atom. We introduce {\it relative
purity} to quantify the correlations between the information about the system
gathered by different observers from their measurements of the environment. We
find that observers gain the most information about the state of the system and
they agree the most about it when they measure the environment observables with
eigenstates most closely correlated with the optimally predictable {\it pointer
basis} of the system.Comment: Updated version: new title and contents. 22 pages, 8 figure
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
A ring of BEC pools as a trap for persistent flow
Mott insulator - superfluid transition in a periodic lattice of Josephson
junctions can be driven by tunneling rate increase. Resulting winding numbers
of the condensate wavefunction decrease with increasing quench time in
accord with the Kibble-Zurek mechanism (KZM). However, in very slow quenches
Bose-Hubbard dynamics rearranges wavefunction phase so that its random walk
cools, decreases and eventually the wavefunction becomes too cold
to overcome potential barriers separating different . Thus, in contrast with
KZM, in very slow quenches is set by random walk with "critical"
step size, independently of .Comment: Decompressed version to appear in Phys. Rev.
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