3,248 research outputs found
Relativistic Harmonic Oscillator Revisited
The familiar Fock space commonly used to describe the relativistic harmonic
oscillator, for example as part of string theory, is insufficient to describe
all the states of the relativistic oscillator. We find that there are three
different vacua leading to three disconnected Fock sectors, all constructed
with the same creation-annihilation operators. These have different spacetime
geometric properties as well as different algebraic symmetry properties or
different quantum numbers. Two of these Fock spaces include negative norm
ghosts (as in string theory) while the third one is completely free of ghosts.
We discuss a gauge symmetry in a worldline theory approach that supplies
appropriate constraints to remove all the ghosts from all Fock sectors of the
single oscillator. The resulting ghost free quantum spectrum in d+1 dimensions
is then classified in unitary representations of the Lorentz group SO(d,1).
Moreover all states of the single oscillator put together make up a single
infinite dimensional unitary representation of a hidden global symmetry
SU(d,1), whose Casimir eigenvalues are computed. Possible applications of these
new results in string theory and other areas of physics and mathematics are
briefly mentioned.Comment: 41 pages, 2 figures, LaTe
Quantum decoherence of the damped harmonic oscillator
In the framework of the Lindblad theory for open quantum systems, we
determine the degree of quantum decoherence of a harmonic oscillator
interacting with a thermal bath. It is found that the system manifests a
quantum decoherence which is more and more significant in time. We also
calculate the decoherence time and show that it has the same scale as the time
after which thermal fluctuations become comparable with quantum fluctuations.Comment: Talk at the XI International Conference on Quantum Optics
(ICQO'2006), May 2006, Minsk (Belarus), 9 page
Storing entanglement of nuclear spins via Uhrig Dynamical Decoupling
Stroboscopic spin flips have already been shown to prolong the coherence
times of quantum systems under noisy environments. Uhrig's dynamical decoupling
scheme provides an optimal sequence for a quantum system interacting with a
dephasing bath. Several experimental demonstrations have already verified the
efficiency of such dynamical decoupling schemes in preserving single qubit
coherences. In this work we describe the experimental study of Uhrig's
dynamical decoupling in preserving two-qubit entangled states using an ensemble
of spin-1/2 nuclear pairs in solution state. We find that the performance of
odd-order Uhrig sequences in preserving entanglement is superior to both
even-order Uhrig sequences and periodic spin-flip sequences. We also find that
there exists an optimal length of the Uhrig sequence at which the decoherence
time gets boosted from a few seconds to about 30 seconds.Comment: 6 pages, 7 figure
The General Correlation Function in the Schwinger Model on a Torus
In the framework of the Euclidean path integral approach we derive the exact
formula for the general N-point chiral densities correlator in the Schwinger
model on a torusComment: 17 pages, misprints corrected, references adde
Distillation by repeated measurements: continuous spectrum case
Repeated measurements on a part of a bipartite system strongly affect the
other part not measured, whose dynamics is regulated by an effective contracted
evolution operator. When the spectrum of this operator is discrete, the latter
system is driven into a pure state irrespective of the initial state, provided
the spectrum satisfies certain conditions. We here show that even in the case
of continuous spectrum an effective distillation can occur under rather general
conditions. We confirm it by applying our formalism to a simple model.Comment: 4 pages, 2 figure
Change of decoherence scenario and appearance of localization due to reservoir anharmonicity
Although coupling to a super-Ohmic bosonic reservoir leads only to partial
dephasing on short time scales, exponential decay of coherence appears in the
Markovian limit (for long times) if anharmonicity of the reservoir is taken
into account. This effect not only qualitatively changes the decoherence
scenario but also leads to localization processes in which superpositions of
spatially separated states dephase with a rate that depends on the distance
between the localized states. As an example of the latter process, we study the
decay of coherence of an electron state delocalized over two semiconductor
quantum dots due to anharmonicity of phonon modes.Comment: 4 pages, 1 figure; moderate changes; auxiliary material added; to
appear in Phys. Rev. Let
Universality of Uhrig dynamical decoupling for suppressing qubit pure dephasing and relaxation
The optimal -pulse dynamical decoupling discovered by Uhrig for a
spin-boson mmodel [Phys. Rev. Lett, {\bf 98}, 100504 (2007)] is proved to be
universal in suppressing to the pure dephasing or the longitudinal
relaxation of a qubit (or spin-1/2) coupled to a generic bath in a short-time
evolution of duration . It is also found that for the purpose of suppressing
the longitudinal relaxation, an ideal Uhrig -pulse sequence can be
generalized to a sequence consisting of the ideal one superimposed with
finite-duration pulses satisfying certain symmetry requirements.Comment: 4 pages, 1 figure
Quantum decoherence in the theory of open systems
In the framework of the Lindblad theory for open quantum systems, we
determine the degree of quantum decoherence of a harmonic oscillator
interacting with a thermal bath. It is found that the system manifests a
quantum decoherence which is more and more significant in time. We calculate
also the decoherence time scale and analyze the transition from quantum to
classical behaviour of the considered system.Comment: 6 pages; talk at the 3rd International Workshop "Quantum Physics and
Communication" (QPC 2005), Dubna, Russia, 200
On the precise connection between the GRW master-equation and master-equations for the description of decoherence
We point out that the celebrated GRW master-equation is invariant under
translations, reflecting the homogeneity of space, thus providing a particular
realization of a general class of translation-covariant Markovian
master-equations. Such master-equations are typically used for the description
of decoherence due to momentum transfers between system and environment.
Building on this analogy we show the exact relationship between the GRW
master-equation and decoherence master-equations, further providing a
collisional decoherence model formally equivalent to the GRW master-equation.
This allows for a direct comparison of order of magnitudes of relevant
parameters. This formal analogy should not lead to confusion on the utterly
different spirit of the two research fields, in particular it has to be
stressed that the decoherence approach does not lead to a solution of the
measurement problem. Building on this analogy however the feasibility of the
extension of spontaneous localization models in order to avoid the infinite
energy growth is discussed. Apart from a particular case considered in the
paper, it appears that the amplification mechanism is generally spoiled by such
modifications.Comment: 9 pages, latex, no figures, to appear on J. Phys.
Collective versus Single--Particle Motion in Quantum Many--Body Systems: Spreading and its Semiclassical Interpretation
We study the interplay between collective and incoherent single-particle
motion in a model of two chains of particles whose interaction comprises a
non-integrable part. In the perturbative regime, but for a general form of the
interaction, we calculate the spectral density for collective excitations. We
obtain the remarkable result that it always has a unique semiclassical
interpretation. We show this by a proper renormalization procedure which allows
us to map our system to a Caldeira-Leggett--type of model in which the bath is
part of the system.Comment: 4 page
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