1,556 research outputs found
Consistency of Semiclassical Gravity
We discuss some subtleties which arise in the semiclassical approximation to
quantum gravity. We show that integrability conditions prevent the existence of
Tomonaga-Schwinger time functions on the space of three-metrics but admit them
on superspace. The concept of semiclassical time is carefully examined. We
point out that central charges in the matter sector spoil the consistency of
the semiclassical approximation unless the full quantum theory of gravity and
matter is anomaly-free. We finally discuss consequences of these considerations
for quantum field theory in flat spacetime, but with arbitrary foliations.Comment: 12 pages, LATEX, Report Freiburg THEP-94/2
Double dot chain as a macroscopic quantum bit
We consider an array of N quantum dot pairs interacting via Coulomb
interaction between adjacent dots and hopping inside each pair. We show that at
the first order in the ratio of hopping and interaction amplitudes, the array
maps in an effective two level system with energy separation becoming
exponentially small in the macroscopic (large N) limit. Decoherence at zero
temperature is studied in the limit of weak coupling with phonons. In this case
the macroscopic limit is robust with respect to decoherence. Some possible
applications in quantum information processing are discussed.Comment: Phys. Rev. A (in press
Properties of 3-manifolds for relativists
In canonical quantum gravity certain topological properties of 3-manifolds
are of interest. This article gives an account of those properties which have
so far received sufficient attention, especially those concerning the
diffeomorphism groups of 3-manifolds. We give a summary of these properties and
list some old and new results concerning them. The appendix contains a
discussion of the group of large diffeomorphisms of the -handle 3-manifold.Comment: 20 pages. Plain-TeX, no figures, 1 Table (A4 format
A Uniqueness Theorem for Constraint Quantization
This work addresses certain ambiguities in the Dirac approach to constrained
systems. Specifically, we investigate the space of so-called ``rigging maps''
associated with Refined Algebraic Quantization, a particular realization of the
Dirac scheme. Our main result is to provide a condition under which the rigging
map is unique, in which case we also show that it is given by group averaging
techniques. Our results comprise all cases where the gauge group is a
finite-dimensional Lie group.Comment: 23 pages, RevTeX, further comments and references added (May 26. '99
Controlling decoherence of a two-level-atom in a lossy cavity
By use of external periodic driving sources, we demonstrate the possibility
of controlling the coherent as well as the decoherent dynamics of a two-level
atom placed in a lossy cavity.
The control of the coherent dynamics is elucidated for the phenomenon of
coherent destruction of tunneling (CDT), i.e., the coherent dynamics of a
driven two-level atom in a quantum superposition state can be brought
practically to a complete standstill. We study this phenomenon for different
initial preparations of the two-level atom. We then proceed to investigate the
decoherence originating from the interaction of the two-level atom with a lossy
cavity mode. The loss mechanism is described in terms of a microscopic model
that couples the cavity mode to a bath of harmonic field modes. A suitably
tuned external cw-laser field applied to the two-level atom slows down
considerably the decoherence of the atom. We demonstrate the suppression of
decoherence for two opposite initial preparations of the atomic state: a
quantum superposition state as well as the ground state. These findings can be
used to the effect of a proficient battling of decoherence in qubit
manipulation processes.Comment: 12 pages including 3 figures, submitted for publicatio
Quantum rings with time dependent spin-orbit coupling: Rabi oscillations, spintronic Schrodinger-cat states, and conductance properties
The strength of the (Rashba-type) spin-orbit coupling in mesoscopic
semiconductor rings can be tuned with external gate voltages. Here we consider
the case of a periodically changing spin-orbit interaction strength as induced
by sinusoidal voltages. In a closed one dimensional quantum ring with weak
spin-orbit coupling, Rabi oscillations are shown to appear. We find that the
time evolution of initially localized wave packets exhibits a series of
collapse and revival phenomena. Partial revivals -- that are typical in
nonlinear systems -- are shown to correspond to superpositions of states
localized at different spatial positions along the ring. These "spintronic
Schrodinger-cat sates" appear periodically, and similarly to their counterparts
in other physical systems, they are found to be sensitive to environment
induced disturbances. The time dependent spin transport problem, when leads are
attached to the ring, is also solved. We show that the "sideband currents"
induced by the oscillating spin-orbit interaction strength can become the
dominant output channel, even in the presence of moderate thermal fluctuations
and random scattering events.Comment: 11 pages, 9 figures, submitted to PR
Exact decoherence to pointer states in free open quantum systems is universal
In this paper it is shown that exact decoherence to minimal uncertainty
Gaussian pointer states is generic for free quantum particles coupled to a heat
bath. More specifically, the paper is concerned with damped free particles
linearly coupled under product initial conditions to a heat bath at arbitrary
temperature, with arbitrary coupling strength and spectral densities covering
the Ohmic, subohmic, and supraohmic regime. Then it is true that there exists a
time t_c such that for times t>t_c the state can always be exactly represented
as a mixture (convex combination) of particular minimal uncertainty Gaussian
states, regardless of and independent from the initial state. This exact
`localisation' is hence not a feature specific to high temperatures and weak
damping limit, but is rather a generic property of damped free particles.Comment: 4 pages, 1 figur
The CWKB particle production and classical condensate in de Sitter spacetime
The complex time WKB approximation is an effective tool in studying particle
production in curved spacetime. We use it in this work to understand the
formation of classical condensate in expanding de Sitter spacetime. The CWKB
leads to the emergence of thermal spectrum that depends crucially on horizons
(as in de Sitter spacetime) or observer dependent horizons (as in Rindler
spacetime). A connection is sought between the horizon and the formation of
classical condensate. We concentrate on de Sitter spacetime and study the
cosmological perturbation of mode with various values of . We find
that for a minimally coupled free scalar field for , the one-mode
occupation number grows more than unity soon after the physical wavelength of
the mode crosses the Hubble radius and soon after diverges as , where . The results substantiates the previous works in this
direction. We also find the correct oscillation and behaviour of at
small from a single expression using CWKB approximation for various values
of . We also discuss decoherence in relation to the formation of
classical condensate. We also find that the squeezed state formalism and CWKB
method give identical results.Comment: 19 pages, revtex, 5 figure
Simultaneity as an Invariant Equivalence Relation
This paper deals with the concept of simultaneity in classical and
relativistic physics as construed in terms of group-invariant equivalence
relations. A full examination of Newton, Galilei and Poincar\'e invariant
equivalence relations in is presented, which provides alternative
proofs, additions and occasionally corrections of results in the literature,
including Malament's theorem and some of its variants. It is argued that the
interpretation of simultaneity as an invariant equivalence relation, although
interesting for its own sake, does not cut in the debate concerning the
conventionality of simultaneity in special relativity.Comment: Some corrections, mostly of misprints. Keywords: special relativity,
simultaneity, invariant equivalence relations, Malament's theore
Reference frames, superselection rules, and quantum information
Recently, there has been much interest in a new kind of ``unspeakable''
quantum information that stands to regular quantum information in the same way
that a direction in space or a moment in time stands to a classical bit string:
the former can only be encoded using particular degrees of freedom while the
latter are indifferent to the physical nature of the information carriers. The
problem of correlating distant reference frames, of which aligning Cartesian
axes and synchronizing clocks are important instances, is an example of a task
that requires the exchange of unspeakable information and for which it is
interesting to determine the fundamental quantum limit of efficiency. There
have also been many investigations into the information theory that is
appropriate for parties that lack reference frames or that lack correlation
between their reference frames, restrictions that result in global and local
superselection rules. In the presence of these, quantum unspeakable information
becomes a new kind of resource that can be manipulated, depleted, quantified,
etcetera. Methods have also been developed to contend with these restrictions
using relational encodings, particularly in the context of computation,
cryptography, communication, and the manipulation of entanglement. This article
reviews the role of reference frames and superselection rules in the theory of
quantum information processing.Comment: 55 pages, published versio
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