114 research outputs found
Localization of Relative-Position of Two Atoms Induced by Spontaneous Emission
We revisit the back-action of emitted photons on the motion of the relative
position of two cold atoms. We show that photon recoil resulting from the
spontaneous emission can induce the localization of the relative position of
the two atoms through the entanglement between the spatial motion of individual
atoms and their emitted photons. The result provides a more realistic model for
the analysis of the environment-induced localization of a macroscopic object.Comment: 8 pages and 4 figure
Extraction of scattering lengths from final-state interactions
A recently proposed method based on dispersion theory, that allows to extract
the scattering length of a hadronic two-body system from corresponding
final-state interactions, is generalized to the situation where the Coulomb
interaction is present. The steps required in a concrete practical application
are discussed in detail. In addition a thorough examination of the accuracy of
the proposed method is presented and a comparison is made with results achieved
with other methods like the Jost-function approach based on the effective-range
approximation. Deficiencies of the latter method are pointed out. The
reliability of the dispersion theory method for extracting also the effective
range is investigated.Comment: 16 pages, 6 figures, some corrections to text, to appear in Phys.
Rev.
Consistent Histories in Quantum Cosmology
We illustrate the crucial role played by decoherence (consistency of quantum
histories) in extracting consistent quantum probabilities for alternative
histories in quantum cosmology. Specifically, within a Wheeler-DeWitt
quantization of a flat Friedmann-Robertson-Walker cosmological model sourced
with a free massless scalar field, we calculate the probability that the
univese is singular in the sense that it assumes zero volume. Classical
solutions of this model are a disjoint set of expanding and contracting
singular branches. A naive assessment of the behavior of quantum states which
are superpositions of expanding and contracting universes may suggest that a
"quantum bounce" is possible i.e. that the wave function of the universe may
remain peaked on a non-singular classical solution throughout its history.
However, a more careful consistent histories analysis shows that for arbitrary
states in the physical Hilbert space the probability of this Wheeler-DeWitt
quantum universe encountering the big bang/crunch singularity is equal to
unity. A quantum Wheeler-DeWitt universe is inevitably singular, and a "quantum
bounce" is thus not possible in these models.Comment: To appear in Foundations of Physics special issue on quantum
foundation
Quantum Dynamical Model for Wave Function Reduction in Classical and Macroscopic Limits
In this papper, a quantum dynamical model describing the quantum measurement
process is presented as an extensive generalization of the Coleman-Hepp model.
In both the classical limit with very large quantum number and macroscopic
limit with very large particle number in measuring instrument, this model
generally realizes the wave packet collapse in quantum measurement as a
consequence of the Schrodinger time evolution in either the exactly-solvable
case or the non-(exactly-)solvable case.
For the latter, its quasi-adiabatic case is explicitly analysed by making use
of the high-order adiabatic approximation method and then manifests the wave
packet collapse as well as the exactly-solvable case. By highlighting these
analysis, it is finally found that an essence of the dynamical model of wave
packet collapse is the factorization of the Schrodinger evolution other than
the exact solvability. So many dynamical models including the well-known ones
before, which are exactly-solvable or not, can be shown only to be the concrete
realizations of this factorizabilityComment: ITP.SB-93-14,19 page
Lambda-N scattering length from the reaction gamma d -> K^+ Lambda n
The perspects of utilizing the strangeness-production reaction gamma d -> K^+
Lambda n for the determination of the Lambda n low-energy scattering parameters
are investigated. The spin observables that need to be measured in order to
isolate the Lambda n singlet (1S0) and triplet (3S1) states are identified.
Possible kinematical regions where the extraction of the Lambda n scattering
lengths might be feasible are discussed.Comment: 8 pages, 4 figure
Dynamical suppression of decoherence in two-state quantum systems
The dynamics of a decohering two-level system driven by a suitable control
Hamiltonian is studied. The control procedure is implemented as a sequence of
radiofrequency pulses that repetitively flip the state of the system, a
technique that can be termed quantum "bang-bang" control after its classical
analog. Decoherence introduced by the system's interaction with a quantum
environment is shown to be washed out completely in the limit of continuous
flipping and greatly suppressed provided the interval between the pulses is
made comparable to the correlation time of the environment. The model suggests
a strategy to fight against decoherence that complements existing quantum
error-correction techniques.Comment: 15 pages, RevTeX style, 3 figures. Submitted to Phys. Rev.
How Events Come Into Being: EEQT, Particle Tracks, Quantum Chaos, and Tunneling Time
In sections 1 and 2 we review Event Enhanced Quantum Theory (EEQT). In
section 3 we discuss applications of EEQT to tunneling time, and compare its
quantitative predictions with other approaches, in particular with
B\"uttiker-Larmor and Bohm trajectory approach. In section 4 we discuss quantum
chaos and quantum fractals resulting from simultaneous continuous monitoring of
several non-commuting observables. In particular we show self-similar,
non-linear, iterated function system-type, patterns arising from quantum jumps
and from the associated Markov operator. Concluding remarks pointing to
possible future development of EEQT are given in section 5.Comment: latex, 27 pages, 7 postscript figures. Paper submitted to Proc.
Conference "Mysteries, Puzzles And Paradoxes In Quantum Mechanics, Workshop
on Entanglement And Decoherence, Palazzo Feltrinelli, Gargnano, Garda Lake,
Italy, 20-25 September, 199
Complex Probabilities on R^N as Real Probabilities on C^N and an Application to Path Integrals
We establish a necessary and sufficient condition for averages over complex
valued weight functions on R^N to be represented as statistical averages over
real, non-negative probability weights on C^N. Using this result, we show that
many path-integrals for time-ordered expectation values of bosonic degrees of
freedom in real-valued time can be expressed as statistical averages over
ensembles of paths with complex-valued coordinates, and then speculate on
possible consequences of this result for the relation between quantum and
classical mechanics.Comment: 4 pages, 0 figure
What is "system": the information-theoretic arguments
The problem of "what is 'system'?" is in the very foundations of modern
quantum mechanics. Here, we point out the interest in this topic in the
information-theoretic context. E.g., we point out the possibility to manipulate
a pair of mutually non-interacting, non-entangled systems to employ
entanglement of the newly defined '(sub)systems' consisting the one and the
same composite system. Given the different divisions of a composite system into
"subsystems", the Hamiltonian of the system may perform in general
non-equivalent quantum computations. Redefinition of "subsystems" of a
composite system may be regarded as a method for avoiding decoherence in the
quantum hardware. In principle, all the notions refer to a composite system as
simple as the hydrogen atom.Comment: 13 pages, no figure
On the origin of the large scale structures of the universe
We revise the statistical properties of the primordial cosmological density
anisotropies that, at the time of matter radiation equality, seeded the
gravitational development of large scale structures in the, otherwise,
homogeneous and isotropic Friedmann-Robertson-Walker flat universe. Our
analysis shows that random fluctuations of the density field at the same
instant of equality and with comoving wavelength shorter than the causal
horizon at that time can naturally account, when globally constrained to
conserve the total mass (energy) of the system, for the observed scale
invariance of the anisotropies over cosmologically large comoving volumes.
Statistical systems with similar features are generically known as glass-like
or lattice-like. Obviously, these conclusions conflict with the widely accepted
understanding of the primordial structures reported in the literature, which
requires an epoch of inflationary cosmology to precede the standard expansion
of the universe. The origin of the conflict must be found in the widespread,
but unjustified, claim that scale invariant mass (energy) anisotropies at the
instant of equality over comoving volumes of cosmological size, larger than the
causal horizon at the time, must be generated by fluctuations in the density
field with comparably large comoving wavelength.Comment: New section added; final version to appear in Physical Review D;
discussion extended and detailed with new calculations to support the claims
of the paper; statistical properties of vacuum fluctuations now discussed in
the context of FRW flat universe; new important conclussions adde
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