3,211 research outputs found
Schulman Replies
This is a reply to a comment of Casati, Chirikov and Zhirov (PRL 85, 896
(2000)) on PRL 83, 5419 (1999).
The suitability of the particlar two-time boundary value problem used in the
earlier PRL is argued
Passage-time distributions from a spin-boson detector model
The passage-time distribution for a spread-out quantum particle to traverse a
specific region is calculated using a detailed quantum model for the detector
involved. That model, developed and investigated in earlier works, is based on
the detected particle's enhancement of the coupling between a collection of
spins (in a metastable state) and their environment. We treat the continuum
limit of the model, under the assumption of the Markov property, and calculate
the particle state immediately after the first detection. An explicit example
with 15 boson modes shows excellent agreement between the discrete model and
the continuum limit. Analytical expressions for the passage-time distribution
as well as numerical examples are presented. The precision of the measurement
scheme is estimated and its optimization discussed. For slow particles, the
precision goes like , which improves previous estimates,
obtained with a quantum clock model.Comment: 11 pages, 6 figures; minor changes, references corrected; accepted
for publication in Phys. Rev.
Relative momentum for identical particles
Possible definitions for the relative momentum of identical particles are
considered
Closed Path Integrals and Renormalisation in Quantum Mechanics
We suggest a closed form expression for the path integral of quantum
transition amplitudes. We introduce a quantum action with renormalized
parameters. We present numerical results for the potential. The
renormalized action is relevant for quantum chaos and quantum instantons.Comment: Revised text, 1 figure added; Text (LaTeX file), 1 Figure (ps file
Analysis of a three-component model phase diagram by Catastrophe Theory
We analyze the thermodynamical potential of a lattice gas model with three
components and five parameters using the methods of Catastrophe Theory. We find
the highest singularity, which has codimension five, and establish its
transversality. Hence the corresponding seven-degree Landau potential, the
canonical form Wigwam or , constitutes the adequate starting point to
study the overall phase diagram of this model.Comment: 16 pages, Latex file, submitted to Phys. Rev.
Sharing Polarization within Quantum Subspaces
Given an ensemble of n spins, at least some of which are partially polarized,
we investigate the sharing of this polarization within a subspace of k spins.
We assume that the sharing results in a pseudopure state, characterized by a
single purity parameter which we call the bias. As a concrete example we
consider ensembles of spin-1/2 nuclei in liquid-state nuclear magnetic
resonance (NMR) systems. The shared bias levels are compared with some current
entanglement bounds to determine whether the reduced subspaces can give rise to
entangled states.Comment: 7 pages, 3 figure
Topological quenching of the tunnel splitting for a particle in a double-well potential on a planar loop
The motion of a particle along a one-dimensional closed curve in a plane is considered. The only restriction on the shape of the loop is that it must be invariant under a twofold rotation about an axis perpendicular to the plane of motion. Along the curve a symmetric double-well potential is present leading to a twofold degeneracy of the classical ground state. In quantum mechanics, this degeneracy is lifted: the energies of the ground state and the first excited state are separated from each other by a slight difference ¿E, the tunnel splitting. Although a magnetic field perpendicular to the plane of the loop does not influence the classical motion of the charged particle, the quantum-mechanical separation of levels turns out to be a function of its strength B. The dependence of ¿E on the field B is oscillatory: for specific discrete values Bn the splitting drops to zero, indicating a twofold degeneracy of the ground state. This result is obtained within the path-integral formulation of quantum mechanics; in particular, the semiclassical instanton method is used. The origin of the quenched splitting is intuitively obvious: it is due to the fact that the configuration space of the system is not simply connected, thus allowing for destructive interference of quantum-mechanical amplitudes. From an abstract point of view this phenomenon can be traced back to the existence of a topological term in the Lagrangian and a nonsimply connected configuration space. In principle, it should be possible to observe the splitting in appropriately fabricated mesoscopic rings consisting of normally conducting metal
The eclipsing X-ray pulsar X-7 in M33
Using our extensive ROSAT X-ray observations of M33, we confirm a 3.45 day
eclipse period for the Einstein source X-7 (Larson & Schulman, 1997) and
discover evidence for a 0.31-s pulse period. The orbital period, pulse period
and observed X-ray luminosity are remarkably similar to SMC X-1. We therefore
suggest M33 X-7 is a neutron star high mass X-ray binary with a 15-40 Msol O/B
companion and a binary separation of 25-33 Rsol if the companion is almost
filling its Roche lobe.Comment: accepted for publication in MNRA
Spectral stochastic processes arising in quantum mechanical models with a non-L2 ground state
A functional integral representation is given for a large class of quantum
mechanical models with a non--L2 ground state. As a prototype the particle in a
periodic potential is discussed: a unique ground state is shown to exist as a
state on the Weyl algebra, and a functional measure (spectral stochastic
process) is constructed on trajectories taking values in the spectrum of the
maximal abelian subalgebra of the Weyl algebra isomorphic to the algebra of
almost periodic functions. The thermodynamical limit of the finite volume
functional integrals for such models is discussed, and the superselection
sectors associated to an observable subalgebra of the Weyl algebra are
described in terms of boundary conditions and/or topological terms in the
finite volume measures.Comment: 15 pages, Plain Te
Subsystem Pseudo-pure States
A critical step in experimental quantum information processing (QIP) is to
implement control of quantum systems protected against decoherence via
informational encodings, such as quantum error correcting codes, noiseless
subsystems and decoherence free subspaces. These encodings lead to the promise
of fault tolerant QIP, but they come at the expense of resource overheads.
Part of the challenge in studying control over multiple logical qubits, is
that QIP test-beds have not had sufficient resources to analyze encodings
beyond the simplest ones. The most relevant resources are the number of
available qubits and the cost to initialize and control them. Here we
demonstrate an encoding of logical information that permits the control over
multiple logical qubits without full initialization, an issue that is
particularly challenging in liquid state NMR. The method of subsystem
pseudo-pure state will allow the study of decoherence control schemes on up to
6 logical qubits using liquid state NMR implementations.Comment: 9 pages, 1 Figur
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