12,236 research outputs found

### Two interacting spins in external fields. Four-level systems

In the present article, we consider the so-called two-spin equation that
describes four-level quantum systems. Recently, these systems attract attention
due to their relation to the problem of quantum computation. We study general
properties of the two-spin equation and show that the problem for certain
external backgrounds can be identified with the problem of one spin in an
appropriate background. This allows one to generate a number of exact solutions
for two-spin equations on the basis of already known exact solutions of the
one-spin equation. Besides, we present some exact solutions for the two-spin
equation with an external background different for each spin but having the
same direction. We study the eigenvalue problem for a time-independent spin
interaction and a time-independent external background. A possible analogue of
the Rabi problem for the two-spin equation is defined. We present its exact
solution and demonstrate the existence of magnetic resonances in two specific
frequencies, one of them coinciding with the Rabi frequency, and the other
depending on the rotating field magnitude. The resonance that corresponds to
the second frequency is suppressed with respect to the first one.Comment: 14 page

### Wigner distribution functions for complex dynamical systems: a path integral approach

Starting from Feynman's Lagrangian description of quantum mechanics, we
propose a method to construct explicitly the propagator for the Wigner
distribution function of a single system. For general quadratic Lagrangians,
only the classical phase space trajectory is found to contribute to the
propagator. Inspired by Feynman's and Vernon's influence functional theory we
extend the method to calculate the propagator for the reduced Wigner function
of a system of interest coupled to an external system. Explicit expressions are
obtained when the external system consists of a set of independent harmonic
oscillators. As an example we calculate the propagator for the reduced Wigner
function associated with the Caldeira-Legett model

### Spin equation and its solutions

The aim of the present article is to study in detail the so-called spin
equation (SE) and present both the methods of generating new solution and a new
set of exact solutions. We recall that the SE with a real external field can be
treated as a reduction of the Pauli equation to the (0+1)-dimensional case.
Two-level systems can be described by an SE with a particular form of the
external field. In this article, we also consider associated equations that are
equivalent or (in one way or another) related to the SE. We describe the
general solution of the SE and solve the inverse problem for this equation. We
construct the evolution operator for the SE and consider methods of generating
new sets of exact solutions. Finally, we find a new set of exact solutions of
the SE.Comment: 29 page

### Generalized Aharonov-Bohm effect, homotopy classes and Hausdorff dimension

We suggest as gedanken experiment a generalization of the Aharonov-Bohm
experiment, based on an array of solenoids. This experiment allows in principle
to measure the decomposition into homotopy classes of the quantum mechanical
propagator. This yields information on the geometry of the average path of
propagation and allows to determine its Hausdorff dimension.Comment: 14 pages, LaTeX + 3 figures, P

### Geometrical Phase Transitions

The geometrical approach to phase transitions is illustrated by simulating
the high-temperature representation of the Ising model on a square lattice.Comment: 5 pages, 3 figures, talk presented at Conference on Computational
Physics 2004, Genoa, 1-4 September 2004; 2nd version: slightly expanded
versio

### J-factors of short DNA molecules

The propensity of short DNA sequences to convert to the circular form is
studied by a mesoscopic Hamiltonian method which incorporates both the bending
of the molecule axis and the intrinsic twist of the DNA strands. The base pair
fluctuations with respect to the helix diameter are treated as path
trajectories in the imaginary time path integral formalism. The partition
function for the sub-ensemble of closed molecules is computed by imposing chain
ends boundary conditions both on the radial fluctuations and on the angular
degrees of freedom. The cyclization probability, the J-factor, proves to be
highly sensitive to the stacking potential, mostly to its nonlinear parameters.
We find that the J-factor generally decreases by reducing the sequence length (
N ) and, more significantly, below N = 100 base pairs. However, even for very
small molecules, the J-factors remain sizeable in line with recent experimental
indications. Large bending angles between adjacent base pairs and anharmonic
stacking appear as the causes of the helix flexibility at short length scales.Comment: The Journal of Chemical Physics - May 2016 ; 9 page

### Interplanetary Particle Environment. Proceedings of a Conference

A workshop entitled the Interplanetary Charged Particle Environment was held at the Jet Propulsion Laboratory (JPL) on March 16 and 17, 1987. The purpose of the Workshop was to define the environment that will be seen by spacecraft operating in the 1990s. It focused on those particles that are involved in single event upset, latch-up, total dose and displacement damage in spacecraft microelectronic parts. Several problems specific to Magellan were also discussed because of the sensitivity of some electronic parts to single-event phenomena. Scientists and engineers representing over a dozen institutions took part in the meeting. The workshop consisted of two major activities, reviews of the current state of knowledge and the formation of working groups and the drafting of their reports

### Quantum initial condition sampling for linearized density matrix dynamics: Vibrational pure dephasing of iodine in krypton matrices

This paper reviews the linearized path integral approach for computing time
dependent properties of systems that can be approximated using a mixed
quantum-classical description. This approach is applied to studying vibrational
pure dephasing of ground state molecular iodine in a rare gas matrix. The
Feynman-Kleinert optimized harmonic approximation for the full system density
operator is used to sample initial conditions for the bath degrees of freedom.
This extremely efficient approach is compared with alternative initial
condition sampling techniques at low temperatures where classical initial
condition sampling yields dephasing rates that are nearly an order of magnitude
too slow compared with quantum initial condition sampling and experimental
results.Comment: 20 pages and 8 figure

### Bose Fluids Above Tc: Incompressible Vortex Fluids and "Supersolidity"

This paper emphasizes that non-linear rotational or diamagnetic
susceptibility is characteristic of Bose fluids above their superfluid Tcs, and
for sufficiently slow rotation or weak B-fields amounts to an incompressible
response to vorticity. The cause is a missing term in the conventionally
accepted model Hamiltonian for quantized vortices in the Bose fluid. The
resulting susceptibility can account for recent observations of Chan et al on
solid He, and Ong et al on cuprate superconductors

### Linear quantum state diffusion for non-Markovian open quantum systems

We demonstrate the relevance of complex Gaussian stochastic processes to the
stochastic state vector description of non-Markovian open quantum systems.
These processes express the general Feynman-Vernon path integral propagator for
open quantum systems as the classical ensemble average over stochastic pure
state propagators in a natural way. They are the coloured generalization of
complex Wiener processes in quantum state diffusion stochastic Schrodinger
equations.Comment: 9 pages, RevTeX, appears in Physics Letters

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