18 research outputs found
On the Relationship Between Complex Potentials and Strings of Projection Operators
It is of interest in a variety of contexts, and in particular in the arrival
time problem, to consider the quantum state obtained through unitary evolution
of an initial state regularly interspersed with periodic projections onto the
positive -axis (pulsed measurements). Echanobe, del Campo and Muga have
given a compelling but heuristic argument that the state thus obtained is
approximately equivalent to the state obtained by evolving in the presence of a
certain complex potential of step-function form. In this paper, with the help
of the path decomposition expansion of the associated propagators, we give a
detailed derivation of this approximate equivalence. The propagator for the
complex potential is known so the bulk of the derivation consists of an
approximate evaluation of the propagator for the free particle interspersed
with periodic position projections. This approximate equivalence may be used to
show that to produce significant reflection, the projections must act at time
spacing less than 1/E, where E is the energy scale of the initial state.Comment: 29 pages, LaTex, 4 figures. Substantial revision
Analytic Examples, Measurement Models and Classical Limit of Quantum Backflow
We investigate the backflow effect in elementary quantum mechanics - the
phenomenon in which a state consisting entirely of positive momenta may have
negative current and the probability flows in the opposite direction to the
momentum. We compute the current and flux for states consisting of
superpositions of gaussian wave packets. These are experimentally realizable
but the amount of backflow is small. Inspired by the numerical results of Penz
et al (M.Penz, G.Gr\"ubl, S.Kreidl and P.Wagner, J.Phys. A39, 423 (2006)), we
find two non-trivial wave functions whose current at any time may be computed
analytically and which have periods of significant backflow, in one case with a
backwards flux equal to about 70 percent of the maximum possible backflow, a
dimensionless number , discovered by Bracken and Melloy
(A.J.Bracken and G.F.Melloy, J.Phys. A27, 2197 (1994)). This number has the
unusual property of being independent of (and also of all other
parameters of the model), despite corresponding to an obviously
quantum-mechanical effect, and we shed some light on this surprising property
by considering the classical limit of backflow. We discuss some specific
measurement models in which backflow may be identified in certain measurable
probabilities.Comment: 33 pages, 14 figures. Minor revisions. Published versio
A review of the decoherent histories approach to the arrival time problem in quantum theory
We review recent progress in understanding the arrival time problem in
quantum mechanics, from the point of view of the decoherent histories approach
to quantum theory. We begin by discussing the arrival time problem, focussing
in particular on the role of the probability current in the expected classical
solution. After a brief introduction to decoherent histories we review the use
of complex potentials in the construction of appropriate class operators. We
then discuss the arrival time problem for a particle coupled to an environment,
and review how the arrival time probability can be expressed in terms of a POVM
in this case. We turn finally to the question of decoherence of the
corresponding histories, and we show that this can be achieved for simple
states in the case of a free particle, and for general states for a particle
coupled to an environment.Comment: 10 pages. To appear in DICE 2010 conference proceeding
Quantum Arrival Time For Open Systems
We extend previous work on the arrival time problem in quantum mechanics, in
the framework of decoherent histories, to the case of a particle coupled to an
environment. The usual arrival time probabilities are related to the
probability current, so we explore the properties of the current for general
open systems that can be written in terms of a master equation of Lindblad
form. We specialise to the case of quantum Brownian motion, and show that after
a time of order the localisation time the current becomes positive. We show
that the arrival time probabilities can then be written in terms of a POVM,
which we compute. We perform a decoherent histories analysis including the
effects of the environment and show that time of arrival probabilities are
decoherent for a generic state after a time much greater than the localisation
time, but that there is a fundamental limitation on the accuracy, ,
with which they can be specified which obeys . We confirm
that the arrival time probabilities computed in this way agree with those
computed via the current, provided there is decoherence. We thus find that the
decoherent histories formulation of quantum mechanics provides a consistent
explanation for the emergence of the probability current as the classical
arrival time distribution, and a systematic rule for deciding when
probabilities may be assigned.Comment: 30 pages, 1 figure. Published versio
Arrival Times, Complex Potentials and Decoherent Histories
We address a number of aspects of the arrival time problem defined using a
complex potential of step function form. We concentrate on the limit of a weak
potential, in which the resulting arrival time distribution function is closely
related to the quantum-mechanical current. We first consider the analagous
classical arrival time problem involving an absorbing potential, and this sheds
some light on certain aspects of the quantum case. In the quantum case, we
review the path decomposition expansion (PDX), in which the propagator is
factored across a surface of constant time, so is very useful for potentials of
step function form. We use the PDX to derive the usual scattering wave
functions and the arrival time distribution function. This method gives a
direct and geometrically appealing account of known results (but also points
the way to how they can be extended to more general complex potentials). We use
these results to carry out a decoherent histories analysis of the arrival time
problem, taking advantage of a recently demonstrated connection between pulsed
measurements and complex potentials. We obtain very simple and plausible
expressions for the class operators (describing the amplitudes for crossing the
origin during intervals of time) and show that decoherence of histories is
obtained for a wide class of initial states (such as simple wave packets and
superpositions of wave packets). We find that the decoherent histories approach
gives results with a sensible classical limit that are fully compatible with
standard results on the arrival time problem. We also find some interesting
connections between backflow and decoherence.Comment: 43 page