5,160 research outputs found
Quantum retrodiction in open systems
Quantum retrodiction involves finding the probabilities for various
preparation events given a measurement event. This theory has been studied for
some time but mainly as an interesting concept associated with time asymmetry
in quantum mechanics. Recent interest in quantum communications and
cryptography, however, has provided retrodiction with a potential practical
application. For this purpose quantum retrodiction in open systems should be
more relevant than in closed systems isolated from the environment. In this
paper we study retrodiction in open systems and develop a general master
equation for the backward time evolution of the measured state, which can be
used for calculating preparation probabilities. We solve the master equation,
by way of example, for the driven two-level atom coupled to the electromagnetic
field.Comment: 12 pages, no figure
Retrodictive states and two-photon quantum imaging
We use retrodictive quantum theory to analyse two-photon quantum imaging
systems. The formalism is particularly suitable for calculating conditional
probability distributions.Comment: 5 pages, 3 figure
Retrodiction as a tool for micromaser field measurements
We use retrodictive quantum theory to describe cavity field measurements by
successive atomic detections in the micromaser. We calculate the state of the
micromaser cavity field prior to detection of sequences of atoms in either the
excited or ground state, for atoms that are initially prepared in the excited
state. This provides the POM elements, which describe such sequences of
measurements.Comment: 20 pages, 4(8) figure
The rich get richer: patterns of plant invasions in the United States
Observations from islands, small-scale experiments, and mathematical models have generally supported the paradigm that habitats of low plant diversity are more vulnerable to plant invasions than areas of high plant diversity. We summarize two independent data sets to show exactly the opposite pattern at multiple spatial scales. More significant, and alarming, is that hotspots of native plant diversity have been far more heavily invaded than areas of low plant diversity in most parts of the United States when considered at larger spatial scales. Our findings suggest that we cannot expect such hotspots to repel invasions, and that the threat of invasion is significant and predictably greatest in these areas
Quantum theory of preparation and measurement
The conventional postulate for the probabilistic interpretation of quantum
mechanics is asymmetric in preparation and measurement, making retrodiction
reliant on inference by use of Bayes' theorem. Here, a more fundamental
symmetric postulate is presented, from which both predictive and retrodictive
probabilities emerge immediately, even where measurement devices more general
than those usually considered are involved. It is shown that the new postulate
is perfectly consistent with the conventional postulate.Comment: 25 pages, No figure
Measurement master equation
We derive a master equation describing the evolution of a quantum system
subjected to a sequence of observations. These measurements occur randomly at a
given rate and can be of a very general form. As an example, we analyse the
effects of these measurements on the evolution of a two-level atom driven by an
electromagnetic field. For the associated quantum trajectories we find Rabi
oscillations, Zeno-effect type behaviour and random telegraph evolution spawned
by mini quantum jumps as we change the rates and strengths of measurement.Comment: 14 pages and 8 figures, Optics Communications in pres
Quantum probability rule : a generalization of the theorems of Gleason and Busch
Buschs theorem deriving the standard quantum probability rule can be regarded as a more general form of Gleasons theorem. Here we show that a further generalization is possible by reducing the number of quantum postulates used by Busch. We do not assume that the positive measurement outcome operators are effects or that they form a probability operator measure. We derive a more general probability rule from which the standard rule can be obtained from the normal laws of probability when there is no measurement outcome information available, without the need for further quantum postulates. Our general probability rule has prediction-retrodiction symmetry and we show how it may be applied in quantum communications and in retrodictive quantum theory
Point source generation of chiral fields:measures of near- and far-field optical helicity
To consider the relationship between different measures of chirality in an optical field, the simplest case is considered: direct spontaneous emission of circularly polarized light by a point source. In the electromagnetic fields radiated from a suitably chiral source, such as a low-symmetry chiral molecule undergoing radiative decay, optical helicity is exhibited in the extent of a difference in left- and right-handed circular polarization components. There are several practical measures for quantifying the emergence of ensuing optical helicity, exhibiting different forms of dependence on the properties of the emitter and the positioning of a detector. By casting each measure in terms of an irreducible helicity density, connections and distinctions can be drawn between results expressible in either classical or quantum form
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