8,978 research outputs found
Non-disturbing quantum measurements
We consider pairs of quantum observables (POVMs) and analyze the relation
between the notions of non-disturbance, joint measurability and commutativity.
We specify conditions under which these properties coincide or
differ---depending for instance on the interplay between the number of outcomes
and the Hilbert space dimension or on algebraic properties of the effect
operators. We also show that (non-)disturbance is in general not a symmetric
relation and that it can be decided and quantified by means of a semidefinite
program.Comment: Minor corrections in v
Characterization of the Sequential Product on Quantum Effects
We present a characterization of the standard sequential product of quantum
effects. The characterization is in term of algebraic, continuity and duality
conditions that can be physically motivated.Comment: 11 pages. Accepted for publication in the Journal of Mathematical
Physic
The Standard Model of Quantum Measurement Theory: History and Applications
The standard model of the quantum theory of measurement is based on an
interaction Hamiltonian in which the observable-to-be-measured is multiplied
with some observable of a probe system. This simple Ansatz has proved extremely
fruitful in the development of the foundations of quantum mechanics. While the
ensuing type of models has often been argued to be rather artificial, recent
advances in quantum optics have demonstrated their prinicpal and practical
feasibility. A brief historical review of the standard model together with an
outline of its virtues and limitations are presented as an illustration of the
mutual inspiration that has always taken place between foundational and
experimental research in quantum physics.Comment: 22 pages, to appear in Found. Phys. 199
Fabrication of thick structures by sputtering
Deposit, 5500-gram of Cu-0.15 wt % Zr alloy, sputtered onto copper cylinder to average thickness of 12.29 mm. Structure was achieved with high-rate sputter deposition for about 100 hours total sputtering time. Material had twice the strength of unsputtered material at temperatures to 723 K and equivalent strength at nearly 873 K
Design, fabrication and evaluation of chalcogenide glass Luneburg lenses for LiNbO3 integrated optical devices
Optical waveguide Luneburg lenses of arsenic trisulfide glass are described. The lenses are formed by thermal evaporation of As2S3 through suitably placed masks onto the surface of LiNbO3:Ti indiffused waveguides. The lenses are designed for input apertures up to 1 cm and for speeds of f/5 or better. They are designed to focus the TM sub 0 guided mode of a beam of wavelength, external to the guide, of 633 nm. The refractive index of the As2S3 films and the changes induced in the refractive index by exposure to short wavelength light were measured. Some correlation between film thickness and optical properties was noted. The short wavelength photosensitivity was used to shorten the lens focal length from the as deposited value. Lenses of rectangular shape, as viewed from above the guide, as well as conventional circular Luneburg lenses, were made. Measurements made on the lenses include thickness profile, general optical quality, focal length, quality of focal spot, and effect of ultraviolet irradiation on optical properties
Low-density, one-dimensional quantum gases in a split trap
We investigate degenerate quantum gases in one dimension trapped in a
harmonic potential that is split in the centre by a pointlike potential. Since
the single particle eigenfunctions of such a system are known for all strengths
of the central potential, the dynamics for non-interacting fermionic gases and
low-density, strongly interacting bosonic gases can be investigated exactly
using the Fermi-Bose mapping theorem. We calculate the exact many-particle
ground-state wave-functions for both particle species, investigate soliton-like
solutions, and compare the bosonic system to the well-known physics of Bose
gases described by the Gross-Pitaevskii equation. We also address the
experimentally important questions of creation and detection of such states.Comment: 7 pages, 5 figure
On localization and position operators in Moebius-covariant theories
Some years ago it was shown that, in some cases, a notion of locality can
arise from the group of symmetry enjoyed by the theory, thus in an intrinsic
way. In particular, when Moebius covariance is present, it is possible to
associate some particular transformations to the Tomita Takesaki modular
operator and conjugation of a specific interval of an abstract circle. In this
context we propose a way to define an operator representing the coordinate
conjugated with the modular transformations. Remarkably this coordinate turns
out to be compatible with the abstract notion of locality. Finally a concrete
example concerning a quantum particle on a line is also given.Comment: 19 pages, UTM 705, version to appear in RM
Relativistic Quantum Mechanics and Relativistic Entanglement in the Rest-Frame Instant Form of Dynamics
A new formulation of relativistic quantum mechanics is proposed in the
framework of the rest-frame instant form of dynamics with its instantaneous
Wigner 3-spaces and with its description of the particle world-lines by means
of derived non-canonical predictive coordinates. In it we quantize the frozen
Jacobi data of the non-local 4-center of mass and the Wigner-covariant relative
variables in an abstract (frame-independent) internal space whose existence is
implied by Wigner-covariance. The formalism takes care of the properties of
both relativistic bound states and scattering ones. There is a natural solution
to the \textit{relativistic localization problem}. The non-relativistic limit
leads to standard quantum mechanics but with a frozen Hamilton-Jacobi
description of the center of mass. Due to the \textit{non-locality} of the
Poincar\'e generators the resulting theory of relativistic entanglement is both
\textit{kinematically non-local and spatially non-separable}: these properties,
absent in the non-relativistic limit, throw a different light on the
interpretation of the non-relativistic quantum non-locality and of its impact
on foundational problems.Comment: 73 pages, includes revision
Approximate joint measurement of qubit observables through an Arthur-Kelly type model
We consider joint measurement of two and three unsharp qubit observables
through an Arthur-Kelly type joint measurement model for qubits. We investigate
the effect of initial state of the detectors on the unsharpness of the
measurement as well as the post-measurement state of the system. Particular
emphasis is given on a physical understanding of the POVM to PVM transition in
the model and entanglement between system and detectors.Two approaches for
characterizing the unsharpness of the measurement and the resulting measurement
uncertainty relations are considered.The corresponding measures of unsharpness
are connected for the case where both the measurements are equally unsharp. The
connection between the POVM elements and symmetries of the underlying
Hamiltonian of the measurement interaction is made explicit and used to perform
joint measurement in arbitrary directions. Finally in the case of three
observables we derive a necessary condition for the approximate joint
measurement and use it show the relative freedom available when the observables
are non-orthogonal.Comment: 22 pages; Late
Maximal Accuracy and Minimal Disturbance in the Arthurs-Kelly Simultaneous Measurement Process
The accuracy of the Arthurs-Kelly model of a simultaneous measurement of
position and momentum is analysed using concepts developed by Braginsky and
Khalili in the context of measurements of a single quantum observable. A
distinction is made between the errors of retrodiction and prediction. It is
shown that the distribution of measured values coincides with the initial state
Husimi function when the retrodictive accuracy is maximised, and that it is
related to the final state anti-Husimi function (the P representation of
quantum optics) when the predictive accuracy is maximised. The disturbance of
the system by the measurement is also discussed. A class of minimally
disturbing measurements is characterised. It is shown that the distribution of
measured values then coincides with one of the smoothed Wigner functions
described by Cartwright.Comment: 12 pages, 0 figures. AMS-Latex. Earlier version replaced with final
published versio
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