670 research outputs found
Industrial Unrest in Canada: A Diagnosis of Recent Experience
To diagnose the recent wave of industrial unrest in Canada, it is first of all necessary to indentify its characteristics. The two major dimensions of this phenomenon concern the source of union militancy and its illegal manifestations
Decoherence in a double-slit quantum eraser
We study and experimentally implement a double-slit quantum eraser in the
presence of a controlled decoherence mechanism. A two-photon state, produced in
a spontaneous parametric down conversion process, is prepared in a maximally
entangled polarization state. A birefringent double-slit is illuminated by one
of the down-converted photons, and it acts as a single-photon two-qubits
controlled not gate that couples the polarization with the transversal momentum
of these photons. The other photon, that acts as a which-path marker, is sent
through a Mach-Zehnder-like interferometer. When the interferometer is
partially unbalanced, it behaves as a controlled source of decoherence for
polarization states of down-converted photons. We show the transition from
wave-like to particle-like behavior of the signal photons crossing the
double-slit as a function of the decoherence parameter, which depends on the
length path difference at the interferometer.Comment: Accepted in Physical Review
Simultaneous minimum-uncertainty measurement of discrete-valued complementary observables
We have made the first experimental demonstration of the simultaneous minimum
uncertainty product between two complementary observables for a two-state
system (a qubit). A partially entangled two-photon state was used to perform
such measurements. Each of the photons carries (partial) information of the
initial state thus leaving a room for measurements of two complementary
observables on every member in an ensemble.Comment: 4 pages, 4 figures, REVTeX, submitted to PR
The modern tools of quantum mechanics (A tutorial on quantum states, measurements, and operations)
This tutorial is devoted to review the modern tools of quantum mechanics,
which are suitable to describe states, measurements, and operations of
realistic, not isolated, systems in interaction with their environment, and
with any kind of measuring and processing devices. We underline the central
role of the Born rule and and illustrate how the notion of density operator
naturally emerges, together the concept of purification of a mixed state. In
reexamining the postulates of standard quantum measurement theory, we
investigate how they may formally generalized, going beyond the description in
terms of selfadjoint operators and projective measurements, and how this leads
to the introduction of generalized measurements, probability operator-valued
measures (POVM) and detection operators. We then state and prove the Naimark
theorem, which elucidates the connections between generalized and standard
measurements and illustrates how a generalized measurement may be physically
implemented. The "impossibility" of a joint measurement of two non commuting
observables is revisited and its canonical implementations as a generalized
measurement is described in some details. Finally, we address the basic
properties, usually captured by the request of unitarity, that a map
transforming quantum states into quantum states should satisfy to be physically
admissible, and introduce the notion of complete positivity (CP). We then state
and prove the Stinespring/Kraus-Choi-Sudarshan dilation theorem and elucidate
the connections between the CP-maps description of quantum operations, together
with their operator-sum representation, and the customary unitary description
of quantum evolution. We also address transposition as an example of positive
map which is not completely positive, and provide some examples of generalized
measurements and quantum operations.Comment: Tutorial. 26 pages, 1 figure. Published in a special issue of EPJ -
ST devoted to the memory of Federico Casagrand
Delta infection without increase in severity of hepatitis.
The findings of increased morbidity of HbsAG positive hepatitis with delta infection in a study by Dr. Smedile et al were contrary to those of studies performed by the authors. A group of 27 and a group of 41 drug abusers were examined serologically and had liver biopsies performed. There was no significant difference in histological findings between delta positive and delta negative patients in the 27 member group. None of the 41 member group showed any increase in severity of illness. Ethnic origin may be an important factor in the pathogenicity of the delta agent
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
Micro-CT yields high image quality in human fetal post-mortem imaging despite maceration
Background
Current clinical post-mortem imaging techniques do not provide sufficiently high-resolution imaging for smaller fetuses after pregnancy loss. Post-mortem micro-CT is a non-invasive technique that can deliver high diagnostic accuracy for these smaller fetuses. The purpose of the study is to identify the main predictors of image quality for human fetal post-mortem micro-CT imaging.
Methods
Human fetuses were imaged using micro-CT following potassium tri-iodide tissue preparation, and axial head and chest views were assessed for image quality on a Likert scale by two blinded radiologists. Simple and multivariable linear regression models were performed with demographic details, iodination, tissue maceration score and imaging parameters as predictor variables.
Results
258 fetuses were assessed, with median weight 41.7 g (2.6–350 g) and mean gestational age 16 weeks (11–24 weeks). A high image quality score (> 6.5) was achieved in 95% of micro-CT studies, higher for the head (median = 9) than chest (median = 8.5) imaging. The strongest negative predictors of image quality were increasing maceration and body weight (p < 0.001), with number of projections being the best positive imaging predictor.
Conclusions
High micro-CT image quality score is achievable following early pregnancy loss despite fetal maceration, particularly in smaller fetuses where conventional autopsy may be particularly challenging. These findings will help establish clinical micro-CT imaging services, addressing the need for less invasive fetal autopsy methods
Joint measurements via quantum cloning
We explore the possibility of achieving optimal joint measurements of
noncommuting observables on a single quantum system by performing conventional
measurements of commuting self adjoint operators on optimal clones of the
original quantum system. We consider the case of both finite dimensional and
infinite dimensional Hilbert spaces. In the former we study the joint
measurement of three orthogonal components of a spin 1/2, in the latter we
consider the case of the joint measurements of any pair of noncommuting
quadratures of one mode of the electromagnetic field. We show that universally
covariant cloning is not ideal for joint measurements, and a suitable non
universally covariant cloning is needed.Comment: 8 page
Witnessing effective entanglement in a continuous variable prepare&measure setup and application to a QKD scheme using postselection
We report an experimental demonstration of effective entanglement in a
prepare&measure type of quantum key distribution protocol. Coherent
polarization states and heterodyne measurement to characterize the transmitted
quantum states are used, thus enabling us to reconstruct directly their
Q-function. By evaluating the excess noise of the states, we experimentally
demonstrate that they fulfill a non-separability criterion previously presented
by Rigas et al. [J. Rigas, O. G\"uhne, N. L\"utkenhaus, Phys. Rev. A 73, 012341
(2006)]. For a restricted eavesdropping scenario we predict key rates using
postselection of the heterodyne measurement results.Comment: 12 pages, 12 figures, 2 table
Optimal estimation of joint parameters in phase space
We address the joint estimation of the two defining parameters of a
displacement operation in phase space. In a measurement scheme based on a
Gaussian probe field and two homodyne detectors, it is shown that both
conjugated parameters can be measured below the standard quantum limit when the
probe field is entangled. We derive the most informative Cram\'er-Rao bound,
providing the theoretical benchmark on the estimation and observe that our
scheme is nearly optimal for a wide parameter range characterizing the probe
field. We discuss the role of the entanglement as well as the relation between
our measurement strategy and the generalized uncertainty relations.Comment: 8 pages, 3 figures; v2: references added and sections added to the
supplemental material; v3: minor changes (published version
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