201 research outputs found
Weak values are quantum: you can bet on it
The outcome of a weak quantum measurement conditioned to a subsequent
postselection (a weak value protocol) can assume peculiar values. These results
cannot be explained in terms of conditional probabilistic outcomes of
projective measurements. However, a classical model has been recently put
forward that can reproduce peculiar expectation values, reminiscent of weak
values. This led the authors of that work to claim that weak values have an
entirely classical explanation. Here we discuss what is quantum about weak
values with the help of a simple model based on basic quantum mechanics. We
first demonstrate how a classical theory can indeed give rise to non-trivial
conditional values, and explain what features of weak values are genuinely
quantum. We finally use our model to outline some main issues under current
research.Comment: 6 pages, 1 figur
Heisenberg scaling with weak measurement: A quantum state discrimination point of view
We examine the results of the paper "Precision metrology using weak
measurements", [Zhang, Datta, and Walmsley, arXiv:1310.5302] from a quantum
state discrimination point of view. The Heisenberg scaling of the photon number
for the precision of the interaction parameter between coherent light and a
spin one-half particle (or pseudo-spin) has a simple interpretation in terms of
the interaction rotating the quantum state to an orthogonal one. In order to
achieve this scaling, the information must be extracted from the spin rather
than from the coherent state of light, limiting the applications of the method
to phenomena such as cross-phase modulation. We next investigate the effect of
dephasing noise, and show a rapid degradation of precision, in agreement with
general results in the literature concerning Heisenberg scaling metrology. We
also demonstrate that a von Neumann-type measurement interaction can display a
similar effect.Comment: 7 pages, 3 figure
Some Aspects of Classical and Quantum Phases
We study classical and quantum phases in the adiabatic Born-Oppenheimer
context. These include a classical astronomical case, the general dual
description of the phases, a new "Paradox" connected to scattering Berry phase
and its resolution and various elaboration of
topological/geometrical/non-abelian phases.Comment: 18 pages, 4 figure
Diffraction-based Interaction-free Measurements
We introduce diffraction-based interaction-free measurements. In contrast with previous work where a set of discrete paths is engaged, good-quality interaction-free measurements can be realized with a continuous set of paths, as is typical of optical propagation. If a bomb is present in a given spatial region—so sensitive that a single photon will set it off—its presence can still be detected without exploding it. This is possible because, by not absorbing the photon, the bomb causes the single photon to diffract around it. The resulting diffraction pattern can then be statistically distinguished from the bomb-free case. We work out the case of single- versus double-slit in detail, where the double-slit arises because of a bomb excluding the middle region
Diffraction-Based Interaction-Free Measurements
We introduce diffraction-based interaction-free measurements. In contrast
with previous work where a set of discrete paths is engaged, good quality
interaction-free measurements can be realized with a continuous set of paths,
as is typical of optical propagation. If a bomb is present in a given spatial
region -- so sensitive that a single photon will set it off -- its presence can
still be detected without exploding it. This is possible because, by not
absorbing the photon, the bomb causes the single photon to diffract around it.
The resulting diffraction pattern can then be statistically distinguished from
the bomb-free case. We work out the case of single- versus double- slit in
detail, where the double-slit arises because of a bomb excluding the middle
region.Comment: 8 pages, 4 figure
On Superoscillations and Supershifts in Several Variables
The aim of this paper is to study a class of superoscillatory functions in several variables, removing some restrictions on the functions that we introduced in a previous paper. Since the tools that we used with our approach are not common knowledge we will give detailed proof for the case of two variables. The results proved for superoscillatory functions in several variables can be further extended to supershifts in several variables
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