24,498 research outputs found
Measurement of a spin-1 system
We derive exact formulas describing an indirect von Neumann measurement of a
spin-1 system. The results hold for any interaction strength and for an
arbitrary output variable \Hat{O}.Comment: 13 pages; comments welcome. V2 25 pages, close to published version,
added two appendices reformulating a result from Dressel and Jordan in terms
of the quantum characteristic functio
Post-selection induced deterministic and probabilistic entanglement with strong and weak interactions
A scheme is proposed to entangle two systems that have not interacted by
using an ancillary particle in a Mach-Zehnder interferometer, by making a
suitable post--selection of the particle followed by a conditional feedback on
one of the subsystems to be entangled. For a strong interaction, the process
works deterministically. For a weaker interaction only the probability of
success is reduced, but the output continues to be a maximally entangled state
Hidden-variable models for the spin singlet. II. Local theories violating Bell and Leggett inequalities
Three classes of local hidden-variable models that violate both Bell and
Leggett inequalities are presented. The models, however, do not reproduce the
quantum mechanical predictions, hence they are experimentally testable. It is
concluded that on one hand neither Bell or Leggett inequality fully captures
the essential counterintuitiveness of quantum mechanics, while on the other
hand the hypothesis of outcome independence and that of locality are
uncorrelated.Comment: 10 pages, no figures, comments welcom
Postselection induced entanglement swapping from a vacuum--excitation entangled state to separate quantum systems
We show that a single particle in a superposition of different paths can
entangle two objects located on each path. The entanglement has its maximum
visibility for intermediate coupling strengths. In particular, when the two
quantum systems with which the particle interacts are detectors that measure
its presence and its polarization, the so-called quantum Cheshire cat is
realized
Beyond Bell's theorem: Admissible hidden-variable models for the spin-singlet
Assuming that quantum mechanics is obeyed exactly after averaging over hidden
variables, and considering models that obey both the hypotheses of free will
and locality, we establish the form of all possible hidden-variable models that
reproduce the spin-singlet.Comment: Based on the oral presentation given at DICE 2012, submitted to J.
Phys. Conf. Ser. No new results compared to arXiv:1105.1286 [quant-ph], just
a different exposition. Reused an appendix reviewing some HV models that was
excised in the final version of 1105.128
Correlations between detectors allow violation of the Heisenberg noise-disturbance principle for position and momentum measurements
Heisenberg formulated a noise-disturbance principle stating that there is a
tradeoff between noise and disturbance when a measurement of position and a
measurement of momentum are performed sequentially, and another principle
imposing a limitation on the product of the uncertainties in a joint
measurement of position and momentum. We prove that the former, the Heisenberg
sequential noise-disturbance principle, holds when the detectors are assumed to
be initially uncorrelated from each other, but that it can be violated for some
properly correlated initial preparations of the detectors.Comment: v2, minor changes. "Resolution" substitutes "sensitivity," according
to current usage; v3, changed title and abstract, simplified derivation,
corrected an error about the joint measurement, extra appendix to appear as
part of a longer version of the pape
Weak values and weak coupling maximizing the output of weak measurements
In a weak measurement, the average output of a probe that
measures an observable of a quantum system undergoing both a
preparation in a state and a postselection in a state
is, to a good approximation, a function of the weak value , a complex number. For a fixed coupling
, when the overlap is very small,
diverges, but stays finite, often tending to zero for
symmetry reasons. This paper answers the questions: what is the weak value that
maximizes the output for a fixed coupling? what is the coupling that maximizes
the output for a fixed weak value? We derive equations for the optimal values
of and , and provide the solutions. The results are independent
of the dimensionality of the system, and they apply to a probe having a Hilbert
space of arbitrary dimension. Using the Schr\"{o}dinger-Robertson uncertainty
relation, we demonstrate that, in an important case, the amplification cannot exceed the initial uncertainty in the observable
, we provide an upper limit for the more general case, and a strategy
to obtain .Comment: v4 close to published version; v3 provides a simpler, more elegant
solution based on geometrical considerations; v2 extends the results for an
arbitrary observable of the probe, which can be even a finite-dimensional
system, and provides an upper bound to the output by exploiting the
Schroedinger-Robertson uncertainty relatio
Comment on `Observation of a quantum Cheshire Cat in a matter-wave interferometer experiment', Nature Comm. 5, 4492
It is shown that a classical experiment using an ordinary cat can reproduce
the same results and it is argued that the quantum nature of the phenomenon
could be revealed instead by making an experiment that detects cross-moments
Does the wavefunction describe individual systems?
We analyze the issue of the interpretation of the wavefunction, namely
whether it should be interpreted as describing individual systems or ensembles
of identically prepared systems. We propose an experiment which can decide the
issue, based on the simultaneous measurement of the same observable with
different detectors, and we discuss the theoretical implications of the
possible experimental outcomes
Measurement of a qubit and measurement with a qubit
Generally, the measurement process consists in coupling a system to a
detector that can give a continuous output. However, it may be interesting to
use as a detector a system with a discrete spectrum, especially in view of
applications to quantum information. Here, we study 1) a two-level system
measuring another two-level system (qubit); 2) a generic system measuring a
qubit; 3) a qubit measuring a generic system. The results include the case when
a postselection on the measured system is made. We provide the exact solution,
and also a controlled expansion in the coupling parameter, giving formulas
valid in the weak measurement regime for arbitrary preparation and
postselection. The concept of generalized Wigner functions is introduced.Comment: 18 pages, 1 figure, 1 table. Comments welcom
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