228 research outputs found
Unusual decoherence in qubit measurements with a Bose-Einstein condensate
We consider an electrostatic qubit located near a Bose-Einstein condensate
(BEC) of noninteracting bosons in a double-well potential, which is used for
qubit measurements. Tracing out the BEC variables we obtain a simple analytical
expression for the qubit's density-matrix. The qubit's evolution exhibits a
slow () damping of the qubit's coherence term, which however
turns to be a Gaussian one in the case of static qubit. This stays in contrast
to the exponential damping produced by most classical detectors. The
decoherence is, in general, incomplete and strongly depends on the initial
state of the qubit.Comment: 5 pages, additional explanations related to experimental realization
are added, typos corrected, Phys. Rev. A, in pres
A Path Intergal Approach to Current
Discontinuous initial wave functions or wave functions with discontintuous
derivative and with bounded support arise in a natural way in various
situations in physics, in particular in measurement theory. The propagation of
such initial wave functions is not well described by the Schr\"odinger current
which vanishes on the boundary of the support of the wave function. This
propagation gives rise to a uni-directional current at the boundary of the
support. We use path integrals to define current and uni-directional current
and give a direct derivation of the expression for current from the path
integral formulation for both diffusion and quantum mechanics. Furthermore, we
give an explicit asymptotic expression for the short time propagation of
initial wave function with compact support for both the cases of discontinuous
derivative and discontinuous wave function. We show that in the former case the
probability propagated across the boundary of the support in time is
and the initial uni-directional current is . This recovers the Zeno effect for continuous detection of a particle
in a given domain. For the latter case the probability propagated across the
boundary of the support in time is and the
initial uni-directional current is . This is an anti-Zeno
effect. However, the probability propagated across a point located at a finite
distance from the boundary of the support is . This gives a decay
law.Comment: 17 pages, Late
Extended Wigner's friend problem and the internal consistency of standard quantum mechanics
The extended Wigner's friend problem deals with two Observers each measuring
a sealed laboratory in which a friend is making a quantum measurement. We
investigate this problem by relying on the basic rules of quantum mechanics as
exposed by Feynman in the well-known "Feynman Lectures on Physics". Although
recent discussions have suggested that the extended Wigner's friend problem
cannot consistently be described by quantum theory, we show here that a
straightforward application of these standard rules results in a non-ambiguous
and consistent account of the measurement outcomes for all agents involved.Comment: 22 pages, 3 figure
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