9,168 research outputs found
Heisenberg uncertainty for qubit measurements
Reports on experiments recently performed in Vienna [Erhard et al, Nature
Phys. 8, 185 (2012)] and Toronto [Rozema et al, Phys. Rev. Lett. 109, 100404
(2012)] include claims of a violation of Heisenberg's error-disturbance
relation. In contrast, we have presented and proven a Heisenberg-type relation
for joint measurements of position and momentum [Phys. Rev. Lett. 111, 160405
(2013)]. To resolve the apparent conflict, we formulate here a new general
trade-off relation for errors in qubit measurements, using the same concepts as
we did in the position-momentum case. We show that the combined errors in an
approximate joint measurement of a pair of +/-1 valued observables A,B are
tightly bounded from below by a quantity that measures the degree of
incompatibility of A and B. The claim of a violation of Heisenberg is shown to
fail as it is based on unsuitable measures of error and disturbance. Finally we
show how the experiments mentioned may directly be used to test our error
inequality.Comment: Version 3 contains further clarifications in our argument refuting
the alleged violation of Heisenberg's error-disturbance relation. Some new
material added on the connection between preparation uncertainty and
approximation error relation
Transport, atom blockade and output coupling in a Tonks-Girardeau gas
Recent experiments have demonstrated how quantum-mechanical impurities can be
created within strongly correlated quantum gases and used to probe the
coherence properties of these systems [S. Palzer, C. Zipkes, C. Sias, and M.
K\"ohl, Phys. Rev. Lett. 103, 150601 (2009).]. Here we present a
phenomenological model to simulate such an output coupler for a Tonks-Girardeau
gas that shows qualitative agreement with the experimental results for atom
transport and output coupling. Our model allows us to explore nonequilibrium
transport phenomena in ultracold quantum gases and leads us to predict a regime
of atom blockade, where the impurity component becomes localized in the parent
cloud despite the presence of gravity. We show that this provides a stable
mixed-species quantum gas in the strongly correlated limit
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