797 research outputs found
Discrimination between evolution operators
Under broad conditions, evolutions due to two different Hamiltonians are
shown to lead at some moment to orthogonal states. For two spin-1/2 systems
subject to precession by different magnetic fields the achievement of
orthogonalization is demonstrated for every scenario but a special one. This
discrimination between evolutions is experimentally much simpler than
procedures proposed earlier based on either sequential or parallel application
of the unknown unitaries. A lower bound for the orthogonalization time is
proposed in terms of the properties of the two Hamiltonians.Comment: 7 pages, 2 figures, REVTe
Detecting non-locality in multipartite quantum systems with two-body correlation functions
Bell inequalities define experimentally observable quantities to detect
non-locality. In general, they involve correlation functions of all the
parties. Unfortunately, these measurements are hard to implement for systems
consisting of many constituents, where only few-body correlation functions are
accessible. Here we demonstrate that higher-order correlation functions are not
necessary to certify nonlocality in multipartite quantum states by constructing
Bell inequalities from one- and two-body correlation functions for an arbitrary
number of parties. The obtained inequalities are violated by some of the Dicke
states, which arise naturally in many-body physics as the ground states of the
two-body Lipkin-Meshkov-Glick Hamiltonian.Comment: 10 pages, 2 figures, 1 tabl
More efficient Bell inequalities for Werner states
In this paper we study the nonlocal properties of two-qubit Werner states
parameterized by the visibility parameter 0<p<1. New family of Bell
inequalities are constructed which prove the two-qubit Werner states to be
nonlocal for the parameter range 0.7056<p<1. This is slightly wider than the
range 0.7071<p<1, corresponding to the violation of the
Clauser-Horne-Shimony-Holt (CHSH) inequality. This answers a question posed by
Gisin in the positive, i.e., there exist Bell inequalities which are more
efficient than the CHSH inequality in the sense that they are violated by a
wider range of two-qubit Werner states.Comment: 7 pages, 1 figur
Maximal violation of the I3322 inequality using infinite dimensional quantum systems
The I3322 inequality is the simplest bipartite two-outcome Bell inequality
beyond the Clauser-Horne-Shimony-Holt (CHSH) inequality, consisting of three
two-outcome measurements per party. In case of the CHSH inequality the maximal
quantum violation can already be attained with local two-dimensional quantum
systems, however, there is no such evidence for the I3322 inequality. In this
paper a family of measurement operators and states is given which enables us to
attain the largest possible quantum value in an infinite dimensional Hilbert
space. Further, it is conjectured that our construction is optimal in the sense
that measuring finite dimensional quantum systems is not enough to achieve the
true quantum maximum. We also describe an efficient iterative algorithm for
computing quantum maximum of an arbitrary two-outcome Bell inequality in any
given Hilbert space dimension. This algorithm played a key role to obtain our
results for the I3322 inequality, and we also applied it to improve on our
previous results concerning the maximum quantum violation of several bipartite
two-outcome Bell inequalities with up to five settings per party.Comment: 9 pages, 3 figures, 1 tabl
A two-qubit Bell inequality for which POVM measurements are relevant
A bipartite Bell inequality is derived which is maximally violated on the
two-qubit state space if measurements describable by positive operator valued
measure (POVM) elements are allowed rather than restricting the possible
measurements to projective ones. In particular, the presented Bell inequality
requires POVMs in order to be maximally violated by a maximally entangled
two-qubit state. This answers a question raised by N. Gisin.Comment: 7 pages, 1 figur
Significant in-medium reduction of the mass of eta' mesons in sqrt(s(NN)) = 200 GeV Au+Au collisions
PHENIX and STAR data on the intercept parameter of the two-pion Bose-Einstein
correlation functions in GeV Au+Au collisions were
analysed in terms of various models of hadronic abundances. To describe these
data, an in-medium mass decrease of at least 200 MeV was needed
in each case.Comment: Dedicated to 60th birthday of Miklos Gyulassy. 2 pages, 4 figures -
To appear in the conference proceedings for Quark Matter 2009, March 30 -
April 4, Knoxville, Tennesse
Semi-device-independent bounds on entanglement
Detection and quantification of entanglement in quantum resources are two key
steps in the implementation of various quantum-information processing tasks.
Here, we show that Bell-type inequalities are not only useful in verifying the
presence of entanglement but can also be used to bound the entanglement of the
underlying physical system. Our main tool consists of a family of
Clauser-Horne-like Bell inequalities that cannot be violated maximally by any
finite-dimensional maximally entangled state. Using these inequalities, we
demonstrate the explicit construction of both lower and upper bounds on the
concurrence for two-qubit states. The fact that these bounds arise from
Bell-type inequalities also allows them to be obtained in a
semi-device-independent manner, that is, with assumption of the dimension of
the Hilbert space but without resorting to any knowledge of the actual
measurements being performed on the individual subsystems.Comment: 8 pages, 2 figures (published version). Note 1: Title changed to
distinguish our approach from the standard device-independent scenario where
no assumption on the Hilbert space dimension is made. Note 2: This paper
contains explicit examples of more nonlocality with less entanglement in the
simplest CH-like scenario (see also arXiv:1011.5206 by Vidick and Wehner for
related results
Bounding the dimension of bipartite quantum systems
Let us consider the set of joint quantum correlations arising from
two-outcome local measurements on a bipartite quantum system. We prove that no
finite dimension is sufficient to generate all these sets. We approach the
problem in two different ways by constructing explicit examples for every
dimension d, which demonstrates that there exist bipartite correlations that
necessitate d-dimensional local quantum systems in order to generate them. We
also show that at least 10 two-outcome measurements must be carried out by the
two parties altogether so as to generate bipartite joint correlations not
achievable by two-dimensional local systems. The smallest explicit example we
found involves 11 settings.Comment: 9 pages, no figures; published versio
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