513 research outputs found
Monogamy of Bell's inequality violations in non-signaling theories
We derive monogamy relations (tradeoffs) between strengths of violations of
Bell's inequalities from the non-signaling condition. Our result applies to
general Bell inequalities with an arbitrary large number of partners, outcomes
and measurement settings. The method is simple, efficient and does not require
linear programming. The results are used to derive optimal fidelity for
asymmetric cloning in nonsignaling theories.Comment: 4 pages, 2 figures, published versio
Why the Tsirelson bound?
Wheeler's question 'why the quantum' has two aspects: why is the world
quantum and not classical, and why is it quantum rather than superquantum,
i.e., why the Tsirelson bound for quantum correlations? I discuss a remarkable
answer to this question proposed by Pawlowski et al (2009), who provide an
information-theoretic derivation of the Tsirelson bound from a principle they
call 'information causality.'Comment: 17 page
Revivals in the attractive BEC in a double-well potential and their decoherence
We study the dynamics of ultracold attractive atoms in a weakly linked two
potential wells. We consider an unbalanced initial state and monitor dynamics
of the population difference between the two wells. The average imbalance
between wells undergoes damped oscillations, like in a classical counterpart,
but then it revives almost to the initial value. We explain in details the
whole behavior using three different models of the system. Furthermore we
investigate the sensitivity of the revivals on the decoherence caused by one-
and three-body losses. We include the dissipative processes using appropriate
master equations and solve them using the stochastic wave approximation method
Effects of Disorder on Superconductivity of Systems with Coexisting Itinerant Electrons and Local Pairs
We study the influence of diagonal disorder (random site energy) of local
pair (LP) site energies on the superconducting properties of a system of
coexisting local pairs and itinerant electrons described by the (hard-core)
boson-fermion model. Our analysis shows that the properties of such a model
with s-wave pairing can be very strongly affected by the diagonal disorder in
LP subsystem (the randomness of the LP site energies). This is in contrast with
the conventional s-wave BCS superconductors, which according to the Anderson's
theorem are rather insensitive to the diagonal disorder (i.e. to nonmagnetic
impurities). It has been found that the disorder effects depend in a crucial
way on the total particle concentration n and the LP level position DELTA_o and
depending on the parameters the system can exhibit various types of
superconducting behaviour, including the LP-like, intermediate (MIXED)and the
'BCS'-like. In the extended range of {n,DELTA_o} the superconducting ordering
is suppressed by the randomness of the LP site energies and the increasing
disorder induces a changeover from the MIXEDlike behaviour to the BCS-like one,
connected with abrupt reduction of T_c and energy gap to zero. However, there
also exist a definite range of {n,DELTA_o} in which the increasing disorder has
a quite different effect: namely it can substantially enhance T_c or even lead
to the phenomenon which can be called disorder induced superconductivity.
Another interesting effect is a possibility of a disorder induced bound pair
formation of itinerant electrons, connected with the change-over to the LP-like
regime.Comment: 18 pages, 12 figure
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