8,243 research outputs found
A generalized structure of Bell inequalities for bipartite arbitrary dimensional systems
We propose a generalized structure of Bell inequalities for arbitrary
d-dimensional bipartite systems, which includes the existing two types of Bell
inequalities introduced by Collins-Gisin-Linden-Massar-Popescu [Phys. Rev.
Lett. 88, 040404 (2002)] and Son-Lee-Kim [Phys. Rev. Lett. 96, 060406 (2006)].
We analyze Bell inequalities in terms of correlation functions and joint
probabilities, and show that the coefficients of correlation functions and
those of joint probabilities are in Fourier transform relations. We finally
show that the coefficients in the generalized structure determine the
characteristics of quantum violation and tightness.Comment: 6 pages, 1 figur
Effects of squeezing on quantum nonlocality of superpositions of coherent states
We analyze effects of squeezing upon superpositions of coherent states (SCSs)
and entangled coherent states (ECSs) for Bell-inequality tests. We find that
external squeezing can always increase the degrees of Bell violations, if the
squeezing direction is properly chosen, for the case of photon parity
measurements. On the other hand, when photon on/off measurements are used, the
squeezing operation can enhance the degree of Bell violations only for moderate
values of amplitudes and squeezing. We point out that a significant improvement
is required over currently available squeezed SCSs in order to directly
demonstrate a Bell-inequality violation in a real experiment.Comment: 7 pages, 4 figures, accepted for publication in Phys. Rev.
Quantum Phase Transitions of Hard-Core Bosons in Background Potentials
We study the zero temperature phase diagram of hard core bosons in two
dimensions subjected to three types of background potentials: staggered,
uniform, and random. In all three cases there is a quantum phase transition
from a superfluid (at small potential) to a normal phase (at large potential),
but with different universality classes. As expected, the staggered case
belongs to the XY universality, while the uniform potential induces a mean
field transition. The disorder driven transition is clearly different from
both; in particular, we find z~1.4, \nu~1, and \beta~0.6.Comment: 4 pages (6 figures); published version-- 2 references added, minor
clarification
High-dimensional Bell test for a continuous variable state in phase space and its robustness to detection inefficiency
We propose a scheme for testing high-dimensional Bell inequalities in phase
space. High-dimensional Bell inequalities can be recast into the forms of a
phase-space version using quasiprobability functions with the complex-valued
order parameter. We investigate their violations for two-mode squeezed states
while increasing the dimension of measurement outcomes, and finally show the
robustness of high-dimensional tests to detection inefficiency.Comment: 8 pages, 2 figures; title and abstract changed, published versio
Ballistic spin field-effect transistors: Multichannel effects
We study a ballistic spin field-effect transistor (SFET) with special
attention to the issue of multi-channel effects. The conductance modulation of
the SFET as a function of the Rashba spin-orbit coupling strength is
numerically examined for the number of channels ranging from a few to close to
100. Even with the ideal spin injector and collector, the conductance
modulation ratio, defined as the ratio between the maximum and minimum
conductances, decays rapidly and approaches one with the increase of the
channel number. It turns out that the decay is considerably faster when the
Rashba spin-orbit coupling is larger. Effects of the electronic coherence are
also examined in the multi-channel regime and it is found that the coherent
Fabry-Perot-like interference in the multi-channel regime gives rise to a
nested peak structure. For a nonideal spin injector/collector structure, which
consists of a conventional metallic ferromagnet-thin insulator-2DEG
heterostructure, the Rashba-coupling-induced conductance modulation is strongly
affected by large resonance peaks that arise from the electron confinement
effect of the insulators. Finally scattering effects are briefly addressed and
it is found that in the weakly diffusive regime, the positions of the resonance
peaks fluctuate, making the conductance modulation signal sample-dependent.Comment: 18 pages, 15 figure
Growth control of oxygen stoichiometry in homoepitaxial SrTiO3 films by pulsed laser epitaxy in high vacuum
In many transition metal oxides (TMOs), oxygen stoichiometry is one of the
most critical parameters that plays a key role in determining the structural,
physical, optical, and electrochemical properties of the material. However,
controlling the growth to obtain high quality single crystal films having the
right oxygen stoichiometry, especially in a high vacuum environment, has been
viewed as a challenge. In this work, we show that through proper control of the
plume kinetic energy, stoichiometric crystalline films can be synthesized
without generating oxygen defects, even in high vacuum. We use a model
homoepitaxial system of SrTiO3 (STO) thin films on single crystal STO
substrates. Physical property measurements indicate that oxygen vacancy
generation in high vacuum is strongly influenced by the energetics of the laser
plume, and it can be controlled by proper laser beam delivery. Therefore, our
finding not only provides essential insight into oxygen stoichiometry control
in high vacuum for understanding the fundamental properties of STO-based thin
films and heterostructures, but expands the utility of pulsed laser epitaxy of
other materials as well
Faithful test of non-local realism with entangled coherent states
We investigate the violation of Leggett's inequality for non-local realism
using entangled coherent states and various types of local measurements. We
prove mathematically the relation between the violation of the
Clauser-Horne-Shimony-Holt form of Bell's inequality and Leggett's one when
tested by the same resources. For Leggett inequalities, we generalize the
non-local realistic bound to systems in Hilbert spaces larger than
bidimensional ones and introduce an optimization technique that allows to
achieve larger degrees of violation by adjusting the local measurement
settings. Our work describes the steps that should be performed to produce a
self-consistent generalization of Leggett's original arguments to
continuous-variable states.Comment: 8 pages, 6 figures, to be published in Phys. Rev.
- …