630 research outputs found
Monogamy of Bell correlations and Tsirelson's bound
We consider three parties, A, B, and C, each performing one of two local measurements on a shared quantum state of arbitrary dimension. We characterize the trade-off between the nonlocality of the Bell correlations observed by AB and of those observed by AC. This generalizes Tsirelson's bound on the quantum value of the CHSH inequality, the latter being recovered when C is completely uncorrelated with AB. We also discuss the trade-off between Bell violations and local expectation values of observables that anticommute with the ones used in the Bell test
Clash of symmetries on the brane
If our 3+1-dimensional universe is a brane or domain wall embedded in a
higher dimensional space, then a phenomenon we term the ``clash of symmetries''
provides a new method of breaking some continuous symmetries. A global
symmetry is spontaneously broken
to , where the continuous subgroup
can be embedded in several different ways in the parent group
, and . A certain
class of topological domain wall solutions connect two vacua that are invariant
under {\it differently embedded} subgroups. There is then
enhanced symmetry breakdown to the intersection of these two subgroups on the
domain wall. This is the ``clash''. In the brane limit, we obtain a
configuration with symmetries in the bulk but the smaller
intersection symmetry on the brane itself. We illustrate this idea using a
permutation symmetric three-Higgs-triplet toy model exploiting the distinct
, and spin U(2) subgroups of U(3). The three disconnected portions
of the vacuum manifold can be treated symmetrically through the construction of
a three-fold planar domain wall junction configuration, with our universe at
the nexus. A possible connection with is discussed.Comment: 30 pages, 9 embedded figure
The Communication Cost of Simulating Bell Correlations
What classical resources are required to simulate quantum correlations? For
the simplest and most important case of local projective measurements on an
entangled Bell pair state, we show that exact simulation is possible using
local hidden variables augmented by just one bit of classical communication.
Certain quantum teleportation experiments, which teleport a single qubit,
therefore admit a local hidden variables model.Comment: 4 pages, 2 figures; reference adde
Sliding Phases in XY-Models, Crystals, and Cationic Lipid-DNA Complexes
We predict the existence of a totally new class of phases in weakly coupled,
three-dimensional stacks of two-dimensional (2D) XY-models. These ``sliding
phases'' behave essentially like decoupled, independent 2D XY-models with
precisely zero free energy cost associated with rotating spins in one layer
relative to those in neighboring layers. As a result, the two-point spin
correlation function decays algebraically with in-plane separation. Our
results, which contradict past studies because we include higher-gradient
couplings between layers, also apply to crystals and may explain recently
observed behavior in cationic lipid-DNA complexes.Comment: 4 pages of double column text in REVTEX format and 1 postscript
figur
Kinetic Roughening in Surfaces of Crystals Growing on Disordered Substrates
Substrate disorder effects on the scaling properties of growing crystalline
surfaces in solidification or epitaxial deposition processes are investigated.
Within the harmonic approach there is a phase transition into a low-temperature
(low-noise) superrough phase with a continuously varying dynamic exponent z>2
and a non-linear response. In the presence of the KPZ nonlinearity the disorder
causes the lattice efects to decay on large scales with an intermediate
crossover behavior. The mobility of the rough surface hes a complex dependence
on the temperature and the other physical parameters.Comment: 13 pages, 2 figures (not included). Submitted to Phys. Rev. Letts.
Use Latex twic
The Hilbertian Tensor Norm and Entangled Two-Prover Games
We study tensor norms over Banach spaces and their relations to quantum
information theory, in particular their connection with two-prover games. We
consider a version of the Hilbertian tensor norm and its dual
that allow us to consider games with arbitrary output alphabet
sizes. We establish direct-product theorems and prove a generalized
Grothendieck inequality for these tensor norms. Furthermore, we investigate the
connection between the Hilbertian tensor norm and the set of quantum
probability distributions, and show two applications to quantum information
theory: firstly, we give an alternative proof of the perfect parallel
repetition theorem for entangled XOR games; and secondly, we prove a new upper
bound on the ratio between the entangled and the classical value of two-prover
games.Comment: 33 pages, some of the results have been obtained independently in
arXiv:1007.3043v2, v2: an error in Theorem 4 has been corrected; Section 6
rewritten, v3: completely rewritten in order to improve readability; title
changed; references added; published versio
Mean-field analysis of a dynamical phase transition in a cellular automaton model for collective motion
A cellular automaton model is presented for random walkers with biologically
motivated interactions favoring local alignment and leading to collective
motion or swarming behavior. The degree of alignment is controlled by a
sensitivity parameter, and a dynamical phase transition exhibiting spontaneous
breaking of rotational symmetry occurs at a critical parameter value. The model
is analyzed using nonequilibrium mean field theory: Dispersion relations for
the critical modes are derived, and a phase diagram is constructed. Mean field
predictions for the two critical exponents describing the phase transition as a
function of sensitivity and density are obtained analytically.Comment: 4 pages, 4 figures, final version as publishe
Bioengineered implantable scaffolds as a tool to study stromal-derived factors in metastatic cancer models
Modeling the hematogenous spread of cancer cells to distant organs poses one of the greatest challenges in the study of human metastasis. Both tumor-cell intrinsic properties as well as interactions with reactive stromal cells contribute to this process, but identification of relevant stromal signals has been hampered by the lack of models allowing characterization of the metastatic niche. Here we describe an implantable bioengineered scaffold, amenable to in vivo imaging, ex vivo manipulation and serial transplantation for the continuous study of human metastasis in mice. Orthotopic or systemic inoculation of tagged human cancer cells into the mouse leads to the release of circulating tumor cells (CTCs) into the vasculature, which seed the scaffold, initiating a metastatic tumor focus. Mouse stromal cells can be readily recovered and profiled, revealing differential expression of cytokines, such as IL-1β, from tumor-bearing versus unseeded scaffolds. Finally, this platform can be used to test the effect of drugs on suppressing initiation of metastatic lesions. This generalizable model to study cancer metastasis may thus identify key stromal-derived factors with important implications for basic and translational cancer research
Nonequilibrium Fluctuations, Travelling Waves, and Instabilities in Active Membranes
The stability of a flexible fluid membrane containing a distribution of
mobile, active proteins (e.g. proton pumps) is shown to depend on the structure
and functional asymmetry of the proteins. A stable active membrane is in a
nonequilibrium steady state with height fluctuations whose statistical
properties are governed by the protein activity. Disturbances are predicted to
travel as waves at sufficiently long wavelength, with speed set by the normal
velocity of the pumps. The unstable case involves a spontaneous, pump-driven
undulation of the membrane, with clumping of the proteins in regions of high
activity.Comment: 4 two-column pages, two .eps figures included, revtex, uses eps
Device-independent quantum key distribution secure against collective attacks
Device-independent quantum key distribution (DIQKD) represents a relaxation
of the security assumptions made in usual quantum key distribution (QKD). As in
usual QKD, the security of DIQKD follows from the laws of quantum physics, but
contrary to usual QKD, it does not rely on any assumptions about the internal
working of the quantum devices used in the protocol. We present here in detail
the security proof for a DIQKD protocol introduced in [Phys. Rev. Lett. 98,
230501 (2008)]. This proof exploits the full structure of quantum theory (as
opposed to other proofs that exploit the no-signalling principle only), but
only holds again collective attacks, where the eavesdropper is assumed to act
on the quantum systems of the honest parties independently and identically at
each round of the protocol (although she can act coherently on her systems at
any time). The security of any DIQKD protocol necessarily relies on the
violation of a Bell inequality. We discuss the issue of loopholes in Bell
experiments in this context.Comment: 25 pages, 3 figure
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