121 research outputs found
The Computational Power of Beeps
In this paper, we study the quantity of computational resources (state
machine states and/or probabilistic transition precision) needed to solve
specific problems in a single hop network where nodes communicate using only
beeps. We begin by focusing on randomized leader election. We prove a lower
bound on the states required to solve this problem with a given error bound,
probability precision, and (when relevant) network size lower bound. We then
show the bound tight with a matching upper bound. Noting that our optimal upper
bound is slow, we describe two faster algorithms that trade some state
optimality to gain efficiency. We then turn our attention to more general
classes of problems by proving that once you have enough states to solve leader
election with a given error bound, you have (within constant factors) enough
states to simulate correctly, with this same error bound, a logspace TM with a
constant number of unary input tapes: allowing you to solve a large and
expressive set of problems. These results identify a key simplicity threshold
beyond which useful distributed computation is possible in the beeping model.Comment: Extended abstract to appear in the Proceedings of the International
Symposium on Distributed Computing (DISC 2015
Sequence correlations shape protein promiscuity
We predict analytically that diagonal correlations of amino acid positions
within protein sequences statistically enhance protein propensity for
nonspecific binding. We use the term 'promiscuity' to describe such nonspecific
binding. Diagonal correlations represent statistically significant repeats of
sequence patterns where amino acids of the same type are clustered together.
The predicted effect is qualitatively robust with respect to the form of the
microscopic interaction potentials and the average amino acid composition. Our
analytical results provide an explanation for the enhanced diagonal
correlations observed in hubs of eukaryotic organismal proteomes [J. Mol. Biol.
409, 439 (2011)]. We suggest experiments that will allow direct testing of the
predicted effect
Correlated Multiphoton Holes
We generate bipartite states of light which exhibit an absence of multiphoton
coincidence events between two modes amid a constant background flux. These
`correlated photon holes' are produced by mixing a coherent state and
relatively weak spontaneous parametric down-conversion using a balanced
beamsplitter. Correlated holes with arbitrarily high photon numbers may be
obtained by adjusting the relative phase and amplitude of the inputs. We
measure states of up to five photons and verify their nonclassicality. The
scheme provides a route for observation of high-photon-number nonclassical
correlations without requiring intense quantum resources.Comment: 4 pages, 3 figures, comments are welcom
Quantum state measurements using multi-pixel photon detectors
The characterization and conditional preparation of multi-photon quantum
states requires the use of photon number resolving detectors. We study the use
of detectors based on multiple avalanche photodiode pixels in this context. We
develop a general model that provides the positive operator value measures for
these detectors. The model incorporates the effect of cross-talk between pixels
which is unique to these devices. We validate the model by measuring coherent
state photon number distributions and reconstructing them with high precision.
Finally, we evaluate the suitability of such detectors for quantum state
tomography and entanglement-based quantum state preparation, highlighting the
effects of dark counts and cross-talk between pixels.Comment: 7 pages, 6 figure
Void Formation and Roughening in Slow Fracture
Slow crack propagation in ductile, and in certain brittle materials, appears
to take place via the nucleation of voids ahead of the crack tip due to plastic
yields, followed by the coalescence of these voids. Post mortem analysis of the
resulting fracture surfaces of ductile and brittle materials on the m-mm
and the nm scales respectively, reveals self-affine cracks with anomalous
scaling exponent in 3-dimensions and in
2-dimensions. In this paper we present an analytic theory based on the method
of iterated conformal maps aimed at modelling the void formation and the
fracture growth, culminating in estimates of the roughening exponents in
2-dimensions. In the simplest realization of the model we allow one void ahead
of the crack, and address the robustness of the roughening exponent. Next we
develop the theory further, to include two voids ahead of the crack. This
development necessitates generalizing the method of iterated conformal maps to
include doubly connected regions (maps from the annulus rather than the unit
circle). While mathematically and numerically feasible, we find that the
employment of the stress field as computed from elasticity theory becomes
questionable when more than one void is explicitly inserted into the material.
Thus further progress in this line of research calls for improved treatment of
the plastic dynamics.Comment: 15 pages, 20 figure
Collective behaviour of partons could be a source of energetic hadrons
We discuss the idea that collective behaviour of the quarks/partons, which
has been intensely discussed for the last 40 years in relativistic
hadron-nuclear and nuclear-nuclear interactions and confirmed by new data
coming from the ultrarelativistic heavy ion collisions, can lead to energetic
particle production. Created from hadronization of the quark/parton (or
quarks/partons), energetic particles could get the energy of grouped partons
from coherent interactions. Therefore, we think that in the centre of some
massive stars, a medium with high density, close to Quantum Chromodynamic one
could be a source of the super high-energy cosmic rays.Comment: 8 pages, 8 figure
Entanglement-enhanced probing of a delicate material system
Quantum metrology uses entanglement and other quantum effects to improve the
sensitivity of demanding measurements. Probing of delicate systems demands high
sensitivity from limited probe energy and has motivated the field's key
benchmark-the standard quantum limit. Here we report the first
entanglement-enhanced measurement of a delicate material system. We
non-destructively probe an atomic spin ensemble by means of near-resonant
Faraday rotation, a measurement that is limited by probe-induced scattering in
quantum-memory and spin-squeezing applications. We use narrowband,
atom-resonant NOON states to beat the standard quantum limit of sensitivity by
more than five standard deviations, both on a per-photon and per-damage basis.
This demonstrates quantum enhancement with fully realistic loss and noise,
including variable-loss effects. The experiment opens the way to ultra-gentle
probing of single atoms, single molecules, quantum gases and living cells.Comment: 7 pages, 8 figures; Nature Photonics, advance online publication, 16
December 201
How Many Cooks Spoil the Soup?
In this work, we study the following basic question: "How much parallelism
does a distributed task permit?" Our definition of parallelism (or symmetry)
here is not in terms of speed, but in terms of identical roles that processes
have at the same time in the execution. We initiate this study in population
protocols, a very simple model that not only allows for a straightforward
definition of what a role is, but also encloses the challenge of isolating the
properties that are due to the protocol from those that are due to the
adversary scheduler, who controls the interactions between the processes. We
(i) give a partial characterization of the set of predicates on input
assignments that can be stably computed with maximum symmetry, i.e.,
, where is the minimum multiplicity of a state in
the initial configuration, and (ii) we turn our attention to the remaining
predicates and prove a strong impossibility result for the parity predicate:
the inherent symmetry of any protocol that stably computes it is upper bounded
by a constant that depends on the size of the protocol.Comment: 19 page
Quantum-Dense Metrology
Quantum metrology utilizes entanglement for improving the sensitivity of
measurements. Up to now the focus has been on the measurement of just one out
of two non-commuting observables. Here we demonstrate a laser interferometer
that provides information about two non-commuting observables, with
uncertainties below that of the meter's quantum ground state. Our experiment is
a proof-of-principle of quantum dense metrology, and uses the additional
information to distinguish between the actual phase signal and a parasitic
signal due to scattered and frequency shifted photons. Our approach can be
readily applied to improve squeezed-light enhanced gravitational-wave detectors
at non-quantum noise limited detection frequencies in terms of a sub shot-noise
veto-channel.Comment: 5 pages, 3 figures; includes supplementary material
Statistical Physics of Fracture Surfaces Morphology
Experiments on fracture surface morphologies offer increasing amounts of data
that can be analyzed using methods of statistical physics. One finds scaling
exponents associated with correlation and structure functions, indicating a
rich phenomenology of anomalous scaling. We argue that traditional models of
fracture fail to reproduce this rich phenomenology and new ideas and concepts
are called for. We present some recent models that introduce the effects of
deviations from homogeneous linear elasticity theory on the morphology of
fracture surfaces, succeeding to reproduce the multiscaling phenomenology at
least in 1+1 dimensions. For surfaces in 2+1 dimensions we introduce novel
methods of analysis based on projecting the data on the irreducible
representations of the SO(2) symmetry group. It appears that this approach
organizes effectively the rich scaling properties. We end up with the
proposition of new experiments in which the rotational symmetry is not broken,
such that the scaling properties should be particularly simple.Comment: A review paper submitted to J. Stat. Phy
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