206 research outputs found
Sub-shot-noise shadow sensing with quantum correlations
The quantised nature of the electromagnetic field sets the classical limit to the sensitivity of position measurements. However, techniques based on the properties of quantum states can be exploited to accurately measure the relative displacement of a physical object beyond this classical limit. In this work, we use a simple scheme based on the split-detection of quantum correlations to measure the position of a shadow at the single-photon light level, with a precision that exceeds the shot-noise limit. This result is obtained by analysing the correlated signals of bi-photon pairs, created in parametric downconversion and detected by an electron multiplying CCD (EMCCD) camera employed as a split-detector. By comparing the measured statistics of spatially anticorrelated and uncorrelated photons we were able to observe a significant noise reduction corresponding to an improvement in position sensitivity of up to 17% (0.8dB). Our straightforward approach to sub-shot-noise position measurement is compatible with conventional shadow-sensing techniques based on the split-detection of light-fields, and yields an improvement that scales favourably with the detector’s quantum efficiency
An Extended Variational Principle for the SK Spin-Glass Model
The recent proof by F. Guerra that the Parisi ansatz provides a lower bound
on the free energy of the SK spin-glass model could have been taken as offering
some support to the validity of the purported solution. In this work we present
a broader variational principle, in which the lower bound, as well as the
actual value, are obtained through an optimization procedure for which
ultrametic/hierarchal structures form only a subset of the variational class.
The validity of Parisi's ansatz for the SK model is still in question. The new
variational principle may be of help in critical review of the issue.Comment: 4 pages, Revtex
Monotonicity of the dynamical activity
The Donsker-Varadhan rate function for occupation-time fluctuations has been
seen numerically to exhibit monotone return to stationary nonequilibrium [Phys.
Rev. Lett. 107, 010601 (2011)]. That rate function is related to dynamical
activity and, except under detailed balance, it does not derive from the
relative entropy for which the monotonicity in time is well understood. We give
a rigorous argument that the Donsker-Varadhan function is indeed monotone under
the Markov evolution at large enough times with respect to the relaxation time,
provided that a "normal linear-response" condition is satisfied.Comment: 19 pages, 1 figure; v3: Section I extended, 3 references adde
What do we learn from the shape of the dynamical susceptibility of glass-formers?
We compute analytically and numerically the four-point correlation function
that characterizes non-trivial cooperative dynamics in glassy systems within
several models of glasses: elasto-plastic deformations, mode-coupling theory
(MCT), collectively rearranging regions (CRR), diffusing defects and
kinetically constrained models (KCM). Some features of the four-point
susceptibility chi_4(t) are expected to be universal. at short times we expect
an elastic regime characterized by a t or sqrt{t} growth. We find both in the
beta, and the early alpha regime that chi_4 sim t^mu, where mu is directly
related to the mechanism responsible for relaxation. This regime ends when a
maximum of chi_4 is reached at a time t=t^* of the order of the relaxation time
of the system. This maximum is followed by a fast decay to zero at large times.
The height of the maximum also follows a power-law, chi_4(t^*) sim t^{*lambda}.
The value of the exponents mu and lambda allows one to distinguish between
different mechanisms. For example, freely diffusing defects in d=3 lead to mu=2
and lambda=1, whereas the CRR scenario rather predicts either mu=1 or a
logarithmic behaviour depending on the nature of the nucleation events, and a
logarithmic behaviour of chi_4(t^*). MCT leads to mu=b and lambda =1/gamma,
where b and gamma are the standard MCT exponents. We compare our theoretical
results with numerical simulations on a Lennard-Jones and a soft-sphere system.
Within the limited time-scales accessible to numerical simulations, we find
that the exponent mu is rather small, mu < 1, with a value in reasonable
agreement with the MCT predictions.Comment: 26 pages, 6 figure
Jamming percolation and glassy dynamics
We present a detailed physical analysis of the dynamical glass-jamming
transition which occurs for the so called Knight models recently introduced and
analyzed in a joint work with D.S.Fisher \cite{letterTBF}. Furthermore, we
review some of our previous works on Kinetically Constrained Models.
The Knights models correspond to a new class of kinetically constrained
models which provide the first example of finite dimensional models with an
ideal glass-jamming transition. This is due to the underlying percolation
transition of particles which are mutually blocked by the constraints. This
jamming percolation has unconventional features: it is discontinuous (i.e. the
percolating cluster is compact at the transition) and the typical size of the
clusters diverges faster than any power law when . These
properties give rise for Knight models to an ergodicity breaking transition at
: at and above a finite fraction of the system is frozen. In
turn, this finite jump in the density of frozen sites leads to a two step
relaxation for dynamic correlations in the unjammed phase, analogous to that of
glass forming liquids. Also, due to the faster than power law divergence of the
dynamical correlation length, relaxation times diverge in a way similar to the
Vogel-Fulcher law.Comment: Submitted to the special issue of Journal of Statistical Physics on
Spin glasses and related topic
Critical properties and finite--size estimates for the depinning transition of directed random polymers
We consider models of directed random polymers interacting with a defect
line, which are known to undergo a pinning/depinning (or
localization/delocalization) phase transition. We are interested in critical
properties and we prove, in particular, finite--size upper bounds on the order
parameter (the {\em contact fraction}) in a window around the critical point,
shrinking with the system size. Moreover, we derive a new inequality relating
the free energy \tf and an annealed exponent which describes extreme
fluctuations of the polymer in the localized region. For the particular case of
a --dimensional interface wetting model, we show that this implies an
inequality between the critical exponents which govern the divergence of the
disorder--averaged correlation length and of the typical one. Our results are
based on on the recently proven smoothness property of the depinning transition
in presence of quenched disorder and on concentration of measure ideas.Comment: 15 pages, 1 figure; accepted for publication on J. Stat. Phy
A compact acoustic spanner to rotate macroscopic objects
Waves can carry both linear and angular momentum. When the wave is transverse (e.g. light), the angular momentum can be characterised by the “spin” angular momentum associated with circular polarisation, and the “orbital” angular momentum (OAM) arising from the phase cross-section of the beam. When the wave is longitudinal (e.g. sound) there is no polarization and hence no spin angular momentum. However, a suitably phase-structured sound beam can still carry OAM. Observing the transfer of OAM from sound to a macroscopic object provides an excellent opportunity to study the exchange of energy between waves and matter. In this paper we show how to build a compact free-space acoustic spanner based on a 3D-printed sound-guiding structure and common electronic components. We first characterise the sound fields by measuring both phase and amplitude maps, and then show a video of our free-space acoustic spanner in action, in which macroscopic objects spin in a circular motion and change direction of rotation according to the handedness of the OAM acoustic field
Quantum Position Measurement of a Shadow: Beating the Classical Limit
The precision with which the position of a shadow can be measured is classically limited by shot-noise. We achieve sub-shot-noise position sensitivity by jointly detecting correlated photons with a simple split-detector scheme
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