3,139 research outputs found
Discrete Breathers in a Realistic Coarse-Grained Model of Proteins
We report the results of molecular dynamics simulations of an off-lattice
protein model featuring a physical force-field and amino-acid sequence. We show
that localized modes of nonlinear origin (discrete breathers) emerge naturally
as continuations of a subset of high-frequency normal modes residing at
specific sites dictated by the native fold. In the case of the small
-barrel structure that we consider, localization occurs on the turns
connecting the strands. At high energies, discrete breathers stabilize the
structure by concentrating energy on few sites, while their collapse marks the
onset of large-amplitude fluctuations of the protein. Furthermore, we show how
breathers develop as energy-accumulating centres following perturbations even
at distant locations, thus mediating efficient and irreversible energy
transfers. Remarkably, due to the presence of angular potentials, the breather
induces a local static distortion of the native fold. Altogether, the
combination of this two nonlinear effects may provide a ready means for
remotely controlling local conformational changes in proteins.Comment: Submitted to Physical Biolog
Experimental quantum cosmology in time-dependent optical media
It is possible to construct artificial spacetime geometries for light by
using intense laser pulses that modify the spatiotemporal properties of an
optical medium. Here we theoretically investigate experimental possibilities
for studying spacetime metrics of the form
. By tailoring the laser
pulse shape and medium properties, it is possible to create a refractive index
variation that can be identified with . Starting from a
perturbative solution to a generalised Hopfield model for the medium described
by an we provide estimates for the number of photons generated by the
time-dependent spacetime. The simplest example is that of a uniformly varying
that therefore describes the Robertson-Walker metric, i.e. a
cosmological expansion. The number of photon pairs generated in experimentally
feasible conditions appears to be extremely small. However, large photon
production can be obtained by periodically modulating the medium and thus
resorting to a resonant enhancement similar to that observed in the dynamical
Casimir effect. Curiously, the spacetime metric in this case closely resembles
that of a gravitational wave. Motivated by this analogy we show that a periodic
gravitational wave can indeed act as an amplifier for photons. The emission for
an actual gravitational wave will be very weak but should be readily observable
in the laboratory analogue.Comment: Version accepted fro publication in New Journal of Physic
The IR-Completion of Gravity: What happens at Hubble Scales?
We have recently proposed an "Ultra-Strong" version of the Equivalence
Principle (EP) that is not satisfied by standard semiclassical gravity. In the
theory that we are conjecturing, the vacuum expectation value of the (bare)
energy momentum tensor is exactly the same as in flat space: quartically
divergent with the cut-off and with no spacetime dependent (subleading) ter ms.
The presence of such terms seems in fact related to some known difficulties,
such as the black hole information loss and the cosmological constant problem.
Since the terms that we want to get rid of are subleading in the high-momentum
expansion, we attempt to explore the conjectured theory by "IR-completing" GR.
We consider a scalar field in a flat FRW Universe and isolate the first
IR-correction to its Fourier modes operators that kills the quadratic (next to
leading) time dependent divergence of the stress energy tensor VEV. Analogously
to other modifications of field operators that have been proposed in the
literature (typically in the UV), the present approach seems to suggest a
breakdown (here, in the IR, at large distances) of the metric manifold
description. We show that corrections to GR are in fact very tiny, become
effective at distances comparable to the inverse curvature and do not contain
any adjustable parameter. Finally, we derive some cosmological implications. By
studying the consistency of the canonical commutation relations, we infer a
correction to the distance between two comoving observers, which grows as the
scale factor only when small compared to the Hubble length, but gets relevant
corrections otherwise. The corrections to cosmological distance measures are
also calculable and, for a spatially flat matter dominated Universe, go in the
direction of an effective positive acceleration.Comment: 27 pages, 2 figures. Final version, references adde
Determining the carrier-envelope phase of intense few-cycle laser pulses
The electromagnetic radiation emitted by an ultra-relativistic accelerated
electron is extremely sensitive to the precise shape of the field driving the
electron. We show that the angular distribution of the photons emitted by an
electron via multiphoton Compton scattering off an intense
(I>10^{20}\;\text{W/cm^2}), few-cycle laser pulse provides a direct way of
determining the carrier-envelope phase of the driving laser field. Our
calculations take into account exactly the laser field, include relativistic
and quantum effects and are in principle applicable to presently available and
future foreseen ultra-strong laser facilities.Comment: 4 pages, 2 figure
Modelling a Particle Detector in Field Theory
Particle detector models allow to give an operational definition to the
particle content of a given quantum state of a field theory. The commonly
adopted Unruh-DeWitt type of detector is known to undergo temporary transitions
to excited states even when at rest and in the Minkowski vacuum. We argue that
real detectors do not feature this property, as the configuration "detector in
its ground state + vacuum of the field" is generally a stable bound state of
the underlying fundamental theory (e.g. the ground state-hydrogen atom in a
suitable QED with electrons and protons) in the non-accelerated case. As a
concrete example, we study a local relativistic field theory where a stable
particle can capture a light quantum and form a quasi-stable state. As
expected, to such a stable particle correspond energy eigenstates of the full
theory, as is shown explicitly by using a dressed particle formalism at first
order in perturbation theory. We derive an effective model of detector (at
rest) where the stable particle and the quasi-stable configurations correspond
to the two internal levels, "ground" and "excited", of the detector.Comment: 13 pages, references added, final versio
Coulomb-Blockade directional coupler
A tunable directional coupler based on Coulomb Blockade effect is presented.
Two electron waveguides are coupled by a quantum dot to an injector waveguide.
Electron confinement is obtained by surface Schottky gates on single
GaAs/AlGaAs heterojunction. Magneto-electrical measurements down to 350 mK are
presented and large transconductance oscillations are reported on both outputs
up to 4.2 K. Experimental results are interpreted in terms of Coulomb Blockade
effect and the relevance of the present design strategy for the implementation
of an electronic multiplexer is underlined.Comment: 4 pages, 4 figures, to be published in Applied Physics Letter
Many-body hierarchy of dissipative timescales in a quantum computer
We show that current noisy quantum computers are ideal platforms for the simulation of quantum many-body dynamics in generic open systems. We demonstrate this using the IBM Quantum Computer as an experimental platform for confirming the theoretical prediction from Wang et al., [Phys. Rev. Lett. 124, 100604 (2020)] of an emergent hierarchy of relaxation timescales of many-body observables involving different numbers of qubits. Using different protocols, we leverage the intrinsic dissipation of the machine responsible for gate errors, to implement a quantum simulation of generic (i.e., structureless) local dissipative interactions
Interaction-Induced Transparency for Strong-Coupling Polaritons
The propagation of light in strongly coupled atomic media takes place through the formation of polaritons-hybrid quasiparticles resulting from a superposition of an atomic and a photonic excitation. Here we consider the propagation under the condition of electromagnetically induced transparency and show that a novel many-body phenomenon can appear due to strong, dissipative interactions between the polaritons. Upon increasing the photon-pump strength, we find a first-order transition between an opaque phase with strongly broadened polaritons and a transparent phase where a long-lived polariton branch with highly tunable occupation emerges. Across this nonequilibrium phase transition, the transparency window is reconstructed via nonlinear interference effects induced by the dissipative polariton interactions. Our predictions are based on a systematic diagrammatic expansion of the nonequilibrium Dyson equations which can be controlled, even in the nonperturbative regime of large single-atom cooperativities, provided the polariton interactions are sufficiently long-ranged. Such a regime can be reached in photonic crystal waveguides thanks to the tunability of interactions, allowing us to observe the interaction-induced transparency transition even at low polariton densities
The final state and thermodynamics of dark energy universe
As it follows from the classical analysis, the typical final state of the
dark energy universe where dominant energy condition is violated is finite
time, sudden future singularity (Big Rip). For a number of dark energy
universes (including scalar phantom and effective phantom theories as well as
specific quintessence model) we demonstrate that quantum effects play the
dominant role near Big Rip, driving the universe out of future singularity (or,
at least, making it milder). As a consequence, the entropy bounds with quantum
corrections become well-defined near Big Rip. Similarly, black holes mass loss
due to phantom accretion is not so dramatic as it was expected: masses do not
vanish to zero due to transient character of phantom evolution stage. Some
examples of cosmological evolution for negative, time-dependent equation of
state are also considered with the same conclusions. The application of
negative entropy (or negative temparature) occurence in the phantom
thermodynamics is briefly discussed.Comment: LaTeX file 36 pages, version to appear in PR
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