99 research outputs found
Minimal Self-Contained Quantum Refrigeration Machine Based on Four Quantum Dots
We present a theoretical study of an electronic quantum refrigerator based on
four quantum dots arranged in a square configuration, in contact with as many
thermal reservoirs. We show that the system implements the basic minimal
mechanism for acting as a self-contained quantum refrigerator, by demonstrating
heat extraction from the coldest reservoir and the cooling of the nearby
quantum-dot.Comment: 5 pages, 3 figure
Tracer particle in a confined correlated medium: an adiabatic elimination method
We present a simple and systematic procedure to determine the effective
dynamics of a Brownian particle coupled to a rapidly fluctuating correlated
medium, modeled as a scalar Gaussian field, under spatial confinement. The
method allows us, in particular, to address the case in which the fluctuations
of the medium are suppressed in the vicinity of the particle, as described by a
quadratic coupling in the underlying Hamiltonian. As a consequence of the
confinement of the correlated medium, the resulting effective Fokker-Planck
equation features spatially dependent drift and diffusion coefficients. We
apply our method to simplified fluid models of binary mixtures and
microemulsions near criticality containing a colloidal particle, and we analyze
the corrections to the stationary distribution of the particle position and the
diffusion coefficient.Comment: 26 pages, 4 figure
Memory-induced oscillations of a driven particle in a dissipative correlated medium
The overdamped dynamics of a particle is in general affected by its
interaction with the surrounding medium, especially out of equilibrium, and
when the latter develops spatial and temporal correlations. Here we consider
the case in which the medium is modeled by a scalar Gaussian field with
relaxational dynamics, and the particle is dragged at constant velocity through
the medium by a harmonic trap. This mimics the setting of an active
microrheology experiment conducted in a near-critical medium. When the particle
is displaced from its average position in the nonequilibrium steady state, its
subsequent relaxation is shown to feature damped oscillations. This is similar
to what has been recently predicted and observed in viscoelastic fluids, but
differs from what happens in the absence of driving or for an overdamped
Markovian dynamics, in which cases oscillations cannot occur. We characterize
these oscillating modes in terms of the parameters of the underlying mesoscopic
model for the particle and the medium, confirming our analytical predictions
via numerical simulations.Comment: 19 pages, 7 figure
Nonequilibrium relaxation of a trapped particle in a near-critical Gaussian field
We study the non-equilibrium relaxational dynamics of a probe particle
linearly coupled to a thermally fluctuating scalar field and subject to a
harmonic potential, which provides a cartoon for an optically trapped colloid
immersed in a fluid close to its bulk critical point. The average position of
the particle initially displaced from the position of mechanical equilibrium is
shown to feature long-time algebraic tails as the critical point of the field
is approached, the universal exponents of which are determined in arbitrary
spatial dimensions. As expected, this behavior cannot be captured by adiabatic
approaches which assume fast field relaxation. The predictions of the analytic,
perturbative approach are qualitatively confirmed by numerical simulations.Comment: 33 pages, 11 figure
Collective response to local perturbations: how to evade threats without losing coherence
Living groups move in complex environments and are constantly subject to
external stimuli, predatory attacks and disturbances. An efficient response to
such perturbations is vital to maintain the group's coherence and cohesion.
Perturbations are often local, i.e. they are initially perceived only by few
individuals in the group, but can elicit a global response. This is the case of
starling flocks, that can turn very quickly to evade predators. In this paper,
we investigate the conditions under which a global change of direction can
occur upon local perturbations. Using minimal models of self-propelled
particles, we show that a collective directional response occurs on timescales
that grow with the system size and it is, therefore, a finite-size effect. The
larger the group is, the longer it will take to turn. We also show that global
coherent turns can only take place if i) the mechanism for information
propagation is efficient enough to transmit the local reaction undamped through
the whole group; and if ii) motility is not too strong, to avoid that the
perturbed individual leaves the group before the turn is complete. No
compliance with such conditions results in the group's fragmentation or in a
non-efficient response.Comment: 23 pages, 7 figure
Quantum Optimization of Fully-Connected Spin Glasses
The Sherrington-Kirkpatrick model with random couplings is programmed
on the D-Wave Two annealer featuring 509 qubits interacting on a Chimera-type
graph. The performance of the optimizer compares and correlates to simulated
annealing. When considering the effect of the static noise, which degrades the
performance of the annealer, one can estimate an improvement on the comparative
scaling of the two methods in favor of the D-Wave machine. The optimal choice
of parameters of the embedding on the Chimera graph is shown to be associated
to the emergence of the spin-glass critical temperature of the embedded
problem.Comment: includes supplemental materia
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