12,793 research outputs found
Sonine approximation for collisional moments of granular gases of inelastic rough spheres
We consider a dilute granular gas of hard spheres colliding inelastically
with coefficients of normal and tangential restitution and ,
respectively. The basic quantities characterizing the distribution function
of linear () and angular
() velocities are the second-degree moments defining the
translational () and rotational () temperatures. The
deviation of from the Maxwellian distribution parameterized by
and can be measured by the cumulants associated
with the fourth-degree velocity moments. The main objective of this paper is
the evaluation of the collisional rates of change of these second- and
fourth-degree moments by means of a Sonine approximation. The results are
subsequently applied to the computation of the temperature ratio
and the cumulants of two paradigmatic states: the
homogeneous cooling state and the homogeneous steady state driven by a
white-noise stochastic thermostat. It is found in both cases that the
Maxwellian approximation for the temperature ratio does not deviate much from
the Sonine prediction. On the other hand, non-Maxwellian properties measured by
the cumulants cannot be ignored, especially in the homogeneous cooling state
for medium and small roughness. In that state, moreover, the cumulant directly
related to the translational velocity differs in the quasi-smooth limit
from that of pure smooth spheres (). This singular
behavior is directly related to the unsteady character of the homogeneous
cooling state and thus it is absent in the stochastic thermostat case.Comment: 14 pages, 8 figures; v2: some parts rewritten, new references added;
published in a special topic decicated to Carlo Cercignan
Continuous Quantum Error Correction Through Local Operations
We propose local strategies to protect global quantum information. The
protocols, which are quantum error correcting codes for dissipative systems,
are based on environment measurements, direct feedback control and simple
encoding of the logical qubits into physical qutrits whose decaying transitions
are indistinguishable and equally probable. The simple addition of one extra
level in the description of the subsystems allows for local actions to fully
and deterministically protect global resources, such as entanglement. We
present codes for both quantum jump and quantum state diffusion measurement
strategies and test them against several sources of inefficiency. The use of
qutrits in information protocols suggests further characterization of
qutrit-qutrit disentanglement dynamics, which we also give together with simple
local environment measurement schemes able to prevent distillability sudden
death and even enhance entanglement in situations in which our feedback error
correction is not possible.Comment: Accepted for publication in Phys. Rev.
Observing different quantum trajectories in cavity QED
The experimental observation of quantum jumps is an example of single open
quantum systems that, when monitored, evolve in terms of stochastic
trajectories conditioned on measurements results. Here we present a proposal
that allows the experimental observation of a much larger class of quantum
trajectories in cavity QED systems. In particular, our scheme allows for the
monitoring of engineered thermal baths that are crucial for recent proposals
for probing entanglement decay and also for entanglement protection. The scheme
relies on the interaction of a three-level atom and a cavity mode that
interchangeably play the roles of system and probe. If the atom is detected the
evolution of the cavity fields follows quantum trajectories and vice-versa.Comment: 5 pages, 2 figure
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