84 research outputs found
Measurements in the L\'{e}vy quantum walk
We study the quantum walk subjected to measurements with a L\'evy
waiting-time distribution. We find that the system has a sub-ballistic behavior
instead of a diffusive one. We obtain an analytical expression for the exponent
of the power law of the variance as a function of the characteristic parameter
of the L\'evy distribution.Comment: 4 pages, 2 figure
Quantum walk, entanglement and thermodynamic laws
We consider an special dynamics of a quantum walk (QW) on a line. Initially,
the walker localized at the origin of the line with arbitrary chirality,
evolves to an asymptotic stationary state. In this stationary state a
measurement is performed and the state resulting from this measurement is used
to start a second QW evolution to achieve a second asymptotic stationary state.
In previous works, we developed the thermodynamics associated with the
entanglement between the coin and position degrees of freedom in the QW. Here
we study the application of the first and second laws of thermodynamics to the
process between the two stationary states mentioned above. We show that: i) the
entropy change has upper and lower bounds that are obtained analytically as a
function of the initial conditions. ii) the energy change is associated to a
heat-transfer process.Comment: It was accepted to publish in Physica
Alternative thermodynamic cycle for the Stirling machine
We develop an alternative thermodynamic cycle for the Stirling machine, where
the polytropic process plays a central role. Analytical expressions for
pressure and temperatures of the working gas are obtained as a function of the
volume and the parameter which characterizes the polytropic process. This
approach achieves a closer agreement with the experimental pressure-volume
diagram and can be adapted to any type of the Stirling engine
Resonant quantum kicked rotor with two internal levels
We develop a system consisting of a quantum kicked rotor with an additional
degree of freedom. This models a single two-level atom with internal ground and
excited states, and it is characterized by its quantum resonances with
ballistic spreading and by the entanglement between the internal and momentum
degrees of freedom. These behaviors establish an equivalence between our model
and the usual quantum walk on the line.Comment: 8 page
Initial-state-dependent thermalization in open qubits
We study, from a thermodynamic perspective, the equilibrium states of a qubit
interacting with an arbitrary environment of dimension N>>2. We show that even
in presence of memory about the initial state, in some cases the qubit can be
considered in a thermal state characterized by an entanglement Hamiltonian,
which encodes the effects of the environment, and an initial-state- dependent
entanglement temperature that measures the degree of entanglement generated
between the system and its environment. Geometrical aspects of the thermal
states are studied, and the results are confirmed for the concrete case of the
Quantum Walk on the Line.Comment: 3 figure
Temperature of a finite-dimensional quantum system
A general expression for the temperature of a finite-dimensional quantum
system is deduced from thermodynamic arguments. At equilibrium, this magnitude
coincides with the standard thermodynamic temperature. Furthermore, it is
well-defined even far from equilibrium. Explicit formulas for the temperature
of two and three-dimensional quantum systems are presented, and some additional
relevant aspects of this quantity are discussed
Relation between the usual and the entanglement temperature, in a simple quantum system
We develop a thermodynamical theory to describe the behavior of the
entanglement between a single two-level atom with a single mode of the
electromagnetic field. The resonant Jaynes-Cummings model is used to study both
the entanglement thermodynamics, in particular the entanglement temperature,
and its connection with the average number of photons in the optical cavity. We
find that this entanglement temperature has a strong dependence with the
initial conditions of the atom. We show that the entanglement temperature
between the photons and the atom defined in this work is the same temperature
obtained within the Jaynes-Cummings model at finite temperature developed in
the Thermo-Field Dynamics formalism.Comment: published in, Physica A 437, 2015, 47
Stirling engine operating at low temperature difference
The paper develops the dynamics and thermodynamics of Stirling engines that
run with temperature differences below 100 0C. The working gas pressure is
analytically expressed using an alternative thermodynamic cycle. The shaft
dynamics is studied using its rotational equation of motion. It is found that
the initial volumes of the cold and hot working gas play a non-negligible role
in the functioning of the engine.Comment: 16 pages, 7 figure
The quantum walk temperature
A thermodynamic theory is developed to describe the behavior of the
entanglement between the coin and position degrees of freedom of the quantum
walk on the line. This theory shows that, in spite of the unitary evolution, a
steady state is established after a Markovian transient stage. This study
suggests that if a quantum dynamics is developed in a composite Hilbert space
(i.e. the tensor product of several sub-spaces) then the behavior of an
operator that only belongs to one of the sub-spaces may camouflage the unitary
character of the global evolution.Comment: 8 pages, 6 figure
The Fluidyne engine
The Fluidyne is a two-part hot-air engine, which has the peculiarity that
both its power piston and displacer are liquids. Both parts operate in tandem
with the common working gas (air) transferring energy from the displacer to the
piston side, from which work is extracted. We describe analytically the
thermodynamics of the Fluidyne engine using the approach previously developed
for the Stirling engine. We obtain explicit expressions for the amplitude of
the power piston movement and for the working gas temperatures and pressure as
functions of the engine parameters. We also study numerically the power and
efficiency of the engine in terms of the phase shift between the motions of
piston and displacer.Comment: 13 pages, 4 figure
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