1,051 research outputs found
A Discrete Four Stroke Quantum Heat Engine Exploring the Origin of Friction
The optimal power performance of a first principle quantum heat engine model
shows friction-like phenomena when the internal fluid Hamiltonian does not
commute with the external control field. The model is based on interacting
two-level-systems where the external magnetic field serves as a control
variable.Comment: 4 pages 3 figure
Improving the Efficiency of an Ideal Heat Engine: The Quantum Afterburner
By using a laser and maser in tandem, it is possible to obtain laser action
in the hot exhaust gases involved in heat engine operation. Such a "quantum
afterburner" involves the internal quantum states of working gas atoms or
molecules as well as the techniques of cavity quantum electrodynamics and is
therefore in the domain of quantum thermodynamics. As an example, it is shown
that Otto cycle engine performance can be improved beyond that of the "ideal"
Otto heat engine.Comment: 5 pages, 3 figure
A quantum-mechanical Maxwell's demon
A Maxwell's demon is a device that gets information and trades it in for
thermodynamic advantage, in apparent (but not actual) contradiction to the
second law of thermodynamics. Quantum-mechanical versions of Maxwell's demon
exhibit features that classical versions do not: in particular, a device that
gets information about a quantum system disturbs it in the process. In
addition, the information produced by quantum measurement acts as an additional
source of thermodynamic inefficiency. This paper investigates the properties of
quantum-mechanical Maxwell's demons, and proposes experimentally realizable
models of such devices.Comment: 13 pages, Te
Properties of optimal gauges in multi-mode cavity QED
Multi-mode cavity quantum electrodynamics (QED) describes, for example, the
coupling between an atom and a multi-mode electromagnetic resonator. The gauge
choice is important for practical calculations in truncated Hilbert spaces,
because the exact gauge-invariance is recovered only in the whole space. An
optimal gauge can be defined as the one predicting the most accurate
observables for the same number of atomic levels and modes. Different metrics
quantifying the gauge performance can be introduced depending on the observable
of interest. In this work we demonstrate that the optimal choice is generally
mode-dependent, i.e., a different gauge is needed for each cavity mode. While
the choice of gauge becomes more important for increasing light-matter
interaction, we also show that the optimal gauge does not correspond to the
situation where the entanglement between light and matter is the smallest.Comment: 8 pages, 5 figure
L\'evy Distribution of Single Molecule Line Shape Cumulants in Low Temperature Glass
We investigate the distribution of single molecule line shape cumulants,
, in low temperature glasses based on the sudden jump,
standard tunneling model. We find that the cumulants are described by L\'evy
stable laws, thus generalized central limit theorem is applicable for this
problem.Comment: 5 pages, 3 figure
Performance of discrete heat engines and heat pumps in finite time
The performance in finite time of a discrete heat engine with internal
friction is analyzed. The working fluid of the engine is composed of an
ensemble of noninteracting two level systems. External work is applied by
changing the external field and thus the internal energy levels. The friction
induces a minimal cycle time. The power output of the engine is optimized with
respect to time allocation between the contact time with the hot and cold baths
as well as the adiabats. The engine's performance is also optimized with
respect to the external fields. By reversing the cycle of operation a heat pump
is constructed. The performance of the engine as a heat pump is also optimized.
By varying the time allocation between the adiabats and the contact time with
the reservoir a universal behavior can be identified. The optimal performance
of the engine when the cold bath is approaching absolute zero is studied. It is
found that the optimal cooling rate converges linearly to zero when the
temperature approaches absolute zero.Comment: 45 pages LaTeX, 25 eps figure
Reply to Comment on "Completely positive quantum dissipation"
This is the reply to a Comment by R. F. O'Connell (Phys. Rev. Lett. 87 (2001)
028901) on a paper written by the author (B. Vacchini, ``Completely positive
quantum dissipation'', Phys.Rev.Lett. 84 (2000) 1374, arXiv:quant-ph/0002094).Comment: 2 pages, revtex, no figure
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