31 research outputs found
Extraction of work from a single thermal bath in the quantum regime
The stationary state of a quantum particle strongly coupled to a quantum
thermal bath is known to be non-gibbsian, due to entanglement with the bath.
For harmonic potentials, where the system can be described by effective
temperatures, thermodynamic relations are shown to take a generalized Gibbsian
form, that may violate the Clausius inequality. For the weakly-anharmonic case
a Fokker-Planck type description is constructed. It is shown that then work can
be extracted from the bath by cyclic variation of a parameter. These apparent
violations of the second law are the consequence of quantum coherence in the
presence of the slightly off-equilibrium nature of the bath.Comment: 5 pages, revtex, 0 figures, to appear in Phys. Rev. Let
Optimizing the classical heat engine
A pair of systems at different temperatures is a classic environment for a
heat engine, which produces work during the relaxation to a common equilibrium.
It is generally believed that a direct interaction between the two systems will
always decrease the amount of the obtainable work, due to inevitable
dissipation. Here a situation is reported where, in some time window, work can
be gained due the direct coupling, while dissipation is relevant only for much
larger times. Thus, the amount of extracted work increases, at the cost of a
change of the final state.Comment: 4 pages, revtex, no figures; to appear in Phys. Rev. Let
Mean-field theory of quantum brownian motion
We investigate a mean-field approach to a quantum brownian particle
interacting with a quantum thermal bath at temperature , and subjected to a
non-linear potential. An exact, partially classical description of quantum
brownian motion is proposed, which uses negative probabilities in its
intermediate steps. It is shown that properties of the quantum particle can be
mapped to those of two classical brownian particles in a common potential,
where one of them interacts with the quantum bath, whereas another one
interacts with a classical bath at zero temperature. Due to damping the system
allows a unique and non-singular classical limit at . For high
the stationary state becomes explicitly classical. The low-temperature case is
studied through an effective Fokker-Planck equation. Non-trivial purely quantum
correlation effects between the two particles are found.Comment: 13 pages, 0 figures, revte
The quantum measurement process: an exactly solvable model
An exactly solvable model for a quantum measurement is discussed, that
integrates quantum measurements with classical measurements.
The z-component of a spin-1/2 test spin is measured with an apparatus, that
itself consists of magnet of N spin-1/2 particles, coupled to a bath. The
initial state of the magnet is a metastable paramagnet, while the bath starts
in a thermal, gibbsian state. Conditions are such that the act of measurement
drives the magnet in the up or down ferromagnetic state, according to the sign
of s_z of the test spin.
The quantum measurement goes in two steps. On a timescale 1/\sqrt{N} the
collapse takes place due to a unitary evolution of test spin and apparatus
spins; on a larger but still short timescale this collapse is made definite by
the bath. Then the system is in a `classical' state, having a diagonal density
matrix. The registration of that state is basically a classical process, that
can already be understood from classical statistical mechanics.Comment: 4 pages, presented at the conference "Anomalies and Strange Behavior
in Physics: Challenging the conventional", Napels, April, 2003. v2:
Elaboration on the statistical interpretation of Q