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 $T$, 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 $\hbar \to 0$. For high $T$ 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