5,983 research outputs found
Entropy and Temperature of a Quantum Carnot Engine
It is possible to extract work from a quantum-mechanical system whose
dynamics is governed by a time-dependent cyclic Hamiltonian. An energy bath is
required to operate such a quantum engine in place of the heat bath used to run
a conventional classical thermodynamic heat engine. The effect of the energy
bath is to maintain the expectation value of the system Hamiltonian during an
isoenergetic expansion. It is shown that the existence of such a bath leads to
equilibrium quantum states that maximise the von Neumann entropy. Quantum
analogues of certain thermodynamic relations are obtained that allow one to
define the temperature of the energy bath.Comment: 4 pages, 1 figur
European productivity gaps: Is R&D the solution?
This paper investigates the potential impact of increased business R&D efforts in Europe on the total factor productivity gap between European and U.S. industry. The paper addresses Europeâs ambition, expressed at the 2000 Lisbon Summit to become "the most competitive and dynamic knowledge-based economy in the world", and the 3% R&D intensity target for Europe formulated at the 2002 Barcelona Summit. Based on existing empirical models from the literature on productivity and R&D expenditures, we provide projections on the expected productivity impacts of increased R&D in manufacturing industries. The results suggest that raising European R&D is not a complete solution to the European productivity backlog relative to the U.S. We also find that the most dramatic impacts may be expected from raising R&D in so-called low-tech sectors
Resonators coupled to voltage-biased Josephson junctions: From linear response to strongly driven nonlinear oscillations
Motivated by recent experiments, where a voltage biased Josephson junction is
placed in series with a resonator, the classical dynamics of the circuit is
studied in various domains of parameter space. This problem can be mapped onto
the dissipative motion of a single degree of freedom in a nonlinear
time-dependent potential, where in contrast to conventional settings the
nonlinearity appears in the driving while the static potential is purely
harmonic. For long times the system approaches steady states which are analyzed
in the underdamped regime over the full range of driving parameters including
the fundamental resonance as well as higher and sub-harmonics. Observables such
as the dc-Josephson current and the radiated microwave power give direct
information about the underlying dynamics covering phenomena as bifurcations,
irregular motion, up- and down conversion. Due to their tunability, present and
future set-ups provide versatile platforms to explore the changeover from
linear response to strongly nonlinear behavior in driven dissipative systems
under well defined conditions.Comment: 12 pages, 11 figure
QualitÀtskontrolle des "On-Line" Bildtransfers zur Verifikation der Therapiefelder in der Radio-Onkologie
A minimal model for spontaneous cell polarization and edge activity in oscillating, rotating and migrating cells
How the cells break symmetry and organize their edge activity to move
directionally is a fun- damental question in cell biology. Physical models of
cell motility commonly rely on gradients of regulatory factors and/or feedback
from the motion itself to describe polarization of edge activity. Theses
approaches, however, fail to explain cell behavior prior to the onset of
polarization. Our analysis using the model system of polarizing and moving fish
epidermal keratocytes suggests a novel and simple principle of
self-organization of cell activity in which local cell-edge dynamics depends on
the distance from the cell center, but not on the orientation with respect to
the front-back axis. We validate this principle with a stochastic model that
faithfully reproduces a range of cell-migration behaviors. Our findings
indicate that spontaneous polarization, persistent motion, and cell shape are
emergent properties of the local cell-edge dynamics controlled by the distance
from the cell center.Comment: 8 pages, 5 figure
Quantum Limits of Measurements Induced by Multiplicative Conservation Laws: Extension of the Wigner-Araki-Yanase Theorem
The Wigner-Araki-Yanase (WAY) theorem shows that additive conservation laws
limit the accuracy of measurements. Recently, various quantitative expressions
have been found for quantum limits on measurements induced by additive
conservation laws, and have been applied to the study of fundamental limits on
quantum information processing. Here, we investigate generalizations of the WAY
theorem to multiplicative conservation laws. The WAY theorem is extended to
show that an observable not commuting with the modulus of, or equivalently the
square of, a multiplicatively conserved quantity cannot be precisely measured.
We also obtain a lower bound for the mean-square noise of a measurement in the
presence of a multiplicatively conserved quantity. To overcome this noise it is
necessary to make large the coefficient of variation (the so-called relative
fluctuation), instead of the variance as is the case for additive conservation
laws, of the conserved quantity in the apparatus.Comment: 8 pages, REVTEX; typo added, to appear in PR
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