126,928 research outputs found
Entropy of a Quantum Oscillator coupled to a Heat Bath and implications for Quantum Thermodynamics
The free energy of a quantum oscillator in an arbitrary heat bath at a
temperature T is given by a "remarkable formula" which involves only a single
integral. This leads to a corresponding simple result for the entropy. The low
temperature limit is examined in detail and we obtain explicit results both for
the case of an Ohmic heat bath and a radiation heat bath. More general heat
bath models are also examined. This enables us to determine the entropy at zero
temperature in order to check the third law of thermodynamics in the quantum
regimeComment: International Conference on "Frontiers of Quantum and Mesoscopic
Thermodynamics
Decoherence in Nanostructures and Quantum Systems
Decoherence phenomena are pervasive in the arena of nanostructures but
perhaps even more so in the study of the fundamentals of quantum mechanics and
quantum computation. Since there has been little overlap between the studies in
both arenas, this is an attempt to bridge the gap. Topics stressed include (a)
wave packet spreading in a dissipative environment, a key element in all
arenas, (b) the definition of a quantitative measure of decoherence, (c) the
near zero and zero temperature limit, and (d) the key role played by initial
conditions: system and environment entangled at all times so that one must use
the density matrix (or Wigner distribution) for the complete system or
initially decoupled system and environment so that use of a reduced density
matrix or reduced Wigner distribution is feasible. Our approach utilizes
generalized quantum Langevin equations and Wigner distributions
Quantum states with negative energy density in the Dirac field and quantum inequalities
Energy densities of the quantum states that are superposition of two
multi-electron-positron states are examined. It is shown that the energy
densities can be negative only when two multi-particle states have the same
number of electrons and positrons or when one state has one more
electron-positron pair than the other. In the cases in which negative energy
could arise, we find that the energy is that of a positive constant plus a
propagating part which oscillates between positive and negative, and the energy
can dip to negative at some places at for a certain period of time if the
quantum states are properly manipulated. It is demonstrated that the negative
energy densities satisfy the quantum inequality. Our results also reveal that
for a given particle content, the detection of negative energy is an operation
that depends on the frame where any measurement is to be performed. This
suggests that the sign of energy density for a quantum state may be a
coordinate-dependent quantity in quantum theory.Comment: Revtex,9 pages, no figures, a couple of typos correcte
Parallel Algorithm for Solving Kepler's Equation on Graphics Processing Units: Application to Analysis of Doppler Exoplanet Searches
[Abridged] We present the results of a highly parallel Kepler equation solver
using the Graphics Processing Unit (GPU) on a commercial nVidia GeForce 280GTX
and the "Compute Unified Device Architecture" programming environment. We apply
this to evaluate a goodness-of-fit statistic (e.g., chi^2) for Doppler
observations of stars potentially harboring multiple planetary companions
(assuming negligible planet-planet interactions). We tested multiple
implementations using single precision, double precision, pairs of single
precision, and mixed precision arithmetic. We find that the vast majority of
computations can be performed using single precision arithmetic, with selective
use of compensated summation for increased precision. However, standard single
precision is not adequate for calculating the mean anomaly from the time of
observation and orbital period when evaluating the goodness-of-fit for real
planetary systems and observational data sets. Using all double precision, our
GPU code outperforms a similar code using a modern CPU by a factor of over 60.
Using mixed-precision, our GPU code provides a speed-up factor of over 600,
when evaluating N_sys > 1024 models planetary systems each containing N_pl = 4
planets and assuming N_obs = 256 observations of each system. We conclude that
modern GPUs also offer a powerful tool for repeatedly evaluating Kepler's
equation and a goodness-of-fit statistic for orbital models when presented with
a large parameter space.Comment: 19 pages, to appear in New Astronom
Effect of an External Field on Decoherence
"Decoherence of quantum superpositions through coupling to engineered
reservoirs" is the topic of a recent article by Myatt et al. [Nature
{\underline{403}}, 269 (2000)] which has attracted much interest because of its
relevance to current research in fundamental quantum theory, quantum
computation, teleportation, entanglement and the quantum-classical interface.
However, the preponderance of theoretical work on decoherence does not consider
the effect of an {\underline{external field}}. Here, we present an analysis of
such an effect in the case of the random delta-correlated force discussed by
Myatt et al
An ergonomics analysis of manual versus chainsaw high ladder pruning of Pinus radiata in New Zealand : a thesis in partial fulfilment of the requirements for the degree of Master of Philosophy at Massey University
Two methods of ladder pruning Pinus radiata from 4.5 - 6.0 metres were compared using a cost-benefit approach within a framework provided by ergonomics. Chainsaw pruning is practiced in areas of New Zealand where large branches occur. The objectives of the research were to compare the costs and benefits of the two pruning techniques and provide recommendations as to whether or not the practice of chainsaw pruning should continue. These objectives were achieved by comparing the risk of injury, the physiological costs, the musculoskeletal costs, the productivity and the quality associated with the use of the two techniques. The general methods used to assess the relative costs and benefits of the two techniques were: l. Numeric descriptions of the 'risk' involved with each method of pruning 2. The use of a relative heart rate index to compare the physiological cost of the two techniques 3. Using questionnaires focusing on musculoskeletal pain and discomfort to assess any relative differences between the two techniques 4. Using continuous time study to quantify any difference in labour productivity between the two techniques 5. Sampling pruned trees to assess differences in the quality of work between manual and chainsaw pruning The research concludes that although both methods of pruning are hazardous, chainsaw pruning is more hazardous than manual pruning. Chainsaw and manual pruning were found to have the same physiological costs. Findings of the research indicate that manual pruning is not associated with a higher prevalence of musculoskeletal discomfort than chainsaw pruning on a yearly basis, although it is associated with a greater relative increase in BPD on a day to day basis and that this may lead to the development of musculoskeletal disease. Chainsaw pruning was found to be significantly more productive than manual pruning, although this was at the cost of quality. The research concludes by recommending that the use of chainsaw pruning should be limited to areas where the branches are demonstrably large. Further research is called for to compare the physiological and musculoskeletal costs of manual pruning in plantation areas of both large and small branch sizes. Further research is called for to compare the safety of two methods of chainsaw pruning with the use of the technique of wrapping one leg around the tree as opposed to not wrapping the leg around the tree. Research to investigate new ladder designs which are safer to use in the New Zealand forest environment is also called for
THE NORMAL BEHAVIOR OF THE SMARANDACHE FUNCTION
Let S(n) be the smallest integer k so that nlk!. This is known as the Smarandache
function and has been studied by many authors
Decoherence at zero temperature
Most discussions of decoherence in the literature consider the
high-temperature regime but it is also known that, in the presence of
dissipation, decoherence can occur even at zero temperature. Whereas most
previous investigations all assumed initial decoupling of the quantum system
and bath, we consider that the system and environment are entangled at all
times. Here, we discuss decoherence for a free particle in an initial
Schr\"{o}dinger cat state. Memory effects are incorporated by use of the single
relaxation time model (since the oft-used Ohmic model does not give physically
correct results)
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