371 research outputs found
Stimulation of Beta Decay due to a Bose-Einstein Condensate
Nuclear processes can be stimulated by the presence of a macroscopic number
of bosons in one of the final states. We describe the conditions necessary to
observe the atom-stimulation of a beta decay process. The stimulation may be
observable if it becomes possible to produce a Bose-Einstein condensate with
the order of atoms in a trap.Comment: 7 pages, LaTeX, uses elsart.cls, home page at
http://online.anu.edu.au/Physics/Welcome.htm
The linewidth of a non-Markovian atom laser
We present a fully quantum mechanical treatment of a single mode atom laser
including pumping and output coupling. By ignoring atom-atom interactions, we
have solved this model without making the Born-Markov approximation. We find
substantially less gain narrowing than is predicted under that approximation.Comment: 4 pages, 1 encapsulated postscript figur
A Quantum-Bayesian Route to Quantum-State Space
In the quantum-Bayesian approach to quantum foundations, a quantum state is
viewed as an expression of an agent's personalist Bayesian degrees of belief,
or probabilities, concerning the results of measurements. These probabilities
obey the usual probability rules as required by Dutch-book coherence, but
quantum mechanics imposes additional constraints upon them. In this paper, we
explore the question of deriving the structure of quantum-state space from a
set of assumptions in the spirit of quantum Bayesianism. The starting point is
the representation of quantum states induced by a symmetric informationally
complete measurement or SIC. In this representation, the Born rule takes the
form of a particularly simple modification of the law of total probability. We
show how to derive key features of quantum-state space from (i) the requirement
that the Born rule arises as a simple modification of the law of total
probability and (ii) a limited number of additional assumptions of a strong
Bayesian flavor.Comment: 7 pages, 1 figure, to appear in Foundations of Physics; this is a
condensation of the argument in arXiv:0906.2187v1 [quant-ph], with special
attention paid to making all assumptions explici
Finite temperature scaling theory for the collapse of Bose-Einstein condensate
We show how to apply the scaling theory in an inhomogeneous system like
harmonically trapped Bose condensate at finite temperatures. We calculate the
temperature dependence of the critical number of particles by a scaling theory
within the Hartree-Fock approximation and find that there is a dramatic
increase in the critical number of particles as the condensation point is
approached.Comment: Published online [6 pages, 3 figures
Facts, Values and Quanta
Quantum mechanics is a fundamentally probabilistic theory (at least so far as
the empirical predictions are concerned). It follows that, if one wants to
properly understand quantum mechanics, it is essential to clearly understand
the meaning of probability statements. The interpretation of probability has
excited nearly as much philosophical controversy as the interpretation of
quantum mechanics. 20th century physicists have mostly adopted a frequentist
conception. In this paper it is argued that we ought, instead, to adopt a
logical or Bayesian conception. The paper includes a comparison of the orthodox
and Bayesian theories of statistical inference. It concludes with a few remarks
concerning the implications for the concept of physical reality.Comment: 30 pages, AMS Late
Nuclear Parity-Violation in Effective Field Theory
We reformulate the analysis of nuclear parity-violation (PV) within the
framework of effective field theory (EFT). To order Q, the PV nucleon-nucleon
(NN) interaction depends on five a priori unknown constants that parameterize
the leading-order, short-range four-nucleon operators. When pions are included
as explicit degrees of freedom, the potential contains additional medium- and
long-range components parameterized by PV piNN couplings. We derive the form of
the corresponding one- and two-pion-exchange potentials. We apply these
considerations to a set of existing and prospective PV few-body measurements
that may be used to determine the five independent low-energy constants
relevant to the pionless EFT and the additional constants associated with
dynamical pions. We also discuss the relationship between the conventional
meson-exchange framework and the EFT formulation, and argue that the latter
provides a more general and systematic basis for analyzing nuclear PV.Comment: 67 Page Latex file with typos correcte
Cosmological parameter estimation using Very Small Array data out to â= 1500
We estimate cosmological parameters using data obtained by the Very Small Array (VSA) in its extended configuration, in conjunction with a variety of other cosmic microwave background (CMB) data and external priors. Within the flat Î cold dark matter (ÎCDM) model, we find that the inclusion of high-resolution data from the VSA modifies the limits on the cosmological parameters as compared to those suggested by the Wilkinson Microwave Anisotropy Probe (WMAP) alone, while still remaining compatible with their estimates. We find that Ωbh2= 0.0234+0.0012â0.0014, Ωdmh2= 0.111+0.014â0.016, h= 0.73+0.09â0.05, nS= 0.97+0.06â0.03, 1010AS= 23+7â3 and Ï= 0.14+0.14â0.07 for WMAP and VSA when no external prior is included. On extending the model to include a running spectral index of density fluctuations, we find that the inclusion of VSA data leads to a negative running at a level of more than 95 per cent confidence ( nrun=â0.069 ± 0.032 ), something that is not significantly changed by the inclusion of a stringent prior on the Hubble constant. Inclusion of prior information from the 2dF galaxy redshift survey reduces the significance of the result by constraining the value of Ωm. We discuss the veracity of this result in the context of various systematic effects and also a broken spectral index model. We also constrain the fraction of neutrinos and find that fÎœ < 0.087 at 95 per cent confidence, which corresponds to mÎœ < 0.32 eV when all neutrino masses are equal. Finally, we consider the global best fit within a general cosmological model with 12 parameters and find consistency with other analyses available in the literature. The evidence for nrun < 0 is only marginal within this model
Mean-field analysis of the stability of a K-Rb Fermi-Bose mixture
We compare the experimental stability diagram of a Fermi-Bose mixture of K-40
and Rb-87 atoms with attractive interaction to the predictions of a mean-field
theoretical model. We discuss how this comparison can be used to give a better
estimate of the interspecies scattering length, which is currently known from
collisional measurements with larger uncertainty.Comment: 5 pages, 4 figure
Rearrangement-free hydroxylation of methylcubanes by a cytochrome P450: the case for dynamical coupling of C-H abstraction and rebound
The highly strained cubylmethyl radical undergoes one of the fastest radical rearrangements known (reported k = 2.9 à 1010 s-1 at 25 °C) through scission of two bonds of the cube. The rearrangement has previously been used as a mechanistic probe to detect radical-based pathways in enzyme-catalyzed C-H oxidations. This paper reports the discovery of highly selective cytochrome P450-catalyzed methylcubane oxidations which notionally proceed via cubylmethyl radical intermediates yet are remarkably free of rearrangement. The bacterial cytochrome P450 CYP101B1 from Novosphingobium aromaticivorans DSM 12444 is found to hydroxylate the methyl group of a range of methylcubane substrates containing a regio-directing carbonyl functionality at C-4. Unlike other reported P450-catalyzed methylcubane oxidations, the designed methylcubanes are hydroxylated with high efficiency and selectivity, giving cubylmethanols in yields of up to 93%. The lack of cubane core ring-opening implies that the cubylmethyl radicals formed during these CYP101B1-catalyzed hydroxylations must have very short lifetimes, of just a few picoseconds, which are too short for them to manifest the side reactivity characteristic of a fully equilibrated P450 intermediate. We propose that the apparent ultrafast radical rebound can be explained by a mechanism in which C-H abstraction and C-O bond formation are merged into a dynamically coupled process, effectively bypassing a discrete radical intermediate. Related dynamical phenomena can be proposed to predict how P450s may achieve various other modes of reactivity by controlling the formation and fate of radical intermediates. In principle, dynamical ideas and two-state reactivity are each individually able to explain apparent ultrashort radical lifetimes in P450 catalysis, but they are best considered together.Md. Raihan Sarkar, Sevan D. Houston, G. Paul Savage, Craig M. Williams, Elizabeth H. Krenske, Stephen G. Bell and James J. De Vos
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