4,521 research outputs found
Single-crystal diamond low-dissipation cavity optomechanics
Single-crystal diamond cavity optomechanical devices are a promising example
of a hybrid quantum system: by coupling mechanical resonances to both light and
electron spins, they can enable new ways for photons to control solid state
qubits. However, realizing cavity optomechanical devices from high quality
diamond chips has been an outstanding challenge. Here we demonstrate
single-crystal diamond cavity optomechanical devices that can enable
photon-phonon-spin coupling. Cavity optomechanical coupling to
frequency () mechanical resonances is observed. In room temperature
ambient conditions, these resonances have a record combination of low
dissipation (mechanical quality factor, ) and high
frequency, with sufficient
for room temperature single phonon coherence. The system exhibits high optical
quality factor () resonances at infrared and visible
wavelengths, is nearly sideband resolved, and exhibits optomechanical
cooperativity . The devices' potential for optomechanical control of
diamond electron spins is demonstrated through radiation pressure excitation of
mechanical self-oscillations whose 31 pm amplitude is predicted to provide 0.6
MHz coupling rates to diamond nitrogen vacancy center ground state transitions
(6 Hz / phonon), and stronger coupling rates to excited state
transitions.Comment: 12 pages, 5 figure
Static Ricci-flat 5-manifolds admitting the 2-sphere
We examine, in a purely geometrical way, static Ricci-flat 5-manifolds
admitting the 2-sphere and an additional hypersurface-orthogonal Killing
vector. These are widely studied in the literature, from different physical
approaches, and known variously as the Kramer - Gross - Perry - Davidson - Owen
solutions. The 2-fold infinity of cases that result are studied by way of new
coordinates (which are in most cases global) and the cases likely to be of
interest in any physical approach are distinguished on the basis of the
nakedness and geometrical mass of their associated singularities. It is argued
that the entire class of solutions has to be considered unstable about the
exceptional solutions: the black string and soliton cases. Any physical theory
which admits the non-exceptional solutions as the external vacuua of a
collapsing object has to accept the possibility of collapse to zero volume
leaving behind the weakest possible, albeit naked, geometrical singularities at
the origin.Finally, it is pointed out that these types of solutions generalize,
in a straightforward way, to higher dimensions.Comment: Generalize, in a straightforward way, to higher dimension
An exact self-similar solution for an expanding ball of radiation
We give an exact solution of the Einstein equations which in 4D can be
interpreted as a spherically symmetric dissipative distribution of matter, with
heat flux, whose effective density and pressure are nonstatic, nonuniform, and
satisfy the equation of state of radiation. The matter satisfies the usual
energy and thermodynamic conditions. The energy density and temperature are
related by the Stefan-Boltzmann law. The solution admits a homothetic Killing
vector in , which induces the existence of self-similar symmetry in 4D,
where the line element as well as the dimensionless matter quantities are
invariant under a simple "scaling" group.Comment: New version expanded and improved. To appear in Int. J. Mod. Phys.
New Wrinkles on an Old Model: Correlation Between Liquid Drop Parameters and Curvature Term
The relationship between the volume and surface energy coefficients in the
liquid drop A^{-1/3} expansion of nuclear masses is discussed. The volume and
surface coefficients in the liquid drop expansion share the same physical
origin and their physical connection is used to extend the expansion with a
curvature term. A possible generalization of the Wigner term is also suggested.
This connection between coefficients is used to fit the experimental nuclear
masses. The excellent fit obtained with a smaller number of parameters
validates the assumed physical connection.Comment: 6 pages, 2 figure
An exact solution of the five-dimensional Einstein equations with four-dimensional de Sitter-like expansion
We present an exact solution to the Einstein field equations which is Ricci
and Riemann flat in five dimensions, but in four dimensions is a good model for
the early vacuum-dominated universe.Comment: 6 pages; to appear in Journal of Mathematical Physics; v2: reference
3 correcte
Stability of Transparent Spherically Symmetric Thin Shells and Wormholes
The stability of transparent spherically symmetric thin shells (and
wormholes) to linearized spherically symmetric perturbations about static
equilibrium is examined. This work generalizes and systematizes previous
studies and explores the consequences of including the cosmological constant.
The approach shows how the existence (or not) of a domain wall dominates the
landscape of possible equilibrium configurations.Comment: 12 pages, 7 figures, revtex. Final form to appear in Phys. Rev.
Adventures in Invariant Theory
We provide an introduction to enumerating and constructing invariants of
group representations via character methods. The problem is contextualised via
two case studies arising from our recent work: entanglement measures, for
characterising the structure of state spaces for composite quantum systems; and
Markov invariants, a robust alternative to parameter-estimation intensive
methods of statistical inference in molecular phylogenetics.Comment: 12 pp, includes supplementary discussion of example
Practical applications of biomechanical principles in resistance training: moments and moment arms
Exercise professionals routinely prescribe resistance training to clients with varied goals. Therefore, they need
to be able to modify the difficulty of a variety of exercises and to understand how such modifications can alter
the relative joint loading on their clients so to maximise the potential for positive adaptation and to minimise
injury risk. This paper is the first in a three part series that will examine how a variety of biomechanical
principles and concepts have direct relevance to the prescription of resistance training for the general and
athletic populations as well as for musculoskeletal injury rehabilitation. In this paper, we start by defining the
terms moment (torque), moment arms, compressive, tensile and shear forces as well as joint stress (pressure).
We then demonstrate how an understanding of moments and moment arms is integral to the exercise
professionals’ ability to develop a systematic progression of variations of common exercises. In particular, we
examine how a variety of factors including joint range of motion, body orientation, type of external loading,
the lifter’s anthropometric proportions and the position of the external load will influence the difficulty of each
exercise variation. We then highlight the primary results of several selected studies which have compared the
resistance moment arms and joint moments, forces or stresses that are encountered during selected variations
of common lower body resistance training exercises. We hope that exercise professionals will benefit from this
knowledge of applied resistance training biomechanics and be better able to systematically progress exercise
difficulty and to modify joint loading as a result. The two remaining articles in this series will focus on the
neuromechanical properties of the human musculoskeletal system and better understanding the biomechanical
implications of a variety of alternative resistance training techniques, respectively
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