2,952 research outputs found
Commutativity properties of Quinn spectra
We give a simple sufficient condition for Quinn's "bordism-type" spectra to
be weakly equivalent to commutative symmetric ring spectra. We also show that
the symmetric signature is (up to weak equivalence) a monoidal transformation
between symmetric monoidal functors, which implies that the Sullivan-Ranicki
orientation of topological bundles is represented by a ring map between
commutative symmetric ring spectra. In the course of proving these statements
we give a new description of symmetric L theory which may be of independent
interest
Facilitation and delay sensitivity of auditory cortex neurons in CF-FM bats, Rhinolopus rouxi and Pteronotus p. parnellii
Unconditional quantum cloning of coherent states with linear optics
A scheme for optimal Gaussian cloning of optical coherent states is proposed
and experimentally demonstrated. Its optical realization is based entirely on
simple linear optical elements and homodyne detection. The optimality of the
presented scheme is only limited by detection inefficiencies. Experimentally we
achieved a cloning fidelity of about 65%, which almost touches the optimal
value of 2/3.Comment: Phys. Rev. Lett. Volume 94, Number 24, p. 24050
Exact solutions in Einstein-Yang-Mills-Dirac systems
We present exact solutions in Einstein-Yang-Mills-Dirac theories with gauge
groups SU(2) and SU(4) in Robertson-Walker space-time , which
are symmetric under the action of the group SO(4) of spatial rotations. Our
approach is based on the dimensional reduction method for gauge and
gravitational fields and relates symmetric solutions in EYMD theory to certain
solutions of an effective dynamical system.
We interpret our solutions as cosmological solutions with an oscillating
Yang-Mills field passing between topologically distinct vacua. The explicit
form of the solution for spinor field shows that its energy changes the sign
during the evolution of the Yang-Mills field from one vacuum to the other,
which can be considered as production or annihilation of fermions.
Among the obtained solutions there is also a static sphaleron-like solution,
which is a cosmological analogue of the first Bartnik-McKinnon solution in the
presence of fermions.Comment: 18 pages, LaTeX 2
Shortcomings of the Bond Orientational Order Parameters for the Analysis of Disordered Particulate Matter
Local structure characterization with the bond-orientational order parameters
q4, q6, ... introduced by Steinhardt et al. has become a standard tool in
condensed matter physics, with applications including glass, jamming, melting
or crystallization transitions and cluster formation. Here we discuss two
fundamental flaws in the definition of these parameters that significantly
affect their interpretation for studies of disordered systems, and offer a
remedy. First, the definition of the bond-orientational order parameters
considers the geometrical arrangement of a set of neighboring spheres NN(p)
around a given central particle p; we show that procedure to select the spheres
constituting the neighborhood NN(p) can have greater influence on both the
numerical values and qualitative trend of ql than a change of the physical
parameters, such as packing fraction. Second, the discrete nature of
neighborhood implies that NN(p) is not a continuous function of the particle
coordinates; this discontinuity, inherited by ql, leads to a lack of robustness
of the ql as structure metrics. Both issues can be avoided by a morphometric
approach leading to the robust Minkowski structure metrics ql'. These ql' are
of a similar mathematical form as the conventional bond-orientational order
parameters and are mathematically equivalent to the recently introduced
Minkowski tensors [Europhys. Lett. 90, 34001 (2010); Phys. Rev. E. 85, 030301
(2012)]
Transverse angular momentum of photons
We develop the quantum theory of transverse angular momentum of light beams.
The theory applies to paraxial and quasi-paraxial photon beams in vacuum, and
reproduces the known results for classical beams when applied to coherent
states of the field. Both the Poynting vector, alias the linear momentum, and
the angular momentum quantum operators of a light beam are calculated including
contributions from first-order transverse derivatives. This permits a correct
description of the energy flow in the beam and the natural emergence of both
the spin and the angular momentum of the photons. We show that for collimated
beams of light, orbital angular momentum operators do not satisfy the standard
commutation rules. Finally, we discuss the application of our theory to some
concrete cases.Comment: 10 pages, 2 figure
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