13,913 research outputs found
Ptychographic reconstruction of attosecond pulses
We demonstrate a new attosecond pulse reconstruction modality which uses an
algorithm that is derived from ptychography. In contrast to other methods,
energy and delay sampling are not correlated, and as a result, the number of
electron spectra to record is considerably smaller. Together with the robust
algorithm, this leads to a more precise and fast convergence of the
reconstruction.Comment: 12 pages, 7 figures, the MATLAB code for the method described in this
paper is freely available at
http://figshare.com/articles/attosecond_Extended_Ptychographyc_Iterative_Engine_ePIE_/160187
Accounting for Convective Blue-Shifts in the Determination of Absolute Stellar Radial Velocities
For late-type non-active stars, gravitational redshifts and convective
blueshifts are the main source of biases in the determination of radial
velocities. If ignored, these effects can introduce systematic errors of the
order of ~ 0.5 km/s. We demonstrate that three-dimensional hydrodynamical
simulations of solar surface convection can be used to predict the convective
blue-shifts of weak spectral lines in solar-like stars to ~ 0.070 km/s. Using
accurate trigonometric parallaxes and stellar evolution models, the
gravitational redshifts can be constrained with a similar uncertainty, leading
to absolute radial velocities accurate to better than ~ 0.1 km/s.Comment: To appear in the proceedings of the Joint Discussion 10, IAU General
Assembly, Rio de Janeiro, August 10-11, 200
Kondo effect in a one-electron double quantum dot: Oscillations of the Kondo current in a weak magnetic field
We present transport measurements of the Kondo effect in a double quantum dot
charged with only one or two electrons, respectively. For the one electron case
we observe a surprising quasi-periodic oscillation of the Kondo conductance as
a function of a small perpendicular magnetic field |B| \lesssim 50mT. We
discuss possible explanations of this effect and interpret it by means of a
fine tuning of the energy mismatch of the single dot levels of the two quantum
dots. The observed degree of control implies important consequences for
applications in quantum information processing
Clues on the Galactic evolution of sulphur from star clusters
(Abridged) The abundances of alpha-elements are a powerful diagnostic of the
star formation history and chemical evolution of a galaxy. Sulphur, being
moderately volatile, can be reliably measured in the interstellar medium (ISM)
of damped Ly-alpha galaxies and extragalactic HII regions. Measurements in
stars of different metallicity in our Galaxy can then be readily compared to
the abundances in external galaxies. Such a comparison is not possible for Si
or Ca that suffer depletion onto dust in the ISM. Furthermore, studying sulphur
is interesting because it probes nucleosynthetic conditions that are very
different from those of O or Mg. The measurements in star clusters are a
reliable tracers of the Galactic evolution of sulphur. We find
NLTE=6.11+/-0.04 for M 4, NLTE=7.17+/-0.02 for NGC 2477, and
NLTE=7.13+/-0.06 for NGC 5822. For the only star studied in Trumpler 5 we
find A(S)NLTE=6.43+/-0.03 and A(S)LTE=6.94+/-0.05. Our measurements show that,
by and large, the S abundances in Galactic clusters trace reliably those in
field stars. The only possible exception is Trumpler 5, for which the NLTE
sulphur abundance implies an [S/Fe] ratio lower by roughly 0.4 dex than
observed in field stars of comparable metallicity, even though its LTE sulphur
abundance is in line with abundances of field stars. Moreover the LTE sulphur
abundance is consistent only with the abundance of another alpha-element, Mg,
in the same star, while the low NLTE value is consistent with Si and Ca. The S
abundances in our sample of stars in clusters imply that the clusters are
chemically homogeneous for S within 0.05 dex.Comment: A&A in pres
Dynamical Semigroup Description of Coherent and Incoherent Particle-Matter Interaction
The meaning of statistical experiments with single microsystems in quantum
mechanics is discussed and a general model in the framework of non-relativistic
quantum field theory is proposed, to describe both coherent and incoherent
interaction of a single microsystem with matter. Compactly developing the
calculations with superoperators, it is shown that the introduction of a time
scale, linked to irreversibility of the reduced dynamics, directly leads to a
dynamical semigroup expressed in terms of quantities typical of scattering
theory. Its generator consists of two terms, the first linked to a coherent
wavelike behaviour, the second related to an interaction having a measuring
character, possibly connected to events the microsystem produces propagating
inside matter. In case these events breed a measurement, an explicit
realization of some concepts of modern quantum mechanics ("effects" and
"operations") arises. The relevance of this description to a recent debate
questioning the validity of ordinary quantum mechanics to account for such
experimental situations as, e.g., neutron-interferometry, is briefly discussed.Comment: 22 pages, latex, no figure
Isotope Spectroscopy
The measurement of isotopic ratios provides a privileged insight both into
nucleosynthesis and into the mechanisms operating in stellar envelopes, such as
gravitational settling. In this article, we give a few examples of how isotopic
ratios can be determined from high-resolution, high-quality stellar spectra. We
consider examples of the lightest elements, H and He, for which the isotopic
shifts are very large and easily measurable, and examples of heavier elements
for which the determination of isotopic ratios is more difficult. The presence
of 6Li in the stellar atmospheres causes a subtle extra depression in the red
wing of the 7Li 670.7 nm doublet which can only be detected in spectra of the
highest quality. But even with the best spectra, the derived Li abundance
can only be as good as the synthetic spectra used for their interpretation. It
is now known that 3D non-LTE modelling of the lithium spectral line profiles is
necessary to account properly for the intrinsic line asymmetry, which is
produced by convective flows in the atmospheres of cool stars, and can mimic
the presence of 6Li. We also discuss briefly the case of the carbon isotopic
ratio in metal-poor stars, and provide a new determination of the nickel
isotopic ratios in the solar atmosphere.Comment: AIP Thinkshop 10 "High resolution optical spectroscopy", invited
talk, AN in pres
Optomechanical circuits for nanomechanical continuous variable quantum state processing
We propose and analyze a nanomechanical architecture where light is used to
perform linear quantum operations on a set of many vibrational modes. Suitable
amplitude modulation of a single laser beam is shown to generate squeezing,
entanglement, and state-transfer between modes that are selected according to
their mechanical oscillation frequency. Current optomechanical devices based on
photonic crystals may provide a platform for realizing this scheme.Comment: 11 pages, 5 figure
Chiral spin liquid and emergent anyons in a Kagome lattice Mott insulator
Topological phases in frustrated quantum spin systems have fascinated
researchers for decades. One of the earliest proposals for such a phase was the
chiral spin liquid put forward by Kalmeyer and Laughlin in 1987 as the bosonic
analogue of the fractional quantum Hall effect. Elusive for many years, recent
times have finally seen a number of models that realize this phase. However,
these models are somewhat artificial and unlikely to be found in realistic
materials. Here, we take an important step towards the goal of finding a chiral
spin liquid in nature by examining a physically motivated model for a Mott
insulator on the Kagome lattice with broken time-reversal symmetry. We first
provide a theoretical justification for the emergent chiral spin liquid phase
in terms of a network model perspective. We then present an unambiguous
numerical identification and characterization of the universal topological
properties of the phase, including ground state degeneracy, edge physics, and
anyonic bulk excitations, by using a variety of powerful numerical probes,
including the entanglement spectrum and modular transformations.Comment: 9 pages, 9 figures; partially supersedes arXiv:1303.696
Preparation of Subradiant States using Local Qubit Control in Circuit QED
Transitions between quantum states by photon absorption or emission are
intimately related to symmetries of the system which lead to selection rules
and the formation of dark states. In a circuit quantum electrodynamics setup,
in which two resonant superconducting qubits are coupled through an on-chip
cavity and driven via the common cavity field, one single-excitation state
remains dark. Here, we demonstrate that this dark state can be excited using
local phase control of individual qubit drives to change the symmetry of the
driving field. We observe that the dark state decay via spontaneous emission
into the cavity is suppressed, a characteristic signature of subradiance. This
local control technique could be used to prepare and study highly correlated
quantum states of cavity-coupled qubits.Comment: 5 pages, 4 figure
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