769 research outputs found
Lower bound for electron spin entanglement from beamsplitter current correlations
We determine a lower bound for the entanglement of pairs of electron spins
injected into a mesoscopic conductor. The bound can be expressed in terms of
experimentally accessible quantities, the zero-frequency current correlators
(shot noise power or cross-correlators) after transmission through an
electronic beam splitter. The effect of spin relaxation (T_1 processes) and
decoherence (T_2 processes) during the ballistic coherent transmission of the
carriers in the wires is taken into account within Bloch theory. The presence
of a variable inhomogeneous magnetic field allows the determination of a useful
lower bound for the entanglement of arbitrary entangled states. The decrease in
entanglement due to thermally mixed states is studied. Both the entanglement of
the output of a source (entangler) and the relaxation (T_1) and decoherence
(T_2) times can be determined.Comment: 4 pages, 3 figure
A discontinuous Galerkin method for the Vlasov-Poisson system
A discontinuous Galerkin method for approximating the Vlasov-Poisson system
of equations describing the time evolution of a collisionless plasma is
proposed. The method is mass conservative and, in the case that piecewise
constant functions are used as a basis, the method preserves the positivity of
the electron distribution function and weakly enforces continuity of the
electric field through mesh interfaces and boundary conditions. The performance
of the method is investigated by computing several examples and error estimates
associated system's approximation are stated. In particular, computed results
are benchmarked against established theoretical results for linear advection
and the phenomenon of linear Landau damping for both the Maxwell and Lorentz
distributions. Moreover, two nonlinear problems are considered: nonlinear
Landau damping and a version of the two-stream instability are computed. For
the latter, fine scale details of the resulting long-time BGK-like state are
presented. Conservation laws are examined and various comparisons to theory are
made. The results obtained demonstrate that the discontinuous Galerkin method
is a viable option for integrating the Vlasov-Poisson system.Comment: To appear in Journal for Computational Physics, 2011. 63 pages, 86
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Fracture strength and Young's modulus of ZnO nanowires
The fracture strength of ZnO nanowires vertically grown on sapphire
substrates was measured in tensile and bending experiments. Nanowires with
diameters between 60 and 310 nm and a typical length of 2 um were manipulated
with an atomic force microscopy tip mounted on a nanomanipulator inside a
scanning electron microscope. The fracture strain of (7.7 +- 0.8)% measured in
the bending test was found close to the theoretical limit of 10% and revealed a
strength about twice as high as in the tensile test. From the tensile
experiments the Young's modulus could be measured to be within 30% of that of
bulk ZnO, contrary to the lower values found in literature.Comment: 5 pages, 3 figures, 1 tabl
Dephasing of Mollow Triplet Sideband Emission of a Resonantly Driven Quantum Dot in a Microcavity
Detailed properties of resonance fluorescence from a single quantum dot in a
micropillar cavity are investigated, with particular focus on emission
coherence in dependence on optical driving field power and detuning.
Power-dependent series over a wide range could trace characteristic Mollow
triplet spectra with large Rabi splittings of GHz. In
particular, the effect of dephasing in terms of systematic spectral broadening
of the Mollow sidebands is observed as a strong fingerprint
of excitation-induced dephasing. Our results are in excellent agreement with
predictions of a recently presented model on phonon-dressed QD Mollow triplet
emission in the cavity-QED regime
Classical and quantum communication without a shared reference frame
We show that communication without a shared reference frame is possible using
entangled states. Both classical and quantum information can be communicated
with perfect fidelity without a shared reference frame at a rate that
asymptotically approaches one classical bit or one encoded qubit per
transmitted qubit. We present an optical scheme to communicate classical bits
without a shared reference frame using entangled photon pairs and linear
optical Bell state measurements.Comment: 4 pages, published versio
Phonon-Assisted Incoherent Excitation of a Quantum Dot and its Emission Properties
We present a detailed study of a phonon-assisted incoherent excitation
mechanism of single quantum dots. A spectrally-detuned laser couples to a
quantum dot transition by mediation of acoustic phonons, whereby excitation
efficiencies up to 20 % with respect to strictly resonant excitation can be
achieved at T = 9 K. Laser frequency-dependent analysis of the quantum dot
intensity distinctly maps the underlying acoustic phonon bath and shows good
agreement with our polaron master equation theory. An analytical solution for
the photoluminescence is introduced which predicts a broadband incoherent
coupling process when electron-phonon scattering is in the strong phonon
coupling (polaronic) regime. Additionally, we investigate the coherence
properties of the emitted light and study the impact of the relevant pump and
phonon bath parameters
On the Move – Do Domestic and Wild Ungulate Species Distributions Overlap in the Mongolian Gobi?
In the Great Gobi B Strictly Protected Area (Great Gobi B), wild and domestic ungulates seasonally share the forage of the semi-desert and desert habitat. Around 130 herder families are grazing their livestock, mainly goats and sheep, in the protected area in winter. Wild ungulates of global significance in Great Gobi B include the reintroduced Przewalski’s horse (Equus ferus przewalskii),which had previously been extinct in the wild. To determine potential habitat overlaps between Przewalski’s horses and livestock, we mapped the movements of 19 livestock herds monitored via GPS collars and ranger observations of Przewalski’s horse herds over a one year period from September 2018 to August 2019. We additionally conducted focus group interviews with nomadic herders about their rangeland management. We found that pasture use in and around the Great Gobi B is still following the nomadic tradition, with herders moving camp locations on average eleven times per year, depending on forage availability. Our results show that the range of Przewalski’s horses and livestock mostly overlap around permanent and ephemeral water points. However, the same resources are used in different seasons. The protected area was recently expanded to twice its size, now also including additional herder households and traditional pastures. For the ongoing discussion about concerning the new zonation of the enlarged protected area it is important to consider both, herder and wildlife movements patterns, to meet the conservation goals of the protected area but also meet the needs of the traditional pastoral herding community
Electron beam induced softening of fused silica
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