2,765 research outputs found
A bridge between the single-photon and squeezed-vacuum state
The two modes of the Einstein-Podolsky-Rosen quadrature entangled state
generated by parametric down-conversion interfere on a beam splitter of
variable splitting ratio. Detection of a photon in one of the beam splitter
output channels heralds preparation of a signal state in the other, which is
characterized using homodyne tomography. By controlling the beam splitting
ratio, the signal state can be chosen anywhere between the single-photon and
squeezed state
Inhibited spontaneous emission of quantum dots observed in a 3D photonic band gap
We present time-resolved emission experiments of semiconductor quantum dots
in silicon 3D inverse-woodpile photonic band gap crystals. A systematic study
is made of crystals with a range of pore radii to tune the band gap relative to
the emission frequency. The decay rates averaged over all dipole orientations
are inhibited by a factor of 10 in the photonic band gap and enhanced up to 2?
outside the gap, in agreement with theory. We discuss the effects of spatial
inhomogeneity, nonradiative decay, and transition dipole orientations on the
observed inhibition in the band gap.Comment: 5 figures, update author lis
Optimal Taylor–Couette flow: radius ratio dependence
Taylor–Couette flow with independently rotating inner (i) and outer (o) cylinders is explored numerically and experimentally to determine the effects of the radius ratio η on the system response. Numerical simulations reach Reynolds numbers of up to Rei=9.5×10^3 and Reo=5×10^3, corresponding to Taylor numbers of up to Ta=10^8 for four different radius ratios η=ri/ro between 0.5 and 0.909. The experiments, performed in the Twente Turbulent Taylor–Couette (T3C) set-up, reach Reynolds numbers of up to Rei=2×10^6 and Reo=1.5×10^6, corresponding to Ta=5×10^12 for η=0.714--0.909. Effective scaling laws for the torque Jω(Ta) are found, which for sufficiently large driving Ta are independent of the radius ratio η. As previously reported for η=0.714, optimum transport at a non-zero Rossby number Ro=ri|ωi−ωo|/[2(ro−ri)ωo] is found in both experiments and numerics. Here Roopt is found to depend on the radius ratio and the driving of the system. At a driving in the range between Ta∼3×10^8 and Ta∼10^10, Roopt saturates to an asymptotic η-dependent value. Theoretical predictions for the asymptotic value of Roopt are compared to the experimental results, and found to differ notably. Furthermore, the local angular velocity profiles from experiments and numerics are compared, and a link between a flat bulk profile and optimum transport for all radius ratios is reported
Observation of sub-Bragg diffraction of waves in crystals
We investigate the diffraction conditions and associated formation of
stopgaps for waves in crystals with different Bravais lattices. We identify a
prominent stopgap in high-symmetry directions that occurs at a frequency below
the ubiquitous first-order Bragg condition. This sub-Bragg diffraction
condition is demonstrated by reflectance spectroscopy on two-dimensional
photonic crystals with a centred rectangular lattice, revealing prominent
diffraction peaks for both the sub-Bragg and first-order Bragg condition. These
results have implications for wave propagation in 2 of the 5 two-dimensional
Bravais lattices and 7 out of 14 three-dimensional Bravais lattices, such as
centred rectangular, triangular, hexagonal and body-centred cubic
Nanoscale superconducting gap variations, strong coupling signatures and lack of phase separation in optimally doped BaFe1.86Co0.14As2
We present tunneling data from optimally-doped, superconducting
BaFe1.86Co0.14As2 and its parent compound, BaFe2As2. In the superconductor,
clear coherence-like peaks are seen across the whole field of view, and their
analysis reveals nanoscale variations in the superconducting gap value, Delta.
The average magnitude of 2Delta is ~7.4 kBTC, which exceeds the BCS weak
coupling value for either s- or d-wave superconductivity. The characteristic
length scales of the deviations from the average gap value, and of an
anti-correlation discovered between the gap magnitude and the zero bias
conductance, match well with the average separation between the Co dopant ions
in the superconducting FeAs planes. The tunneling spectra themselves possess a
peak-dip-hump lineshape, suggestive of a coupling of the superconducting
electronic system to a well-defined bosonic mode of energy 4.7 kBTC, such as
the spin resonance observed recently in inelastic neutron scattering.Comment: 4 figures, corrected typos, reduced size of image
Wall roughness induces asymptotic ultimate turbulence
Turbulence is omnipresent in Nature and technology, governing the transport
of heat, mass, and momentum on multiple scales. For real-world applications of
wall-bounded turbulence, the underlying surfaces are virtually always rough;
yet characterizing and understanding the effects of wall roughness for
turbulence remains a challenge, especially for rotating and thermally driven
turbulence. By combining extensive experiments and numerical simulations, here,
taking as example the paradigmatic Taylor-Couette system (the closed flow
between two independently rotating coaxial cylinders), we show how wall
roughness greatly enhances the overall transport properties and the
corresponding scaling exponents. If only one of the walls is rough, we reveal
that the bulk velocity is slaved to the rough side, due to the much stronger
coupling to that wall by the detaching flow structures. If both walls are
rough, the viscosity dependence is thoroughly eliminated in the boundary layers
and we thus achieve asymptotic ultimate turbulence, i.e. the upper limit of
transport, whose existence had been predicted by Robert Kraichnan in 1962
(Phys. Fluids {\bf 5}, 1374 (1962)) and in which the scalings laws can be
extrapolated to arbitrarily large Reynolds numbers
Controlling the quality factor of a tuning-fork resonance between 9 K and 300 K for scanning-probe microscopy
We study the dynamic response of a mechanical quartz tuning fork in the
temperature range from 9 K to 300 K. Since the quality factor Q of the
resonance strongly depends on temperature, we implement a procedure to control
the quality factor of the resonance. We show that we are able to dynamically
change the quality factor and keep it constant over the whole temperature
range. This procedure is suitable for applications in scanning probe
microscopy.Comment: 5 pages, 6 figure
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