18,929 research outputs found
Sub-Poissonian Shot Noise In A Diffusive Conductor
A review is given of the shot-noise properties of metallic, diffusive
conductors. The shot noise is one third of the Poisson noise, due to the
bimodal distribution of transmission eigenvalues. The same result can be
obtained from a semiclassical calculation. Starting from Oseledec's theorem it
is shown that the bimodal distribution is required by Ohm's law.Comment: 9 pages, LaTeX, including 2 figure
Doubled Shot Noise In Disordered Normal-Metal-Superconductor Junctions
The low-frequency shot-noise power of a normal-metal-superconductor junction
is studied for arbitrary normal region. Through a scattering approach, a
formula is derived which expresses the shot-noise power in terms of the
transmission eigenvalues of the normal region. The noise power divided by the
current is enhanced by a factor two with respect to its normal-state value, due
to Cooper-pair transport in the superconductor. For a disordered normal region,
it is still smaller than the Poisson noise, as a consequence of noiseless open
scattering channels.Comment: 4 pages, RevTeX v3.0, including 1 figure, Submitted to Physical
Review
Classical Optimizers for Noisy Intermediate-Scale Quantum Devices
We present a collection of optimizers tuned for usage on Noisy Intermediate-Scale Quantum (NISQ) devices. Optimizers have a range of applications in quantum computing, including the Variational Quantum Eigensolver (VQE) and Quantum Approximate Optimization (QAOA) algorithms. They are also used for calibration tasks, hyperparameter tuning, in machine learning, etc. We analyze the efficiency and effectiveness of different optimizers in a VQE case study. VQE is a hybrid algorithm, with a classical minimizer step driving the next evaluation on the quantum processor. While most results to date concentrated on tuning the quantum VQE circuit, we show that, in the presence of quantum noise, the classical minimizer step needs to be carefully chosen to obtain correct results. We explore state-of-the-art gradient-free optimizers capable of handling noisy, black-box, cost functions and stress-test them using a quantum circuit simulation environment with noise injection capabilities on individual gates. Our results indicate that specifically tuned optimizers are crucial to obtaining valid science results on NISQ hardware, and will likely remain necessary even for future fault tolerant circuits
VLT/VIMOS Observations of an Occulting Galaxy Pair: Redshifts and Effective Extinction Curve
We present VLT/VIMOS IFU observations of an occulting galaxy pair previously
discovered in HST observations. The foreground galaxy is a low-inclination
spiral disk, which causes clear attenuation features seen against the bright
bulge and disk of the background galaxy. We find redshifts of and z=0.065 for the foreground and background galaxy respectively.
This relatively small difference does not rule out gravitational interaction
between the two galaxies. Emission line ratios point to a star-forming, not
AGN-dominated foreground galaxy.
We fit the Cardelli, Clayton & Mathis (CCM) extinction law to the spectra of
individual fibres to derive slope () and normalization (). The
normalization agrees with the HST attenuation map and the slope is lower than
the Milky Way relation (), which is likely linked to the spatial
sampling of the disk. We speculate that the values of point to either
coherent ISM structures in the disk larger than usual ( kpc) or higher
starting values of , indicative of recent processing of the dust.
The foreground galaxy is a low stellar mass spiral () with a high dust content (). The dust disk geometry visible in the HST image would explain the
observed SED properties of smaller galaxies: a lower mean dust temperature, a
high dust-to-stellar mass ratio but relatively little optical attenuation.
Ongoing efforts to find occulting pairs with a small foreground galaxies will
show how common this geometry is.Comment: 16 pages, 3 tables, 13 figures, accepted for publication in MNRA
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