19,372 research outputs found
Dreaming of atmospheres
Here we introduce the RobERt (Robotic Exoplanet Recognition) algorithm for
the classification of exoplanetary emission spectra. Spectral retrievals of
exoplanetary atmospheres frequently requires the preselection of
molecular/atomic opacities to be defined by the user. In the era of
open-source, automated and self-sufficient retrieval algorithms, manual input
should be avoided. User dependent input could, in worst case scenarios, lead to
incomplete models and biases in the retrieval. The RobERt algorithm is based on
deep belief neural (DBN) networks trained to accurately recognise molecular
signatures for a wide range of planets, atmospheric thermal profiles and
compositions. Reconstructions of the learned features, also referred to as
`dreams' of the network, indicate good convergence and an accurate
representation of molecular features in the DBN. Using these deep neural
networks, we work towards retrieval algorithms that themselves understand the
nature of the observed spectra, are able to learn from current and past data
and make sensible qualitative preselections of atmospheric opacities to be used
for the quantitative stage of the retrieval process.Comment: ApJ accepte
Two-dimensional quantum walk under artificial magnetic field
We introduce the Peierls substitution to a two-dimensional discrete-time
quantum walk on a square lattice to examine the spreading dynamics and the
coin-position entanglement in the presence of an artificial gauge field. We use
the ratio of the magnetic flux through the unit cell to the flux quantum as a
control parameter. For a given flux ratio, we obtain faster spreading for a
small number of steps and the walker tends to be highly localized around the
origin. Moreover, the spreading of the walk can be suppressed and decreased
within a limited time interval for specific rational values of flux ratio. When
the flux ratio is an irrational number, even for a large number of steps, the
spreading exhibit diffusive behavior rather than the well-known ballistic one
as in the classical random walk and there is a significant probability of
finding the walker at the origin. We also analyze the coin-position
entanglement and show that the asymptotic behavior vanishes when the flux ratio
is different from zero and the coin-position entanglement become nearly maximal
in a periodic manner in a long time range.Comment: 7 pages, 5 figures, sections 3 and 4 revise
Relating the Lorentzian and exponential: Fermi's approximation,the Fourier transform and causality
The Fourier transform is often used to connect the Lorentzian energy
distribution for resonance scattering to the exponential time dependence for
decaying states. However, to apply the Fourier transform, one has to bend the
rules of standard quantum mechanics; the Lorentzian energy distribution must be
extended to the full real axis instead of being bounded from
below (``Fermi's approximation''). Then the Fourier transform
of the extended Lorentzian becomes the exponential, but only for times , a time asymmetry which is in conflict with the unitary group time evolution
of standard quantum mechanics. Extending the Fourier transform from
distributions to generalized vectors, we are led to Gamow kets, which possess a
Lorentzian energy distribution with and have exponential
time evolution for only. This leads to probability predictions
that do not violate causality.Comment: 23 pages, no figures, accepted by Phys. Rev.
Field Theory Entropy, the -theorem and the Renormalization Group
We consider entropy and relative entropy in Field theory and establish
relevant monotonicity properties with respect to the couplings. The relative
entropy in a field theory with a hierarchy of renormalization group fixed
points ranks the fixed points, the lowest relative entropy being assigned to
the highest multicritical point. We argue that as a consequence of a
generalized theorem Wilsonian RG flows induce an increase in entropy and
propose the relative entropy as the natural quantity which increases from one
fixed point to another in more than two dimensions.Comment: 25 pages, plain TeX (macros included), 6 ps figures. Addition in
title. Entropy of cutoff Gaussian model modified in section 4 to avoid a
divergence. Therefore, last figure modified. Other minor changes to improve
readability. Version to appear in Phys. Rev.
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