16,392 research outputs found
Quantum theory of electron tunneling into intersubband cavity polariton states
Through a non-perturbative quantum theory, we investigate how the
quasi-electron excitations of a two-dimensional electron gas are modified by
strong coupling to the vacuum field of a microcavity. We show that the
electronic dressed states originate from a Fano-like coupling between the bare
electron states and the continuum of intersubband cavity polariton excitations.
In particular, we calculate the electron spectral function modified by
light-matter interactions and its impact on the electronic injection of
intersubband cavity polaritons. The domain of validity of the present
theoretical results is critically discussed. We show that resonant electron
tunneling from a narrow-band injector can selectively excite superradiant
states and produce efficient intersubband polariton electroluminescence
Updating constraint preconditioners for KKT systems in quadratic programming via low-rank corrections
This work focuses on the iterative solution of sequences of KKT linear
systems arising in interior point methods applied to large convex quadratic
programming problems. This task is the computational core of the interior point
procedure and an efficient preconditioning strategy is crucial for the
efficiency of the overall method. Constraint preconditioners are very effective
in this context; nevertheless, their computation may be very expensive for
large-scale problems, and resorting to approximations of them may be
convenient. Here we propose a procedure for building inexact constraint
preconditioners by updating a "seed" constraint preconditioner computed for a
KKT matrix at a previous interior point iteration. These updates are obtained
through low-rank corrections of the Schur complement of the (1,1) block of the
seed preconditioner. The updated preconditioners are analyzed both
theoretically and computationally. The results obtained show that our updating
procedure, coupled with an adaptive strategy for determining whether to
reinitialize or update the preconditioner, can enhance the performance of
interior point methods on large problems.Comment: 22 page
Vetera rerum exempla. La cultura antiquaria fra Bologna e l’Europa nei secoli XV-XVII
Alcuni dotti di Bologna, attivi presso la corte bentivolesca, indagarono le fasi più remote della loro città , con forzature fantasiose ma anche con un’attenzione per i resti materiali di epoca romana. Il vivo interesse per le origini della propria città e per le testimonianze antiche della sua storia (fra le quali assumevano un’importanza crescente i documenti epigrafici) era condiviso da molti altri intellettuali in Germania e nei Paesi Bassi, molti dei quali almeno in parte si erano formati presso lo Studio bolognese. Se è difficile stabilire quale sia stato lo specifico apporto bolognese nella maturazione dell’interesse per l’antico di questi personaggi, a Bologna può in ogni caso essere riconosciuto un fondamentale ruolo di crocevia nella circolazione degli interessi antiquari che attraversarono l’Europa rinascimentale
Double dot chain as a macroscopic quantum bit
We consider an array of N quantum dot pairs interacting via Coulomb
interaction between adjacent dots and hopping inside each pair. We show that at
the first order in the ratio of hopping and interaction amplitudes, the array
maps in an effective two level system with energy separation becoming
exponentially small in the macroscopic (large N) limit. Decoherence at zero
temperature is studied in the limit of weak coupling with phonons. In this case
the macroscopic limit is robust with respect to decoherence. Some possible
applications in quantum information processing are discussed.Comment: Phys. Rev. A (in press
Modulation of galactic protons in the heliosphere during the unusual solar minimum of 2006 to 2009
The last solar minimum activity period, and the consequent minimum modulation
conditions for cosmic rays, was unusual. The highest levels of galactic protons
were recorded at Earth in late 2009 in contrast to expectations. Proton spectra
observed for 2006 to 2009 from the PAMELA cosmic ray detector on-board the
Resurs-DK1 satellite are presented together with the solutions of a
comprehensive numerical model for the solar modulation of cosmic rays. The
model is used to determine what mechanisms were mainly responsible for the
modulation of protons during this period, and why the observed spectrum for
2009 was the highest ever recorded. From mid-2006 until December 2009 we find
that the spectra became significantly softer because increasingly more low
energy protons had reached Earth. To simulate this effect, the rigidity
dependence of the diffusion coefficients had to decrease significantly below ~3
GeV. The modulation minimum period of 2009 can thus be described as relatively
more "diffusion dominated" than previous solar minima. However, we illustrate
that drifts still had played a significant role but that the observable
modulation effects were not as well correlated with the waviness of the
heliospheric current sheet as before. Protons still experienced global gradient
and curvature drifts as the heliospheric magnetic field had decreased
significantly until the end of 2009, in contrast to the moderate decreases
observed during previous minimum periods. We conclude that all modulation
processes contributed to the observed increases in the proton spectra for this
period, exhibiting an intriguing interplay of these major mechanisms
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