1,124 research outputs found
Generalized Entropies
We study an entropy measure for quantum systems that generalizes the von
Neumann entropy as well as its classical counterpart, the Gibbs or Shannon
entropy. The entropy measure is based on hypothesis testing and has an elegant
formulation as a semidefinite program, a type of convex optimization. After
establishing a few basic properties, we prove upper and lower bounds in terms
of the smooth entropies, a family of entropy measures that is used to
characterize a wide range of operational quantities. From the formulation as a
semidefinite program, we also prove a result on decomposition of hypothesis
tests, which leads to a chain rule for the entropy.Comment: 21 page
Room-temperature transverse-electric polarized intersubband electroluminescence from InAs/AlInAs quantum dashes
We report the observation of transverse electric polarized
electroluminescence from InAs/AlInAs quantum dash quantum cascade structures up
to room temperature. The emission is attributed to the electric field confined
along the shortest lateral dimension of the dashes, as confirmed by its
dependence on crystallographic orientation both in absorption measurements on a
dedicated sample and from electroluminescence itself. From the absorption we
estimate a dipole moment for the observed transition of =1.7 nm. The
electroluminescence is peaked at around 110 meV and increases with applied
bias. Its temperature dependence shows a decrease at higher temperatures
limited by optical phonon emission.Comment: 15 pages, 4 figures, submitted to Applied Physics Letter
Consequences of above-ground invasion by non-native plants into restored vernal pools do not prompt same changes in below-ground processes
Given the frequent overlap between biological plant invasion and ecological restoration efforts it is important to investigate their interactions to sustain desirable plant communities and modify long-Term legacies both above-and below-ground. To address this relationship, we used natural reference, invaded and created vernal pools in the Central Valley of California to examine potential changes in direct and indirect plant effects on soils associated with biological invasion and active restoration ecosystem disturbances. Our results showed that through a shift in vegetation composition and changes in the plant community tissue chemistry, invasion by non-native plant species has the potential to transform plant inputs to soils in vernal pool systems. In particular, we found that while invasive plant litter decomposition was driven by seasonal and interannual variability, associated with changes in precipitation, the overall decomposition rates for invasive litter was drastically lower than native species. This shift has important implications for long-Term alterations in plant-based inputs to soils in an amplifying feedback to nutrient cycling. Moreover, these results were independent of historic active restoration efforts. Despite the consistent shift in plant litter decomposition rates and community composition, we did not detect associated shifts in below-ground function associated with invasion by non-native plants. Instead, soil C:N ratios and microbial biomass did not differ between invaded and naturally occurring reference pools but were reduced in the manipulated created pools independent of invasion levels. Our results suggest that while there is an observed invasive amplifying feedback above-ground this trajectory is not represented below-ground, and restoration legacies dominated 10 years after practices were applied. Restoration practices that limit invasive plant feedbacks and account for soil legacy recovery, therefore offer the best solution for disturbed ephemeral ecosystems
Fano Lineshapes Revisited: Symmetric Photoionization Peaks from Pure Continuum Excitation
In a photoionization spectrum in which there is no excitation of the discrete
states, but only the underlying continuum, we have observed resonances which
appear as symmetric peaks, not the commonly expected window resonances.
Furthermore, since the excitation to the unperturbed continuum vanishes, the
cross section expected from Fano's configuration interaction theory is
identically zero. This shortcoming is removed by the explicit introduction of
the phase shifted continuum, which demonstrates that the shape of a resonance,
by itself, provides no information about the relative excitation amplitudes to
the discrete state and the continuum.Comment: 4 pages, 3 figure
Quantum dot occupation and electron dwell time in the cotunneling regime
We present comparative measurements of the charge occupation and conductance
of a GaAs/AlGaAs quantum dot. The dot charge is measured with a capacitively
coupled quantum point contact sensor. In the single-level Coulomb blockade
regime near equilibrium, charge and conductance signals are found to be
proportional to each other. We conclude that in this regime, the two signals
give equivalent information about the quantum dot system. Out of equilibrium,
we study the inelastic-cotunneling regime. We compare the measured differential
dot charge with an estimate assuming a dwell time of transmitted carriers on
the dot given by h/E, where E is the blockade energy of first-order tunneling.
The measured signal is of a similar magnitude as the estimate, compatible with
a picture of cotunneling as transmission through a virtual intermediate state
with a short lifetime
Positive Cross Correlations in a Normal-Conducting Fermionic Beam Splitter
We investigate a beam splitter experiment implemented in a normal conducting
fermionic electron gas in the quantum Hall regime. The cross-correlations
between the current fluctuations in the two exit leads of the three terminal
device are found to be negative, zero or even positive depending on the
scattering mechanism within the device. Reversal of the cross-correlations sign
occurs due to interaction between different edge-states and does not reflect
the statistics of the fermionic particles which `antibunch'.Comment: 4 pages, 4 figure
Intersubband gain in a Bloch oscillator and Quantum cascade laser
The link between the inversion gain of quantum cascade structures and the
Bloch gain in periodic superlattices is presented. The proposed theoretical
model based on the density matrix formalism is able to treat the gain mechanism
of the Bloch oscillator and Quantum cascade laser on the same footing by taking
into account in-plane momentum relaxation. The model predicts a dispersive
contribution in addition to the (usual) population-inversion-dependent
intersubband gain in quantum cascade structures and - in the absence of
inversion - provides the quantum mechanical description for the dispersive gain
in superlattices. It corroborates the predictions of the semi-classical
miniband picture, according to which gain is predicted for photon energies
lower than the Bloch oscillation frequency, whereas net absorption is expected
at higher photon energies, as a description which is valid in the
high-temperature limit. A red-shift of the amplified emission with respect to
the resonant transition energy results from the dispersive gain contribution in
any intersubband transition, for which the population inversion is small.Comment: 10 pages, 6 figure
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