789 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
Graphene–Metamaterial Photodetectors for Integrated Infrared Sensing
PublishedIn this work we study metamaterial-enhanced graphene photodetectors operating in the mid-IR to THz. The detector element consists of a graphene ribbon embedded within a dual-metal split ring resonator, which acts like a cavity to enhance the absorption of electromagnetic radiation by the graphene ribbon, while the asymmetric metal contacts enable photothermoelectric detection. Detectors designed for the mid-IR demonstrate peak responsivity (referenced to total power) of ∼120 mV/W at 1500 cm–1 and are employed in the spectroscopic evaluation of vibrational resonances, thus demonstrating a key step toward a platform for integrated surface-enhanced sensing.The authors thank Johanna Wolf for providing the QCL used for the detector characterization. This research was supported by the European Union under the FET-open grant GOSFEL and the Swiss National Science Foundation through NCCR QSIT. G.R.N. also gratefully acknowledges the support of the UK Engineering and Physical Sciences Research Council through a fellowship in Frontier Manufacturing (Grant No. EP/J018651/1)
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
Quantum dot admittance probed at microwave frequencies with an on-chip resonator
We present microwave frequency measurements of the dynamic admittance of a
quantum dot tunnel coupled to a two-dimensional electron gas. The measurements
are made via a high-quality 6.75 GHz on-chip resonator capacitively coupled to
the dot. The resonator frequency is found to shift both down and up close to
conductance resonance of the dot corresponding to a change of sign of the
reactance of the system from capacitive to inductive. The observations are
consistent with a scattering matrix model. The sign of the reactance depends on
the detuning of the dot from conductance resonance and on the magnitude of the
tunnel rate to the lead with respect to the resonator frequency. Inductive
response is observed on a conductance resonance, when tunnel coupling and
temperature are sufficiently small compared to the resonator frequency.Comment: 8 pages, 4 figure
Digital alloy interface grading of an InAlAs/InGaAs quantum cascade laser structure studied by cross-sectional scanning tunneling microscopy
We have studied an InGaAs/InAlAs quantum cascade laser structure with cross-sectional scanning tunneling microscopy. In the quantum cascade laser structure digital alloy grading was used to soften the barriers of the active region. We show that due to alloy fluctuations, softening of the barriers occurs even without the digital gradin
Strong coupling in the far-infrared between graphene plasmons and the surface optical phonons of silicon dioxide
This is the author accepted manuscript. The final version is available from American Chemical Society via the DOI in this record.We study plasmonic resonances in electrostatically gated graphene nanoribbons on silicon dioxide substrates. Absorption spectra are measured in the mid-far infrared and reveal multiple peaks, with width-dependent resonant frequencies. We calculate the dielectric function within the random phase approximation and show that the observed spectra can be explained by surface-plasmon-phonon-polariton modes, which arise from coupling of the graphene plasmon to three surface optical phonon modes in the silicon dioxide.This research was supported by the UK Engineering and Physical Sciences Research Council, via the award of a Fellowship in Frontier Manufacturing (EP/J018651/1) to G.N., and the European Union under the FET-open grant GOSFEL
Transcriptional and post-translational patterns of interleukin-6 (IL-6) synthesis following massive trauma and during sepsis syndrome
Electrically Switchable Photonic Molecule Laser
We have studied the coherent intercavity coupling of the evanescent fields of
the whispering gallery modes of two terahertz quantum-cascade lasers
implemented as microdisk cavities. The electrically pumped single-mode
operating microcavities allow to electrically control the coherent mode
coupling for proximity distances of the cavities up to 30-40 \mu\m. The optical
emission of the strongest coupled photonic molecule can be perfectly switched
by the electrical modulation of only one of the coupled microdisks. The
threshold characteristics of the strongest coupled photonic molecule
demonstrates the linear dependence of the gain of a quantum-cascade laser on
the applied electric field.Comment: 4 pages, 4 figure
Resonant Photon-Assisted Tunneling Through a Double Quantum Dot: An Electron Pump From Spatial Rabi Oscillations
The time average of the fully nonlinear current through a double quantum dot,
subject to an arbitrary combination of ac and dc voltages, is calculated
exactly using the Keldysh nonequilibrium Green function technique. When driven
on resonance, the system functions as an efficient electron pump due to Rabi
oscillation between the dots. The pumping current is maximum when the coupling
to the leads equals the Rabi frequency.Comment: 6 pages, REVTEX 3.0, 3 postscript figure
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