713 research outputs found
Jet transport performance in thunderstorm wind shear conditions
Several hours of three dimensional wind data were collected in the thunderstorm approach-to-landing environment, using an instrumented Queen Air airplane. These data were used as input to a numerical simulation of aircraft response, concentrating on fixed-stick assumptions, while the aircraft simulated an instrument landing systems approach. Output included airspeed, vertical displacement, pitch angle, and a special approach deterioration parameter. Theory and the results of approximately 1000 simulations indicated that about 20 percent of the cases contained serious wind shear conditions capable of causing a critical deterioration of the approach. In particular, the presence of high energy at the airplane's phugoid frequency was found to have a deleterious effect on approach quality. Oscillations of the horizontal wind at the phugoid frequency were found to have a more serious effect than vertical wind. A simulation of Eastern flight 66, which crashed at JFK in 1975, served to illustrate the points of the research. A concept of a real-time wind shear detector was outlined utilizing these results
Adiabatic steering and determination of dephasing rates in double dot qubits
We propose a scheme to prepare arbitrary superpositions of quantum states in
double quantum--dots irradiated by coherent microwave pulses. Solving the
equations of motion for the dot density matrix, we find that dephasing rates
for such superpositions can be quantitatively infered from additional electron
current pulses that appear due to a controllable breakdown of coherent
population trapping in the dots.Comment: 5 pages, 4 figures. To appear in Phys. Rev.
Mechanical Mixing in Nonlinear Nanomechanical Resonators
Nanomechanical resonators, machined out of Silicon-on-Insulator wafers, are
operated in the nonlinear regime to investigate higher-order mechanical mixing
at radio frequencies, relevant to signal processing and nonlinear dynamics on
nanometer scales. Driven by two neighboring frequencies the resonators generate
rich power spectra exhibiting a multitude of satellite peaks. This nonlinear
response is studied and compared to -order perturbation theory and
nonperturbative numerical calculations.Comment: 5 pages, 7 figure
Superposition of photon- and phonon- assisted tunneling in coupled quantum dots
We report on electron transport through an artificial molecule formed by two
tunnel coupled quantum dots, which are laterally confined in a two-dimensional
electron system of an AlGaAs/GaAs heterostructure. Coherent
molecular states in the coupled dots are probed by photon-assisted tunneling
(PAT). Above 10 GHz, we observe clear PAT as a result of the resonance between
the microwave photons and the molecular states. Below 8 GHz, a pronounced
superposition of phonon- and photon-assisted tunneling is observed. Coherent
superposition of molecular states persists under excitation of acoustic
phonons.Comment: 5 pages, 4 figure
Nanomechanical resonators operating as charge detectors in the nonlinear regime
We present measurements on nanomechanical resonators machined from
Silicon-on-Insulator substrates. The resonators are designed as freely
suspended Au/Si beams of lengths on the order of 1 - 4 um and a thickness of
200 nm. The beams are driven into nonlinear response by an applied modulation
at radio frequencies and a magnetic field in plane. The strong hysteresis of
the magnetomotive response allows sensitive charge detection by varying the
electrostatic potential of a gate electrode.Comment: 8 pages, 6 figure
Determination of the complex microwave photoconductance of a single quantum dot
A small quantum dot containing approximately 20 electrons is realized in a
two-dimensional electron system of an AlGaAs/GaAs heterostructure. Conventional
transport and microwave spectroscopy reveal the dot's electronic structure. By
applying a coherently coupled two-source technique, we are able to determine
the complex microwave induced tunnel current. The amplitude of this
photoconductance resolves photon-assisted tunneling (PAT) in the non-linear
regime through the ground state and an excited state as well. The out-of-phase
component (susceptance) allows to study charge relaxation within the quantum
dot on a time scale comparable to the microwave beat period.Comment: 5.5 pages, 6 figures, accepted by Phys. Rev. B (Jan. B15 2001
Non-Markovian dynamics of double quantum dot charge qubits due to acoustic phonons
We investigate the dynamics of a double quantum dot charge qubit which is
coupled to piezoelectric acoustic phonons, appropriate for GaAs
heterostructures. At low temperatures, the phonon bath induces a non-Markovian
dynamical behavior of the oscillations between the two charge states of the
double quantum dot. Upon applying the numerically exact quasiadiabatic
propagator path-integral scheme, the reduced density matrix of the charge qubit
is calculated, thereby avoiding the Born-Markov approximation. This allows a
systematic study of the dependence of the Q-factor on the lattice temperature,
on the size of the quantum dots, as well as on the interdot coupling. We
calculate the Q-factor for a recently realized experimental setup and find that
it is two orders of magnitudes larger than the measured value, indicating that
the decoherence due to phonons is a subordinate mechanism.Comment: 5 pages, 7 figures, replaced with the version to appear in Phys. Rev.
Josephson Junctions defined by a Nano-Plough
We define superconducting constrictions by ploughing a deposited Aluminum
film with a scanning probe microscope. The microscope tip is modified by
electron beam deposition to form a nano-plough of diamond-like hardness, what
allows the definition of highly transparent Josephson junctions. Additionally a
dc-SQUID is fabricated to verify appropriate functioning of the junctions. The
devices are easily integrated in mesoscopic devices as local radiation sources
and can be used as tunable on-chip millimeter wave sources
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