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 $n^{th}$-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 Al$_x$Ga$_{1-x}$As/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