6 research outputs found
Fourier synthesis of radio frequency nanomechanical pulses with different shapes
The concept of Fourier synthesis is heavily employed in both consumer
electronic products and fundamental research. In the latter, pulse shaping is
key to dynamically initialize, probe and manipulate the state of classical or
quantum systems. In nuclear magnetic resonance, for instance, shaped pulses
have a long-standing tradition and the underlying fundamental concepts have
subsequently been successfully extended to optical frequencies and even to
implement quantum gate operations. Transferring these paradigms to
nanomechanical systems requires tailored nanomechanical waveforms. Here, we
report on an additive Fourier synthesizer for nanomechanical waveforms based on
monochromatic surface acoustic waves. As a proof of concept, we electrically
synthesize four different elementary nanomechanical waveforms from a
fundamental surface acoustic wave at MHz using a superposition
of up to three discrete harmonics . We employ these shaped pulses to
interact with an individual sensor quantum dot and detect their deliberately
and temporally modulated strain component via the opto-mechanical quantum dot
response. Importantly, and in contrast to the direct mechanical actuation by
bulk piezoactuators, surface acoustic waves provide much higher frequencies (>
20 GHz) to resonantly drive mechanical motion. Thus, our technique uniquely
allows coherent mechanical control of localized vibronic modes of
optomechanical crystals, even in the quantum limit when cooled to the
vibrational ground state.Comment: 18 pages - final manuscript and supporting materia
Kelvin-Helmholtz instability in the presence of variable viscosity for mudflow resuspension in estuaries
The temporal stability of a parallel shear flow of miscible fluid layers of dif- ferent density and viscosity is investigated through a linear stability analysis and direct numerical simulations. The geometry and rheology of this Newto- nian fluid mixing can be viewed as a simplified model of the behavior of mud- flow at the bottom of estuaries for suspension studies. In this study, focus is on the stability and transition to turbulence of an initially laminar configuration. A parametric analysis is performed by varying the values of three control pa- rameters, namely the viscosity ratio, the Richardson and Reynolds numbers, in the case of initially identical thickness of the velocity, density and viscosity profiles. The range of parameters has been chosen so as to mimic a wide variety of real configurations. This study shows that the Kelvin-Helmholtz instability is controlled by the local Reynolds and Richardson numbers of the inflection point. In addition, at moderate Reynolds number, viscosity strat- ification has a strong influence on the onset of instability, the latter being enhanced at high viscosity ratio, while at high Reynolds number, the influ- ence is less pronounced. In all cases, we show that the thickness of the mixing layer (and thus resuspension) is increased by high viscosity stratification, in particular during the non-linear development of the instability and especially pairing processes. This study suggests that mud viscosity has to be taken into account for resuspension parameterizations because of its impact on the inflec- tion point Reynolds number and the viscosity ratio, which are key parameters for shear instabilities