We present a theory of the collective excitation spectrum in the fractional quantum Hall effect which is closely analogous to Feynman's theory of superfluid helium. The predicted spectrum has a large gap at k=O and a deep magneto-roton minimum at finite wave vector, in excellent quantitative agreement with recent numerical calculations. We demonstrate that the magneto-roton minimum is a precursor to the gap collapse associated with the Wigner crystal instability occurring near v= 7. In addition to providing a simple physical picture of the collective excitation modes, this theory allows one to compute rather easily and accurately experimentally relevant quantities such as the susceptibihty and the ac conductivity.

No abstract

A quasi-two-dimensional set of electrons (1 < N < 10(9)) in vacuum, trapped in one-dimensional hydrogenic levels above a micrometer-thick film of liquid helium, is proposed as an easily manipulated strongly interacting set of quantum bits. Individual electrons are laterally confined by micrometer-sized metal pads below the helium. Information is stored in the lowest hydrogenic levels. With electric fields, at temperatures of 10(-2) kelvin, changes in the wave function can be made in nanoseconds. Wave function coherence times are 0.1 millisecond. The wave function is read out with an inverted dc voltage, which releases excited electrons from the surface.

Periodic intensity variations in the measured Compton profile anisotropies of ordinary ice Ih correspond to distances of 1.72 and 2.85 Å, which are close to the hydrogen bond length and the nearestneighbor O-O distance, respectively. We interpret this result as direct evidence for the substantial covalent nature of the hydrogen bond. Very good quantitative agreement between the data and a fully quantum mechanical bonding model for ice Ih and the disagreement with a purely electrostatic (classical) bonding model support this interpretation and demonstrate how exquisitely sensitive Compton scattering is to the phase of the electronic wave function. [S0031-9007(98)08227-1]

No abstract