Quantum Computing Using Electrons Floating on Liquid Helium
  The system of electrons trapped in vacuum above the liquid helium surface displays the highest mobilities known in condensed matter physics. We provide a brief summary of the experimental and theoretical results obtained for this system. We then show that a quasi-2D set of N > 10^8 electrons in vacuum trapped in 1D hydrogenic levels above a micron-thick helium film can be used as an easily manipulated strongly interacting set of quantum bits. Individual electrons are laterally confined by micron sized metal pads below the helium. Information is stored in the lowest hydrogenic levels. Using electric fields at temperatures of 10 mK, changes in the wave function can be made in nanoseconds. Wave function coherence times are .1 millisecond. The wave function is read out using an inverted dc voltage which releases excited electrons from the surface, or using SETs attached to the metal pads which control the electrons. 
M. I. Dykman and P. M. Platzman