A Single-Level Electron Turnstile
A new design for an electron “turnstile” uses a quantum dot sandwiched between superconductors to shuffle one electron through at a time. The advantage of this device over previous turnstiles is that only one energy level in the dot is available for electrons to pass. This restricted access prevents electrons from cutting in line and ensures that the electrons that do flow have a single energy, making the device ideal for quantum metrology applications, such as defining the ampere.
The basic electron turnstile uses a small conducting island connected through tunnel junctions to entrance and exit leads. Tunneling is controlled by a periodically varying voltage that works like a revolving door, letting electrons first tunnel into the island and then, later, out. In principle, only one electron passes at a time because the charge of the electron on the island repels others from following. In practice, however, extra electrons can sneak through.
The new turnstile made by David van Zanten from the University of Grenoble, France, and his collaborators is based on a currently popular design in which a metal island sits between superconducting leads. Superconductors have gaps in their electron energy distribution that help limit the tunneling, but the metal has a continuum of energy levels that lets unwanted electrons through. To prevent this, the authors essentially shrunk the metal island to a quantum dot, which has discrete energy levels. The spacing of these levels is large enough that electrons can only occupy the dot’s ground level. The team showed that this single-level access produces single-electron flow (at one energy) with less than 1% error from electron interlopers.
This research is published in Physical Review Letters.