Quantum point contact sustains 5/2 quantum Hall state

If the theorists are right, the quasiparticle excitations of the 5/2 quantum Hall state share a remarkable collective degeneracy that can be exploited for fault-tolerant quantum computing (see Physics Today, October 2005, page 21 ). But before experimenters can build even a single logic gate, they need to confirm the 5/2 state’s true nature. And before they do that, they need to trap the 5/2 state within the confines of a gated quantum dot or quantum point contact. That preliminary step has just been carried out. Jeff Miller of Harvard University and his collaborators from Alcatel-Lucent’s Bell Labs, Harvard, and MIT fabricated seven different-sized QPCs out of a semiconductor heterostructure made from gallium arsenide and aluminum gallium arsenide (see figure). Using clever tunings of voltages and magnetic fields, Miller and his collaborators observed the 5/2 state not only in the gallium arsenide bulk of the heterostructure but also between the QPCs. In addition, their measurements reveal comforting, but not conclusive, evidence of tunneling by edge states across the QPC. That tunneling is a vital ingredient for harnessing the quantum computing potential of the 5/2 state. (J. B. Miller et al. , Nature Physics , in press.)