We Share Science

Video abstract for the article 'Anyonic interferometry without anyons: how a flux qubit can read out a topological qubit ' by F Hassler, A R Akhmerov, C-Y Hou and C W J Beenakker (F Hassler et al 2010 New J. Phys. 12 125002) Read the full article in New Journal of Physics at http://iopscience.iop.org/1367-2630/12/12/125002/fulltext/. Featured in our 'Focus on topological quantum computation' issue GENERAL SCIENTIFIC SUMMARY Introduction and background. A quantum computer uses the superposition principle from quantum physics to perform at speed calculations that would take a normal computer thousands of years to complete. The zeroes and ones of a quantum computation can be stored in the direction of the current circulating in a superconducting ring: a clockwise current represents 0, counterclockwise represents 1. The coherent superposition of 0 and 1 that is allowed by the laws of quantum mechanics is called the 'quantum bit', or 'qubit'. Unfortunately, this superposition turns out to be very fragile; it quickly becomes incoherent as a result of small disturbances. Main results. We propose a way to store qubits in a manner that makes them insensitive to external disturbances. The direction of the circulating current is stored in a semiconducting wire of indium-arsenide, running over the superconductor. An old but little-known effect, the so-called Aharonov--Casher effect, is invoked to couple the ring to the wire. The Aharonov--Casher effect has a topological origin (like the more familiar Aharonov--Bohm effect) and therefore does not depend on the microscopic details of the system. It is this independence that protects the qubit from decoherence. Wider implications. The topological nature of the Aharonov--Casher effect offers a way to realize topological quantum computation, where topology protects the qubit from decoherence.