The central element of any quantum computation scheme is the qubit, a two-state system analogous to the spin-1⁄2 electron. In the course of a computation, the qubit experiences various changes in its state; once the calculation is completed, one needs to determine whether the qubit is “up” or “down.” Unfortunately, the qubit also has to interact with its thermal environment, which can destroy, or decohere, its quantum state. Any useful qubit, then, must be able to resist decoherence for as long as it takes to do a worthwhile sequence of quantum manipulations.
Several systems could conceivably meet that criterion, from Josephson junctions (see PHYSICS TODAY, July 2009, page 14) to quantum dots (see PHYSICS TODAY, March 2011, page 19). Of the solid-state candidates, however, nearly all must be chilled to cryogenic temperatures to obtain suitable coherence times
One exception is an atomic-scale diamond impurity known as an NV...
