A single-atom qubit in silicon

The up or down spin of an electron makes it a natural qubit to use in an eventual quantum computer. One difficulty in any qubit system is preserving the qubits’ fragile phase coherence long enough to perform a sequence of quantum calculations. Another is scalability. For instance, qubits made from isolated atoms offer long coherence times but are hard to scale up into macroscopic devices, whereas those made from bulk semiconductors are scalable but usually suffer from high decoherence rates (see Physics Today, March 2006, page 16). A research group led by Andrea Morello and Andrew Dzurak (both from the University of New South Wales) have now combined the advantages of both architectures by fabricating a qubit based on a single atom’s electron spin. Ordinarily, a phosphorus atom embedded in silicon donates an electron that enhances Si’s electrical conductivity, but at cryogenic temperatures the electron becomes trapped around the P nucleus. The qubit formed by the spin of the electron is protected from decoherence thanks to weak spin–orbit coupling and a near absence of nuclear spin in the surrounding Si lattice. After implanting P in a Si chip, the researchers also fashioned on the chip a transistor to initialize and read out the qubit’s spin state. Between those operations they used microwave pulses resonant with the spin transition frequency to coherently manipulate the qubit’s state over about 200 µs; that’s long enough to enable more than 1000 operations. (J. J. Pla et al., Nature 489, 541, 2012.)—R. Mark Wilson