Of the obstacles to building a quantum computer, two remain especially tough to surmount: scaling up from a few-qubit logic gate to a many-qubit device and sustaining the qubits’ coherence long enough to do a worthwhile and accurate calculation.
In principle, both obstacles could be tackled by basing a quantum computer on electron spins in semiconductors. Intel, Samsung, and other chipmakers already integrate tens of millions of logic gates on dime-sized wafers of silicon. And the up-or-down spin of an electron, a natural qubit, couples to the surrounding lattice with coherence-preserving weakness.
But weak coupling has the defect of its virtue. In quantum computation one needs to manipulate individual qubits. Grasping a particular spin by its puny magnetic field, setting its initial state, flipping it on cue, and reading its final state are beyond today’s technology.
Fortunately, the electron’s charge provides another, firmer handle. Eight years ago, Daniel Loss of...
