To enable that, it’s generally thought that the qubits have to be entangled. This means that the quantum state of one of them depends on the states of the others – even though these states aren’t actually assigned until they are measured. In other words, if you entangle a pair of spins that have opposite orientations, and measure one of them as being “up”, the other instantly becomes “down”, no matter how far away it is. Some early quantum theorists, including Einstein, thought this would be impossible, but this entanglement is now a well-established fact.
But here’s the rub: like most quantum properties, entanglement seems to be very delicate. Amid all the jostling of other atoms, a pair of entangled particles can lose their special connection so that their states become independent of each other. Sustaining entanglement has tended to mean cooling the particles down close to absolute zero to remove that jostling. But a quantum computer that needs to be so cold won’t ever find much of a market.
Dolde and colleagues have shown, however, that two nitrogen atoms trapped in diamond tens of nanometres apart can be kept entangled at room temperature for more than a millisecond (thousandth of a second), which could be long enough to perform quantum calculations. They used microwave photons to nudge the atoms into an entangled state, by firing a beam of nitrogen ions (charged atoms) at a diamond film though a mask with holes about 20 nanometres apart.
The case for nitrogen-doped diamond quantum computers is boosted further by a paper from Martin Plenio of the University of Ulm in Germany and his co-workers, who have shown that in theory – no more than that yet – such a system could be used as a “quantum simulator”: a kind of quantum computer that can calculate how other quantum systems will behave. The mathematics needed to predict quantum behaviour is complicated, and ordinary computers struggle to accommodate it. But a quantum simulator, working by quantum rules, already has the “quantum-ness” built in to its components, and so can carry out such calculations much more easily. Diamond, of all things, could take the hardness out of the problem.