Idealog

Where do you think is the safest place in the universe, the place where things change least and there are fewest surprises? Well, if you happen to be a quantum dot, the answer would appear to be inside a diamond… I suppose I’d better explain.

I’ve written about the problems of constructing a quantum computer before (www.pcpro.co.uk/links/160idealog) and the biggest of those problems is that the entangled quantum states used for quantum computation are normally destroyed by contact with external matter. Fabricating a quantum chip without using matter is strictly Harry Potter territory, but it now turns out this may not be necessary, thanks to the good old diamond.

Diamond is the hardest natural substance (competitors like boron nitride being strictly man-made), and the reason it’s so hard is that it’s so stable. It consists of nothing but carbon atoms bonding to other carbon atoms, in their lowest-energy, tetrahedral configuration. The carbon atoms in a diamond lattice are so happy to be there that it takes a great deal of energy to separate them or force them to budge. However, nothing is perfect, and that goes for diamonds, too – they often contain interlopers in the shape of the odd atom of another element squatting awkwardly in the lattice. Unless that alien atom has the same valency (that is, number of bonds) as carbon, namely four, then it can’t fit seamlessly into the lattice but must cram itself in untidily, causing a glitch like a ladder in a stocking.

These imperfections are responsible for the colour of some diamonds – red, yellow, green or blue – and they’re also the key to using diamonds to build quantum computers. The particular impurity atom of most interest is nitrogen, because it’s next to carbon in the Periodic Table and so fits into the diamond lattice with ease. It has only three bonds available instead of four, which leaves a dangling spare bond on one of its carbon neighbours, and that gap may be the answer to practical quantum computing.

I’ve also written about spintronics (www.pcpro.co.uk/links/160idealog2), the promising new science that instead of manipulating moving electrons, as in electronics, manipulates the spins of individual particles. There are already spintronic devices on the market, including the giant magnetorestrictive read/write heads used in modern hard disks and magnetic RAM memory chips. These use magnetic fields to change the spins of electrons. It rules them out for quantum computing since magnetic fields destroy quantum entanglements.

What makes the inside of a diamond such a sparkling place for quantum dots to live is that Carbon-12, the isotope that makes up 99% of natural carbon, has zero nuclear spin, so the lattice is a very non-magnetic place indeed. Moreover, a defect in the diamond in the shape of a single nitrogen atom and its neighbouring vacancy (called an N-V centre) does have a spin, and that spin can be polarised (switched from “up” to “down”) using optical-wavelength light at room temperature, which is just about a quantum computer designer’s idea of heaven. As if that weren’t enough, N-V centres fluoresce when illuminated and one of their two spin states fluoresces far more brightly than the other, so you can read the spin states directly, bright meaning one and dim meaning zero.

Perhaps there’s a God and he’s into quantum computing. The reason for this benign behaviour need not invoke Our Maker, though: it’s just that while normal diamond is one of the best known insulators, when doped with nitrogen or other impurities it becomes a semiconductor like silicon, but the greater stability of its lattice makes the “band gap” between the valence band and the first empty conduction band huge: 5.5 electron volts, or five times greater than that in silicon. The energies of photons of visible light fit comfortably within this gap, so a diamond-based quantum computer could slip straight into the existing technologies of optical computing, such as optic fibres and solid-state lasers.