DIAMONDS AND SPIN

Dick Pountain/13 November 2007/13:22/Idealog 160

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 that's going to need a bit of further explanation (sigh). I've written here before about the problems that face constructing a quantum computer - see for example "The Physics of Computing" PC Pro June 2003 - and the biggest of those problems is that the entangled quantum states used for quantum computation are normally destroyed by contact with external matter. But fabricating a quantum chip without using matter is strictly Harry Potter territory. However it now turns out that this may not be necessary, thanks to the good old diamond.

Diamond is the hardest substance that occurs in nature (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 are 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 little difficulty. It's roughly the same size as a carbon atom but has only three bonds available instead of four, which leaves a dangling spare bond on one of its carbon neighbours, and that gap or vacancy in the lattice may prove the answer to practical quantum computing.

I've also written here before (PC Pro 123, Nov 2004) about spintronics, the promising new science which instead of manipulating moving electrons, as in electronics,  manipulates the spins of individual particles. There are already some spintronic devices on the market, including the Giant Magnetorestrictive read/write heads used in modern hard drives and MRAM (magnetic RAM) memory chips. These use magnetic fields to change the spins of electrons, which requires the presence of ferromagnetic materials like barium titanate that are very hard to incorporate into standard CMOS chip fabrication techniques. It also rules them out for quantum computing since magnetic fields also destroy quantum entanglements.

What makes the inside of a diamond such a sparkling place for quantum dots to live in 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 were not enough, N-V centres fluoresce (emit visible light) 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 1 and dim meaning 0. Perhaps there is 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 insulators known, 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, solid-state lasers and so on.

Several groups worldwide, including David D. Awschalom and his team at the Center for Spintronics and Quantum Computing, University of California Santa Barbara, are working to produce such a diamond-based quantum chip, though the remaining obstacles are formidable. Instead of natural diamond they use synthetic diamond deposited as a thin film, a few nanometres thick, by decomposing methane with intense microwaves. They etch this coating to form optical cavities in which light can form standing waves, and ideally each cavity would contain a single N-V centre, but the technology for precisely placing nitrogen atoms is one of the biggest challenges. At present they must use randomly occuring N-V centres and build a cavity around them. Even so, for the first time room-temperature quantum computing feels truly within grasp, which is why giants like Hewlett Packard are now piling into diamond technologies. Shine on.