Quantum mechanics isn’t what it used to be. Several decades ago it was all about how, at the very small scales of atoms, energy comes in chunks or “quanta”: not continuous, like water, but discrete, like money. Even light is grainy, divided up into little packets of energy called photons.
But never mind all that. Today, quantum physicists aren’t really talking about quanta, they’re talking about information. They suspect that at its root quantum mechanics is a theory about what can and can’t be known about the world. The famous uncertainty principle, and the idea that quantum objects might be either here or there, are examples of that idea.
It’s not all theory, though. The new view offers potential applications in the form of so-called quantum information technology: ways of storing, transmitting and manipulating information that work using quantum rules rather than the “classical” rules of our everyday world. The most celebrated manifestation of this technology is the quantum computer, which could exploit quantum principles to achieve far greater power than the devices on which I’m writing and you are reading.
Although it’s clear to those in the field how quantum computers should work, no one knows how to make one. Scientists have made “toy” quantum computers with just a handful of bits (compared to the billions in your smart phone), and some companies are even starting to offer primitive versions for sale – to the scepticism of some experts. But despite tantalising reports of incremental breakthroughs over the past few years, there’s still no prospect that you’ll have a useful quantum laptop in the coming future.
However, scientists in Germany have just reported what could be a significant step forward. They say that the ideal material for a quantum computer could be diamond.
Don’t despair – that doesn’t mean they will cost the earth. The very thin films of diamond needed for such devices don’t have to be mined; they can be made artificially from carbon-rich gases such as methane. It’s not exactly cheap, but neither are the methods needed to make semiconductor films for a host of existing electronic devices.
Both conventional and quantum computers work by encoding and manipulating information in binary form; as “bits”, represented as zeroes and ones. Florian Dolde at the University of Stuttgart and his colleagues think the ideal elements that will store this information on a quantum computer are individual nitrogen atoms implanted into a diamond film. Nitrogen atoms have one more electron than the carbon atoms in diamond, and this spare electron can exist in two different quantum states thanks to a property called spin. Rather like the poles of a magnet (which are used to store information in magnetic disks and tapes), an electron spin can be considered to point either “up” or “down”.
That much has been known for some time, and others have experimented with nitrogen-doped diamond for quantum computing. The advance made by Dolde and colleagues is to show how they can place these spins in nitrogen electrons without having to cool the diamond to very low temperatures.
In a spin
The reason quantum computers could be so powerful is that a collection of bits could exist in many more different states than the same number of “classical” bits. That’s because quantum particles can exist in two or more different states at the same time – in a so-called superposition of states. So each quantum bit (qubit) can be not just a 1 or a 0 but mixtures of both. As a result, a group of qubits could perform many different calculations at once, rather than having to do them sequentially like an ordinary computer.
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.