Physicists have performed the first quantum calculations to be carried out using individual atoms sitting on a surface.
The technique, described on 5 October in Science1, controls titanium atoms by beaming microwave signals from the tip of a scanning tunnelling microscope (STM). It is unlikely to compete any time soon with the leading approaches to quantum computing, including those adopted by Google and IBM, as well as by many start-up companies. But the tactic could be used to study quantum properties in a variety of other chemical elements or even molecules, say the researchers who developed it.
At some level, everything in nature is quantum and can, in principle, perform quantum computations. The hard part is to isolate quantum states called qubits — the quantum equivalent of the memory bits in a classical computer — from environmental disturbances, and to control them finely enough for such calculations to be achieved.
Andreas Heinrich at the Institute for Basic Science in Seoul and his collaborators worked with nature’s ‘original’ qubit — the spin of the electron. Electrons act like tiny compass needles, and measuring the direction of their spin can yield only two possible values, ‘up’ or ‘down’, which correspond to the ‘0’ and ‘1’ of a classical bit. But before it is measured, electron spin can exist in a continuum of possible intermediate states, called superpositions. This is the key to performing quantum computations.
The researchers started by scattering titanium atoms on a perfectly flat surface made of magnesium oxide. They then mapped the atoms’ positions using the STM, which has atomic resolution. They used the tip of the STM probe to move the titanium atoms around, arranging three of them into a triangle.
Using microwave signals emitted from the STM tip, the researchers were able to control the spin of a single electron in one of the titanium atoms. By tuning the frequencies of the microwaves appropriately, they could also make its spin interact with the spins in the other two titanium atoms, similarly to how multiple compass needles can influence each other through their magnetic fields. By doing this, the team was able to set up a simple two-qubit quantum operation, and also to read out its results. The operation took just nanoseconds — faster than is possible with most other types of qubit.
Heinrich says that it will be fairly straightforward to extend the technique to perhaps 100 qubits, possibly by manipulating spins in a combination of individual atoms and molecules. It might be difficult to push it much beyond that, however — and the leading qubit technologies are already being scaled up to hundreds of qubits. “We are more on the basic-science side,” Heinrich says, although he adds that multiple STM quantum computers could one day be linked to form a bigger one.