‘Tractor beams’ build atom-by-atom assembly

Have you ever tried to catch a speck of dust between your fingers? That’s challenging enough, but what about catching a single atom?

Controlling the position of individual atoms is vital for quantum computers, which use individual atoms as “qubits” – the quantum version of the “bits” of regular computers.

Usually atom assembly is a painstaking process, and can only be done one at a time.

In a new paper uploaded to the Arxiv (prior to peer review), physicists at Harvard, Caltech and MIT have teamed up to manipulate up to 50 individual rubidium atoms using an array of 100 optical tweezers.

The technique works a bit like the tractor beam from Star Trek. The atoms float around in a cloud within a vacuum chamber, and the tweezers pluck them out of mid-air (or, perhaps we should say, out of mid-vacuum).

The system then automatically arranges the atoms into a precise formation in less than half a second.

Optical tweezers are tightly focused beams of light able to hold microscopic particles, or even single atoms, in three dimensions. It works by focusing two laser beams on to the same spot.

An atom caught in the crossbeam stops dead, like a deer in headlights, because it is attracted to the strong electric field right at the center of the beam.

When the beam is moved, the atom is dragged with it.

Usually optical tweezers can only control one atom at a time. Now a team of American researchers, led by Mikhail Lukin at Harvard University and Manuel Endres at the California Institute of Technology, have found a way to upscale the process to control 50 atoms at once.

The advance hinges on the team’s ability to split their laser source into 50 separate beams, and then control each beam individually.

The team starts off with a cloud of rubidium atoms cooled to less than a degree above absolute zero, floating around in a vacuum chamber. When the scientists switch on the optical tweezers array, they create a line of 50 atom traps within the cloud.

Most, but not all, of the traps usually succeed in catching an atom, and to check which ones are successful the researchers snap a picture using a special camera that can detect how a single atom fluoresces when trapped.

Empty tweezers are simply switched off, while those that are holding atoms are manipulated to drag the atoms a desired pattern. Then another picture confirms it.

If the pattern does not match up with what has been programmed, the system can use any remaining empty tweezers to grab a few more atoms and bring them over.

For 50 atoms, this whole process takes about 400 milliseconds. For smaller arrays, it takes even less time.

So far the technique only works for making a single line of atoms stretching about a tenth of a millimetre across. But the team plans to scale up the process to make a two dimensional array of optical tweezers.

They write that the “robust creation of defect-free arrays of hundreds of atoms is feasible”.

Another problem is how long the pattern can be held. At the moment, the limit is about 10 seconds. A quantum computer, meanwhile, would require holding times on the order of 100 seconds.

The researchers expect that using a better vacuum, and an improved laser system, might get them at least to the one-minute mark.

After that, they’ll need to think of other tricks so that their pattern is not gone in 60 seconds.