Nanotubes Isolate Quantum 'Bits' for Computing

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Quantum computers promise to be the next step in computing power, able to outperform conventional machines. One of the challenges to building such a computer is finding a way to measure, control and store qubits, the units of information akin to the 0′s and 1′s in conventional computers. Unlike conventional electronic bits that can be either a 0 or a 1, though, qubits can be either or both simultaneously — a characteristic called superposition. But they’re easily disturbed if anyone tries to observe them and can lose their superposition.

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At the Delft University of Technology in the Netherlands physicists used carbon nanotubes, hollow cylinders of carbon only a few millionths of a millimeter in diameter, to isolate and control qubits from the surrounding environment so they could be observed. The advance could get scientists one step closer to building an actual multi-bit quantum computer. The experiments were detailed in the July 28 issue of Nature Nanotechnology.

For their experiment, the Delft team bent a carbon nanotube into a “U” shape and put two electrons on the surface. They used electrons because the information that describes the state of an electron’s spin, whether it’s pointing up or down, is one kind of qubit. (There are others, but for simplicity’s sake, we’ll stick with electron and spin). They limited the number of electrons to two because it would have been harder to see the spin state of only one, while three would have been more difficult to control.

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Next, the scientists showered the nanotube in an electric field, rapidly switching from positive to negative, to spiral the electrons up and down the tube, inside and out. A positive field made the electrons move in one direction and a negative one forced them in the opposite direction.

When the electrons encountered the “U” bend, they flipped over into another spin state, either from up to down or from down to up. That’s because the curve pinched the electric field and made it uneven. The electron flip was the equivalent of turning a switch in a transistor from on to off or from off to on, essentially making a 0 or 1.

Even though the electrons were zooming around a nanotube, the team hadn’t measured the spin states yet. That meant those spin states were still in superposition. Although the superposition state only lasted for about 65 billionths of a second, it showed the possibility of turning the qubits on and off. The next step, Fei Pei said, is to add more qubits to the system.

Credit: Delft University of Technology