Vibrating carbon nanotubes offer one route toward the ultimate computer -- and may even reveal how reality itself emerges from uncertainty
By Kate Becker
Take a one-atom-thick sheet of carbon, roll it into a hollow cylinder, and you have a carbon nanotube: a remarkably strong material so thin that you could fit 100,000 nanotubes across the width of a human hair. Last month, electrical engineers built the first working computer from these tiny cylindrical molecules -- an important step towards creating smaller, faster and lower-powered machines. But materials scientist Edward Laird has a more ambitious aim: He is setting up a new lab at Oxford University to grow these tubes and investigate how they could be used to make a new breed of quantum computers. Plucking them may even reveal how the reality we see around us emerges from a fuzzy and uncertain quantum underworld.
Laird was first attracted to researching quantum mechanics because of its many bizarre qualities. For instance, before you measure them, quantum particles can exist in a “superposition state”, holding multiple properties at the same time -- an ability that lies at the heart of quantum computers. Traditional computers store and process information in bits, each of which represents either a one or a zero. By contrast, quantum computers operate on quantum bits, or qubits, that can take the value of zero or one, or a superposition of both at once. This allows them to perform many calculations in parallel, running exponentially faster than a conventional computer. “Quantum theory began as a set of rules, like the uncertainty principle, that limit what we can do", explains Laird, "But we've come to understand that by harnessing the quantum world we can achieve what is otherwise practically impossible”.
An electron makes a handy qubit because, before it is measured, its angular momentum, known as “spin”, exists as a superposition of being both “up” (the equivalent of a zero bit) and “down” (the equivalent of a 1). But keeping qubits in superposition is a delicate business. Like a soap bubble that pops as soon as it is touched, superposition breaks down the moment the qubit interacts with the outside environment. For practical quantum computing, physicists must find a way to protect spins for at least several thousand processor cycles.
Today’s leading spin qubit material, the semiconductor gallium arsenide, can hold a superposition for about 200 microseconds, but only with difficulty, because the atomic nuclei within the compound act like tiny magnets, disturbing the electrons. This is where carbon nanotubes come in: Because the carbon nuclei of the nanotubes are not magnetized, Laird and his colleagues hope that they may be able to maintain a superposition in a nanotube for as long as ten seconds.
Laird and his colleagues Leo Kouwenhoven and Fei Pei, both of the Delft University of Technology in the Netherlands, demonstrated the first nanotube qubits earlier this year (Laird et al, Nature Nanotechnology 8 565-568 (2013)). Unfortunately, they could only hold their electrons in superposition for about 65 nanoseconds -- less than a billionth of the most optimistic theoretical predictions. One of Laird’s first goals at his new lab is to extend this time, using an ultra-pure form of carbon that has been cooled to within a fraction of a degree above absolute zero and rigorously stripped of stray magnetized nuclei. The photograph shows the core of a dilution fridge, one of the systems that is essential to Laird's work -- these state-of-the-art facilities were funded in part by generous philanthropic donations.
To make a functional quantum computer, you also need a way to control the state of the qubits. Carbon nanotubes, again, offer up a useful solution. Though the carbon nuclei are demagnetized, the nanotube itself carries an internal magnetic field, pointing along the direction of the nanotube, which is felt by electrons moving in the tube. Laird and his colleagues have developed a technique for stamping a bend into the nanotube, causing the magnetic field to change direction and providing a means by which to manipulate the qubits as they move across the kink.
Carbon nanotubes are more than just tools for building better computers, though. They could help explore one of the deepest mysteries in physics: how the strange laws of quantum mechanics transform into the ordinary rules of physics that we experience on human scales. In the micro domain of atoms, energy is “quantized”. That is, atoms jump from one discrete energy level to another, and can’t land anywhere in between.
Laird plans to investigate whether nanotubes, which bridge the size gap between the atomic and macro regimes, follow similar rules. One way to check this is by using an electric field to “pluck” nanotubes “like little guitar strings,” he explains. This will determine whether the nanotubes act like macroscopic objects, vibrating with any arbitrary amplitude. If, however, they skip from one amplitude to the next without any crescendo in between, then they are exhibiting quantum behaviour -- and Laird will be a step nearer to understanding how big objects can get, while still obeying quantum rules. Maybe then, he and his colleagues will be able to reveal how the strange notes of quantum mechanics combine to create the familiar melody of the everyday world.