Heisenberg’s uncertainty principle limits what we can know about the quantum world. Now the uncertainty principle is being harnessed to see if it is possible to identify a point at which matter begins to exhibit weird quantum behavior.

According to the uncertainty principle, measuring the position of an object always disturbs its momentum in an unpredictable way. Physicists ordinarily see this so-called "back action" as a nuisance, but a team led by Keith Schwab of the University of Maryland, College Park, decided to put it to good use.

Schwab’s team fabricated a nanoscale resonator – the equivalent of a tiny pendulum – on a silicon chip, which oscillates at 20 megahertz. On the same chip, they created a single-electron transistor and electrically coupled it to the resonator in such a way that any change in the resonator’s position caused a change in the transistor’s current.

Measuring the current should cause back action in the resonator – and it did (Nature, vol 443, p 193). In most cases, the back action caused the resonator to get noisier or "hotter" than it would have if the measurement hadn’t taken place. But when the team set the transistor voltage to a value that let electrons tunnel through the device, allowing the transistor to absorb energy, they found that the resonator cooled from the ambient temperature of about 500 millikelvin down to about 300 millikelvin.

By cooling the resonator in this way – to temperatures out of reach of conventional technology – Schwab hopes to put it into a state called quantum superposition, where it is in two states at once. The resonator would be the largest object placed in such a state. By monitoring if and when the superposition vanishes, the team aims to probe the boundary between the quantum and classical worlds.