Sand-grain-sized drum extends reach of quantum theory

The banging of a tiny drum heralds the intrusion of the weird world of quantum mechanics into our everyday experience. Though no bigger than a grain of sand, the drum is the largest object ever to have been caught obeying the uncertainty principle, a central idea in quantum theory.

As well as extending the observed reach of quantum theory, the finding could complicate the hunt for elusive gravitational waves : it suggests that the infinitesimal motion caused by these still-hypothetical ripples in spacetime could be overwhelmed by quantum effects.

The uncertainty principle says that you cannot simultaneously determine both a particle's exact position and momentum. For example, bouncing a photon off an electron will tell you where it is, but it will also change the electron's motion, creating fresh uncertainty in its speed.

This idea limits our ability to measure the properties of very small objects, such as electrons and atoms. The principle should also apply to everyday, macroscopic objects, but this has not been tested ? for larger objects, the principle's effects tend to be swamped by other uncertainties in measurement, due to random noise, say.

Quantum drum

To extend the known reach of the uncertainty principle, Tom Purdy and colleagues of the University of Colorado, Boulder, created a drum by tightly stretching a 40-nanometre-thick sheet of silicon nitride over a square frame with sides of half a millimetre ? about the width of a grain of sand. They placed the drum inside a vacuum chamber cooled to a few degrees above absolute zero, minimising any interference by random noise.

By continuously firing a stream of photons at the drum they were able to get increasingly precise measurements of the position of the skin at any moment. However, this also caused the skin to vibrate at an unknown speed. When they attempted to determine its momentum, the error in their measurement had increased ? just as the uncertainty principle predicts.

"You don't usually have to think about quantum mechanics for objects you can hold in your hand," says Purdy.

That the uncertainty principle holds sway at such a large scale could affect the hunt for gravitational waves, which are predicted by Einstein's theory of general relativity but have never been detected.

Mitigation strategy

Gravitational wave detectors look for very slight changes in the distance between two test masses caused by passing spacetime ripples. Purdy says his team's experiment confirms long-held suspicions that quantum uncertainty could overwhelm these very small changes.

Now he and others can use the drum to explore more advanced measurement techniques to mitigate the effects. For example, uncertainty in an object's momentum could lead to future uncertainty in its position and there should be ways to minimise such knock-on effects. "You can't avoid the uncertainty principle, but you can in some clever ways make it [such that] increasing the momentum doesn't add back to the uncertainty in position at a later time," says Purdy.

His experiment is a neat demonstration of the breakdown of the traditional notion that the atomic world is quantum while the macroscopic world is classic, says Gerard Milburn of the University of Queensland in Brisbane, Australia, who was not involved in the work. Previous, attempts to blur the quantum-classical divide have involved entangling diamonds and demonstrating quantum superposition in a strip of metal.

Despite these feats, Milburn doesn't rule out the prospect of a breakdown on really large scales. "Of course maybe one day we will see quantum mechanics fail at some scale. Testing it to destruction is a good motivation for going down this path," he says.

Journal reference: Science, DOI: 10.1126/science.1231282

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