Heisenberg's Uncertainty Principle in doubt

Posted by Emma Woollacott

Heisenberg's Uncertainty Principle is looking a little, well, uncertain, with the discovery that observation needn't disturb systems as much as thought.

Over a century ago, physicist Werner Heisenberg came up with a formula for one of the most important aspects of quantum physics: that not all properties of a quantum particle can be measured with complete accuracy.

When measuring the location and momentum of an electron, for example, increased certainty in the first measurement automatically leads to less in the second.

Now, though, researchers from the University of Toronto say they have gathered experimental evidence that Heisenberg's original formulation is wrong.

They set up an experiment to measure the polarization of a pair of entangled photons, aiming to quantify how much the act of measuring the polarization disturbed the photons.

They did by this by observing the light particles both before and after the measurement. But there's an obvious problem here: if the 'before' observation disturbs the system, the 'after' observation would be tainted.

However, the researchers found a way around this Catch-22 by using techniques from quantum measurement theory to sneak peeks at the photons before their polarization was measured.

"If you interact very weakly with your quantum particle, you won't disturb it very much," says Lee Rozema, a PhD candidate in quantum optics research at the University of Toronto.

But by taking a number of such weak interactions, says Rozema, it's possible to build up a comparatively clear picture.

By comparing thousands of 'before' and 'after' views of the photons, the researchers found that their precise measurements disturbed the system much less than predicted by the original Heisenberg formula.

The results provide the first direct experimental evidence for a new measurement-disturbance relationship, mathematically computed by physicist Masanao Ozawa, at Nagoya University in 2003.

"Precision quantum measurement is becoming a very important topic, especially in fields like quantum cryptography where we rely on the fact that measurement disturbs the system in order to transmit information securely," says Rozema.

"In essence, our experiment shows that we are able to make more precise measurements and give less disturbance than we had previously thought."