Quantum uncertainty not all in the measurement

A common interpretation of Heisenberg's uncertainty principle is proven false.

 

Geoff Brumfiel 11 September 2012

 

The uncertainty principle limits what we can know about a quantum

system, and that fuzziness is not entirely caused by the act of

measurement.

 

Contrary to what many students are taught, quantum uncertainty may not

always be in the eye of the beholder. A new experiment shows that

measuring a quantum system does not necessarily introduce uncertainty.

The study overthrows a common classroom explanation of why the quantum

world appears so fuzzy, but the fundamental limit to what is knowable

at the smallest scales remains unchanged.

 

At the foundation of quantum mechanics is the Heisenberg uncertainty

principle. Simply put, the principle states that there is a

fundamental limit to what one can know about a quantum system. For

example, the more precisely one knows a particle's position, the less

one can know about its momentum, and vice versa. The limit is

expressed as a simple equation that is straightforward to prove

mathematically.

 

Heisenberg sometimes explained the uncertainty principle as a problem

of making measurements. His most well-known thought experiment

involved photographing an electron. To take the picture, a scientist

might bounce a light particle off the electron's surface. That would

reveal its position, but it would also impart energy to the electron,

causing it to move. Learning about the electron's position would

create uncertainty in its velocity; and the act of measurement would

produce the uncertainty needed to satisfy the principle.

 

Physics students are still taught this measurement-disturbance version

of the uncertainty principle in introductory classes, but it turns out

that it's not always true. Aephraim Steinberg of the University of

Toronto in Canada and his team have performed measurements on photons

(particles of light) and showed that the act of measuring can

introduce less uncertainty than is required by Heisenberg's

principle1. The total uncertainty of what can be known about the

photon's properties, however, remains above Heisenberg's limit.

 

Delicate measurement

Steinberg's group does not measure position and momentum, but rather

two different inter-related properties of a photon: its polarization

states. In this case, the polarization along one plane is

intrinsically tied to the polarization along the other, and by

Heisenberg's principle, there is a limit to the certainty with which

both states can be known.

 

The researchers made a 'weak' measurement of the photon's polarization

in one plane - not enough to disturb it, but enough to produce a rough

sense of its orientation. Next, they measured the polarization in the

second plane. Then they made an exact, or 'strong', measurement of the

first polarization to see whether it had been disturbed by the second

measurement.

 

When the researchers did the experiment multiple times, they found

that measurement of one polarization did not always disturb the other

state as much as the uncertainty principle predicted. In the strongest

case, the induced fuzziness was as little as half of what would be

predicted by the uncertainty principle.

 

Don't get too excited: the uncertainty principle still stands, says

Steinberg: 'In the end, there's no way you can know [both quantum

states] accurately at the same time.' But the experiment shows that

the act of measurement isn't always what causes the uncertainty. 'If

there's already a lot of uncertainty in the system, then there doesn't

need to be any noise from the measurement at all,' he says.

 

The latest experiment is the second to make a measurement below the

uncertainty noise limit. Earlier this year, Yuji Hasegawa, a physicist

at the Vienna University of Technology in Austria, measured groups of

neutron spins and derived results well below what would be predicted

if measurements were inserting all the uncertainty into the system2.

 

But the latest results are the clearest example yet of why

Heisenberg's explanation was incorrect. "This is the most direct

experimental test of the Heisenberg measurement-disturbance

uncertainty principle," says Howard Wiseman, a theoretical physicist

at Griffith University in Brisbane, Australia "Hopefully it will be

useful for educating textbook writers so they know that the naive

measurement-disturbance relation is wrong."

 

Shaking the old measurement-uncertainty explanation may be difficult,

however. Even after doing the experiment, Steinberg still included a

question about how measurements create uncertainty on a recent

homework assignment for his students. "Only as I was grading it did I

realize that my homework assignment was wrong," he says. "Now I have

to be more careful."

Journal name:

Nature

DOI:

doi:10.1038/nature.2012.11394

References

1.Rozema, L. A. et al. Phys. Rev. Lett. 109, 100404 (2012).

 

ArticleShow context

2.Erhart, J. et al. Nature Phys. 8, 185-189 (2012).