June 5, 2011

Experiment ‘Bends’ Quantum Mechanics Rule

Researchers have changed one of the most basic rules of quantum mechanics: observing light behaving as both a wave and a particle -- a "complementarity" rule that asserts that it is impossible to do, even though it is strictly both.

In an experiment reported in the journal Science, researchers say the feat "pulls back the veil" on quantum reality in a way that was thought to be prohibited by theory.

Quantum mechanics, a counterintuitive branch of physics that deals with atomic-scale interactions, has created and continues to fuel the debate about the nature of what we can see and measure, and what nature keeps hidden.

For example: a well-known rule called the Heisenberg uncertainty principle asserts that for some pairs of measurements, high precision in one necessarily reduces the precision that can be achieved in the other.

One incarnation of this idea lies in a "two-slit interferometer", in which light can pass through one of two slits and is viewed on a screen. Let a number of the units of light called photons through the slits, and an interference pattern develops, like waves overlapping in a pond. However, keeping a close eye on which photons went through which slits - what may be termed a "strong measurement" - destroys the pattern.

Aephraim Steinberg of the University of Toronto, lead author of the study, worked with colleagues to sidestep this limitation by undertaking "weak measurements" of the photons' momentum.

The team of researchers allowed the photons to pass through a thin sliver of calcite which gave each photon a tiny nudge in its path, with the amount of deviation dependent on which slit it passed through. By averaging over a great many photons passing through the apparatus, and only measuring the light patterns on a camera, the researchers were able to infer what paths the photons had taken.

While observing the interference pattern indicative of the wave nature of light was easy enough, the team was able to see from which slit's the photons had come, a sure sign of their particle nature.

The trajectories of the photons within the experiment - forbidden in a sense by the laws of physics - have been laid bare. On one level, the experiment appears to violate a central rule of quantum mechanics, but Professor Steinberg said this was not the case.

"While the uncertainty principle does indeed forbid one from knowing the position and momentum of a particle exactly at the same time, it turns out that it is possible to ask "Ëœwhat was the average momentum of the particles which reached this position?'" Steinberg explained to BBC News.

"You can't know the exact value for any single particle, but you can talk about the average," he said.

"It's a beautiful series of measurements by an excellent group, the likes of which I've not seen before," said Marlan Scully of Texas A&M University, a quantum physicist who has published his own work on this subject.

"This paper is probably the first that has really put this weak measurement idea into a real experimental realization, and it also gave us the trajectories," Scully said, adding the work would -- inevitably -- raise philosophical issues as well.

"The exact way to think about what they're doing will be researched for some time, and the weak measurement concept itself will be a matter of controversy - but now we have a very pretty experiment with these weak measurements," he added.

Steinberg believes that the result reduces a limitation not on quantum physics but on physicists themselves. "I feel like we're starting to pull back a veil on what nature really is."

"The trouble with quantum mechanics is that while we've learned to calculate the outcomes of all sorts of experiments, we've lost much of our ability to describe what is really happening in any natural language," said Steinberg. "I think that this has really hampered our ability to make progress, to come up with new ideas and see intuitively how new systems ought to behave."


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