Quantum mechanics rule 'bent' in classic experiment

Water ripples Light can interfere with itself just as water ripples can add to or cancel one another

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Researchers have bent one of the most basic rules of quantum mechanics, a counterintuitive branch of physics that deals with atomic-scale interactions.

Its "complementarity" rule asserts that it is impossible to observe light behaving as both a wave and a particle, though it is strictly both.

In an experiment reported in Science, researchers have now done exactly that.

They say the feat "pulls back the veil" on quantum reality in a way that was thought to be prohibited by theory.

Quantum mechanics has spawned and continues to fuel spirited debates about the nature of what we can see and measure, and what nature keeps hidden - debates that often straddle the divide between the physical and the philosophical.

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

One embodiment 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.

Young's two-slit experiment

Diffraction pattern from two-slit experiment
  • A central idea in quantum mechanics is that light and matter can behave as both particle and wave
  • However, the idea of "complementarity" prevents observation of both behaviours simultaneously
  • In the two-slit experiment, light is passed through two tiny holes and is then viewed on a screen
  • The two beams interfere with each other, forming a rippled "diffraction pattern" - as if the light were made of a number of waves adding or cancelling
  • However, if one of the holes is blocked, the light can be seen as a single beam on the screen - as if light were made of particles
  • The new work, for the first time, observes both kinds of behaviour at the same time

Now, Aephraim Steinberg of the University of Toronto and his colleagues have sidestepped this limitation by undertaking "weak measurements" of the photons' momentum.

The team allowed the photons to pass through a thin sliver of the mineral 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 team was able to infer what paths the photons had taken.

While they were able to easily observe the interference pattern indicative of the wave nature of light, they were able also to see from which slits 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.

He explained to BBC News that "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 'what was the average momentum of the particles which reached this position?'" .

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

Philosophical beginnings

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

"This paper is probably the first that has really put this weak measurement idea into a real experimental realisation, and it also gave us the trajectories."

He said that 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.

For his part, Professor 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," he said.

"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.

"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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