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