Heisenberg uncertainty principle stressed in new test

By Jason Palmer
Science and technology reporter, BBC News

image captionThe experiment requires preparing pairs of "entangled" photons, the particles from which light is made

Pioneering experiments have cast doubt on a founding idea of the branch of physics called quantum mechanics.

The Heisenberg uncertainty principle is in part an embodiment of the idea that in the quantum world, the mere act of measuring can affect the result.

But the idea had never been put to the test, and a team writing in Physical Review Letters says "weak measurements" prove the rule was never quite right.

That could play havoc with "uncrackable codes" of quantum cryptography.

Quantum mechanics has since its very inception raised a great many philosophical and metaphysical debates about the nature of nature itself.

Heisenberg's uncertainty principle, as it came to be known later, started as an assertion that when trying to measure one aspect of a particle precisely, say its position, experimenters would necessarily "blur out" the precision in its speed.

"Uncertainty" itself is somewhat ill-defined in this sense; Heisenberg's original word for it translates better as "indeterminacy"; the idea raised the spectre of a physical world whose nature was, beyond some fundamental level, unknowable.

This problem with the act of measuring is not confined to the quantum world, explained senior author of the new study, Aephraim Steinberg of the University of Toronto.

"You find a similar thing with all sorts of waves," he told BBC News. "A more familiar example is sound: if you've listened to short clips of audio recordings you realise if they get too short you can't figure out what sound someone is making, say between a 'p' and a 'b'.

"If I really wanted to say as precisely as possible, 'when did you make that sound?', I wouldn't also be able to ask what sound it was, I'd need to listen to the whole recording."

The problem with Heisenberg's theory was that it vastly predated any experimental equipment or approaches that could test it at the quantum level: it had never been proven in the lab.

"Heisenberg had this intiuition about the way things ought to be, but he never really proved anything very strict about the value," said Prof Steinberg.

"Later on, people came up with the mathematical proof of the exact value."

'Full of uncertainty'

Prof Steinberg and his team are no stranger to bending quantum mechanics' rules; in 2011, they carried out a version of a classic experiment on photons - the smallest indivisible packets of light energy - that plotted out the ways in which they are both wave and particle, something the rules strictly preclude.

This time, they aimed to use so-called weak measurements on pairs of photons, putting into practice an idea first put forward in a 2010 paper in the New Journal of Physics.

Photons can be prepared in pairs which are inextricably tied to one another, in a delicate quantum state called entanglement, and the weak measurement idea is to infer information about them as they pass, before and after carrying out a formal measurement.

What the team found was that the act of measuring did not appreciably "blur out" what could be known about the pairs.

It remains true that there is a fundamental limit of knowability, but it appears that, in this case, just trying to look at nature does not add to that unavoidably hidden world.

Or, as the authors put it: "The quantum world is still full of uncertainty, but at least our attempts to look at it don't have to add as much uncertainty as we used to think!"

Whether the finding made much practical difference was an open question, said Prof Steinberg.

"The jury is still out on that. It's certainly more than a footnote in the textbooks; it will certainly change the way I teach quantum mechanics and I think a lot of textbooks.

"But there's actually a lot of technology that relies on quantum uncertainty now, and the main one is quantum cryptography - using quantum systems to convey our information securely - and that mostly boils down to the uncertainty principle."

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