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## Using Averages To Bend the Uncertainty Principle112

Posted by Soulskill
from the i-hear-heisenberg-was-a-good-cook dept.
summerbreeze writes "Researchers at the University of Toronto have conducted a two-slit experiment, published in Science, that uses 'weak measurement' on photons to push back the boundaries of what can be known about them, given the Heisenberg Uncertainty Principle. Jason Palmer does a great job reporting this experiment to us mere mortals in a BBC article: '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."
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## Using Averages To Bend the Uncertainty Principle

• #### Another one!?!?! (Score:4, Interesting)

on Saturday June 04, 2011 @02:31PM (#36337674) Homepage Journal

Canada certainly does punch above it's weight in many areas...

But this is a really interesting experiment! It really does turn the classic double slit experiment on it's ear!

• #### Experiment is about "Measurement" not UP (Score:5, Interesting)

on Saturday June 04, 2011 @03:19PM (#36337880) Homepage

Averaging over many measurements won't allow you to "defeat" uncertainty principle, as uncertainty principle tells you the width of the distribution (of measurements). If you wanted to get a precise measurement of the center of that distribution, yes, you can take many averages and reduce the error on that (see error of the mean [wikipedia.org]), but the width of the distribution (given by uncertainty principle), remains unchanged.

A consequence of the quantum mechanical uncertainty principle is that one may not discuss the path or “trajectory” that a quantum particle takes, because any measurement of position irrevocably disturbs the momentum, and vice versa. Using weak measurements, however, it is possible to operationally define a set of trajectories for an ensemble of quantum particles. We sent single photons emitted by a quantum dot through a double-slit interferometer and reconstructed these trajectories by performing a weak measurement of the photon momentum, postselected according to the result of a strong measurement of photon position in a series of planes. The results provide an observationally grounded description of the propagation of subensembles of quantum particles in a two-slit interferometer.

It looks like the goal of experiment is to nail down (or get further in nailing down) what constitutes "measurement [wikipedia.org]". But I'm still trying to figure out how this experiment is different from the standard QND [wikipedia.org] (which doesn't claim not to collapse the wavefunction as all measurements ought to).

• #### Re:I don't get it (Score:5, Interesting)

on Saturday June 04, 2011 @03:48PM (#36338000) Homepage

The fun thing is that you can do this with photons which were gravitational lensed around both sides of a galaxy and *still* collapse the wave function. Your measurement instantly changes something which happened a billion years ago (the lensing).

• #### Re:I don't get it (Score:4, Interesting)

on Saturday June 04, 2011 @04:14PM (#36338140)

Quantum mechanics is a statistical theory, valid only in the statistical limit of an infinite number of measurements and looking at the ensemble. It actually places no inherent limits on a single measurement, only on an ensemble of measurements. Hence, you have no violation of the uncertainty principle because you are tracking individual photons or a very small number of them. The Stern-Gerlach experiment back in the day observed individual particle strikes but when viewed as a large average you had the interference pattern characteristic of wave phenomena, while the individual flashes on the phosphor screen indicated a particle nature.

That's absurd. The interference patterns in the two-slit experiment are still created even when the intensity is reduced to the point that there is never more than one photon traversing the slits at a time. The QM rules apply to every wavicle, not just to aggregations.

You are misinterpreting Stern-Gerlach which also shows that each particle has quantized values for angular momentum and hence meets QM predictions.

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