Using Averages To Bend the Uncertainty Principle

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."

Comments

Experiment is about "Measurement" not UP

[bkpark]

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), but the width of the distribution (given by uncertainty principle), remains unchanged.

Reading the paper abstract:

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". But I'm still trying to figure out how this experiment is different from the standard QND (which doesn't claim not to collapse the wavefunction as all measurements ought to).

Re:I don't get it

[Joce640k]

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).