Two Nature Articles - Quick Questions

[StevieTNZ]

First, in regard to this article:
http://physicsworld.com/cws/article/news/45535 (http://www.nature.com/nature/journal/v471/n7339/full/nature09778.html)
Does the Heisenberg limit apply to single particle systems, rather than N amount of particles? I draw that conclusion from

Napolitano is keen to point out that this result does not imply that the Heisenberg uncertainty principle is wrong, but rather it shows that we do not properly understand how to scale that principle up to multiple-particle systems.

The second, and last:
http://www.pas.rochester.edu/~jordan/nature_steinberg.pdf
A weak measurement causes us to only gain little information of a quantum system?
Which would explain this? http://www.physorg.com/news137245970.html
A weak measurement doesn't cause a full collapse of the quantum system, only partial like in the PhysOrg article? So its not so much that a complete collapse has occurred, then been undone?

Katz, however, says being able to reverse the collapse "tells us that we really can't assume that measurements create reality because it is possible to erase the effects of a measurement and start again."

But only a weak measurement was done? Any other measurement than a weak measurement would cause the superposition to collapse to yield a definite state?

 

[DrChinese]

But only a weak measurement was done? Any other measurement than a weak measurement would cause the superposition to collapse to yield a definite state?

I don't believe that is the case. According to Eberly, it should be possible to reconstruct a superposition by reassembling the outputs of a polarizing beamsplitter in precisely such a fashion that which path information is erased.

http://www.optics.rochester.edu/~stroud/cqi/rochester/UR19.pdf

I don't know that this experiment has actually been performed, but I believe it follows theory in all respects.

 

[StevieTNZ]

Ah, ok. So the experiment can be thought of in terms of delayed-choice quantum eraser experiments where performing a measurement, then erasing the result (ie. undoing the measurement) causes a return of the superposition? Thanks! I wouldn't have thought of that. I guess what I'm going off is the article saying a 'partial collapse'.