Going through the news, I came across this article, suggesting that some Canadian researchers had “overcome some important challenges of the Principle”. The rest of the article promptly went zipping right over my head.
I know that science reporting can be rather dodgy, and headlines especially often mislead. So: could one of you fine smart people bring this down to my level, and tell me whether we really are about to know where something is and where it’s going at the very same time?
I’ll give it a go:
The first thing to say is that the Heisenberg uncertainity principle is not violated, ratehr a clever concept called weak measurement is used.
Assume we’re trying to measure position and momentum at the same time, which we are forbidden from doing by basic QM as any measurement of one after measuring the other will necessarily change the quantum state. So instead we measure something called the weak value which is a complex number whose real part corresponds to the position and whose imaginary part corresponds to the momentum. Taking a single weak measurement won’t tell us much though as there is necessarily a large amount of uncertainty in any weak measurement (if the uncertainty is too small it becomes a normal QM ‘strong measurement’).
However if we repeatedly run an experiment on a system which we prepare so that it always has the same initial state and we post-select so that it always has the same final state and in between the initial and final states taking a weak measurement, we can average out the uncertainty in our measurement of the weak value so we can can obtain an arbitrarily accurate value for the average weak value in our repeated experiment and thus arbitrarily accurate values for for position and momentum.
This at first sight may appear to violate the HUP, but it doesn’t because at no stage in the weak measurement are you strongly measuring the position and the momentum and the values have a different physical meaning from the ones you would obtain from an experiment that demonstrated the uncertainty principle.
BTW for anyone feeling brave enough to read the actual paper which the article is about, it is available on arXiv:
As I try to slowly decompose this article by dripping my weak intellectual acids over it, how important is this work? The paper says it’s pathbreaking–a not unusual claim. What-all will be done with it? (Of course I’m quering Doper physicists to elaborate on the conclusions of the authors.)
It’s fairly important in that it demonstrates a facet of weak measurement, an idea that has been around since the 1980s, but has only recently started to be demonstrated experimentally.
Weak measurement is a subject that is quite popular at the moment and it could have implications for quantum computing, as well as being interesting from an ontological point of view, particularly how it relates to Bohmian mechanics and many worlds theory.
So it’s a 2010’s style “Heisenberg Compensator”?
That’s really the crucial part: any single weak measurement is completely uninformative, in the sense that, say, the distance some pointer (i.e. something like the needle on a scale) moves is completely lost in the noise. It’s only in the limit of many measurements that you can recover (or perhaps, reconstruct) information about the ensemble. But this opens up a lot of questions, few of which (to my knowledge) have so far been resolved, among them: what, exactly, is a weak value the value of? The problem is, weak values have some pretty strange properties—they may be complex, lead to negative probabilities (what’s less than zero chance?), and many other such niceties. Weak measurements are often claimed to resolve some long-standing ‘paradoxes’ of quantum theory; but I think that they, taken at face value—as some property possessed by a quantum state, or at least an ensemble thereof—, probably introduce as many puzzles as they solve. And if we don’t take them at face value, then as what should we take them?
On the other hand, I can’t claim to have reviewed the literature to any great extent, so if there’s some clarification I’m missing, I’d be very grateful for some pointers (‘understand weak measurement’ has been on my to-do list for at least a year now, but somehow I always find something else to occupy myself with).