Polarization states "directly measured": What did this experiment do? I ran across, on phys.org, this fairly pop-sciencey RIT press release: http://www.rochester.edu/news/show.php?id=5692 It describes an experiment which sounds very interesting, but the way the experiment is described is colloquial and in general I do not trust university PR departments. I was wondering if someone could either explain the experiment or help me find a more hard-science source. The paper is in Nature Photonics according to the press release but I can't seem to find it listed in the current issue on their website. The press release describes "a recently developed technique to directly measure for the first time the polarization states of light", with applications to quantum computing. Reading this, they seem to be saying they measured two conjugate variables simultaneously, i.e., both the magnetic and the electric components of a photon's polarization at once. The article suggests this particular measurement was not previously possible (what is it you normally measure when measuring polarization-- angle?). They then go one step further though. They say they directly measured the "wavefunction" (of what?) and claim that previously, this particular measurement would have been considered not just impossible in current practice but rather impossible in theory due to the Heisenberg uncertainty principle. They describe their process as "getting around" the Heisenberg uncertainty principle. Here is the clearest statement of this I find in the press release: Now, my amateur's understanding is that the uncertainty principle does *not* say you cannot measure two conjugate variables, but rather only that there is a accuracy lower bound on measuring any particular system of conjugate variables. Moreover, when they describe what they actually did, it seems consistent with the idea that both variables were measured but neither perfectly. They describe their technique as taking several "weak" measurements of one variable, followed by a "strong" measurement of another variable (are these technical or colloquial terms?). This seems consistent with the possibility that each "weak" measurement of A fuzzed the value of B slightly, such that both were eventually measured but both with some amount of uncertainty. So this seems to me quite consistent with the HUP. However, Prof. Leach as quoted by RIT PR above says the HUP previously ruled [whatever they did] impossible. So here is what I wonder: - What exactly did they measure? Am I correct that they measured the E and M components of a single photon at once? What would it even mean to "measure a wavefunction"? - Did they actually violate the conventional understanding of the Heisenberg Uncertainty Principle in doing this? Before the technique was outlined/tried, would anyone have actually claimed that the HUP excluded this measurement? - If they are within the Heisenberg Uncertainty Principle's bounds, it seems this means that in order to get the level of accuracy they did they must have had a trade-off where they get some certainty on these two variables but sacrifice certainty somewhere else. Correct? Where then do they make this uncertainty sacrifice?