"We designed an apparatus to measure a property -- the polarization -- of a single photon. We then needed to measure how much that apparatus disturbed that photon," says Lee Rozema, a Ph.D. candidate in Professor Aephraim Steinberg's quantum optics research group at U of T, and lead author of a study published this week in Physical Review Letters.Hmm, it seems to me like the claim of having beaten Heisenberg may be an overstatement here. By repeating the experiment many times, they are in effect averaging out the randomness added by the weak measurements. What we see in the macro world is what happens when all the uncertainty is averaged out by the large number of events. Hence, we can effectively know exactly where a car is and how fast it's going at the same time without running into any uncertainty. Averaging many single events has the same effect.
"To do this, we would need to measure the photon before the apparatus but that measurement would also disturb the photon," Rozema says.
In order to overcome this hurdle, Rozema and his colleagues employed a technique known as weak measurement wherein the action of a measuring device is weak enough to have an imperceptible impact on what is being measured. Before each photon was sent to the measurement apparatus, the researchers measured it weakly and then measured it again afterwards, comparing the results. They found that the disturbance induced by the measurement is less than Heisenberg's precision-disturbance relation would require.
"Each shot only gave us a tiny bit of information about the disturbance, but by repeating the experiment many times we were able to get a very good idea about how much the photon was disturbed," says Rozema.
Sunday, September 9, 2012
Scientists Uncertain About Uncertainty
Scientists Cast Doubt On Heisenberg's Uncertainty Principle