2 responses

1. 

   Martin humby

   July 19, 2011 at 4:59 pm

   Hi Dave,

   Thanks for the interesting posting! Looks like Edinburgh was a first class event. Always liked Scottish curry, curry houses, and from my experience Indian waiters with broad Scottish accents – at least in Glasgow: a survival trait possibly. Really disappointed not to have heard you speak.

   Can’t say I managed to get a full grasp of the logic behind the theory from the slides but mostly my knowledge of quantum mechanics is limited to Richard P Feynman’s QED book and in general have never been able to understand why some quantum effects should be considered in any way surprising.

   In the double slit experiment for example why should it sometimes be put forward as a surprise that detecting which slit a particle goes through destroys the effect – even the statement is a misrepresentation. How can the passing of a photon or even an electron be detected without affecting the system? Clearly it can’t but that is not the point, what is this non-intrusive detector, often referred to but never defined, that might have been thought capable of doing this? For me this is a puzzlement of many years and have always been hoping to contact someone who could explain. Could you spare a minute to help me out on this one?

   For me the really exciting part is the proven possibility of extracting information from a system with out destroying the required effects making quantum computation possible – real Sci-Fi stuff. Anyway, thanks again for the posting and the slides, all the best,

   Martin.

   Reply
2. 

   davemcmahon81

   July 19, 2011 at 10:35 pm

   Hi Martin,

   “How can the passing of a photon or even an electron be detected without affecting the system?” . You’ve got to the root of the issue here. Contrary to what the popular press try to push out, quantum theory is rooted in the practicalities of measurement and the realities of the universe (as is relativity), and builds a universe based upon rigorous mathematics which has yet to be confounded by any experiment. There is no such thing as a perfect measurement,as you rightly point out, you always interact with a system to measure it, thereby changing the system, and it’s self-evident as you imply.

   Take the simple Qubit system I talk about, from this experiment it can be plainly seen that trying to detect which route the photon takes will affect the outcome. If you detect the photon coming off of the ‘North’ mirror, then the expectation value of the system is then 1. Applying a beam splitter will then put the system into expectation value of 0, and both East and North detectors pick up the photons. If NO photon comes off of the North mirror, it must have come off the Eat one, so the expectation value is -1. Put that through a beam splitter and the expectation value is 0 again. Reality and theory match.

   This interpretation makes use of this impossible, imaginary ‘non-invasive’ detector, but the result is the same when you consider an invasive detector as you would in real life.

   The reality is if you detect the photons coming East after the mirror, then they will no longer take part in the system and only the photons going north are left, so naturally the output will be photons at both detectors!

   It’s great stuff, the logic is there, pure and simple, it’s just the results are not always intuitive and are hard to interpret in terms of everyday experience. The key difference is that the mathematics of continuums as with classical physics is different to that of discreet packets or ‘quanta’. Naturally the results will differ. Quantum Theory has always won over classical theory, because it is a better representation of reality – whether we like it or not … perhaps a better theory will come along someday which will both match reality and our expectations of reality …

   Glad you liked the slides … not going to make it to Aberdeen then :-)?

   Reply