Sunday, 22 July 2007

Quantum Mechanics again: point for experimentalists

Understanding the basic principles of QM is really difficult. The philosophical and theoretical discussions, even those coupled with a lot of more or less developed mathematics are only adding to the confusion. Of course - they are needed - because thanks to such theoretical musings as EPR's or Bell's the path may be opened for experimental evidence. And such evidence, very often, is more surprising than we could expect.

A fine example is provided by
Jacques, V.; Wu, E.; Grosshans, F.; Treussart, F.; Grangier, P.; Aspect, A. & Roch, J. Experimental realization of Wheeler's delayed-choice gedanken experiment. Science, 2007, 315, 966-968

Let me just quote here the conclusions of the paper:
Our realization of Wheeler’s delayed choice Gedanken Experiment demonstrates beyond any doubt that the behavior of the photon in the interferometer depends on the choice of the observable which is measured, even when that choice is made at a position and a time such that it is separated from the entrance of the photon in the interferometer by a space-like interval. In Wheeler’s words, since no signal traveling at a velocity less than that of light can connect these two events, “we have a strange inversion of the normal order of time. We, now, by moving the mirror in or out have an unavoidable effect on what we have a right to say about the already past history of that photon”. Once more, we find that Nature behaves in agreement with the predictions of Quantum Mechanics even in surprising situations where a tension with Relativity seems to. appear

I wonder if this result will hold on repetitions. If yes, then this would reaffirm that we have a lot to understand yet. Especially about time in Quantum Mechanics.


Xawer said...

Few comments from ex-experimentalist:

1. The presented experiment is not quite equivalent to Wheelers's Gedankenexperiment.

Wheeler had output beamsplitter inserted or not, while here we have the output beamsplitter always in place, but sometimes rotated by 45°. Actually, it is even a little bit more complicated. The expected result should be the same, but, if we believe in such 'shoulds' (depending on our belief in QED) we should stay with cheap Gedankenexperimenten.

2. This is where I see the misunderstanding leading to paradox:
Fig.1: In the open configuration, detectors D1 and D2 allow one to unambiguously determine which path has been followed by the photon. In the closed configuration, detection probabilities at D1 and D2 depend on the phase-shift between the two interfering arms.
So the paradox is only in minds of authors of the article (and Wheeler, Einstein...) They think about the experiment as like they had Newton and Huygens invited for help, so they need to move backward in time to ask either one of them to flash his torch.

3. What this experiment shows to me are two phenomena:

A. Light interferes if we do not play with polarization, but the interference vanishes when we rotate the polarization on one of the paths by 90°.
Nothing new for 300 years.

B. Light interferes even if we deal with just single photons at a time.
And here we have the philosophical difficulty: the light behaves as waves, but interacts as particles. It is really hard to internalize, most of us can't believe it or intuitively rejects such idea. But this is how the Nature works, so we should humbly accept it...


Thanks for reading on my old question - how to generate single photons!

Xawer said...

Ex-experimentalist's remarks, cont'd.

After a while later than I wrote the previous comments, my subconsciousness started to kick my mind: in abstract from Wheeler's philosophical questions, something must be totally wrong about French experiment! My critics is not related to philosophical implications of Wheeler's Gednakenexperiment at all.

Frenchmen call their strange setup 'Mach-Zender interferometer'. (See
False! Mach used half-silvered mirrors to split the beam. Frenchmen use the polarizing beamsplitters instead. It was not enough for them to spoil Wheeler's idea - they had to put λ/2-plate as well.
In effect they managed to build funny, adjustable (by changing Path_1-Path_2 optical difference, which is not depicted on their drawing, but referred in the text as Φ) polarizator. This is a good exercise at high-school student level - show, that depending on Φ outcoming light will range from not polarized at all (Φ=0, Φ=180°) to fully polarized as \ (Φ=90°) or as / (Φ=270°).
Then we have EOM - adjustable polarization plane rotator.

If we do not rotate the outgoing beam at all and we analyze slanted light splitting to vertical and horizontal part - we always get half of the photons going each way.
But if we rotate the polarization detector by 45° using EOM, we can see results like authors got.

So - what really this experiment shows is not very revelational: if we put the polarization aware detector in partially polarized beam of light, one photomultiplier (avalanche diode) would count more hits than the perpendicular one, but if we rotate the detector at 45° angle to the beam polarization plane – both would count the same number of hits.

What this experiment can teach us about philosophy of Quantum Mechanics?