To illustrate the idea behind the test, imagine two stacks of playing cards. One contains only red cards; the other contains only aces. You're given a card and asked to identify which deck it came from, says Martin Ringbauer, a physicist also at the University of Queensland. If it is a red ace, he says, there's an overlap and you won't be able to say where it came from. But if you know how many of each type of card is in each deck, you can at least calculate how often such ambiguous situations will arise.
Out on a limb
A similar ambiguity occurs in quantum systems. It is not always possible for a single measurement in the lab to distinguish how a photon is polarized, for example. In real life, it's pretty easy to tell west from slightly south of west, but in quantum systems, it's not that simple, says White. According to the standard Copenhagen interpretation, there is no point in asking what the polarization is because the question does not have an answer or at least, not until another measurement can determine that answer precisely.
But according to the wavefunction-as-ignorance models, the question is perfectly meaningful; it is just that the experimenters like the card-game player do not have enough information from that one measurement to answer. As with the cards, it is possible to estimate how much ambiguity can be explained by such ignorance, and compare it with the larger amount of ambiguity allowed by standard theory.
That is essentially what Fedrizzi's team tested. The group measured polarization and other features in a beam of photons and found a level of overlap that could not be explained by the ignorance models. The results support the alternative view that, if objective reality exists, then the wavefunction is real. It's really impressive that the team was able to address a profound issue, with what's actually a very simple experiment, says Andrea Alberti, a physicist at the University of Bonn in Germany.
The conclusion is still not ironclad, however: because the detectors picked up only about one-fifth of the photons used in the test, the team had to assume that the lost photons were behaving in the same way
7. That is a big assumption, and the group is currently working on closing the sampling gap to produce a definitive result. In the meantime, Maroney's team at Oxford is collaborating with a group at the University of New South Wales in Australia, to perform similar tests with ions, which are easier to track than photons. Within the next six months we could have a watertight version of this experiment, says Maroney.
But even if their efforts succeed and the wavefunction-as-reality models are favoured, those models come in a variety of flavours and experimenters will still have to pick them apart.
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