Less than zero photons?

Quantum physics and reality | I'm not looking, honest! | The Economist

Not all that many places picked up on this report that appeared in The Economist, of all places, probably because it is hard to understand what it means.

It seems that a team of clever Japanese physicists have confirmed another team's experiment in which they, in effect, directly observed a quantum paradox (called Hardy's paradox. No, I hadn't heard of it before either.)

The implications, according to the magazine are:

They managed to do what had previously been thought impossible: they probed reality without disturbing it. Not disturbing it is the quantum-mechanical equivalent of not really looking. So they were able to show that the universe does indeed exist when it is not being observed.

Well, that's encouraging, I suppose.

But the physical meaning of what they observed seems very unclear:

What the several researchers found was that there were more photons in some places than there should have been and fewer in others. The stunning result, though, was that in some places the number of photons was actually less than zero. Fewer than zero particles being present usually means that you have antiparticles instead. But there is no such thing as an antiphoton (photons are their own antiparticles, and are pure energy in any case), so that cannot apply here.

The only mathematically consistent explanation known for this result is therefore Hardy’s. The weird things he predicted are real and they can, indeed, only be seen by people who are not looking. Dr Yokota and his colleagues went so far as to call their results “preposterous”.

That word appears in the abstract to the paper:

Unlike Hardy's original argument in which the contradiction is inferred by retrodiction, our experiment reveals its paradoxical nature as preposterous values actually read out from the meter.

The Science Daily version of the story doesn't add much.

This is all very interesting, but it seems to me that no one is doing a good job of explaining what it means from the point of view of the understanding of quantum physics and reality. I have some questions:

1. Is this relevant to the question of whether the Copenhagen interpretation of quantum mechanics is correct?

2. Is it relevant to the more famous quantum paradox of Schrodinger's cat? (My initial hunch is that it tells us the cat is really there, and it is both dead and alive!)

Philosophers of the quantum world, get to it!