
Scientists reveal 'quantum pigeonhole principle' - jonbaer
http://www.dailymail.co.uk/sciencetech/article-3409392/Forget-Schrodingers-cat-researchers-reveal-quantum-pigeonhole-principle-say-tests-basic-notion-physics.html
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acqq
The text by the New Scientist, now retold by the Daily Mail (why now? just the
"content spam" I guess), is from July 2014 (!):

[https://www.newscientist.com/article/mg22329802-300-pigeon-p...](https://www.newscientist.com/article/mg22329802-300-pigeon-
paradox-reveals-quantum-cosmic-connections/)

It would be surprising that DM covers such a subject first?

The PDF of the work:

[http://arxiv.org/abs/1407.3194](http://arxiv.org/abs/1407.3194)

The first and the third listed authors:

[https://en.wikipedia.org/wiki/Yakir_Aharonov](https://en.wikipedia.org/wiki/Yakir_Aharonov)

[http://www.bristol.ac.uk/physics/people/sandu-
popescu/](http://www.bristol.ac.uk/physics/people/sandu-popescu/)

~~~
mratzloff
I don't quite understand the pigeonhole concept as it relates to the pigeons.
Is the thought experiment saying that if they check each pigeonhole they'll
only ever find one pigeon? Even with infinite pigeons, they'll only find one?
(And presumably all the other pigeons will occupy the other roost?) I'm not
quite following.

~~~
acqq
The start of the PDF of the authors, better than the articles:

"The pigeonhole principle: "If you put three pigeons in two pigeonholes at
least two of the pigeons end up in the same hole" is an obvious yet
fundamental principle of Nature as it captures the very essence of counting.
Here however we show that in quantum mechanics this is not true! We find
instances when three quantum particles are put in two boxes, yet no two
particles are in the same box."

That's quantum physics. Our experiences from the "big world" don't match
what's going on there. The "electron" is a particle (called so for our
convenience, like other particles) but it doesn't behave like anything from
the macroworld. And take care finding what the "boxes" actually in this case
are.

------
Strilanc
I've tried to figure out what exactly the paper is saying.

They start by noting that this circuit:

    
    
        │0〉-H─•───√X-[=1!]─
        │0〉─H─┼─•─√X─[=1!]─
        │0〉─H─┼─┼─√X─[=1!]─
        │0〉───X─X────────── (will now =1 when measured)
    

Which puts three qubits each into the state │0〉+ │1〉, stashes the parity of
two of those qubits' values onto a scratch line, then performs a post-
selection along the Y axis of all three qubits. The interesting thing is that
the stashed parity ends up always equal to 1, indicating that the two qubits
had opposite value.

The authors then start talking about this meaning that "all the qubits are
unequal" because no matter which parity you stash, you end up discovering the
two qubits were not equal. I think that's a bit of a stretch, framing-wise.
It's the wrong way to think about it. For example, it stops working if you
stash two of the parities instead of just one (you'll start getting "they were
equal" results half of the time).

Instead of talking about the qubits all being unequal, I prefer to think of
this in terms of winning a game against a referee. You get three coins and can
choose if each is set to heads or set to tails. Then a referee secretly picks
two of the coins at random, measures their parity, and otherwise does nothing
except pass the coins back to you. Then you do whatever you want to the coins,
but have to choose whether or not to "keep" this run or "discard" it. You win
if, whenever you say "keep", the referee wrote down "the coins differed".
(Also you can't just never say "keep".)

Classically, there's no strategy that wins the game 100% of the time. But if
you're using qubits instead of coins, quantum 2-level systems instead of
classical 2-level systems, you _can_ win 100% of the time (by using the
circuit described above). In fact, the strategy works even if you have _a
million_ coins/qubits instead of just three!

------
macawfish
Daily Mail science articles crack me up... especially when they use the word
"boffins" (sadly they didn't in this particular article.)

