"Experimental delayed-choice entanglement swapping"; quant. steering to past" (!)...."--(!), or (?)

Is the following abstract ($32 for full) important and exciting?

Can someone comment or background on this? I’ve been to the Wik but it’s much more fun to listen to you rap…

Experimental delayed-choice entanglement swapping

Xiao-song Ma Stefan Zotter Johannes Kofler Rupert Ursin Thomas Jennewein Časlav Brukner Anton Zeilinger
Nature Physics (2012) doi:10.1038/nphys2294
Received 23 September 2011 Accepted 16 March 2012 Published online 22 April 2012 Corrected online 26 April 2012
Motivated by the question of which kind of physical interactions and processes are needed for the production of quantum entanglement, Peres has put forward the radical idea of delayed-choice entanglement swapping. There, entanglement can be ‘produced a posteriori, after the entangled particles have been measured and may no longer exist’. Here, we report the realization of Peres’s gedanken experiment. Using four photons, we can actively delay the choice of measurement—implemented through a high-speed tunable bipartite-state analyser and a quantum random-number generator—on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one of two mutually exclusive quantum states in which the photons are either entangled (quantum correlations) or separable (classical correlations). This can also be viewed as ‘quantum steering into the past’.

Yeah, that’s some pretty hot stuff.

That wasn’t a whoosh, was it?

Well, Nature Physics claims to have the highest impact score of any physics journal, so we can at least say it was among the most important papers in physics that month. (Or week. I don’t know how frequently it publishes.)

Other than that, I got nothin’.

But I share your curiosity on what people who know what they’re talking about think!

I’ve got only time for a brief answer, but I’ll elaborate if any questions are left open…

Basically, what they’re doing is nothing new, in the sense that it’s not surprising – essentially, they’ve realized a thought experiment from Asher Peres from 1999, I believe, and nobody was having much doubt that things would work out as predicted. (By the way, here’s the article in free preprint form, and here’s Peres’ original work.)

What they’re testing, however, is a somewhat counterintuitive effect (well, a lot of quantum effects are), and is unfortunately liable to over-interpretation. The key point in their setup is the delayed-choice component, which basically says that in the quantum world, you can determine (in some sense) a measurement result after the measurement has taken place – for instance, in the familiar double slit experiment, you can decide whether or not to observe a photon after it has passed the barrier, thus leading either to the observation of wave- or particle-like behaviour (roughly). This is Wheeler’s original delayed-choice experiment.

However, big caveats need to be put into place here. Most importantly, this doesn’t really lend itself to ‘influencing the past’-type interpretations; in particular, as always with entanglement, you couldn’t send any message with it, nor do anything else that’s causally efficacious. Indeed, from the point of view of entanglement the effect isn’t all that surprising: measuring one of a pair of entangled particles leads to instantaneous ‘collapse’ of the other into some definite state (if you like to talk that way, which I usually don’t), where instantaneous here means without delay across arbitrary distances – but according to special relativity, of course, this is in some sense equal to action ‘backwards into the past’ (if two parties, A and B, share an entangled pair of qubits, and both measure at the same time in their shared frame of reference, there are inertial observers seeing both A measure first, causing the collapse of B’s particle, and seeing B measure first).

So in that sense, it’s not really so weird after all – at least not weirder than however weird you consider entanglement to be.

The additional twist they put onto the story is that of entanglement swapping, which is a different, counterintuitive effect in quantum mechanics. Basically, you have three parties, A, B, and C, and create two pairs of entangled photons, AC1, and BC2. A gets one of the AC1-entangled pair, while C gets the other; similarly, B gets one of the BC2 entangled pair, while C gets the other. So the situation is somewhat like this:

o~~~~~~o  o~~~~~~o
A      C1 C2     B

Now, C can make an operation on his two photons, entangling them – and as a result, A and B end up entangled with one another, as well, even though they have never interacted with one another! In the end, the situation is thus this one:

A      C1 C2    B

Now, of course, A and B can, through measurement and comparing their results afterwards, determine whether their photons were entangled.

But, and here’s the twist, C can decide whether or not to entangle the photons after both B and A have already carried out their measurements, thus entangling them a posteriori!

However, the whole shebang only becomes noticeable once all three observers meet and compare notes – in particular, C needs to tell A and B in which cases she entangled their photons. Otherwise, there are many possible ways to select subsets of ‘entangled’ states from A’s and B’s combined data, even if they have in fact only undertaken random measurements on uncorrelated systems. So the only ‘active’ role C plays, ultimately, is just that of a selector of a certain subset of A’s and B’s measurements – like in the case above, not quite the ‘influence on the past’ kind of thing that is often suggested (but still, really neat).

Seems this got a little longer than anticipated…

Interesting. Could somebody please explain what is wrong with the following train of thought?

If the delays in the Zeilinger experiment could be lengthened out a bit, then there could be time for A and B to do a long series of measurements on their particles, and form an estimate of approximately what proportion k of their particles were entangled, before C has decided what that proportion is going to be.

And it would seem that A and B could then transmit their estimate of the value of k to C; and C could then decide to make a very different from k proportion of their particles entangled!

This seems impossible, but presumably I am missing something vital.


I think, though I am not certain, that the nature of the entanglement between A’s and B’s particles depends on the nature of the entanglement-transference that C performs, and that A and B can therefore not determine what proportion are entangled until they learn what C has done. But this stuff always hurts my brain.

I wasn’t aware that the Time Cube site had a forum, but I think this thread was supposed to be there…

That’s right. To A and B, the measurements seem completely uncorrelated and random; only when C tells them what she did, and more importantly, what results she got (essentially, she does Bell measurements on her photon pair), can they group their measurement results in such a way as to recognize the entangled ones.

And I don’t think that a brain has yet evolved which this stuff doesn’t hurt.

Oh, there are plenty of brains it doesn’t hurt. I explained it to my mom’s dog, and he just sniffed around a bit and licked my face.