Quantum entanglement is a state where two particles have correlated properties: when you make a measurement on one, it constrains the outcome of the measurement on the second, even if the two particles are widely separated. It's also possible to entangle more than two particles, and even to spread out the entanglements over time, so that a system that was only partly entangled at the start is made fully entangled later on.

This sequential process goes under the clunky name of "delayed-choice entanglement swapping." And, as described in a Nature Physics article by Xiao-song Ma et al., it has a rather counterintuitive consequence. You can take a measurement before the final entanglement takes place, but the measurement's results depend on whether or not you subsequently perform the entanglement.

Delayed-choice entanglement swapping consists of the following steps. (I use the same names for the fictional experimenters as in the paper for convenience, but note that they represent acts of measurement, not literal people.)

Two independent sources (labeled I and II) produce pairs photons such that their polarization states are entangled. One photon from I goes to Alice, while one photon from II is sent to Bob. The second photon from each source goes to Victor. (I'm not sure why the third party is named "Victor".) Alice and Bob independently perform polarization measurements; no communication passes between them during the experiment—they set the orientation of their polarization filters without knowing what the other is doing. At some time after Alice and Bob perform their measurements, Victor makes a choice (the "delayed choice" in the name). He either allows his two photons from I and II to travel on without doing anything, or he combines them so that their polarization states are entangled. A final measurement determines the polarization state of those two photons.

The results of all four measurements are then compared. If Victor did not entangle his two photons, the photons received by Alice and Bob are uncorrelated with each other: the outcome of their measurements are consistent with random chance. (This is the "entanglement swapping" portion of the name.) If Victor entangled the photons, then Alice and Bob's photons have correlated polarizations—even though they were not part of the same system and never interacted.

The practicalities of delayed-choice entanglement swapping bears many similarities to other entanglement experiments. Ma et al. sent pulsed light from an ultraviolet laser through two separate beta-barium borate (BBO) crystals, which respond by emitting two photons with entangled polarizations, but equal wavelength. The BBO crystals acted as the sources labeled I and II above; the oppositely polarized photons they produced were sent down separate paths. One path for each BBO crystal led to a polarization detector ("Alice" and "Bob"), while the other passed through a fiber-optic cable 104 meters long before arriving at the "Victor" apparatus.

That little bit of cabling was enough to ensure that anything that happened at Victor occurred after Alice and Bob had done their measurements.

The choice about entangling the photons at the Victor apparatus was made by a random-number generator, and passed through a tunable bipartite state analyzer (BiSA). The BiSA contained two beam-splitters that select photons' paths depending on their polarization, along with a device that rotated the polarization of the photons. Depending on the "choice" to entangle or not, the polarization of the photons from I and II were made to correlate or left alone. Finally, the polarization of both photons at Victor were measured, and compared with the results from Alice and Bob.

Due to the 104-meter fiber-optic cable, Victor's measurements occurred at least 14 billionths of a second after those of Alice and Bob, precluding the idea that the setting of the BiSA caused the polarization results to change. While comparatively few photons made it all the way through every step of the experiment, this is due to the difficulty of measurements with so few photons, rather than a problem with the results.

Ma et al. found to a high degree of confidence that when Victor selected entanglement, Alice and Bob found correlated photon polarizations. This didn't happen when Victor left the photons alone.

Suffice it to say that facile explanations about information passing between Alice's and Bob's photons lead to violations of causality, since Alice and Bob perform their polarization measurement before Victor makes his choice about whether to entangle his photons or not. (Similarly, if you think that all the photons come from a single laser source, they must be correlated from the start, and you must answer how they "know" what Victor is going to do before he does it.)

The picture certainly looks like future events influence the past, a view any right-minded physicist would reject. The authors conclude with some strong statements about the nature of physical reality that I'm not willing to delve into (the nature of physical reality is a bit above my pay grade).

As always with entanglement, it's important to note that no information is passing between Alice, Bob, and Victor: the settings on the detectors and the BiSA are set independently, and there's no way to communicate faster than the speed of light. Nevertheless, this experiment provides a realization of one of the fundamental paradoxes of quantum mechanics: that measurements taken at different points in space and time appear to affect each other, even though there is no mechanism that allows information to travel between them.

Nature Physics, 2012. DOI: 10.1038/nphys2294 (About DOIs).