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A recent experiment shows that it is possible to entangle two particles that never co-existed in time.


Time line diagram. (I) Birth of photons 1 and 2, (II) detection of photon 1, (III) birth of photons 3 and 4, (IV) Bell projection of photons 2 and 3, (V) detection of photon 4. Source:

Entanglement Swapping between Photons that have Never Coexisted. E. Megidish et al. Phys. Rev. Lett. 110, 210403 (2013), arXiv:1209.4191.

and its summary in (B. Yirka, 28 May 2013).

In this experiment:

To prove it, the researchers first used a laser to cause entanglement between a pair of photons, P1, P2. They then measured the polarization of P1, which was immediately followed by the entangling of another pair of photons, P3, P4. This was followed by measuring P2 and P3 simultaneously and causing them to become entangled with one another—a process known as projective measurement. Then, P4 was measured. Measuring P1 caused its demise of course—before P4 was born—but the measurement of P4 showed that it had become entangled with P1 nevertheless, if only for a very short period of time.

The researchers suggest that the outcome of their experiment shows that entanglement is not a truly physical property, at least not in a tangible sense. To say that two photons are entangled, they write, doesn't mean they have to exist at the same time. It shows that quantum events don't always have a parallel in the observable world.

While I don't claim to understand how entanglement works across space, I'm even more confused at how it could work across time. Is this an example of entanglement swapping? Or in a broader sense is entanglement both time and space independent? Could someone try the impossible and help me understand the idea?

Related: Entanglement in Time

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I have not tried to understand the experiment, but have just skimmed this commentary… ... The author says yes, it is entanglement swapping. He also says it could be viewed as "a quantum teleportation kind of thing, teleporting a state into the future via a chain of entanglement"... which might be what was discussed at – Mitchell Porter Sep 13 '13 at 22:38

1 Answer 1

The title of the article and the abstract of the article both say it is entanglement swapping and you described entanglement swapping, so it seems like entanglement swapping.

The authors also mention that one interpretation is the measuring the first particle on the first entangled pair steers the dynamics of the second pair. In fact, this is normal, if you have two maximally entangled particles and you measure one of them, you have now placed the other one into a definite state.

So effectively you have a photon in a definite polarization state and then you do a projection (measurement) onto a bell state. The result of the bell state measurement is to find out which bell state it is now in. And it performs an entanglement swap.

If the first pair were entangled to have the same polarization as was the second, then when you do a measurement of one of each pair into a bell state you entangle the other two.

For instance you could get a bell state where the two particles measured into the bell state have the same polarization so now the other two have the same as each other. If instead you get them in a bell state where they have opposing polarization then the other two now have opposite polarizations.

This should sound entirely undeep. And it isn't deep, it is a regular entanglement swap, and even the authors say that you can interpret it as the measurement of the first particle influencing the latter results. It isn't like the polarization measurement result on the first particle determined everything, it doesn't tell us which bell state you get, but if you did the experiment with matter then there is a well defined probability current and which part of the beam ends up where is influenced by "irrelevant" factors like if you adjust the spacing for the bell measurement to give an extra half wavelength to one of the paths to adjust which detector corresponds to which bell state.

And all the interesting quantum correlations come not from a single measurement, but from correlations of different kinds of measurements on particles 1 and 4.

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