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. Credit: Phys. Rev. Lett. 110, 210403 (2013) DOI: 10.1103/PhysRevLett.110.210403 (source article is pay-walled)
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