Entanglement Out of Time
A practical pointer to time as an emergent property occurs by entangling particles that don’t exist at the same time. Nonlocality can also be nontemporal.
A scheme termed entanglement swapping – chaining entanglement through time between subatomic particle pairs – has been experimentally demonstrated using 4 photons.
First, entangled photons 1 and 2 are created by zapping a special crystal with laser light. The polarization of photon 1 is measured while 1 and 2 are entangled. Photon 1 is destroyed by the measurement.
Then the entangled pair of 3 and 4 are created. Next, an entangling measurement of photons 2 and 3 is made even as it absorbs and destroys them. Finally, the polarization of photon 4 is measured.
Thanks to unavoidable uncertainty, unobserved photons on the fly are simultaneously polarized vertically and horizontally. Measuring a photon collapses its uncertainty wave function such that it will always be found to be either horizontally or vertically polarized.
Even though there is no moment in time when photons 1 and 4 coexist, they show entanglement by their measured polarization matching.
There is no moment in time in which the 2 photons coexist, so you cannot say that the system is entangled at this or that moment. ~ Israeli physicist Hagai Eisenberg