Quantum entanglement just got more exciting.
An experiment by Hagai Eisenberg extends the idea of "spooky action at a distance" to time by entangling a pair of photons that never coexisted.
If you have two pairs of entangled photons, taking one photon from each pair and entangling them disengages the two original pairs, and creates a second, fresh entanglement between the two, left out photons. Eisenberg's team used the swap to entangle a photon with one that no longer existed.
They started with an entangled pair of photons, 1 and 2, and then measured the quantum state of photon 1, which destroys the particle. Photon 2, however, lived on and, about 100 nanoseconds later, the team created a new pair of entangled photons, 3 and 4.
When the team entangled photon 2 with newborn photon 3, photon 4 also became entangled with photon 1 - even though 1 was by then "dead" (see diagram).
The team knew 4 was entangled with 1 by measuring 4's state, which depended on the states measured for 1, 2 and 3 (arxiv.org/abs/1209.4191v1). "Without the idea of entanglement, you cannot explain it," says von Zanthier, who was not involved in the latest experiment. "The future photon, which is not born, is strongly influenced by a photon that is already dead."
That's very interesting.