Einstein mockingly called it "spooky action at a distance": the finding that quantum particles can influence each other regardless of how far apart they are. We can only imagine his horror at a new experiment that extends the idea to time by entangling a pair of photons that never coexisted. As well as expanding the reach of quantum theory's baffling implications, the experiment could improve long-distance cryptography.
At the heart of the phenomena is entanglement, in which the quantum states of two entities become linked. The implications of this for spatially distant particles stumped even Einstein, but things got still stranger last year. Joachim von Zanthier of the University of Erlangen-Nuremberg in Germany and his colleagues showed that, in principle, entanglement could also work for particles that have never existed at the same time (Optics Letters, doi.org/bdwpsj).
Now Hagai Eisenberg of the Hebrew University of Jerusalem in Israel and colleagues have done the experiment, via a process called an entanglement swap.
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.
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