OK, here is some pretty wild stuff about entangled photons. In an experiment reported in the Preprint Archives last week, a team of distinguished physicists are continuing to push the bounds of what we can understand as "entanglement". They demonstrate entanglement of 2 photons which have never interacted in the past! Here is the abstract: Article: Entangling Independent Photons by Time Measurement by Matthaeus Halder, Alexios Beveratos, Nicolas Gisin, Valerio Scarani, Christoph Simon, Hugo Zbinden "A quantum system composed of two or more subsystems can be in an entangled state, i.e. a state in which the properties of the global system are well defined but the properties of each subsystem are not. Entanglement is at the heart of quantum physics, both for its conceptual foundations and for applications in information processing and quantum communication. Remarkably, entanglement can be "swapped": if one prepares two independent entangled pairs A1-A2 and B1-B2, a joint measurement on A1 and B1 (called a "Bell-State Measurement", BSM) has the effect of projecting A2 and B2 onto an entangled state, although these two particles have never interacted or shared any common past[1,2]. Experiments using twin photons produced by spontaneous parametric down-conversion (SPDC) have already demonstrated entanglement swapping[3-6], but here we present its first realization using continuous wave (CW) sources, as originally proposed. The challenge was to achieve sufficiently sharp synchronization of the photons in the BSM. Using narrow-band filters, the coherence time of the photons that undergo the BSM is significantly increased, exceeding the temporal resolution of the detectors. Hence pulsed sources can be replaced by CW sources, which do not require any synchronization[6,7], allowing for the first time the use of completely autonomous sources. Our experiment exploits recent progress in the time precision of photon detectors, in the efficiency of photon pair production by SPDC with waveguides in nonlinear crystals, and in the stability of narrow-band filters. This approach is independent of the form of entanglement; we employed time-bin entangled photons at telecom wavelengths. Our setup is robust against thermal or mechanical fluctuations in optical fibres thanks to cm-long coherence lengths. " In other words: A1 and A2 share a common past, B1 and B2 share a common past, A1 and B1 are made to interact at a later time in such a way as to make them indistinguishable, and the result is that space-like separated A2 and B2 are now entangled (at least partially, but to a level that can be demonstrated). It really makes one think about what entanglement means, as if things weren't strange enough already! And by the way, the results are fully in keeping with Quantum Mechanics.