Transforming Resonance Fluorescence into Maximally Entangled Photon Pairs Using Minimal Resources

Entanglement is a fundamental concept in quantum mechanics, describing two or more quantum systems that exhibit strong correlations beyond the classical limits at the expense of losing their individual properties. More recently, it has become a cornerstone of quantum technologies, promising revolutionary advancements in fields like quantum communication, sensing, and computation. For these reasons, the generation of technologically useful entangled states is key to progress in these fields. Here, we experimentally demonstrate that resonance fluorescence from a weakly coupled two-level emitter can be transformed, using beam splitters, delay lines, and post-selection only, into a stream of pairs of photons that are maximally entangled in the time-bin basis. We verify the entanglement via a CHSH-type Bell inequality test, yielding an S-parameter of 2.80 \pm 0.19, i.e., a clear 4{\sigma} violation of the classical bound. Our results pave the way for realising efficient sources of bandwidth-limited time-bin entangled photon pairs.