Measuring photonic Bell states is a fundamental building block of many quantum communication and metrology protocols, as it certifies the presence of entanglement and it unleashes its power. However, Bell state measurements are tricky to perform in practice, especially if we want to limit the complexity of the measurement apparatus to just linear optical elements. In addition to that, the usual setup begins with interfering two photons at a single beam splitter, which creates an asymmetry between two parties who want to participate equally (unless we introduce a trusted third party, at the cost of also introducing trust). Well, no more. Kim and colleagues have realized that as the reason for an initial beam splitter is just to exchange two amplitudes, one can devise a modified Bell state analyzer where two parties exchange an amplitude (a photon path in this case) directly and infer the presence or absence of a Bell state from their joint measurement outcomes. Both parties are symmetrically involved and there is no need for trusted third parties. Remarkably, by removing the detectors, they can use this device to produce Bell states collaboratively (with 50% success probability) and part with the ubiquitous "entanglement source in the middle." This work is a rare novelty in the design of quantum optical setups.
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