Experimental proof of nonlocal wavefunction collapse for a single particle using homodyne measurements

A single quantum particle can be described by a wave function that spreads over arbitrarily large distances; however, it is never detected in two (or more) places. Here we rigorously demonstrate the nonlocal collapse of the wave function, with no efficiency loophole. We split a single photon between two laboratories and experimentally test whether the choice of measurement in one laboratory causes a change in the local quantum state in the other laboratory. To this end, we use homodyne measurements with six different measurement settings and quantitatively verify Einstein’s "spooky action" by violating an Einstein–Podolsky–Rosen-steering inequality by 0.042±0.006. Moreover, we show theoretically that the best previous experimental results involving a split single particle could not possibly be used to demonstrate EPR-steering. Our experiment is thus the first that verifies the entanglement of the split single photon even when one side is untrusted.