Proc. Natl Acad. Sci. http://doi.org/j76 (2013)

The 'quantum eraser' is a quirky twist to the double-slit experiment: knowing which path the photon took through the slits destroys the interference pattern; conversely, by erasing the 'which path' information the interference pattern is recovered — and that remains true whether the decision to erase (or not) is made using a past or a future measurement on a distant entangled particle. Such delayed-choice arrangements do not, however, exclude the possibility of hidden (although not faster-than-light) communication between the choice and interference event.

Xiao-Song Ma and colleagues have demonstrated a quantum-eraser experiment in which the decision is causally disconnected — communication between choice and interference is impossible because the entangled photons are so far separated that any information transfer would have to be superluminal. This was tested in different scenarios using an interferometric set-up, with entangled photon pairs distributed over 55 m of optical fibre or 144 km of free space. Their results suggest that we should abandon our classically rooted view of causally connected quantum events, as these seem to be independent of space-time.