localityloophole

The locality loophole is a potential shortcoming in Bell-test experiments that could allow correlations between distant measurement outcomes to be explained by subluminal communication rather than by quantum entanglement. In such tests, two observers (often called Alice and Bob) measure parts of an entangled system at separate locations. If the choices of measurement settings or the measurement events themselves are not spacelike separated, signals confined to the light cone could travel between sites and influence results, offering a local realistic explanation for the observed correlations.

In practice, closing the locality loophole requires that the choice of what to measure at each site and the actual measurement event occur so that no signal moving at or below the speed of light could connect them within the relevant time window. This typically involves large separations, fast and independent random-number generators to select measurement settings, rapid measurement switching, and precise timing coordination. The aim is to ensure that the measurement at one site cannot be causally influenced by events at the other site during the measurement interval.

Notable progress includes experiments that sought to close the locality loophole by achieving spacelike separation between setting choices and detections. Some tests also addressed other loopholes, such as low detector efficiency, leading to “loophole-free” or “device-independent” Bell tests when both the locality and detection loopholes are simultaneously closed. These efforts have reinforced the view that quantum correlations cannot be easily explained by local hidden-variable theories under properly designed conditions.

See also: Bell test, detection loophole, loophole-free Bell test, spacelike separation.