Some pesky loopholes no longer plague a crucial test for assessing the weirdness of quantum mechanics. Experiments reported in 2015 definitively demonstrate that the quantum world violates locality, the principle that events sufficiently separated in spacetime must be independent. “It’s a landmark result,” says Matthew Leifer, a quantum physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada.
The experiments execute a test proposed by physicist John Bell in 1964 to evaluate locality by performing the quantum equivalent of repeatedly flipping two coins simultaneously. If locality applies to the microscopic world, then seeing one coin land heads offers no insight into the landing face of the other coin. There would be a limit to how often one coin’s face corresponded with the other’s. But Bell showed that if the coins were entangled — if they had that mysterious non-local connection that Einstein referred to as “spooky action at a distance” — then the limit would no longer hold.
Rather than observing coins, Bell tests seek correspondences between measurements of properties such as particles’ spins. Despite dozens of Bell experiments that have found locality violation, ardent skeptics weren’t all convinced. No single test had used detectors efficient enough to measure almost every pair of particles that had become entangled and had ruled out the possibility that the particles were communicating at light speed. Until now.
European researchers this year performed an experiment with electrons on opposite sides of a university campus, nearly 1.3 kilometers apart (SN: 9/19/15, p. 12). In trials lasting 18 days, the team coaxed the electrons into an entangled state 245 times, reliably measuring the electrons’ spins every time. The results showed a clear nonlocal connection.
Two subsequent loophole-free tests measured photons rather than electrons and came to the same conclusion but with stronger data, having measured far more entangled particles than the first team (SN Online: 11/13/15).
Such tests will continue: One team plans to eliminate the possibility of another local effect by using light from distant galaxies to determine which properties of the particles will be measured.