The latest test in the validity of one of the fundamental postulates of quantum mechanics has been done, and QM wins once again!
"In principle, this experiment is very simple," says Gregor Weihs “and we were quite surprised to find that nobody hadn’t performed this experiment before." However, the physicists were struggling with measurement errors, which they were eventually able to overcome during their two year long Sisyphean task. "Our measurements show that we can rule out the existence of third-order interference up to a certain bound," says a happy experimental physicist Weihs. His next step will be to considerably lower the bound with an improved experiment.
U.Sinha, et al., Science v.329, p.418 (2010).
More review of this work at Nature (link open for free only for a limited time) and at PhysicsWorld.