Remember learning about interference through the double-slit experiment? A century after it was first used to probe the duality of waves and particles, a modified version of the experimentâ€”using three slitsâ€”has placed a limit on the accuracy of one of the laws of quantum mechanics.

Urbasi Sinha and colleagues at the Institute for Quantum Computing of the University of Waterloo (Ontario, Canada) tested the quantum interference from the slits to see whether it occurred due to pairs of paths, as predicted by an axiom known as Born's rule, or whether it produced higher-order interference that violated the axiom.

Born's rule, named after the physicist Max Born (1882-1970), predicts the probability of a certain outcome in quantum mechanics. Specifically, the team found that the probability of the higher-order interference was less than 10^{-2} of the expected interference from pairs of paths (Science **329**, 418).

The researchers created three slits 30 µm wide and 300 µm long, spaced 100 µm apart, and capable of being masked in varying combinations. Through a single-mode fiber, the team sent light from a heralded single-photon source to the slits. (“Heralded” means that the photons from a 405-nm laser pass through a nonlinear crystal, and through parametric downconversion they become two red photons. A beam splitter sends one red photon to the slits and another goes directly to the detector.)

When one is trying to measure an expected null result, one won't actually measure “zero,” but will measure something very small, Sinha said. Analyzing the systemic and random errors was a painstaking six-month-long task.

Since the double-slit experiment has been studied so thoroughly, why has no one added a third slit before? Perhaps someone did but never published their work. “Even if someone did multi-slit, they really didn't report about it to the community,” Sinha said.

The work has broader implications beyond taking an undergraduate experiment to the next level. Physicists are still trying to unify the theories of quantum mechanics and general relativity, and if Born's rule had needed tweaking, the modification could have large repercussions in studies of quantum gravity and quantum computing.

Patricia Daukantas is the senior writer/editor of *Optics & Photonics News*.