When waves — regardless of whether light or sound — collide, they overlap, creating interferences. Austrian and Canadian quantum physicists now have been able to rule out the existence of higher-order interferences experimentally and thereby have confirmed an axiom in quantum physics: Born’s rule. Austrian and Canadian quantum physicists have now been able to rule out the existence of higher-order interferences experimentally and thereby confirmed an axiom in quantum physics: Born’s rule. (Images: © IQC) In quantum mechanics, many propositions are expressed as probabilities. In 1926, German physicist Max Born postulated that the probability of finding a quantum object in a certain place at a certain time equals the square of its wave function. A direct consequence of this rule is the interference pattern shown in the double-slit diffraction experiment. Born’s rule is one of the key laws in quantum mechanics, and it proposes that interference occurs in pairs of possibilities. Interferences of higher order are ruled out. There was no experimental verification of this proposition until now, when the research group led by Gregor Weihs, professor of photonics at the University of Innsbruck, in collaboration with the University of Waterloo, confirmed the accuracy of Born’s law in a triple-slit experiment. “The existence of third-order interference terms would have tremendous theoretical repercussions — it would shake quantum mechanics to the core,” said Weihs. The impetus for this experiment was the suggestion made by physicists to generalize either quantum mechanics or gravitation — the two pillars of modern physics — to achieve unification, thereby arriving at one all-encompassing theory. “Our experiment thwarts these efforts once again,” he said. Triple-slit experiment Weihs and his team are investigating new light sources to be used for transmitting quantum information. He developed a single-photon source, which served as the basis for testing Born’s rule. Photons were sent through a steel membrane mask that has 3 µm-sized slits cut into it. Measurements were performed with the slits closed individually, resulting in eight independent slit combinations. The data taken was then used to calculate whether Born’s rule applies. “In principle, this experiment is very simple, and we were quite surprised to find that no one had performed this experiment before,” said Weihs. However, the physicists were struggling with measurement errors, which eventually they were able to overcome during their two-yearlong Sisyphean task. “Our measurements show that we can rule out the existence of third-order interference up to a certain bound,” added Weihs. His next step will be to considerably lower the bound with an improved experiment. Master of light particles The experiment was performed at the Institute for Quantum Computing at the University of Waterloo in Ontario, where Weihs worked before his appointment at the University of Innsbruck. Since 2008 he has been setting up his own research group at the Institute for Experimental Physics in Innsbruck, which now comprises 12 members. The international team investigates the development of novel single-photon sources and entangled photon pairs from semiconductor nanostructures. The researchers’ ultimate goal is to integrate quantum optical experiments with functions on semiconductor chips. They have published their findings in the scientific journal Science. For more information, visit: www.uibk.ac.at or www.uwaterloo.ca