A counterintuitive facet of the physics of photon interference has been uncovered by three researchers of Université libre de Bruxelles, Belgium. In an article published this month in Nature Photonics, they have proposed a thought experiment that utterly contradicts common knowledge on the so-called bunching property of photons. The observation of this anomalous bunching effect seems to be within reach of today’s photonic technologies and, if achieved, would strongly impact on our understanding of multiparticle quantum interferences.

One of the cornerstones of quantum physics is Niels Bohr’s complementarity principle, which, roughly speaking, states that objects may behave either like particles or like waves. These two mutually exclusive descriptions are well illustrated in the iconic double-slit experiment, where particles are impinging on a plate containing two slits. If the trajectory of each particle is not watched, one observes wave-like interference fringes when collecting the particles after going through the slits. But if the trajectories are watched, then the fringes disappear and everything happens as if we were dealing with particle-like balls in a classical world.

As coined by physicist Richard Feynman, the interference fringes originate from the absence of “which-path” information, so that the fringes must necessarily vanish as soon as the experiment allows us to learn that each particle has taken one or the other path through the left or right slit.

A team of physicists, including University of Massachusetts assistant professor Tigran Sedrakyan, recently announced in the journal Nature that they have discovered a new phase of matter. Called the “chiral Bose-liquid state,” the discovery opens a new path in the age-old effort to understand the nature of the physical world.

Under everyday conditions, matter can be a solid, liquid or gas. But once you venture beyond the everyday—into temperatures approaching absolute zero, things smaller than a fraction of an atom or which have extremely low states of energy—the world looks very different. “You find quantum states of matter way out on these fringes,” says Sedrakyan, “and they are much wilder than the three classical states we encounter in our everyday lives.”

Sedrakyan has spent years exploring these wild quantum states, and he is particularly interested in the possibility of what physicists call “band degeneracy,” “moat bands” or “kinetic frustration” in strongly interacting quantum matter.