Quantum-optics tricks for topological photonics

Topology is one of the ‘hot topics’ in physics these days, and was placed into the spotlight when David Thouless, Duncan Haldane and Michael Kosterlitz have been awarded the Nobel Prize in Physics 2016, "for theoretical discoveries of topological phase transitions and topological phases of matter". Much of that current interest stems form the discovery of topological insulators. These are materials that have an electrically insulating bulk but can conduct electrons on their surface — and not only that, the conducting surface states have a built-in robustness against disorder, known as ‘topological protection’. This sort of protection is based on fundamental symmetries and makes the surface of topological insulators excellent conductors even when the material is not perfectly pure.

In recent years, a number of works have established that topological protection is not unique to electrons moving, according to the laws of quantum mechanics, in crystals. Analogue phenomena have also been demonstrated in classical systems, such as optical fibres, mechanical pendula or electrical circuits. Oded Zilberberg of the Institute for Theoretical Physics at ETH Zurich, together with colleagues from the Royal Melbourne Institute of Technology (RMIT) University in Australia and the Politecnico di Milano in Italy, has now found a way to ‘put back the quantum’ into one of these classical platforms. As they report today in the journal Science Advances, they constructed a photonic device that both behaves in many ways like a topological insulator and, when fed with single photons, displays one of quintessential effects of quantum optics, the Hong–Ou–Mandel effect.

A topological beam splitter

The team designed a device consisting of ten optical fibres that are coupled to one another in a way that light can only flow through the fibres at the boundary, but not through the central fibres. Confined photonic boundary states of this type have been realised before, but Zilberberg and his co-workers found now a way that states at opposite edges of the device can be controllably ‘spread out’ (or, delocalized). The boundary states then reach into the bulk of the device, where they can overlap and thus interfere with one another. Specifically, the researchers carefully configured their device such that incoming light on one edge is split evenly between the two boundaries when it exits the device (see the figure, left and central panel).

But something rather magical happened when this ‘topological beam splitter’ was operated in the extreme regime where the input to the device consisted of a pair of single photons, one entering through each of the edges. At the exit, the two photons then appeared not separately one on each side, but essentially always left the device together on one of the sides (see the figure, right panel). This behaviour is due to quantum interference between the two photons — an effect that, in this form, does not appear for ‘normal’ light, but only for single photons that are identical in all of their properties.

Bright prospects for topological photonics

The phenomenon, the Hong–Ou–Mandel effect, is well understood and constitutes one of the textbook examples of quantum behaviour in optics. But having now a device that combines behaviour grounded in topology on the one hand and in quantum optics on the other promises a novel and versatile new platform both for exploring fundamental topological phenomena and for developing novel quantum technologies exploiting them.