Quantum vortices of strongly interacting photons

Have you ever tried dragging a plate through the surface of a water pool? Quite excitingly, you would see a pair of vortex and antivortex moving steadily through the surface of the water.

These vortices are a hallmark of topologically nontrivial dynamics in nonlinear physics. In optics, vortices manifest as phase twists of the electromagnetic field, commonly formed by the interaction of light and matter. Quantum vortices – phase singularities of the wavefunction – are typically expected in strongly interacting systems of many particles, primarily superfluid. Formation of vortices by effective interaction of light with itself requires strong optical nonlinearity and has, therefore, been confined until now to the classical regime.

We report on the realization of quantum vortices of photons that result from a strong photon-photon interaction in a quantum nonlinear optical medium. The interaction causes faster phase accumulation for copropagating photons, producing a quantum vortex-antivortex pair within the two-photon wave function.

For three photons, the strong interaction leads to an even richer topological structure of the three-photon wavefunction. The point vortices lead to the formation of vortex lines, and a central vortex ring attests to a genuine three-photon interaction.

The wavefunction topology, governed by two- and three-photon bound states, imposes a conditional phase shift of π per photon, a potential resource for deterministic quantum logic operations.