Quantum Vortex dynamics of strongly interacting photons*

Vortices appear in optics as phase twists in the electromagnetic field resulting from light-matter interactions. Quantum vortices, characterized by phase singularities in the wavefunction, are typically associated with strongly interacting many-particle systems. However, the emergence of vortices through the effective interaction of light with itself, a phenomenon requiring strong optical nonlinearity, was previously limited to the classical regime until recent advancements.

We recently observed quantum photon vortices resulting 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 function¹.

Further, we show that the evolution of the n-photon wavefunction is governed by a multiband, Dirac-like dispersion with one massive mode and n − 1-degenerate modes. The resulting band structure features an n-fold rotational symmetry, including an n-fold warped light cone. For three photons, the band dispersion breaks the symmetry between the situations of a photon pair propagating ahead or behind a single photon. We experimentally confirm these findings by measuring the three-photon phase and intensity correlation functions. We observed the effect of trigonal warping on the quantum vortex ring generated by the three-photon interaction².

A non-trivial vortex structure is observed for counter-propagating photons. Depending on the interaction strength, the diffraction-like structure of the vortex is observed.  In the intensity correlations, structures like higher-order bound states appear instead of the co-propagating photons.