Quantum vortices of strongly interacting photons

Have you ever tried dragging a plate through the surface of a water pool?

https://www.youtube.com/watch?v=pnbJEg9r1o8

Quite excitingly, you would form a pair of vortex and antivortex, which would propagate steadily on the water’s surface!

In optics, vortices manifest as phase twists of the electromagnetic field, usually formed by the interaction with matter. Vortex formation due to light interacting with light requires strong optical nonlinearity and has therefore been confined, until now, to the classical regime.

We will discuss the experimental realization of strong, effective photon-photon interaction in a quantum nonlinear medium based on ultracold Rydberg atoms. This interaction results in a faster phase accumulation for copropagating photon pairs. Similarly to a plate pushing water, the accumulation of localized excess phase produces a quantum vortex-antivortex pair within the two-photon wavefunction. The “conditional” π phase localized between these vortices can be used for deterministic quantum logic operations. Moreover, triplets of photons produce vortex lines and a vortex ring, giving rise to a 2π conditional phase. The deviation from the 3π conditional phase, expected for a quantum Kerr-nonlinear medium, attests to genuine three-photon interaction.

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