Quantized fractional Thouless pumping of solitons

In many contexts, interaction between particles gives rise to emergent phenomena. An example is the fractional quantum Hall effect, where the interaction between electrons leads to fractionally quantized Hall conductance. In photonic systems, the nonlinear response of an ambient medium mediates the interaction between photons, and, in the mean-field limit, these dynamics are described by the nonlinear Schrödinger (also called Gross–Pitaevskii) equation. It was recently shown that at weak nonlinearity, soliton motion in nonlinear Thouless pumps—a dimensionally reduced implementation of a Chern insulator—could be quantized to the Chern number, because solitons track the single-band Wannier function throughout the pumping cycle. Here using arrays of coupled optical waveguides, we show that a sufficiently strong nonlinearity fractionally quantizes the motion of solitons. Specifically, we find that the soliton follows maximally localized multi-band Wannier functions and therefore returns to itself only after multiple cycles of the Thouless pump—but displaced by an integer number of unit cells—leading to a rich fractional plateau structure describing soliton motion. Our results represent an example of emergent behaviour in topologically non-trivial systems in the presence of interactions.