Theory of two-dimensional nonlinear spectroscopy for the Kitaev spin liquid

Experimental identification of fractionalized quasiparticles in quantum spin liquids is a long-sought goal in correlated topological phases. Since conventional spectroscopic probes (e.g., neutron scattering) cannot directly couple to each individual fractionalized excitation, one can only measure a broad continuum in energy due to a multitude of fractionalized quasiparticles decaying from a single spin-flip. In this talk, we discuss how two-dimensional nonlinear spectroscopy can sharply reveal distinctive signatures of fractionalized excitations in quantum spin liquids by considering the example of the exactly solvable Kitaev honeycomb model. We demonstrate the existence of a number of salient features of the Majorana fermions and fluxes in two-dimensional nonlinear spectrum, which provide crucial information about such excitations.