  Quantum path interferences occur whenever multiple equivalent and coherent
transitions result in a common final state. Such interferences strongly modify
the probability of a particle to be found in that final state, a key concept of
quantum coherent control. When multiple nonlinear and energy-degenerate
transitions occur in a system, the multitude of possible quantum path
interferences is hard to disentangle experimentally. Here, we analyze quantum
path interferences during the nonlinear emission of electrons from hybrid
plasmonic and photonic fields using time-resolved photoemission electron
microscopy. We experimentally distinguish quantum path interferences by
exploiting the momentum difference between photons and plasmons and through
balancing the relative contributions of their respective fields. Our work
provides a fundamental understanding of the nonlinear photon-plasmon-electron
interaction. Distinguishing emission processes in momentum space, as introduced
here, will ultimately allow nano-optical quantum-correlations to be studied
without destroying the quantum path interferences.
