  We use orthogonally polarized two-colour (OTC) laser pulses to separate
quantum paths in multiphoton ionization of Ar atoms. Our OTC pulses consist of
400~nm and 800~nm light at a relative intensity ratio of 10:1. We find a
hitherto unobserved interference in the photoelectron momentum distribution,
which exhibits a strong dependence on the relative phase of the OTC pulse.
Analysis of model calculations reveal that the interference is caused by
quantum pathways from non-adjacent quarter cycles.
