  Strongly-driven nonlinear optical processes such as spontaneous parametric
down-conversion and spontaneous four-wave mixing can produce multiphoton
nonclassical beams of light which have applications in quantum information
processing and sensing. In contrast to the low-gain regime, new physical
effects arise in a high-gain regime due to the interactions between the
nonclassical light and the strong pump driving the nonlinear process. Here, we
describe and experimentally observe a gain-induced group delay between the
multiphoton pulses generated in a high-gain type-II spontaneous parametric
down-conversion source. Since the group delay introduces distinguishability
between the generated photons, it will be important to compensate for it when
designing quantum interference devices in which strong optical nonlinearities
are required.
