  The superradiant amplification in the scattering from a rotating medium was
first elucidated by Sir Roger Penrose over 50 years ago as a means by which
particles could gain energy from rotating black holes. Despite this fundamental
process being ubiquitous also in wave physics, it has only been observed once
experimentally, in a water tank, and never in an astrophysical setting. Here,
we measure this amplification for a nonlinear optics experiment in the
superfluid regime. In particular, by focusing a weak optical beam carrying
orbital angular momentum onto the core of a strong pump vortex beam, negative
norm modes are generated and trapped inside the vortex core, allowing for
amplification of a reflected beam. Our experiment demonstrates amplified
reflection due to a novel form of nonlinear optical four-wave mixing, whose
phase-relation coincides with the Zel'dovich-Misner condition for Penrose
superradiance in our photon superfluid, and unveil the role played by negative
frequency modes in the process.}
