  Particle accelerators and storage rings have been transformative instruments
of discovery, and, for many applications, innovations in particle-beam cooling
have been a principal driver of that success1. Beam cooling reduces the spread
in particle positions and momenta, while keeping the number of particles
constant, and combats diffusive effects, thereby enabling particle accumulation
and the production and preservation of intense beams. In the case of particle
colliders, cooling increases the likelihood of observing rare physics events.
One of the most important conceptual and technological advances in this area
was stochastic cooling (SC), which was instrumental in the discovery of the W
and Z bosons at CERN and the top quark at Fermilab2-6. SC reduces the random
motion of the beam particles through granular sampling and correction of the
beams phase-space structure, thus bearing resemblance to a Maxwells demon. The
extension of SC from the microwave regime up to optical frequencies and
bandwidths has long been pursued as it could increase the achievable cooling
rates by three to four orders of magnitude and provide a powerful new tool for
future accelerators. First proposed nearly thirty years ago, Optical Stochastic
Cooling (OSC) replaces the conventional microwave elements of SC with
optical-frequency analogs and is, in principle, compatible with any species of
charged-particle beam7,8. Here we describe the first experimental demonstration
of OSC in a proof-of-principle experiment9 at the Fermi National Accelerator
Laboratorys Integrable Optics Test Accelerator10.
