  Collision phenomena are ubiquitous and of importance in determining the
microscopic structures and intermolecular interactions of atoms and molecules.
The existing approaches are mostly based on atomic or molecular scatterings,
which are hindered by the inconvenience of using ultra-high vacuum and low
temperature systems. Here we demonstrate a new spin-noise spectroscopic
approach by measuring optical polarization rotation noise of the probe light,
which operates with simple apparatus and ambient conditions. Our approach
features tens of gigahertz bandwidth and one part-per-million resolution,
outperforming existing spin-noise techniques. Enabled by the new technique, we
observe the collision-induced spin noise of alkali atoms, and precisely
determine key collision parameters, such as collision diameter, well depth, and
dominant interaction type. Our work provides a new tool to study a broad range
of collision phenomena under ambient conditions.
