Collision phenomena are ubiquitous and highly useful 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 ultrahigh vacuum and low-temperature systems. Here we demonstrate a spin-noise spectroscopic approach by measuring optical polarization rotation noise of the probe light with simple apparatus and ambient conditions. Our approach features tens of gigahertz bandwidth and one part-per-million resolution, outperforming previous spin-noise techniques. Enabled by the technique, we observe the collision-sensitive 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 tool to study a broad range of collision phenomena under ambient conditions.

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• Cold and ultracold molecules
• Fluctuations & noise
• Spin dynamics
• Spin fluctuations
• Atomic gases
• Molecules
• Magnetic moment
• Spin noise spectroscopy