Interferometric measurements of an optically trapped exciton-polariton condensate reveal a regime where the condensate pseudo-spin precesses persistently within the driving optical pulse. For a single $20\mu \mathrm{s}$ optical pulse, the condensate pseudo-spin undergoes over ${10}^5$ full precessions with striking frequency stability. The emergence of the precession is traced to polariton nonlinear interactions that give rise to a self-induced out-of-plane magnetic field, which in turn drives the system spin dynamics. The Larmor precession frequency and trajectory are directly influenced by the condensate density, enabling the control of this effect with optical means. Our results accentuate the system’s potential for the realization of magnetometry devices and can lead to the emergence of spin-squeezed polariton condensates.

• Received 24 June 2020
• Revised 1 July 2022
• Accepted 22 August 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.155301