The strong band edge exciton-phonon interactions in metal halide perovskite quantum dots (QDs) offer a unique platform to explore many-body phenomena. Employing $\mathrm{Cs}\mathrm{Pb}{\mathrm{Br}}_3$ QDs as a perovskite model system, we report the observation of spin-selective polaronic biexcitons using collective excitations of two circularly polarized ultrafast lasers of a duration that is two orders of magnitude shorter than the exciton lifetime and one order of magnitude shorter than the spin relaxation time. The intervalley polaron pairing of charge carriers determines the anomalously strong exciton-exciton interactions, where the Haynes factor is an order of magnitude larger than the bulk and five times larger than the two-dimensional and quantum well semiconductors, demonstrating a very robust correlation of excitons. Our findings reveal a mechanism of generating highly stable biexciton states even at room temperature to realize higher-order correlations of charge carriers such as quantum droplets and Bose-Einstein condensates.

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• Nanocrystals
• Nanophotonics
• Transient absorption spectroscopy
• Ultrafast pump-probe spectroscopy