Quantum mechanics suggests a persistent lattice vibration even at zero temperature; however, this elusive zero-point motion is difficult to measure for further analyses and exploitations. Here, we have performed near-resonant excitations on single perovskite ${\mathrm{CsPbI}}_3$ nanocrystals (NCs) at cryogenic temperatures, showing that pronounced up-converted photoluminescence (UC-PL) still takes place even at $\sim 4\mathrm{K}$ despite the vanishing phonon-assisted absorption. This surprising result can be explained in terms of a dynamic electric field caused by the zero-point motion of polar optical phonons, which gives rise to an absorption tail as in the Franz-Keldysh electroabsorption process. Furthermore, laser cooling is demonstrated in the $\sim 4\mathrm{K}$ UC-PL process of a single ${\mathrm{CsPbI}}_3$ NC, which opens a novel research avenue for the realization of even lower temperatures beyond the limit set forth by the helium-based cryostat.

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• Excitons
• Luminescence
• Phonons
• Photoluminescence