Charged quantum dots provide an important platform for a range of emerging quantumtechnologies. Colloidal quantum dots in particular offer unique advantages for suchapplications (facile synthesis, manipulation and compatibility with a wide range of environments), especially if stable charged states can be harnessed in these materials. Here weengineer the CdSe nanocrystal core and shell structure to efficiently ionize at cryogenictemperatures, resulting in trion emission with a single sharp zero-phonon line and a monoexponential decay. Magneto-optical spectroscopy enables direct determination of electronand hole g-factors. Spin relaxation is observed in high fields, enabling unambiguous identi-fication of the trion charge. Importantly, we show that spin flips are completely inhibited forZeeman splittings below the low-energy bound for confined acoustic phonons. This reveals acharacteristic unique to colloidal quantum dots that will promote the use of these versatilematerials in challenging quantum technological applications.
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