  Quantum photonics technologies like wavelength division multiplexing (WDM)
for high-rate quantum key distribution require narrowband, spectrally tunable
single photon emitters. Physical methods that rely on the application of large
mechanical strain to epitaxial quantum dots or electric and magnetic fields to
color centers in 2D metal dichalcogenides provide limited spectral tunability.
Here we adopt a chemical approach to synthesize a family of colloidal
mixed-cation perovskite quantum dots
($\text{Cs}_{1-x}\text{FA}_{x}\text{PbBr}_3$) that show highly photo-stable,
compositionally tunable single photon emission at room temperature - spanning
more than 30 nm in the visible wavelength spectral range. We find that,
tailoring the stoichiometry of the organic formamidinium (FA) cation in
all-inorganic cesium lead bromide ($\text{CsPbBr}_3$) perovskite quantum dots
detunes the electronic band structure while preserving their excellent single
photon emission characteristics. We argue that the mixed-cation perovskite
quantum dots studied in this work offer a new platform for the realization of
color-tunable single photon emitters that could be readily integrated in a
diversity of quantum photonic devices.
