  Chemically prepared colloidal semiconductor quantum dots have long been
proposed as scalable and color-tunable single emitters in quantum optics, but
they have typically suffered from prohibitively incoherent emission. We now
demonstrate that individual colloidal lead halide perovskite quantum dots
(PQDs) display highly efficient single photon emission with optical coherence
times as long as 80 ps, an appreciable fraction of their 210 ps radiative
lifetimes. These measurements suggest that PQDs should be explored as building
blocks in sources of indistinguishable single photons and entangled photon
pairs. Our results present a starting point for the rational design of lead
halide perovskite-based quantum emitters with fast emission, wide
spectral-tunability, scalable production, and which benefit from the
hybrid-integration with nano-photonic components that has been demonstrated for
colloidal materials.
