  Ultranarrow bandwidth single-photon sources operating at room-temperature are
of vital importance for viable optical quantum technologies at scale, including
quantum key distribution, cloud based quantum information processing networks,
and quantum metrology. Here we show a room-temperature ultranarrow bandwidth
single-photon source generating polarised photons at a rate of 5MHz based on an
inorganic CsPbI3 perovskite quantum dot embedded in a tunable open-access
optical microcavity. When coupled to an optical cavity mode, the quantum dot
room-temperature emission becomes single-mode and the spectrum narrows down to
just 1 nm. The low numerical aperture of the optical cavities enables efficient
collection of high-purity single-mode single-photon emission at
room-temperature, offering promising performance for photonic and quantum
technology applications. We measure 94% pure single-photon emission into a
single-mode under pulsed and continuous-wave (CW) excitation.
