  Coherent manipulation of the exciton wave function in a single semiconductor
colloidal nanocrystal (NC) has been actively pursued in the past decades
without any success, mainly due to the bothersome existences of the spectral
diffusion and the photoluminescence (PL) blinking effects. Such optical
deficiencies can be naturally avoided in the newly-developed colloidal NCs of
perovskite CsPbI3, leading to the PL spectrum with a stable intensity at the
single-particle level. Meanwhile, from the first-order photon correlation
measurement, a PL linewidth smaller than 20 {\mu}eV is estimated for the
emission state of the neutral excitons in a single CsPbI3 NC. Moreover, a
dephasing time of about 10 ps can be extracted from the quantum interference
measurement on the absorption state of the charged excitons. This first
demonstration of a coherent optical feature will advance single colloidal NCs
into the quantum information regime, opening up an alternative yet prospective
research direction beyond their traditional applications such as in
optoelectronic devices and bioimaging.
