  Producing advanced quantum states of light is a priority in quantum
information technologies. While remarkable progress has been made on single
photons and photon pairs, multipartite correlated photon states are usually
produced in purely optical systems by post-selection or cascading, with
extremely low efficiency and exponentially poor scaling. Multipartite states
enable improved tests of the foundations of quantum mechanics as well as
implementations of complex quantum optical networks and protocols. It would be
favorable to directly generate these states using solid state systems, for
better scaling, simpler handling, and the promise of reversible transfer of
quantum information between stationary and flying qubits. Here we use the
ground states of two optically active coupled quantum dots to directly produce
photon triplets. The wavefunctions of photogenerated excitons localized in
these ground states are correlated via molecular hybridization and Coulomb
interactions. The formation of a triexciton leads to a triple cascade
recombination and sequential emission of three photons with strong
correlations. The quantum dot molecule is embedded in an epitaxially grown
nanowire engineered for single-mode waveguiding and improved extraction
efficiency at the emission wavelength. We record 65.62 photon triplets per
minute, surpassing rates of all earlier reported sources, in spite of the
moderate efficiency of our detectors. Our structure and data represent a
breakthrough towards implementing multipartite photon entanglement and
multi-qubit readout schemes in solid state devices, suitable for integrated
quantum information processing.
