  A non-classical light source emitting pairs of identical photons represents a
versatile resource of interdisciplinary importance with applications in quantum
optics and quantum biology. Emerging research fields, which benefit from such
type of quantum light source, include quantum-optical spectroscopy or
experiments on photoreceptor cells sensitive to photon statistics. To date,
photon twins have mostly been generated using parametric downconversion
sources, relying on Poissonian number distributions, or atoms, exhibiting low
emission rates. Here, we propose and experimentally demonstrate the efficient,
triggered generation of photon twins using the energy-degenerate
biexciton-exciton radiative cascade of a single semiconductor quantum dot.
Deterministically integrated within a microlens, this nanostructure emits
highly-correlated photon pairs, degenerate in energy and polarization, at a
rate of up to (234 $\pm$ 4) kHz. Furthermore, we verify a significant degree of
photon-indistinguishability and directly observe twin-photon emission by
employing photon-number-resolving detectors, which enables the reconstruction
of the emitted photon number distribution.
