  Generating correlated photon pairs at the nanoscale is a prerequisite to
creating highly integrated optoelectronic circuits that perform quantum
computing tasks based on heralded single-photons. Here we demonstrate
fulfilling this requirement with a generic tip-surface metal junction. When the
junction is luminescing under DC bias, inelastic tunneling events of single
electrons produce a photon stream in the visible spectrum whose super-bunching
index is 17 when measured with a 53 picosecond instrumental resolution limit.
These photon bunches contain true photon pairs of plasmonic origin, distinct
from accidental photon coincidences. The effect is electrically rather than
optically driven - completely absent are pulsed lasers, down-conversions, and
four-wave mixing schemes. This discovery has immediate and profound
implications for quantum optics and cryptography, notwithstanding its
fundamental importance to basic science and its ushering in of heralded photon
experiments on the nanometer scale.
