A semiconductor source of triggered entangled photon pairs

Entangled photon pairs are an important resource in quantum optics1, and are essential for quantum information2 applications such as quantum key distribution3,4 and controlled quantum logic operations5. The radiative decay of biexcitons—that is, states consisting of two bound electron–hole pairs—in a quantum dot has been proposed as a source of triggered polarization-entangled photon pairs6. To date, however, experiments have indicated that a splitting of the intermediate exciton energy yields only classically correlated emission7,8,9. Here we demonstrate triggered photon pair emission from single quantum dots suggestive of polarization entanglement. We achieve this by tuning the splitting to zero, through either application of an in-plane magnetic field or careful control of growth conditions. Entangled photon pairs generated ‘on demand’ have significant fundamental advantages over other schemes10,11,12,13, which can suffer from multiple pair emission, or require post-selection techniques or the use of photon-number discriminating detectors. Furthermore, control over the pair generation time is essential for scaling many quantum information schemes beyond a few gates. Our results suggest that a triggered entangled photon pair source could be implemented by a simple semiconductor light-emitting diode14.