1. Toshiba Research Europe Limited, 260 Cambridge Science Park, Cambridge CB4 0WE, UK
2. Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK

Correspondence to: R. M. Stevenson¹ Correspondence and requests for materials should be addressed to R.M.S. (Email: mark.stevenson@crl.toshiba.co.uk).

Abstract

Entangled photon pairs are an important resource in quantum optics¹, and are essential for quantum information² applications such as quantum key distribution^{3, 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 emission^{7, 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 schemes^{10, 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.

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