A near-perfect entangled photon source is an important piece of technology required for several quantum photonic information protocols that have applications in computing, sensing, and communication. To date, the workhorse platform to generate entangled photons for many quantum information experiments has been based on spontaneous parametric down conversion (SPDC) and other related non-linear optical techniques. These sources have an imperfection: highly entangled photons pairs are only emitted at low efficiencies because pumping at higher power results in an increase in the probability of multi-photon emission. The ideal source would be one with near-unity efficiency and near-unity entanglement fidelity. Semiconductor quantum dots have emerged as a promising candidate to realize this ideal source. However, it was believed that quantum dots could not reach near-perfect entanglement because of dephasing induced by the “noisy” solid-state environment surrounding the emitter. Here, we show that photon pairs emitted from an InP/InAsP quantum dot embedded in a tapered photonic nanowire exhibit very high degrees of peak entanglement (>97% fidelity, >95% concurrence), with average fidelity values greater than 93% over the lifetime of the emission. These results were obtained by employing resonant excitation of the quantum dot that results in low multiphoton emission ($g^{(2)}_{XX} = 0.01$), and by using state of the art single photon detectors with excellent timing resolution (approx. 20ps) and a very low dark count rate (approx. 1/sec). These results further support existing experimental and theoretical findings which challenge the assumption that quantum dots will never reach unity entanglement because of the effects of dephasing from the solid-state environment.

This is joint work with M. Zeeshan, A. Fogini, M. E. Reimer, P. Poole, D. Dalacu, K. Jöns, and V. Zwiller.