Quantum dots for single and entangled photon emitters

Abstract

Efficient generation of polarized single or entangled photons is a crucial requirement for the implementation of quantum key distribution (QKD) systems. Self-organized semiconductor quantum dots (QDs) are capable of emitting one polarized photon or an entangled photon pair at a time using appropriate electrical current injection. We realized highly efficient single photon sources (SPS) based on well established semiconductor technology: In a pin structure a single electron and a single hole are funneled into a single InAs quantum dot using a submicron AlOx current aperture. Efficient radiative recombination leads to emission of single polarized photons with an all-time record purity of the spectrum. Non-classicality of the emitted light without using additional spectral filtering is demonstrated. Out-coupling efficiency and emission rate are increased by embedding the SPS into a micro-cavity of Q = 140. The design of the micro-cavity is based on detailed modeling to optimize its performance. The resulting resonant single-QD diode generates single polarized photons at a repetition rate of 1 GHz exhibiting a second order correlation function of g(2)(0) = 0. Eventually, QDs grown on (111) oriented substrate are proposed as source of entangled photon pairs. Intrinsic symmetry-lowering effects leading to the splitting of the exciton bright states are shown to be absent for this substrate orientation. As a result the XX → X → 0 recombination cascade of a QD can be used for the generation of entangled photons without further tuning of the finestructure splitting via QD size and/or shape. We present first micro-photoluminescence studies on QDs grown on (111) GaAs, demonstrating a fine structure splitting less than the spectral resolution of our set-up.