Single quantum dots as photon pair emitters

Summary form only given. A realization of time-bin entanglement obtained from quantum dots would join the strength of long distance transmission that characterizes this type of entanglement with the single photon state purity of the quantum dot emission. Compared with polarization entanglement from quantum dots this scheme does not require elimination of the fine structure splitting responsible for partial distinguishability of the quantum dot cascades [1].Here, we present our results on coherent and resonant excitation of quantum dots, creation of photon pairs, scattering free emission and measurement of the high purity of the emitted photons [2, 3]. In addition, we will show our measurements of the generated time-bin entanglement. To excite a single quantum dot we performed two-photon resonant excitation of the biexciton state. Here, we exploited the biexciton binding energy in order to use the laser light which was not resonant to any photon emitted from the quantum dot (Fig. 1a). Our measurements were performed on a single self-assembled InAs/GaAs quantum dot. We confirmed the resonant nature of the excitation by observing Rabi oscillations (Fig. 1b). Additionally, we performed Ramsey interference measurement and determined the coherence time of the ground-biexciton state superposition. These measurements show that we can coherently transfer the phase of the excitation laser onto the quantum dot system, a necessary requirement to obtain time-bin entanglement.To test the statistics of the emitted state we measured the auto-correlation of the emitted photons (Fig. 3c). Here, the auto-correlation parameter at zero delay was measured to be 0.012(1) without and 0.0073(8) with background subtraction. In addition, we performed a measurement to characterize the state emitted from a quantum dot using a witness-based criterion described in [4]. With this measurement we experimentally confirmed that the resonantly excited quantum dot emits a non-Gaussian state of light (Fig. 1d). This measurement is the first of this nature ever performed on a semiconductor single photon emitter.