题目：Entangled-Photon Sources Based on Self-Assembled Quantum Dots
报告人：Dr.Jiaxiang Zhang（Ludwig-Maximilians- niversit?t München, Germany）
时  间：2017年4月20日(周四)下午2:00
地  点：北京理工大学 中心教学楼610
Abstract:
    Self-assembled quantum dots (QDs) are among the most promising entangled-photon sources. They offer many key features towards practical implementation of quantum communication technologies, including high brightness, high indistinguishability and easy integration with a diode structure to realize electrical excitation. In practice, however, self-assembled QDs suffer from a random growth process, which results in the presence of fine structure splitting (FSS) and large inhomogeneous energy broadening for the majority of QDs. As a result, realization of QDs based entangled-photon sources requires suitable post-growth tuning techniques to control the FSS and the energy of QDs deterministically [1].
In this talk, I will demonstrate how to employ strain to control the optical properties of QDs so as to address the above mentioned challenges. I will first present a strain tunable entangled-light-emitting diode to achieve on-demand control over the FSS of QDs. The demonstrated device consists of a diode nanomembrane containing InGaAs QDs integrated onto a piezoelectric crystal capable of delivering a uniaxial stress to QDs. The application of such uniaxial stress enables a capability of tuning the FSS of QDs effectively. We show (i) that the FSS of QDs can be eliminated with the elastic strain fields solely without affecting the electrical injection of the operation of the ELEDs; (ii) that up to 30% of the QDs are tuned to be suitable for the generation of entangled-photon pairs (more than an order of magnitude more than in previous devices) and (iii) the highest operation speed ever reported so far for an entangled-photon source (i.e., 400 MHz). This unique set of properties paves the way towards the real exploitation of ELEDs in high data-rate quantum computation involving a large numbers of all-electrically operated entangled-photon sources [2].
In the second part of my talk, I will demonstrate two viable schemes for developing QDs-based scalable entangled-photon sources. In the first scheme, an in-plane stress tensor obtained from a thin film PMN-PT/silicon micro-electromechanical system was used, with which the FSS of QDs can be eliminated with one uniaxial stress whilst their exciton emission energy is tuning via the second orthogonal stress [3]. In the second scheme, we employ a combination of uniaxial stress and electric field to achieve simultaneous control over the FSS and the energy of the exciton photon emission [4]. Backed up by a two-level bright exciton Hamiltonian incorporating stress-dependent and the quantum confined Stark effect, we find that, by aligning the uniaxial stress axis and selecting the exciton polarization direction of QDs along the GaAs [110] (or [1-10]) direction, the critical uniaxial stress used to eliminate the fine-structure-splitting of QDs can be linearly shifted by either the uniaxial stress or the vertical electric field. These allows direct realization of electric-field (or strain field) induced energy tuning of entangled-photon emission from QDs. Experimentally, a broad energy tuning of ~ 5 meV for polarization entangled-photon emission from a QD has been achieved in both schemes, and high degree of entanglement-fidelities have been obtained for tuned energies in response to the externally applied stress or electric field.
References:
1. Shields, A. J. Nat. Photon. 1, 215–223 (2007).
2. Zhang, J-X. et al. Nat. Commun. 6, 10067 (2015).
3. Chen, Y and Zhang, J.-X. et al, Nat. Commun. 7, 10387 (2016).
4. Zhang, J-X. et al. Nano Letters, 17, 501 (2017).
简历：
    Dr. Jiaxiang Zhang received his PhD from Chemnitz University of Technology, Germany in 2015, and conducted his works at Leibnitz Institute for Solid State and Material Research. His research focuses on the development of ultrafast electrically triggered single and entangled-photon sources for photonic quantum applications. Currently, he is working as a senior scientist in the Center for NanoScience (CeNS) at Ludwig Maximilians University of Munich. His current research activities focus on experimental quantum optics, quantum light sources, and optoelectronic devices based on III-V semiconductor materials and the 2D semiconducting materials.