Physics and devices of quanutm light emission from semicoductor self-assembled quantum Dots

Abstract

Self-assembled semiconductor single quantum dots (QDs), as a good candidate of solid-state real single photon (SP) emitters in high purity and counting rate, have attracted great attention in recent two decades, promising for quantum information, optical quantum computation, quantum storage, and quantum coherent manipulation. To isolate single QD from the other QDs surrounding, 1) dilute QD density is well controlled during epitaxy; 2) micro-pillars or nanowires individually in space as hosts are fabricated. To enhance their uni-directional emission, GaAs/AlAs distributed Bragg reflector (DBR) planar cavity is integrated. To improve the system (i.e. confocal microscope, traditionally) stability and its optical collection efficiency, a near-field fiber coupling by adhering a micro-pillar chip to fiber facets directly is used. To enhance the coherence of QD spontaneous emission, resonant excitation technique is applied. In this article, we review our research progress in self-assembled QD SP emission, including SP emission from InAs or GaAs QDs on Ga droplet-self-catalyzed GaAs nanowires (with g2(0) of 0.031 or 0.18, respectively), SP emission from InAs/GaAs QDs coupled with high-Q (1000-5000) DBR micro-pillar cavities and their fiber-coupled device fabrication with SP fiber output rate ~1.8 MHz, single QD resonant fluorescence with inter-dot coherent visibility of 40%, strain-coupled bilayer InAs QDs to extend their emission wavelength to 1320 nm and parametric down conversion of 775 nm SP emission from single QD in nanowire to realize entangled photon pairs at 1550 nm (entanglement fidelity of 91.8%) for telecomm application, and definite quantum storage of InAs QD SPs at 879 nm in ion-doped solid (at most 100 time-bins). In future, there will be still several urgent things to do, including 1) puring the environment of a single QD (e.g. growing GaAs QDs to avoid the wetting layer, and optimizing QD growth to avoid smaller QDs) to reduce its spectral diffusion and developing a high-symmetric QD (e.g. GaAs QD) to reduce the fine structure splitting of its emission; 2) positioning single QD precisely for a good alignment of single QD to a micro-cavity or fiber cone (single mode with high numerical aperture) to increase optical excitation efficiency and SP collection efficiency; 3) developing optical quantum integrated chip, including hybrid structures of active micro-cavity and passive waveguide, and high-transmission waveguide beamsplitter or Mach-Zender interferometer to improve SP extraction (micro-cavity), collection (optical setup) and counting rate (at avalanched photon detectors and coincidence counting module).