Browsing by Author "Manfra, Michael J."

Now showing 1 - 2 of 2
  • Thumbnail Image
    Coherent Multispin Exchange Coupling in a Quantum-Dot Spin Chain
    Qiao, Haifeng; Kandel, Yadav P.; Deng, Kuangyin; Fallahi, Saeed; Gardner, Geoffrey C.; Manfra, Michael J.; Barnes, Edwin Fleming; Nichol, John M. (2020-07-08)
    Heisenberg exchange coupling between neighboring electron spins in semiconductor quantum dots provides a powerful tool for quantum information processing and simulation. Although so far unrealized, extended Heisenberg spin chains can enable long-distance quantum information transfer and the generation of nonequilibrium quantum states. In this work, we implement simultaneous, coherent exchange coupling between all nearest-neighbor pairs of spins in a quadruple quantum dot. The main challenge in implementing simultaneous exchange couplings is the nonlinear and nonlocal dependence of the exchange couplings on gate voltages. Through a combination of electrostatic simulation and theoretical modeling, we show that this challenge arises primarily due to lateral shifts of the quantum dots during gate pulses. Building on this insight, we develop two models that can be used to predict the confinement gate voltages for a desired set of exchange couplings. Although the model parameters depend on the number of exchange couplings desired (suggesting that effects in addition to lateral wave-function shifts are important), the models are sufficient to enable simultaneous and independent control of all three exchange couplings in a quadruple quantum dot. We demonstrate two-, three-, and four-spin exchange oscillations, and our data agree with simulations.
  • Thumbnail Image
    Photoluminescence Study of Carrier Localization and Recombination in Nearly Strain-Balanced Nonpolar InGaN/AlGaN Quantum Wells
    Cao, Yang; Dzuba, Brandon; Magill, Brenden A.; Senichev, Alexander; Trang Nguyen; Diaz, Rosa E.; Manfra, Michael J.; McGill, Stephen; Garcia, Carlos; Khodaparast, Giti A.; Malis, Oana (Wiley-V C H, 2022-02-16)
    Temperature-dependent continuous-excitation and time-resolved photoluminescence are studied to probe carrier localization and recombination in nearly strain-balanced m-plane In0.09Ga0.91N/Al0.19Ga0.81N multi-quantum wells grown by plasma-assisted molecular-beam epitaxy. An average localization depth of 21 meV is estimated for the undoped sample. This depth is much smaller than the reported values in polar structures and m-plane InGaN quantum wells. As part of this study, temperature and magnetic field dependence of time-resolved photoluminescence is performed. At 2 K, an initial fast decay time of approximate to 0.3 ns is measured for both undoped and doped structures. The undoped sample also exhibits a slow decay component with a time scale of 2.2 ns. The existence of two relaxation paths in the undoped structure can be attributed to different localization centers. The fast relaxation decays are relatively insensitive to external magnetic fields, while the slower relaxation time constant decreases significantly with increasing magnetic fields. The fast decay time scale in the undoped sample is likely due to indium fluctuations in the quantum well. The slow decay time may be related to carrier localization in the barriers. The addition of doping leads to a single fast decay time likely due to stronger exciton localization in the InGaN quantum wells.