Electrical control of quantum bits could pave the way for scalable quantum computation. An acceptor spin qubit in Si, based on spin-3/2 holes, can be controlled by electrical means using a gate electrode, which offers fast one- and two-qubit rotations and long coherence times at certain sweet spots. The relaxation time T1, while allowing >10^5 operations, is the primary limiting factor. I will show that, due to the interplay of the Td symmetry of the acceptor in the Si lattice and the spin-3/2 characteristic of hole systems, an applied in-plane magnetic field strongly enhances the performance and coherence properties of the qubit. An appropriate choice of magnetic field orientation leads to a near-total suppression of spin relaxation as well as full tunability of two-qubit operations in a parameter regime in which dephasing due to charge fluctuations can be eliminated. Interestingly for spintronic applications, an extreme in-plane anisotropy exists such that the in-plane g-factor can vanish under certain circumstances.
Title: Cavity Quantum Electrodynamics with mesoscopic topological superconductors
In this talk, I will present our recent studies on zero energy edge states in finite-length carbon nanotubes from topological viewpoint.
An effective one-dimensional (1D) lattice model for a single-wall carbon nanotube (SWNT) is introduced to analyze the quantum system with boundary . Open-ended SWNTs commonly host edge states whose energies lie in the bulk band gap. By analyzing the 1D lattice model a bulk-edge correspondence is proven, which shows that the number of edge states and a winding number topological invariant defined in the corresponding bulk system are identical . Manipulation of the edge states by a magnetic field is suggested in terms of the topological phase transition .
We extend these studies to investigate the nature of edge states which arise when the SWNT is proximity coupled to a superconductor . The zero energy edge states emerge as a combined effect of curvature-induced Dirac point shift and strong superconducting coupling between nearest-neighbor sites. A 1D continuum model reveals the topological origin of the zero energy edge states.
 W. Izumida, R. Okuyama, R. Saito, Phys. Rev. B 91, 235442 (2015)
 W. Izumida, R. Okuyama, A. Yamakage, R. Saito, Phys. Rev. B 93, 195442 (2016)
 R. Okuyama, W. Izumida, M. Eto, J. Phys. Soc. Jpn. 86, 013702 (2017)
 W. Izumida, L. Milz, M. Marganska, M. Grifoni, arXiv:1707.02934