Enhancement of vortex pinning in superconductor/ferromagnet bilayers via angled demagnetization

We use local and global magnetometry measurements to study the influence of magnetic domain width w on the domain-induced vortex pinning in superconducting/ferromagnetic bilayers, built of a Nb film and a ferromagnetic Co/Pt multilayer with perpendicular magnetic anisotropy, with an insulating layer to eliminate proximity effect. The quasi-periodic domain patterns with different and systematically adjustable width w, as acquired by a special demagnetization procedure, exert tunable vortex pinning on a superconducting layer. The largest enhancement of vortex pinning, by a factor of more than 10, occurs when w ~ 310 nm is close to the magnetic penetration depth.Comment: 5 pages, 3 figures, accepted to Phys. Rev. B, Rapid Communication

Probing non-Abelian statistics of Majorana fermions in ultracold atomic superfluid

We propose an experiment to directly probe the non-Abelian statistics of Majorana fermions by braiding them in an s-wave superfluid of ultracold atoms. We show different orders of braiding operations give orthogonal output states that can be distinguished through Raman spectroscopy. Realization of Majorana bound states in an s-wave superfluid requires strong spin-orbital coupling and a controllable Zeeman field in the perpendicular direction. We present a simple laser configuration to generate the artificial spin-orbital coupling and the required Zeeman field in the dark state subspace.Comment: 4 pages; Add detailed discussion of feasibility of the scheme;add ref

Topological Bose-Mott Insulators in a One-Dimensional Optical Superlattice

We study topological properties of the Bose-Hubbard model with repulsive interactions in a one-dimensional optical superlattice. We find that the Mott insulator states of the single-component (two-component) Bose-Hubbard model under fractional fillings are topological insulators characterized by a nonzero charge (or spin) Chern number with nontrivial edge states. For ultracold atomic experiments, we show that the topological Chern number can be detected through measuring the density profiles of the bosonic atoms in a harmonic trap.Comment: 5 pages, published versio

Implementation of universal quantum gates based on nonadiabatic geometric phases

We propose an experimentally feasible scheme to achieve quantum computation based on nonadiabatic geometric phase shifts, in which a cyclic geometric phase is used to realize a set of universal quantum gates. Physical implementation of this set of gates is designed for Josephson junctions and for NMR systems. Interestingly, we find that the nonadiabatic phase shift may be independent of the operation time under appropriate controllable conditions. A remarkable feature of the present nonadiabatic geometric gates is that there is no intrinsic limitation on the operation time, unlike adiabatic geometric gates. Besides fundamental interest, our results may simplify the implementation of geometric quantum computation based on solid state systems, where the decoherence time may be very short.Comment: 5 pages, 2 figures; the version published in Phys. Rev. Let

Performance analysis of full bridge, boost half bridge and half bridge topologies for application in phase shift converters

In this paper, performances of three topologies of bidirectional ports applicable to phase shift converters are compared. The proposed topologies include full bridge, half bridge and boost-half bridge that are commonly used as bi-directional port in various topologies of DC converters. The proposed analyses based on several indicators and characteristics of the topologies including reliability factor, switching loss, current ripple, cost, size, efficiency, range of power flow versus phase shift angle and control complexity. A phase shift converter based on proposed topologies was simulated using P-SIM. The analysis shows that considering all effective factors, full bridge topology provides better characteristics compared with others and can be selected for a phase shift converter. Also in some applications other topologies still remain a favorite choice. © 2013 IEEE

Implementation of quantum gates based on geometric phases accumulated in the eigenstates of periodic invariant operators

We propose a new strategy to physically implement a universal set of quantum gates based on geometric phases accumulated in the nondegenerate eigenstates of a designated invariant operator in a periodic physical system. The system is driven to evolve in such a way that the dynamical phase shifts of the invariant operator eigenstates are the same (or {\it mod} $2\pi$) while the corresponding geometric phases are nontrivial. We illustrate how this strategy to work in a simple but typical NMR-type qubit system.Comment: 4 page