Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: We study the implications of spin-orbit interaction (SOI) for two-qubit gates (TQGs) in semiconductor spin qubit platforms. The exchange interaction governing qubit pairs is anisotropic under SOI, posing a problem for conventional TQGs derived under the Heisenberg exchange. After developing a concise form of the effective two-qubit Hamiltonian under SOI, we use it to derive properties of rotating-frame evolution. Two main observations are made. First, in contrary to past belief, we find that an appropriate amount of SOI can significantly enhance the controlled-phase gate fidelity compared to the no-SOI case. Second, SOI enables novel two-qubit dynamics, that are conventionally inaccessible through DC evolution, such as the reflection gate and the controlled-not gate.

Abstract: We generalize the nested Wilson loop formalism, which has been playing an important role in the study of topological quadrupole insulators, to two-dimensional (2D) and 3D nonsymmorphic materials with higher-order topology. In particular, certain 3D Dirac semimetals exhibit 1D higher-order Fermi arc (HOFA) states localizing on hinges where two surfaces meet and connecting the projection of the bulk Dirac points. We discover that the generalized nested Berry phase (gNBP) derived from this formalism is the bulk topological indicator determining the existence/absence of HOFAs, revealing a direct bulk-hinge correspondence in 3D Dirac semimetals. Finally, we study the Dirac semimetals NaCuSe and KMgBi based on first-principles calculations and explicitly show that the change in gNBP adjacent to the Dirac point corresponds to the termination of HOFAs at the projection of Dirac points on the hinge. Our findings not only improve the understanding of the bulk-hinge correspondence in topological Dirac semimetals but also provide a general formalism for studying the higher-order topology in nonsymmorphic systems.

Abstract: Within the Floquet theory of periodically driven quantum systems, we demonstrate that a circularly polarized off-resonant electromagnetic field can destroy the electron states bound by three-dimensional attractive potentials. As a consequence, the optically induced delocalization of bound electrons appears. The effect arises from the changing of topological structure of a potential landscape under a circularly polarized off-resonant electromagnetic field which turns simply connected potentials into doubly connected ones. Possible manifestations of the effect are discussed for conduction electrons in condensed-matter structures.