4,698 research outputs found
Quantum simulation of topological Majorana bound states and their universal quantum operations using charge-qubit arrays
Majorana bound states have been a focus of condensed matter research for
their potential applications in topological quantum computation. Here we
utilize two charge-qubit arrays to explicitly simulate a DIII class
one-dimensional superconductor model where Majorana end states can appear.
Combined with one braiding operation, universal single-qubit operations on a
Majorana-based qubit can be implemented by a controllable inductive coupling
between two charge qubits at the ends of the arrays. We further show that in a
similar way, a controlled-NOT gate for two topological qubits can be simulated
in four charge-qubit arrays. Although the current scheme may not truly realize
topological quantum operations, we elaborate that the operations in
charge-qubit arrays are indeed robust against certain local perturbations.Comment: 5 pages, 3 figure
Modeling Light-Extraction Characteristics of Packaged Light-Emitting Diodes
We employ a Monte Carlo ray-tracing technique to model light-extraction characteristics of light-emitting diodes. By relaxing restrictive assumptions on photon traversal history, our method improves upon available analytical models for estimating light-extraction efficiencies from bare LED chips, and enhances modeling capabilities by realistically treating the various processes which photons can encounter in a packaged LED. Our method is not only capable of calculating extraction efficiencies, but can also provide extensive statistical information on photon extraction processes, and predict LED spatial emission characteristics
Interface roughness effects on transport in tunnel structures
Direct simulations of interface roughness effects on transport properties of tunnel structures are performed using the planar supercell stack method. The method allows for the inclusion of realistic three-dimensional rough interfacial geometries in transport calculations. For double barrier resonant tunneling structures, we used our method to analyze the effect of roughness at each of the four interfaces, and to test for sensitivity of transport properties to island size and height. Our simulations yields the following conclusions: (1) We find that scattering of off-resonance states into on-resonance states provides the dominant contribution to interface roughness assisted tunneling. Analyses of scattering strength sensitivity to interface layer configurations reveals preferential scattering into Delta k parallel to approximate to 2 pi/lambda states, where lambda is the island size. (2) We find that roughness at interfaces adjacent to the quantum well can cause lateral localization of wave functions, which increases with island size and depth. Lateral localization can result in the broadening and shifting of transmission resonances, and the introduction of preferential transmission paths. In structures with wide and tall islands, it is possible to find localization over "islands" as well as localization over "oceans." (3) The leading rough interface is the strongest off-resonance scatterer, while rough interfaces adjacent to quantum well are the strongest on-resonance scatterers. The trailing interface is the weakest scatterer
Correlation evolution and monogamy of two geometric quantum discords in multipartite systems
We explore two different geometric quantum discords defined respectively via
the trace norm (GQD-1) and Hilbert-Schmidt norm (GQD-2) in multipartite
systems. A rigorous hierarchy relation is revealed for the two GQDs in a class
of symmetric two-qubit -shape states. For multiqubit pure states, it is
found that both GQDs are related to the entanglement concurrence, with the
hierarchy relation being saturated. Furthermore, we look into a four-partite
dynamical system consisting of two cavities interacting with independent
reservoirs. It is found that the GQD-2 can exhibit various sudden change
behaviours, while the GQD-1 only evolves asymptotically, with the two GQDs
exhibiting different monogamous properties.Comment: 5 pages, 3 figure
Spin-charge separation: From one hole to finite doping
In the presence of nonlocal phase shift effects, a quasiparticle can remain
topologically stable even in a spin-charge separation state due to the
confinement effect introduced by the phase shifts at finite doping. True
deconfinement only happens in the zero-doping limit where a bare hole can lose
its integrity and decay into holon and spinon elementary excitations. The Fermi
surface structure is completely different in these two cases, from a large
band-structure-like one to four Fermi points in one-hole case, and we argue
that the so-called underdoped regime actually corresponds to a situation in
between.Comment: 4 pages, 2 figures, presented in M2S-HTSC-VI conference (2000
Description of bulk inversion asymmetry in the effective-bond-orbital model
We have extended the effective-bond-orbital model (EBOM) method [Y. C. Chang, Phys. Rev. B 37, 8215 (1988)] to include the effects of the bulk inversion asymmetry (BIA) present in zinc blendes. This is accomplished without adding to the number of basis states or extending the range of interaction. We have also investigated a variant form of the EBOM proposed in the original formulation that offers improved zone-center behavior, but may also generate spurious solutions in heterostructure calculations due to poor description of bulk zone-boundary band structure. We offer suggestions for avoiding this problem so that this variant form of EBOM may be used safely. In general, we find that the addition of BIA effects in EBOM results in improved descriptions of zone-center band structure, but also in a loss of accuracy far from the Brillouin-zone center. We illustrate the use of the BIA extension with band-structure calculations for bulk GaSb. We show that the spin splitting predicted by the extended EBOM method for an AlSb/GaSb superlattice is in good agreement with k·p calculations that include BIA effects
Numerical spurious solutions in the effective mass approximation
We have characterized a class of spurious solutions that appears when using the finite difference method to solve the effective mass approximation equations. We find that the behavior of these solutions as predicted by our model shows excellent agreement with numerical results. Using this interpretation we find a set of analytical expressions for conditions that the Luttinger parameters must satisfy to avoid spurious solutions. Finally, we use these conditions to check commonly used sets of parameters for their potential for generating this class of spurious solutions
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