1,330 research outputs found
Lifetime of Majorana qubits in Rashba nanowires with non-uniform chemical potential
We study the lifetime of topological qubits based on Majorana bound states
hosted in a one-dimensional Rashba nanowire (NW) with proximity-induced
superconductivity and non-uniform chemical potential needed for manipulation
and read-out. If nearby gates tune the chemical potential locally so that part
of the NW is in the trivial phase, Andreev bound states (ABSs) can emerge which
are localized at the interface between topological and trivial phases with
energies significantly less than the gap. The emergence of such subgap states
strongly decreases the Majorana qubit lifetime at finite temperatures due to
local perturbations that can excite the system into these ABSs. Using Keldysh
formalism, we study such excitations caused by fluctuating charges in
capacitively coupled gates and calculate the corresponding Majorana lifetimes
due to thermal noise, which are shown to be much shorter than those in NWs with
uniform chemical potential.Comment: 9 pages, 8 figure
Conductance fluctuations in diffusive rings: Berry phase effects and criteria for adiabaticity
We study Berry phase effects on conductance properties of diffusive
mesoscopic conductors, which are caused by an electron spin moving through an
orientationally inhomogeneous magnetic field. Extending previous work, we start
with an exact, i.e. not assuming adiabaticity, calculation of the universal
conductance fluctuations in a diffusive ring within the weak localization
regime, based on a differential equation which we derive for the diffuson in
the presence of Zeeman coupling to a magnetic field texture. We calculate the
field strength required for adiabaticity and show that this strength is reduced
by the diffusive motion. We demonstrate that not only the phases but also the
amplitudes of the h/2e Aharonov-Bohm oscillations are strongly affected by the
Berry phase. In particular, we show that these amplitudes are completely
suppressed at certain magic tilt angles of the external fields, and thereby
provide a useful criterion for experimental searches. We also discuss Berry
phase-like effects resulting from spin-orbit interaction in diffusive
conductors and derive exact formulas for both magnetoconductance and
conductance fluctuations. We discuss the power spectra of the
magnetoconductance and the conductance fluctuations for inhomogeneous magnetic
fields and for spin-orbit interaction.Comment: 18 pages, 13 figures; minor revisions. To appear in Phys. Rev.
Quantum charge pumping through fractional Fermions in charge density modulated quantum wires and Rashba nanowires
We study the phenomenon of adiabatic quantum charge pumping in systems
supporting fractionally charged fermionic bound states, in two different
setups. The first quantum pump setup consists of a charge-density-modulated
quantum wire, and the second one is based on a semiconducting nanowire with
Rashba spin-orbit interaction, in the presence of a spatially oscillating
magnetic field. In both these quantum pumps transport is investigated in a
N-X-N geometry, with the system of interest (X) connected to two normal-metal
leads (N), and the two pumping parameters are the strengths of the effective
wire-lead barriers. Pumped charge is calculated within the scattering matrix
formalism. We show that quantum pumping in both setups provides a unique
signature of the presence of the fractional-fermion bound states, in terms of
asymptotically quantized pumped charge. Furthermore, we investigate shot noise
arising due to quantum pumping, verifying that quantized pumped charge
corresponds to minimal shot noise.Comment: This is the published versio
Wheat experimental results
The development of wheat relative to frost susceptibility, 87NA10. Optimum flowering times of wheat, 87NA11 and 87WH8. Grain growth and development of a historical set of wheats
Factors affecting frost damage to wheat in Western Australia
It may at first seem strange that precious research funds are being channelled into a project concerned with frost damage in a country where high temperatures and moisture stress limit the growth of plants for a large portion of the year. But cereal crops are only grown in the temperate zones of the continent during winter and spring when they may be exposed to low diurnal temperatures. In many areas cold damage is irregular and rare,however it limits yields not only by causing actual damage but also by restricting the most effective period for flowering. For muck of the Australian wheat crop excessive risk of frost damage sets the earliest date for flowering and the beginning of grain filling at a time when other conditions are at their most favourable for carbohydrate assimilation. The latest date for flowering is determined by high temperatures and low water availability towards the end of the grain filling phase. An optimum flowering time and maximised yield in the long term are achieved when a compromise between the effects of frost and drought is reached
Quantum Computation and Spin Electronics
In this chapter we explore the connection between mesoscopic physics and
quantum computing. After giving a bibliography providing a general introduction
to the subject of quantum information processing, we review the various
approaches that are being considered for the experimental implementation of
quantum computing and quantum communication in atomic physics, quantum optics,
nuclear magnetic resonance, superconductivity, and, especially, normal-electron
solid state physics. We discuss five criteria for the realization of a quantum
computer and consider the implications that these criteria have for quantum
computation using the spin states of single-electron quantum dots. Finally, we
consider the transport of quantum information via the motion of individual
electrons in mesoscopic structures; specific transport and noise measurements
in coupled quantum dot geometries for detecting and characterizing
electron-state entanglement are analyzed.Comment: 28 pages RevTeX, 4 figures. To be published in "Quantum Mesoscopic
Phenomena and Mesoscopic Devices in Microelectronics," eds. I. O. Kulik and
R. Ellialtioglu (NATO Advanced Study Institute, Turkey, June 13-25, 1999
Degeneracy lifting of Majorana bound states due to electron-phonon interactions
We study theoretically how electron-phonon interaction affects the energies
and level broadening (inverse lifetime) of Majorana bound states (MBSs) in a
clean topological nanowire at low temperatures. At zero temperature, the energy
splitting between the right and left MBSs remains exponentially small with
increasing nanowire length . At finite temperatures, however, the absorption
of thermal phonons leads to the broadening of energy levels of the MBSs that
does not decay with system length, and the coherent absorption/emission of
phonons at opposite ends of the nanowire results in MBSs energy splitting that
decays only as an inverse power-law in . Both effects remain exponential in
temperature. In the case of quantized transverse motion of phonons, the
presence of Van Hove singularities in the phonon density of states causes
additional resonant enhancement of both the energy splitting and the level
broadening of the MBSs. This is the most favorable case to observe the
phonon-induced energy splitting of MBSs as it becomes much larger than the
broadening even if the topological nanowire is much longer than the coherence
length. We also calculate the charge and spin associated with the energy
splitting of the MBSs induced by phonons. We consider both a spinless
low-energy continuum model, which we evaluate analytically, as well as a
spinful lattice model for a Rashba nanowire, which we evaluate numerically
Lower bound for electron spin entanglement from beamsplitter current correlations
We determine a lower bound for the entanglement of pairs of electron spins
injected into a mesoscopic conductor. The bound can be expressed in terms of
experimentally accessible quantities, the zero-frequency current correlators
(shot noise power or cross-correlators) after transmission through an
electronic beam splitter. The effect of spin relaxation (T_1 processes) and
decoherence (T_2 processes) during the ballistic coherent transmission of the
carriers in the wires is taken into account within Bloch theory. The presence
of a variable inhomogeneous magnetic field allows the determination of a useful
lower bound for the entanglement of arbitrary entangled states. The decrease in
entanglement due to thermally mixed states is studied. Both the entanglement of
the output of a source (entangler) and the relaxation (T_1) and decoherence
(T_2) times can be determined.Comment: 4 pages, 3 figure
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