191 research outputs found
Interplay of chiral and helical states in a Quantum Spin Hall Insulator lateral junction
We study the electronic transport across an electrostatically-gated lateral
junction in a HgTe quantum well, a canonical 2D topological insulator, with and
without applied magnetic field. We control carrier density inside and outside a
junction region independently and hence tune the number and nature of 1D edge
modes propagating in each of those regions. Outside the 2D gap, magnetic field
drives the system to the quantum Hall regime, and chiral states propagate at
the edge. In this regime, we observe fractional plateaus which reflect the
equilibration between 1D chiral modes across the junction. As carrier density
approaches zero in the central region and at moderate fields, we observe
oscillations in resistance that we attribute to Fabry-Perot interference in the
helical states, enabled by the broken time reversal symmetry. At higher fields,
those oscillations disappear, in agreement with the expected absence of helical
states when band inversion is lifted.Comment: 5 pages, 4 figures, supp. ma
Hrp Mutant of Pseudomonas syringae pv phaseolicola Induces Cell Wall Alterations but Not Membrane Damage Leading to the Hypersensitive Reaction in Lettuce
Electron-nuclear interaction in 13C nanotube double quantum dots
For coherent electron spins, hyperfine coupling to nuclei in the host
material can either be a dominant source of unwanted spin decoherence or, if
controlled effectively, a resource allowing storage and retrieval of quantum
information. To investigate the effect of a controllable nuclear environment on
the evolution of confined electron spins, we have fabricated and measured
gate-defined double quantum dots with integrated charge sensors made from
single-walled carbon nanotubes with a variable concentration of 13C (nuclear
spin I=1/2) among the majority zero-nuclear-spin 12C atoms. Spin-sensitive
transport in double-dot devices grown using methane with the natural abundance
(~ 1%) of 13C is compared with similar devices grown using an enhanced (~99%)
concentration of 13C. We observe strong isotope effects in spin-blockaded
transport, and from the dependence on external magnetic field, estimate the
hyperfine coupling in 13C nanotubes to be on the order of 100 micro-eV, two
orders of magnitude larger than anticipated theoretically. 13C-enhanced
nanotubes are an interesting new system for spin-based quantum information
processing and memory, with nuclei that are strongly coupled to gate-controlled
electrons, differ from nuclei in the substrate, are naturally confined to one
dimension, lack quadrupolar coupling, and have a readily controllable
concentration from less than one to 10^5 per electron.Comment: supplementary discussion at http://marcuslab.harvard.edu/13CSupp.pd
PREDICT-PD: Identifying risk of Parkinson's disease in the community: methods and baseline results
To present methods and baseline results for an online screening tool to identify increased risk for Parkinson's disease (PD) in the UK population
Toxicity of unsaturated fatty acids to the biohydrogenating ruminal bacterium, Butyrivibrio fibrisolvens
Peer reviewedPublisher PD
Relaxation and Dephasing in a Two-Electron 13C Nanotube Double Quantum Dot
We use charge sensing of Pauli blockade (including spin and isospin) in a
two-electron 13C nanotube double quantum dot to measure relaxation and
dephasing times. The relaxation time, T1, first decreases with parallel
magnetic field then goes through a minimum in a field of 1.4 T. We attribute
both results to the spin-orbit-modified electronic spectrum of carbon
nanotubes, which suppresses hyperfine mediated relaxation and enhances
relaxation due to soft phonons. The inhomogeneous dephasing time, T2*, is
consistent with previous data on hyperfine coupling strength in 13C nanotubes.Comment: related papers at http://marcuslab.harvard.ed
Introduction to topological superconductivity and Majorana fermions
This short review article provides a pedagogical introduction to the rapidly
growing research field of Majorana fermions in topological superconductors. We
first discuss in some details the simplest "toy model" in which Majoranas
appear, namely a one-dimensional tight-binding representation of a p-wave
superconductor, introduced more than ten years ago by Kitaev. We then give a
general introduction to the remarkable properties of Majorana fermions in
condensed matter systems, such as their intrinsically non-local nature and
exotic exchange statistics, and explain why these quasiparticles are suspected
to be especially well suited for low-decoherence quantum information
processing. We also discuss the experimentally promising (and perhaps already
successfully realized) possibility of creating topological superconductors
using semiconductors with strong spin-orbit coupling, proximity-coupled to
standard s-wave superconductors and exposed to a magnetic field. The goal is to
provide an introduction to the subject for experimentalists or theorists who
are new to the field, focusing on the aspects which are most important for
understanding the basic physics. The text should be accessible for readers with
a basic understanding of quantum mechanics and second quantization, and does
not require knowledge of quantum field theory or topological states of matter.Comment: 21 pages, 5 figure
Demonstration of Universal Parametric Entangling Gates on a Multi-Qubit Lattice
We show that parametric coupling techniques can be used to generate selective
entangling interactions for multi-qubit processors. By inducing coherent
population exchange between adjacent qubits under frequency modulation, we
implement a universal gateset for a linear array of four superconducting
qubits. An average process fidelity of is estimated for
three two-qubit gates via quantum process tomography. We establish the
suitability of these techniques for computation by preparing a four-qubit
maximally entangled state and comparing the estimated state fidelity against
the expected performance of the individual entangling gates. In addition, we
prepare an eight-qubit register in all possible bitstring permutations and
monitor the fidelity of a two-qubit gate across one pair of these qubits.
Across all such permutations, an average fidelity of
is observed. These results thus offer a path to a scalable architecture with
high selectivity and low crosstalk
- …