181 research outputs found
Roadmap for gallium arsenide spin qubits
Gate-defined quantum dots in gallium arsenide (GaAs) have been used
extensively for pioneering spin qubit devices due to the relative simplicity of
fabrication and favourable electronic properties such as a single conduction
band valley, a small effective mass, and stable dopants. GaAs spin qubits are
readily produced in many labs and are currently studied for various
applications, including entanglement, quantum non-demolition measurements,
automatic tuning, multi-dot arrays, coherent exchange coupling, and
teleportation. Even while much attention is shifting to other materials, GaAs
devices will likely remain a workhorse for proof-of-concept quantum information
processing and solid-state experiments.Comment: This section is part of a roadmap on quantum technologies and
comprises 4 pages with 2 figure
Anisotropic magnetoresistance and anisotropic tunneling magnetoresistance due to quantum interference in ferromagnetic metal break junctions
We measure the low-temperature resistance of permalloy break junctions as a
function of contact size and the magnetic field angle, in applied fields large
enough to saturate the magnetization. For both nanometer-scale metallic
contacts and tunneling devices we observe large changes in resistance with
angle, as large as 25% in the tunneling regime. The pattern of
magnetoresistance is sensitive to changes in bias on a scale of a few mV. We
interpret the effect as a consequence of conductance fluctuations due to
quantum interference.Comment: 4 pages, 4 figures. Changes in response to reviewer comments. New
data provide information about the mechanism causing the AMR and TAM
Metal-nanoparticle single-electron transistors fabricated using electromigration
We have fabricated single-electron transistors from individual metal
nanoparticles using a geometry that provides improved coupling between the
particle and the gate electrode. This is accomplished by incorporating a
nanoparticle into a gap created between two electrodes using electromigration,
all on top of an oxidized aluminum gate. We achieve sufficient gate coupling to
access more than ten charge states of individual gold nanoparticles (5-15 nm in
diameter). The devices are sufficiently stable to permit spectroscopic studies
of the electron-in-a-box level spectra within the nanoparticle as its charge
state is varied.Comment: 3 pages, 3 figures, submitted to AP
From ballistic transport to tunneling in electromigrated ferromagnetic breakjunctions
We fabricate ferromagnetic nanowires with constrictions whose cross section
can be reduced gradually from 100 nm to the atomic scale and eventually to the
tunneling regime by means of electromigration. These devices are mechanically
stable against magnetostriction and magnetostatic effects. We measure
magnetoresistances ~ 0.3% for 100*30 nm^2 constrictions, increasing to a
maximum of 80% for atomic-scale widths. These results are consistent with a
geometrically-constrained domain wall trapped at the constriction. For the
devices in the tunneling regime we observe large fluctuations in MR, between
-10 and 85%.Comment: 4 pages, 5 figure
Giant spin rotation under quasiparticle-photoelectron conversion: Joint effect of sublattice interference and spin-orbit coupling
Spin- and angular-resolved photoemission spectroscopy is a basic experimental
tool for unveiling spin polarization of electron eigenstates in crystals. We
prove, by using spin-orbit coupled graphene as a model, that photoconversion of
a quasiparticle inside a crystal into a photoelectron can be accompanied with a
dramatic change in its spin polarization, up to a total spin flip. This
phenomenon is typical of quasiparticles residing away from the Brillouin zone
center and described by higher rank spinors, and results in exotic patterns in
the angular distribution of photoelectrons.Comment: 5 pages, 4 figures, improved presentation and figures, added
reference
A Semiconductor Nanowire-Based Superconducting Qubit
We introduce a hybrid qubit based on a semiconductor nanowire with an
epitaxially grown superconductor layer. Josephson energy of the transmon-like
device ("gatemon") is controlled by an electrostatic gate that depletes
carriers in a semiconducting weak link region. Strong coupling to an on-chip
microwave cavity and coherent qubit control via gate voltage pulses is
demonstrated, yielding reasonably long relaxation times (0.8 {\mu}s) and
dephasing times (1 {\mu}s), exceeding gate operation times by two orders of
magnitude, in these first-generation devices. Because qubit control relies on
voltages rather than fluxes, dissipation in resistive control lines is reduced,
screening reduces crosstalk, and the absence of flux control allows operation
in a magnetic field, relevant for topological quantum information
Quantum transport in carbon nanotubes
Carbon nanotubes are a versatile material in which many aspects of condensed
matter physics come together. Recent discoveries, enabled by sophisticated
fabrication, have uncovered new phenomena that completely change our
understanding of transport in these devices, especially the role of the spin
and valley degrees of freedom. This review describes the modern understanding
of transport through nanotube devices.
Unlike conventional semiconductors, electrons in nanotubes have two angular
momentum quantum numbers, arising from spin and from valley freedom. We focus
on the interplay between the two. In single quantum dots defined in short
lengths of nanotube, the energy levels associated with each degree of freedom,
and the spin-orbit coupling between them, are revealed by Coulomb blockade
spectroscopy. In double quantum dots, the combination of quantum numbers
modifies the selection rules of Pauli blockade. This can be exploited to read
out spin and valley qubits, and to measure the decay of these states through
coupling to nuclear spins and phonons. A second unique property of carbon
nanotubes is that the combination of valley freedom and electron-electron
interactions in one dimension strongly modifies their transport behaviour.
Interaction between electrons inside and outside a quantum dot is manifested in
SU(4) Kondo behavior and level renormalization. Interaction within a dot leads
to Wigner molecules and more complex correlated states.
This review takes an experimental perspective informed by recent advances in
theory. As well as the well-understood overall picture, we also state clearly
open questions for the field. These advances position nanotubes as a leading
system for the study of spin and valley physics in one dimension where
electronic disorder and hyperfine interaction can both be reduced to a very low
level.Comment: In press at Reviews of Modern Physics. 68 pages, 55 figure
Hole Spin Coherence in a Ge/Si Heterostructure Nanowire
Relaxation and dephasing of hole spins are measured in a gate-defined Ge/Si
nanowire double quantum dot using a fast pulsed-gate method and dispersive
readout. An inhomogeneous dephasing time
exceeds corresponding measurements in III-V semiconductors by more than an
order of magnitude, as expected for predominately nuclear-spin-free materials.
Dephasing is observed to be exponential in time, indicating the presence of a
broadband noise source, rather than Gaussian, previously seen in systems with
nuclear-spin-dominated dephasing.Comment: 15 pages, 4 figure
Observation and Spectroscopy of a Two-Electron Wigner Molecule in an Ultra-Clean Carbon Nanotube
Coulomb interactions can have a decisive effect on the ground state of
electronic systems. The simplest system in which interactions can play an
interesting role is that of two electrons on a string. In the presence of
strong interactions the two electrons are predicted to form a Wigner molecule,
separating to the ends of the string due to their mutual repulsion. This
spatial structure is believed to be clearly imprinted on the energy spectrum,
yet to date a direct measurement of such a spectrum in a controllable
one-dimensional setting is still missing. Here we use an ultra-clean suspended
carbon nanotube to realize this system in a tunable potential. Using tunneling
spectroscopy we measure the excitation spectra of two interacting carriers,
electrons or holes, and identify seven low-energy states characterized by their
spin and isospin quantum numbers. These states fall into two multiplets
according to their exchange symmetries. The formation of a strongly-interacting
Wigner molecule is evident from the small energy splitting measured between the
two multiplets, that is quenched by an order of magnitude compared to the
non-interacting value. Our ability to tune the two-electron state in space and
to study it for both electrons and holes provides an unambiguous demonstration
of the fundamental Wigner molecule state.Comment: SP and FK contributed equally to this wor
- …