5 research outputs found
Superconducting Gatemon Qubit based on a Proximitized Two-Dimensional Electron Gas
The coherent tunnelling of Cooper pairs across Josephson junctions (JJs)
generates a nonlinear inductance that is used extensively in quantum
information processors based on superconducting circuits, from setting qubit
transition frequencies and interqubit coupling strengths, to the gain of
parametric amplifiers for quantum-limited readout. The inductance is either set
by tailoring the metal-oxide dimensions of single JJs, or magnetically tuned by
parallelizing multiple JJs in superconducting quantum interference devices
(SQUIDs) with local current-biased flux lines. JJs based on
superconductor-semiconductor hybrids represent a tantalizing all-electric
alternative. The gatemon is a recently developed transmon variant which employs
locally gated nanowire (NW) superconductor-semiconductor JJs for qubit control.
Here, we go beyond proof-of-concept and demonstrate that semiconducting
channels etched from a wafer-scale two-dimensional electron gas (2DEG) are a
suitable platform for building a scalable gatemon-based quantum computer. We
show 2DEG gatemons meet the requirements by performing voltage-controlled
single qubit rotations and two-qubit swap operations. We measure qubit
coherence times up to ~2 us, limited by dielectric loss in the 2DEG host
substrate
Evidence of topological superconductivity in planar Josephson junctions
Majorana zero modes are quasiparticle states localized at the boundaries of
topological superconductors that are expected to be ideal building blocks for
fault-tolerant quantum computing. Several observations of zero-bias conductance
peaks measured in tunneling spectroscopy above a critical magnetic field have
been reported as experimental indications of Majorana zero modes in
superconductor/semiconductor nanowires. On the other hand, two dimensional
systems offer the alternative approach to confine Ma jorana channels within
planar Josephson junctions, in which the phase difference {\phi} between the
superconducting leads represents an additional tuning knob predicted to drive
the system into the topological phase at lower magnetic fields. Here, we report
the observation of phase-dependent zero-bias conductance peaks measured by
tunneling spectroscopy at the end of Josephson junctions realized on a InAs/Al
heterostructure. Biasing the junction to {\phi} ~ {\pi} significantly reduces
the critical field at which the zero-bias peak appears, with respect to {\phi}
= 0. The phase and magnetic field dependence of the zero-energy states is
consistent with a model of Majorana zero modes in finite-size Josephson
junctions. Besides providing experimental evidence of phase-tuned topological
superconductivity, our devices are compatible with superconducting quantum
electrodynamics architectures and scalable to complex geometries needed for
topological quantum computing.Comment: main text and extended dat
On wealth and the diversity of friendships: High social class people around the world have fewer international friends
This is the final published version. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.paid.2015.07.040Having international social ties carries many potential advantages, including access to novel ideas and greater commercial opportunities. Yet little is known about who forms more international friendships. Here, we propose social class plays a key role in determining people’s internationalism. We conducted two studies to test whether social class is related positively to internationalism (the building social class hypothesis) or negatively to internationalism (the restricting social class hypothesis). In Study 1, we found that among individuals in the United States, social class was negatively related to percentage of friends on Facebook that are outside the United States. In Study 2, we extended these findings to the global level by analyzing country-level data on Facebook friends formed in 2011 (nearly 50 billion friendships) across 187 countries. We found that people from higher social class countries (as indexed by GDP per capita) had lower levels of internationalism—that is, they made more friendships domestically than abroad
Anodic Oxidation of Epitaxial Superconductor-Semiconductor Hybrids
We demonstrate a new fabrication process for hybrid
semiconductor-superconductor heterostructures based on anodic oxidation (AO),
allowing controlled thinning of epitaxial Al films. Structural and transport
studies of oxidized epitaxial Al films grown on insulating GaAs substrates
reveal spatial non-uniformity and enhanced critical temperature and magnetic
fields. Oxidation of epitaxial Al on hybrid InAs heterostructures with a
conducting quantum well show similarly enhanced superconducting properties
transferred to the two-dimensional electron gas (2DEG) by proximity effect,
with critical perpendicular magnetic fields up to 3.5 T. An insulating AlOx
film, that passivates the heterostructure from exposure to air, is obtained by
complete oxidation of the Al. It simultaneously removes the need to strip Al
which damages the underlying semiconductor. AO passivation yielded 2DEG
mobilities two times higher than similar devices with Al removed by wet
etching. An AO-passivated Hall bar showed quantum Hall features emerging at a
transverse field of 2.5 T, below the critical transverse field of thinned
films, eventually allowing transparent coupling of quantum Hall effect and
superconductivity. AO thinning and passivation are compatible with standard
lithographic techniques, giving lateral resolution below <50 nm. We demonstrate
local patterning of AO by realizing a semiconductor-based Josephson junction
operating up to 0.3 T perpendicular