113 research outputs found
Coulomb-assisted braiding of Majorana fermions in a Josephson junction array
We show how to exchange (braid) Majorana fermions in a network of
superconducting nanowires by control over Coulomb interactions rather than
tunneling. Even though Majorana fermions are charge-neutral quasiparticles
(equal to their own antiparticle), they have an effective long-range
interaction through the even-odd electron number dependence of the
superconducting ground state. The flux through a split Josephson junction
controls this interaction via the ratio of Josephson and charging energies,
with exponential sensitivity. By switching the interaction on and off in
neighboring segments of a Josephson junction array, the non-Abelian braiding
statistics can be realized without the need to control tunnel couplings by gate
electrodes. This is a solution to the problem how to operate on topological
qubits when gate voltages are screened by the superconductor
Top-transmon: hybrid superconducting qubit for parity-protected quantum computation
Qubits constructed from uncoupled Majorana fermions are protected from
decoherence, but to perform a quantum computation this topological protection
needs to be broken. Parity-protected quantum computation breaks the protection
in a minimally invasive way, by coupling directly to the fermion parity of the
system --- irrespective of any quasiparticle excitations. Here we propose to
use a superconducting charge qubit in a transmission line resonator (a socalled
transmon) to perform parity-protected rotations and read-out of a topological
(top) qubit. The advantage over an earlier proposal using a flux qubit is that
the coupling can be switched on and off with exponential accuracy, promising a
reduced sensitivity to charge noise.Comment: 7 pages, 5 figure
Ballistic Josephson junctions in edge-contacted graphene
Hybrid graphene-superconductor devices have attracted much attention since
the early days of graphene research. So far, these studies have been limited to
the case of diffusive transport through graphene with poorly defined and modest
quality graphene-superconductor interfaces, usually combined with small
critical magnetic fields of the superconducting electrodes. Here we report
graphene based Josephson junctions with one-dimensional edge contacts of
Molybdenum Rhenium. The contacts exhibit a well defined, transparent interface
to the graphene, have a critical magnetic field of 8 Tesla at 4 Kelvin and the
graphene has a high quality due to its encapsulation in hexagonal boron
nitride. This allows us to study and exploit graphene Josephson junctions in a
new regime, characterized by ballistic transport. We find that the critical
current oscillates with the carrier density due to phase coherent interference
of the electrons and holes that carry the supercurrent caused by the formation
of a Fabry-P\'{e}rot cavity. Furthermore, relatively large supercurrents are
observed over unprecedented long distances of up to 1.5 m. Finally, in the
quantum Hall regime we observe broken symmetry states while the contacts remain
superconducting. These achievements open up new avenues to exploit the Dirac
nature of graphene in interaction with the superconducting state.Comment: Updated version after peer review. Includes supplementary material
and ancillary file with source code for tight binding simulation
The Aharonov-Bohm effect for massless Dirac fermions and the spectral flow of Dirac type operators with classical boundary conditions
We compute, in topological terms, the spectral flow of an arbitrary family of
self-adjoint Dirac type operators with classical (local) boundary conditions on
a compact Riemannian manifold with boundary under the assumption that the
initial and terminal operators of the family are conjugate by a bundle
automorphism. This result is used to study conditions for the existence of
nonzero spectral flow of a family of self-adjoint Dirac type operators with
local boundary conditions in a two-dimensional domain with nontrivial topology.
Possible physical realizations of nonzero spectral flow are discussed.Comment: 15 pages, 6 figures. Submitted to Theoretical and Mathematical
Physics. v2: A change has been made to the paragraph describing the previous
work of M. Prokhorov
How to realize a robust practical Majorana chain in a quantum dot-superconductor linear array
Semiconducting nanowires in proximity to superconductors are promising
experimental systems for Majorana fermions, which may ultimately be used as
building blocks for topological quantum computers. A serious challenge in the
experimental realization of the Majorana fermions is the supression of
topological superconductivity by disorder. We show that Majorana fermions
protected by a robust topological gap can occur at the ends of a chain of
quantum dots connected by s-wave superconductors. In the appropriate parameter
regime, we establish that the quantum dot/superconductor system is equivalent
to a 1D Kitaev chain, which can be tuned to be in a robust topological phase
with Majorana end modes even in the case where the quantum dots and
superconductors are both strongly disordered. Such a spin-orbit coupled quantum
dot - s-wave superconductor array provides an ideal experimental platform for
the observation of non-Abelian Majorana modes.Comment: 8 pages; 3 figures; version 2: Supplementary material updated to
include more general proof for localized Majorana fermion
Strained graphene structures: from valleytronics to pressure sensing
Due to its strong bonds graphene can stretch up to 25% of its original size
without breaking. Furthermore, mechanical deformations lead to the generation
of pseudo-magnetic fields (PMF) that can exceed 300 T. The generated PMF has
opposite direction for electrons originating from different valleys. We show
that valley-polarized currents can be generated by local straining of
multi-terminal graphene devices. The pseudo-magnetic field created by a
Gaussian-like deformation allows electrons from only one valley to transmit and
a current of electrons from a single valley is generated at the opposite side
of the locally strained region. Furthermore, applying a pressure difference
between the two sides of a graphene membrane causes it to bend/bulge resulting
in a resistance change. We find that the resistance changes linearly with
pressure for bubbles of small radius while the response becomes non-linear for
bubbles that stretch almost to the edges of the sample. This is explained as
due to the strong interference of propagating electronic modes inside the
bubble. Our calculations show that high gauge factors can be obtained in this
way which makes graphene a good candidate for pressure sensing.Comment: to appear in proceedings of the NATO Advanced Research Worksho
Edge States of Monolayer and Bilayer Graphene Nanoribbons
On the basis of tight-binding lattice model, the edge states of monolayer and
bilayer graphene nanoribbons (GNRs) with different edge terminations are
studied. The effects of edge-hopping modulation, spin-orbital coupling (SOC),
and bias voltage on bilayer GNRs are discussed. We observe the following: (i)
Some new extra edge states can be created by edge-hopping modulation for
monolayer GNRs. (ii) Intralayer Rashba SOC plays a role in depressing the band
energy gap opened by intrinsic SOC for both monolayer and bilayer GNRs.
An almost linear dependent relation, i.e., , is found. (iii)
Although the bias voltage favors a bulk energy gap for bilayer graphene without
intrinsic SOC, it tends to reduce the gap induced by intrinsic SOC. (iv) The
topological phase of the quantum spin Hall effect can be destroyed completely
by interlayer Rashba SOC for bilayer GNRs.Comment: 6 pages, 6 figure
Detecting non-Abelian statistics of Majorana fermions in quantum nanowire networks
We propose a scheme in semiconducting quantum nanowires structure to
demonstrate the non-Abelian statistics for Majorana fermions in terms of braid
group. The Majorana fermions are localized at the endpoints of semiconducting
wires, which are deposited on an \emph{s}-wave superconductor. The non-Abelian
nature of Majorana fermion is manifested by the fact that the output of the
different applied orders of two operations, constructed by the braid group
elements, are different. In particular, the difference can be unambiguously
imprinted on the quantum states of a superconducting flux qubit.Comment: V1, accepted for publication at JETP Letters; v2, typos correcte
Zero-voltage conductance peak from weak antilocalization in a Majorana nanowire
We show that weak antilocalization by disorder competes with resonant Andreev
reflection from a Majorana zero-mode to produce a zero-voltage conductance peak
of order e^2/h in a superconducting nanowire. The phase conjugation needed for
quantum interference to survive a disorder average is provided by particle-hole
symmetry - in the absence of time-reversal symmetry and without requiring a
topologically nontrivial phase. We identify methods to distinguish the Majorana
resonance from the weak antilocalization effect.Comment: 13 pages, 8 figures. Addendum, February 2014: Appendix B shows
results for weak antilocalization in the circular ensemble. (This appendix is
not in the published version.
Long-term follow-up of a female patient with Weber–Christian panniculitis
Weber–Christian disease is a rare form of lobular panniculitis characterized by painful subcutaneous nodules on the trunk and limbs commonly concurrent with systemic manifestations and a tendency to relapse. Known are only sporadic cases of a long-term (more than 5-year) followup of patients with Weber–Christian disease. For this reason, there are neither prognostic criteria for the disease no proven conventional treatment option now. The paper describes a clinical case of a female patient with a verified diagnosis of Weber–Christian disease who has a sustained subclinical remission due to low-dose prednisone use and basic antirecurrent therapy
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