6,548 research outputs found
Braiding of non-Abelian anyons using pairwise interactions
The common approach to topological quantum computation is to implement
quantum gates by adiabatically moving non-Abelian anyons around each other.
Here we present an alternative perspective based on the possibility of
realizing the exchange (braiding) operators of anyons by adiabatically varying
pairwise interactions between them rather than their positions. We analyze a
system composed by four anyons whose couplings define a T-junction and we show
that the braiding operator of two of them can be obtained through a particular
adiabatic cycle in the space of the coupling parameters. We also discuss how to
couple this scheme with anyonic chains in order to recover the topological
protection.Comment: 8 pages, 7 figures. Errors corrected, clarifications and comments
adde
Single fermion manipulation via superconducting phase differences in multiterminal Josephson junctions
We show how the superconducting phase difference in a Josephson junction may
be used to split the Kramers degeneracy of its energy levels and to remove all
the properties associated with time reversal symmetry. The superconducting
phase difference is known to be ineffective in two-terminal short Josephson
junctions, where irrespective of the junction structure the induced Kramers
degeneracy splitting is suppressed and the ground state fermion parity must
stay even, so that a protected zero-energy Andreev level crossing may never
appear. Our main result is that these limitations can be completely avoided by
using multi-terminal Josephson junctions. There the Kramers degeneracy breaking
becomes comparable to the superconducting gap, and applying phase differences
may cause the change of the ground state fermion parity from even to odd. We
prove that the necessary condition for the appearance of a fermion parity
switch is the presence of a "discrete vortex" in the junction: the situation
when the phases of the superconducting leads wind by . Our approach
offers new strategies for creation of Majorana bound states as well as spin
manipulation. Our proposal can be implemented using any low density, high
spin-orbit material such as InAs quantum wells, and can be detected using
standard tools.Comment: Source code available as ancillary files. 10 pages, 7 figures. v2:
minor changes, published versio
Thermal conductance as a probe of the non-local order parameter for a topological superconductor with gauge fluctuations
We investigate the effect of quantum phase slips on a helical quantum wire
coupled to a superconductor by proximity. The effective low-energy description
of the wire is that of a Majorana chain minimally coupled to a dynamical
gauge field. Hence the wire emulates a matter-coupled gauge
theory, with fermion parity playing the role of the gauged global symmetry.
Quantum phase slips lift the ground state degeneracy associated with unpaired
Majorana edge modes at the ends of the chain, a change that can be understood
as a transition between the confined and the Higgs-mechanism regimes of the
gauge theory. We identify the quantization of thermal conductance at the
transition as a robust experimental feature separating the two regimes. We
explain this result by establishing a relation between thermal conductance and
the Fredenhagen-Marcu string order-parameter for confinement in gauge theories.
Our work indicates that thermal transport could serve as a measure of non-local
order parameters for emergent or simulated topological quantum order.Comment: 5 pages, 2 figures; v2: different introduction, added references,
updated figure 2; published version to appear in PR
Minimal circuit for a flux-controlled Majorana qubit in a quantum spin-Hall insulator
We construct a minimal circuit, based on the top-transmon design, to rotate a
qubit formed out of four Majorana zero-modes at the edge of a two-dimensional
topological insulator. Unlike braiding operations, generic rotations have no
topological protection, but they do allow for a full characterization of the
coherence times of the Majorana qubit. The rotation is controlled by variation
of the flux through a pair of split Josephson junctions in a Cooper pair box,
without any need to adjust gate voltages. The Rabi oscillations of the Majorana
qubit can be monitored via oscillations in the resonance frequency of the
microwave cavity that encloses the Cooper pair box.Comment: Contribution for the proceedings of the Nobel Symposium on
topological insulators. 8 pages, 6 figure
Large Surveys in Cosmology: The Changing Sociology
Galaxy redshift surveys and Cosmic Microwave Background experiments are
undertaken with larger and larger teams, in a fashion reminiscent of particle
physics experiments and the human genome projects. We discuss the role of young
researchers, the issue of multiple authorship, and ways to communicate
effectively in teams of tens to hundreds of collaborators.Comment: Invited article for "Organizations and Strategies in Astronomy II",
ed. A. Heck, Kluwer Acad. Publ., in press (7 pages, no figures
Effects of disorder on Coulomb-assisted braiding of Majorana zero modes
Majorana zero modes in one-dimensional topological superconductors obey
non-Abelian braiding statistics. Braiding manipulations can be realized by
controlling Coulomb couplings in hybrid Majorana-transmon devices. However,
strong disorder may induce accidental Majorana modes, which are expected to
have detrimental effects on braiding statistics. Nevertheless, we show that the
Coulomb-assisted braiding protocol is efficiently realized also in the presence
of accidental modes. The errors occurring during the braiding cycle are small
if the couplings of the computational Majorana modes to the accidental ones are
much weaker than the maximum Coulomb coupling.Comment: 7 pages, 4 figures, this is the final, published versio
Topological blockade and measurement of topological charge
The fractionally charged quasiparticles appearing in the 5/2 fractional
quantum Hall plateau are predicted to have an extra non-local degree of
freedom, known as topological charge. We show how this topological charge can
block the tunnelling of these particles, and how such 'topological blockade'
can be used to readout their topological charge. We argue that the short time
scale required for this measurement is favorable for the detection of the
non-Abelian anyonic statistics of the quasiparticles. We also show how
topological blockade can be used to measure braiding statistics, and to couple
a topological qubit with a conventional one.Comment: Published version: one additional paragraph (on the 331 state); Figs.
1 and 4 modified; Ref. 46 adde
- …