84 research outputs found
Topological quantum buses: coherent quantum information transfer between topological and conventional qubits
We propose computing bus devices that enable quantum information to be
coherently transferred between topological and conventional qubits. We describe
a concrete realization of such a topological quantum bus acting between a
topological qubit in a Majorana wire network and a conventional semiconductor
double quantum dot qubit. Specifically, this device measures the joint
(fermion) parity of these two different qubits by using the Aharonov-Casher
effect in conjunction with an ancilliary superconducting flux qubit that
facilitates the measurement. Such a parity measurement, together with the
ability to apply Hadamard gates to the two qubits, allows one to produce states
in which the topological and conventional qubits are maximally entangled and to
teleport quantum states between the topological and conventional quantum
systems.Comment: 5 pages, 2 figures; v2: minor revision
Search for Majorana fermions in multiband semiconducting nanowires
We study multiband semiconducting nanowires proximity-coupled with an s-wave
superconductor. We show that when odd number of subbands are occupied the
system realizes non-trivial topological state supporting Majorana modes
localized at the ends. We study the topological quantum phase transition in
this system and analytically calculate the phase diagram as a function of the
chemical potential and magnetic field. Our key finding is that multiband
occupancy not only lifts the stringent constraint of one-dimensionality but
also allows to have higher carrier density in the nanowire and as such
multisubband nanowires are better-suited for observing the Majorana particle.
We study the robustness of the topological phase by including the effects of
the short- and long-range disorder. We show that in the limit of strong
interband mixing there is an optimal regime in the phase diagram ("sweet spot")
where the topological state is to a large extent insensitive to the presence of
disorder.Comment: 4 pages, 3 figures, expanded version includes new results; accepted
for publication in PR
Topological Protection of Majorana Qubits
We study the stability of the topological quantum computation proposals
involving Majorana fermions against thermal fluctuations. We use a minimal
realistic model of a spinless px+ipy superconductor and consider effect of
excited midgap states localized in the vortex core as well as of transitions
above the bulk superconducting gap on the quasiparticle braiding,
interferometry-based qubit read-out schemes, and quantum coherence of the
topological qubits. We find that thermal occupation of the midgap states does
not affect adiabatic braiding operations but leads to a reduction in the
visibility of the interferometry measurements. We also consider quantum
decoherence of topological qubits at finite temperatures and calculate their
decay rate which is associated with the change of the fermion parity and, as
such, is exponentially suppressed at temperatures well below the bulk
excitation gap. Our conclusion is that the Majorana-based topological quantum
computing schemes are indeed protected by the virtue of the quantum
non-locality of the stored information and the presence of the bulk
superconducting gap.Comment: 8 pages, 1 figur
Spontaneous interlayer superfluidity in bilayer systems of cold polar molecules
Quantum degenerate cold-atom gases provide a remarkable opportunity to study
strongly interacting systems. Recent experimental progress in producing
ultracold polar molecules with a net electric dipole moment opens up new
possibilities to realize novel quantum phases governed by the long-range and
anisotropic dipole-dipole interactions. In this work we predict the existence
of experimentally observable novel broken-symmetry states with spontaneous
interlayer coherence in cold polar molecules. These exotic states appear due to
strong repulsive interlayer interactions and exhibit properties of superfluids,
ferromagnets and excitonic condensates.Comment: 7 pages, 5 figures, final versio
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