12 research outputs found
Topological superfluids on a lattice with non-Abelian gauge fields
Two-component fermionic superfluids on a lattice with an external non-Abelian
gauge field give access to a variety of topological phases in presence of a
sufficiently large spin imbalance. We address here the important issue of
superfluidity breakdown induced by spin imbalance by a self-consistent
calculation of the pairing gap, showing which of the predicted phases will be
experimentally accessible. We present the full topological phase diagram, and
we analyze the connection between Chern numbers and the existence of
topologically protected and non-protected edge modes. The Chern numbers are
calculated via a very efficient and simple method.Comment: 6 pages, 5 figures to be published in Europhysics Letter
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
Observation of Majorana Fermions in a Nb-InSb Nanowire-Nb Hybrid Quantum Device
We report on the observation of excitation of Majorana fermions in a Nb-InSb
nanowire quantum dot-Nb hybrid system. The InSb nanowire quantum dot is formed
between the two Nb contacts by weak Schottky barriers and is thus in the regime
of strong couplings to the contacts. Due to the proximity effect, the InSb
nanowire segments covered by superconductor Nb contacts turn to superconductors
with a superconducting energy gap . Under an applied magnetic field
larger than a critical value for which the Zeeman energy in the InSb nanowire
is , the entire InSb nanowire is found to be in a nontrivial
topological superconductor phase, supporting a pair of Majorana fermions, and
Cooper pairs can transport between the superconductor Nb contacts via the
Majorana fermion states. This transport process will be suppressed when the
applied magnetic field becomes larger than a second critical value at which the
transition to a trivial topological superconductor phase occurs in the system.
This physical scenario has been observed in our experiment. We have found that
the measured zero-bias conductance for our hybrid device shows a conductance
plateau in a range of the applied magnetic field in quasi-particle Coulomb
blockade regions.Comment: 7 pages, 4 figures, supplementary materials of 3 pages and 1 figur
Spin-resolved Andreev levels and parity crossings in hybrid superconductor–semiconductor nanostructures
The physics and operating principles of hybrid superconductor–semiconductor devices rest ultimately on the magnetic properties of their elementary subgap excitations, usually called Andreev levels. Here we report a direct measurement of the Zeeman effect on the Andreev levels of a semiconductor quantum dot with large electron g-factor, strongly coupled to a conventional superconductor with a large critical magnetic field. This material combination allows spin degeneracy to be lifted without destroying superconductivity. We show that a spin-split Andreev level crossing the Fermi energy results in a quantum phase transition to a spin-polarized state, which implies a change in the fermionic parity of the system. This crossing manifests itself as a zero-bias conductance anomaly at finite magnetic field with properties that resemble those expected for Majorana modes in a topological superconductor. Although this resemblance is understood without evoking topological superconductivity, the observed parity transitions could be regarded as precursors of Majorana modes in the long-wire limit.Chemistry and Chemical Biolog