123 research outputs found
Nonzero temperature effects on antibunched photons emitted by a quantum point contact out of equilibrium
Electrical current fluctuations in a single-channel quantum point contact can
produce photons (at frequency omega close to the applied voltage V x e/hbar)
which inherit the sub-Poissonian statistics of the electrons. We extend the
existing zero-temperature theory of the photostatistics to nonzero temperature
T. The Fano factor F (the ratio of the variance and the average photocount) is
1 for T>T_c (bunched photons). The
crossover temperature T_c ~ Deltaomega x hbar/k_B is set by the band width
Deltaomega of the detector, even if hbar x Deltaomega << eV. This implies that
narrow-band detection of photon antibunching is hindered by thermal
fluctuations even in the low-temperature regime where thermal electron noise is
negligible relative to shot noise.Comment: 4 pages, 2 pages appendix, 3 figure
Geometrically protected triple-point crossings in an optical lattice
We show how to realize topologically protected crossings of three energy
bands, integer-spin analogs of Weyl fermions, in three-dimensional optical
lattices. Our proposal only involves ultracold atom techniques that have
already been experimentally demonstrated and leads to isolated triple-point
crossings (TPCs) which are required to exist by a novel combination of lattice
symmetries. The symmetries also allow for a new type of topological object, the
type-II, or tilted, TPC. Our Rapid Communication shows that spin-1 Weyl points,
which have not yet been observed in the bandstructure of crystals, are within
reach of ultracold atom experiments.Comment: 5 pages, 2 figures + 3 pages, 3 figures supplemental material. Added
appendix on model symmetries, fixed typos and added references. This is the
final, published versio
Extended topological group structure due to average reflection symmetry
We extend the single-particle topological classification of insulators and
superconductors to include systems in which disorder preserves average
reflection symmetry. We show that the topological group structure of bulk
Hamiltonians and topological defects is exponentially extended when this
additional condition is met, and examine some of its physical consequences.
Those include localization-delocalization transitions between topological
phases with the same boundary conductance, as well as gapless topological
defects stabilized by average reflection symmetry.Comment: 8 pages, 5 figures, 1 table; improved section 4 "Extended topological
classification" incl. example of stacked QSH layer
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
Scattering formula for the topological quantum number of a disordered multi-mode wire
The topological quantum number Q of a superconducting or chiral insulating
wire counts the number of stable bound states at the end points. We determine Q
from the matrix r of reflection amplitudes from one of the ends, generalizing
the known result in the absence of time-reversal and chiral symmetry to all
five topologically nontrivial symmetry classes. The formula takes the form of
the determinant, Pfaffian, or matrix signature of r, depending on whether r is
a real matrix, a real antisymmetric matrix, or a Hermitian matrix. We apply
this formula to calculate the topological quantum number of N coupled dimerized
polymer chains, including the effects of disorder in the hopping constants. The
scattering theory relates a topological phase transition to a conductance peak,
of quantized height and with a universal (symmetry class independent) line
shape. Two peaks which merge are annihilated in the superconducting symmetry
classes, while they reinforce each other in the chiral symmetry classes.Comment: 8 pages, 3 figures, this is the final, published versio
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