1,363 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
Flat-lens focusing of electrons on the surface of a topological insulator
We propose the implementation of an electronic Veselago lens on the
conducting surface of a three-dimensional topological insulator (such as
Bi2Te3). The negative refraction needed for such a flat lens results from the
sign change of the curvature of the Fermi surface, changing from a circular to
a snowflake-like shape across a sufficiently large electrostatic potential
step. No interband transition (as in graphene) is needed. For this reason, and
because the topological insulator provides protection against backscattering,
the potential step is able to focus a broad range of incident angles. We
calculate the quantum interference pattern produced by a point source,
generalizing the analogous optical calculation to include the effect of a
noncircular Fermi surface (having a nonzero conic constant).Comment: 6 pages, 6 figure
Anyonic interferometry without anyons: How a flux qubit can read out a topological qubit
Proposals to measure non-Abelian anyons in a superconductor by quantum
interference of vortices suffer from the predominantly classical dynamics of
the normal core of an Abrikosov vortex. We show how to avoid this obstruction
using coreless Josephson vortices, for which the quantum dynamics has been
demonstrated experimentally. The interferometer is a flux qubit in a Josephson
junction circuit, which can nondestructively read out a topological qubit
stored in a pair of anyons --- even though the Josephson vortices themselves
are not anyons. The flux qubit does not couple to intra-vortex excitations,
thereby removing the dominant restriction on the operating temperature of
anyonic interferometry in superconductors.Comment: 7 pages, 3 figures; Added an Appendix on parity-protected
single-qubit rotations; problem with Figure 3 correcte
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
Quantized conductance at the Majorana phase transition in a disordered superconducting wire
Superconducting wires without time-reversal and spin-rotation symmetries can
be driven into a topological phase that supports Majorana bound states. Direct
detection of these zero-energy states is complicated by the proliferation of
low-lying excitations in a disordered multi-mode wire. We show that the phase
transition itself is signaled by a quantized thermal conductance and electrical
shot noise power, irrespective of the degree of disorder. In a ring geometry,
the phase transition is signaled by a period doubling of the magnetoconductance
oscillations. These signatures directly follow from the identification of the
sign of the determinant of the reflection matrix as a topological quantum
number.Comment: 7 pages, 4 figures; v3: added appendix with numerics for long-range
disorde
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
Topological quantum number and critical exponent from conductance fluctuations at the quantum Hall plateau transition
The conductance of a two-dimensional electron gas at the transition from one
quantum Hall plateau to the next has sample-specific fluctuations as a function
of magnetic field and Fermi energy. Here we identify a universal feature of
these mesoscopic fluctuations in a Corbino geometry: The amplitude of the
magnetoconductance oscillations has an e^2/h resonance in the transition
region, signaling a change in the topological quantum number of the insulating
bulk. This resonance provides a signed scaling variable for the critical
exponent of the phase transition (distinct from existing positive definite
scaling variables).Comment: 6 pages, 9 figure
Coulomb stability of the 4\pi-periodic Josephson effect of Majorana fermions
The Josephson energy of two superconducting islands containing Majorana
fermions is a 4\pi-periodic function of the superconducting phase difference.
If the islands have a small capacitance, their ground state energy is governed
by the competition of Josephson and charging energies. We calculate this ground
state energy in a ring geometry, as a function of the flux -\Phi- enclosed by
the ring, and show that the dependence on the Aharonov-Bohm phase 2e\Phi/\hbar
remains 4\pi-periodic regardless of the ratio of charging and Josephson
energies - provided that the entire ring is in a topologically nontrivial
state. If part of the ring is topologically trivial, then the charging energy
induces quantum phase slips that restore the usual 2\pi-periodicity.Comment: 4 pages, 4 figures; v2: more references, improved phase-slip formula,
and a discussion of the effect of overlapping Majorana'
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
Partitioning of on-demand electron pairs
We demonstrate the high fidelity splitting of electron pairs emitted on
demand from a dynamic quantum dot by an electronic beam splitter. The fidelity
of pair splitting is inferred from the coincidence of arrival in two detector
paths probed by a measurement of the partitioning noise. The emission
characteristic of the on-demand electron source is tunable from electrons being
partitioned equally and independently to electron pairs being split with a
fidelity of 90%. For low beam splitter transmittance we further find evidence
of pair bunching violating statistical expectations for independent fermions
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