2,019 research outputs found
Topological minigap in quasi-one-dimensional spin-orbit-coupled semiconductor Majorana wires
The excitation gap above the Majorana fermion (MF) modes at the ends of 1D
topological superconducting (TS) semiconductor wires scales with the bulk
quasiparticle gap E_{qp}. This gap, also called minigap, facilitates
experimental detection of the pristine TS state and MFs at experimentally
accessible temperatures T << E_{qp}. Here we show that the linear scaling of
minigap with E_{qp} can fail in quasi-1D wires with multiple confinement bands
when the applied Zeeman field is greater than or equal to about half of the
confinement-induced bandgap. TS states in such wires have an approximate chiral
symmetry supporting multiple near zero energy modes at each end leading to a
minigap which can effectively vanish. We show that the problem of small minigap
in such wires can be resolved by forcing the system to break the approximate
chirality symmetry externally with a second Zeeman field. Although experimental
signatures such as zero bias peak from the wire ends is suppressed by the
second Zeeman field above a critical value, such a field is required in some
important parameter regimes of quasi-1D wires to isolate the topological
physics of end state MFs. We also discuss the crucial difference of our minigap
calculations from the previously reported minigap results appropriate for
idealized spinless p-wave superconductors and explain why the clustering of
fermionic subgap states around the zero energy Majorana end state with
increasing chemical potential seen in the latter system does not apply to the
experimental TS states in spin-orbit coupled nanowires.Comment: Crucial difference of the present results with previously reported
results for idealized spinless p-wave wires discussed (see conclusion); new
references added; Title changed in response to Editor comment; new version as
accepted in PR
A practical phase gate for producing Bell violations in Majorana wires
The Gottesman-Knill theorem holds that operations from the Clifford group,
when combined with preparation and detection of qubit states in the
computational basis, are insufficient for universal quantum computation.
Indeed, any measurement results in such a system could be reproduced within a
local hidden variable theory, so that there is no need for a quantum mechanical
explanation and therefore no possibility of quantum speedup. Unfortunately,
Clifford operations are precisely the ones available through braiding and
measurement in systems supporting non-Abelian Majorana zero modes, which are
otherwise an excellent candidate for topologically protected quantum
computation. In order to move beyond the classically simulable subspace, an
additional phase gate is required. This phase gate allows the system to violate
the Bell-like CHSH inequality that would constrain a local hidden variable
theory. In this article, we both demonstrate the procedure for measuring Bell
violations in Majorana systems and introduce a new type of phase gate for the
already existing semiconductor-based Majorana wire systems. We conclude with an
experimentally feasible schematic combining the two, which should potentially
lead to the demonstration of Bell violation in a Majorana experiment in the
near future. Our work also naturally leads to a well-defined platform for
universal fault-tolerant quantum computation using Majorana zero modes, which
we describe.Comment: 11 pages, 13 figures; Title and references update
Amplification of Fluctuations in a Spinor Bose Einstein Condensate
Dynamical instabilities due to spin-mixing collisions in a Rb F=1
spinor Bose-Einstein condensate are used as an amplifier of quantum spin
fluctuations. We demonstrate the spectrum of this amplifier to be tunable, in
quantitative agreement with mean-field calculations. We quantify the
microscopic spin fluctuations of the initially paramagnetic condensate by
applying this amplifier and measuring the resulting macroscopic magnetization.
The magnitude of these fluctuations is consistent with predictions of a
beyond-mean-field theory. The spinor-condensate-based spin amplifier is thus
shown to be nearly quantum-limited at a gain as high as 30 dB
Probing a topological quantum critical point in semiconductor-superconductor heterostructures
Quantum ground states on the non-trivial side of a topological quantum
critical point (TQCP) have unique properties that make them attractive
candidates for quantum information applications. A recent example is provided
by s-wave superconductivity on a semiconductor platform, which is tuned through
a TQCP to a topological superconducting (TS) state by an external Zeeman field.
Despite many attractive features of TS states, TQCPs themselves do not break
any symmetries, making it impossible to distinguish the TS state from a regular
superconductor in conventional bulk measurements. Here we show that for the
semiconductor TQCP this problem can be overcome by tracking suitable bulk
transport properties across the topological quantum critical regime itself. The
universal low-energy effective theory and the scaling form of the relevant
susceptibilities also provide a useful theoretical framework in which to
understand the topological transitions in semiconductor heterostructures. Based
on our theory, specific bulk measurements are proposed here in order to
characterize the novel TQCP in semiconductor heterostructures.Comment: 8+ pages, 5 figures, Revised version as accepted in PR
Phase diagram and excitations of a Shiba molecule
We analyze the phase diagram associated with a pair of magnetic impurities
trapped in a superconducting host. The natural interplay between Kondo
screening, superconductivity and exchange interactions leads to a rich array of
competing phases, whose transitions are characterized by discontinuous changes
of the total spin. Our analysis is based on a combination of numerical
renormalization group techniques as well as semi-classical analytics. In
addition to the expected screened and unscreened phases, we observe a new
molecular doublet phase where the impurity spins are only partially screened by
a single extended quasiparticle. Direct signatures of the various Shiba
molecule states can be observed via RF spectroscopy.Comment: 13 pages, 7 figure
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