46 research outputs found

### Leakage and dephasing in $^{28}$Si-based exchange-only spin qubits

Exchange-only spin qubits hosted in $^{28}$Si-based triple quantum dots do
not suffer from decoherence caused by randomly fluctuating nuclear-spin
ensembles and can be relatively robust against electrical noise when operated
at a sweet spot. Remaining sources of decoherence are qubit relaxation, leakage
out of the qubit subspace, and dephasing due to residual effects of charge
noise, the latter two of which are the focus of this work. We investigate
spin-orbit-mediated leakage rates to the three-spin ground state accompanied by
virtual (i) tunneling, (ii) orbital excitation, and (iii) valley excitation of
an electron. We find different power-law dependencies on the applied magnetic
field $B$ for the three mechanisms as well as for the two leakage rates,
ranging from $\propto B^5$ to $\propto B^{11}$, and identify the sweet spot as
a point of minimal leakage. We also revisit the role of electrical noise at the
sweet spot, and show that it causes a decay of coherent qubit oscillations that
follows a power law $\propto 1/t$ (as opposed to the more common exponential
decay) and introduces a $\pi/2$ phase shift.Comment: 10 pages, three figures. Minor changes with respect to the previous
version. The supplemental material is now included as appendice

### Multi-level interference resonances in strongly-driven three-level systems

We study multi-photon resonances in a strongly-driven three-level quantum
system, where one level is periodically swept through a pair of levels with
constant energy separation $E$. Near the multi-photon resonance condition
$n\hbar\omega = E$, where $n$ is an integer, we find qualitatively different
behavior for $n$ even or odd. We explain this phenomenon in terms of families
of interfering trajectories of the multi-level system. Remarkably, the behavior
is insensitive to fluctuations of the energy of the driven level, and survives
deep into the strong dephasing regime. The setup can be relevant for a variety
of solid state and atomic or molecular systems. In particular, it provides a
clear mechanism to explain recent puzzling experimental observations in
strongly-driven double quantum dots.Comment: 4 pages, 3 figure

### Semiclassical theory of persistent current fluctuations in ballistic chaotic rings

The persistent current in a mesoscopic ring has a Gaussian distribution with
small non-Gaussian corrections. Here we report a semiclassical calculation of
the leading non-Gaussian correction, which is described by the three-point
correlation function. The semiclassical approach is applicable to systems in
which the electron dynamics is ballistic and chaotic, and includes the
dependence on the Ehrenfest time. At small but finite Ehrenfest times, the
non-Gaussian fluctuations are enhanced with respect to the limit of zero
Ehrenfest time.Comment: 9 pages, 3 figures; submitted as invited contribution to a special
issue in Physica E in memory of Markus Buettike

### Spin-wave-induced correction to the conductivity of ferromagnets

We calculate the correction to the conductivity of a disordered ferromagnetic
metal due to spin-wave-mediated electron--electron interactions. This
correction is the generalization of the Altshuler-Aronov correction to
spin-wave-mediated interactions. We derive a general expression for the
conductivity correction to lowest order in the spin-wave-mediated interaction
and for the limit that the exchange splitting $\Delta$ is much smaller than the
Fermi energy. For a "clean" ferromagnet with $\Delta\tau_{\rm el}/\hbar \gg 1$,
with $\tau_{\rm el}$ the mean time for impurity scattering, we find a
correction $\delta \sigma \propto -T^{5/2}$ at temperatures $T$ above the spin
wave gap. In the opposite, "dirty" limit, $\Delta\tau_{\rm el}/\hbar \ll 1$,
the correction is a non-monotonous function of temperature.Comment: 9 pages, 6 figure

### Conductance spectroscopy on Majorana wires and the inverse proximity effect

Recent experimental searches for signatures of Majorana-like excitations in
proximitized semiconducting nanowires involve conductance spectroscopy, where
the evidence sought after is a robust zero-bias peak (in longer wires) and its
characteristic field-dependent splitting (in shorter wires). Although
experimental results partially confirm the theoretical predictions, commonly
observed discrepancies still include (i) a zero-bias peak that is significantly
lower than the predicted value of $2e^2/h$ and (ii) the absence of the expected
"Majorana oscillations" of the lowest-energy modes at higher magnetic fields.
Here, we investigate how the inevitable presence of a normal drain lead
connected to the hybrid wire can affect the conductance spectrum of the hybrid
wire. We present numerical results using a one-band model for the proximitized
nanowire, where the superconductor is considered to be in the diffusive regime,
described by semi-classical Green functions. We show how the presence of the
normal drain could (at least partially) account for the observed discrepancies,
and we complement this with analytic results providing more insights in the
underlying physics.Comment: 10 pages, 7 figure

### Phase-tunable Majorana bound states in a topological N-SNS junction

We theoretically study the differential conductance of a one-dimensional
normal-superconductor-normal-superconductor (N-SNS) junction with a phase bias
applied between the two superconductors. We consider specifically a junction
formed by a spin-orbit coupled semiconducting nanowire with regions of the
nanowire having superconducting pairing induced by a bulk $s$-wave
superconductor. When the nanowire is tuned into a topologically non-trivial
phase by a Zeeman field, it hosts zero-energy Majorana modes at its ends as
well as at the interface between the two superconductors. The phase-dependent
splitting of the Majorana modes gives rise to features in the differential
conductance that offer a clear distinction between the topologically trivial
and non-trivial phases. We calculate the transport properties of the junction
numerically and also present a simple analytical model that captures the main
properties of the predicted tunneling spectroscopy.Comment: 11 pages, 7 figure

### Spin-wave-induced corrections to the electronic density of states in metallic ferromagnets

We calculate the correction to the electronic density of states in a
disordered ferromagnetic metal induced by spin-wave mediated interaction
between the electrons. Our calculation is valid for the case that the exchange
splitting in the ferromagnet is much smaller than the Fermi energy, but we make
no assumption on the relative magnitude of the exchange splitting and the
elastic electronic scattering time. In the "clean limit", where the exchange
splitting is much larger than the electronic scattering rate, we find a
correction with a T^{d/2} temperature dependence, where d is the effective
dimensionality of the ferromagnet. In the opposite "dirty limit" the
density-of-states correction is a non-monotonous function of energy and
temperature.Comment: 8 pages, 4 figure

### Time scales for Majorana manipulation using Coulomb blockade in gate-controlled superconducting nanowires

We numerically compute the low-energy spectrum of a gate-controlled
superconducting topological nanowire segmented into two islands, each
Josephson-coupled to a bulk superconductor. This device may host two pairs of
Majorana bound states and could provide a platform for testing Majorana fusion
rules. We analyze the crossover between (i) a charge-dominated regime
utilizable for initialization and readout of Majorana bound states, (ii) a
single-island regime for dominating inter-island Majorana coupling, (iii) a
Josephson-plasmon regime for large coupling to the bulk superconductors, and
(iv) a regime of four Majorana bound states allowing for topologically
protected Majorana manipulations. From the energy spectrum, we derive
conservative estimates for the time scales of a fusion-rule testing protocol
proposed recently [arXiv:1511.05153]. We also analyze the steps needed for
basic Majorana braiding operations in branched nanowire structures

### Interaction effects on proximity-induced superconductivity in semiconducting nanowires

We investigate the effect of electron-electron interactions on
proximity-induced $s$-wave superconductivity in one-dimensional nanowires. We
treat the interactions on a self-consistent mean-field level, and find an
analytic expression for the effective pairing potential in the presence of
interactions, valid for a weakly tunnel coupled wire. We show that for a set of
two nanowires placed in parallel on a superconducting substrate, the
interaction-induced reduction of the pairing energy could result in the
effective interwire pairing potential exceeding the intrawire potential, which
is one of the requirements for creating a time-reversal symmetric topological
superconducting state in such a two-wire system.Comment: 7 pages, 1 figur