653 research outputs found
Dual approach to circuit quantization using loop charges
The conventional approach to circuit quantization is based on node fluxes and
traces the motion of node charges on the islands of the circuit. However, for
some devices, the relevant physics can be best described by the motion of
polarization charges over the branches of the circuit that are in general
related to the node charges in a highly nonlocal way. Here, we present a
method, dual to the conventional approach, for quantizing planar circuits in
terms of loop charges. In this way, the polarization charges are directly
obtained as the differences of the two loop charges on the neighboring loops.
The loop charges trace the motion of fluxes through the circuit loops. We show
that loop charges yield a simple description of the flux transport across
phase-slip junctions. We outline a concrete construction of circuits based on
phase-slip junctions that are electromagnetically dual to arbitrary planar
Josephson junction circuits. We argue that loop charges also yield a simple
description of the flux transport in conventional Josephson junctions shunted
by large impedances. We show that a mixed circuit description in terms of node
fluxes and loop charges yields an insight into the flux decompactification of a
Josephson junction shunted by an inductor. As an application, we show that the
fluxonium qubit is well approximated as a phase-slip junction for the
experimentally relevant parameters. Moreover, we argue that the - qubit
is effectively the dual of a Majorana Josephson junction.Comment: 20 pages, 11 figures. Version accepted for publication in PRB.
Changes: introduction has become less technical and an example for the
inclusion of offset charges has been adde
Majorana-assisted nonlocal electron transport through a floating topological superconductor
The nonlocal nature of the fermionic mode spanned by a pair of Majorana bound
states in a one-dimensional topological superconductor has inspired many
proposals aiming at demonstrating this property in transport. In particular,
transport through the mode from a lead attached to the left bound state to a
lead attached to the right will result in current cross-correlations. For ideal
zero modes on a grounded superconductor, the cross-correlations are however
completely suppressed in favor of purely local Andreev reflection. In order to
obtain a non-vanishing cross-correlation, previous studies have required the
presence of an additional global charging energy. Adding nonlocal terms in the
form of a global charging energy to the Hamiltonian when testing the intrinsic
nonlocality of the Majorana modes seems to be conceptually troublesome. Here,
we show that a floating superconductor allows to observe nonlocal current
correlations in the absence of charging energy. We show that the
non-interacting and the Coulomb-blockade regime have the same peak conductance
but different shot-noise power; while the shot noise is sub-Poissonian
in the Coulomb-blockade regime in the large bias limit, Poissonian shot noise
is generically obtained in the non-interacting case
Supersymmetry in the Majorana Cooper-Pair Box
Over the years, supersymmetric quantum mechanics has evolved from a toy model
of high energy physics to a field of its own. Although various examples of
supersymmetric quantum mechanics have been found, systems that have a natural
realization are scarce. Here, we show that the extension of the conventional
Cooper-pair box by a 4pi-periodic Majorana-Josephson coupling realizes
supersymmetry for certain values of the ratio between the conventional
Josephson and the Majorana- Josephson coupling strength. The supersymmetry we
find is a "hidden" minimally bosonized supersymmetry that provides a
non-trivial generalization of the supersymmetry of the free particle and relies
crucially on the presence of an anomalous Josephson junction in the system. We
show that the resulting degeneracy of the energy levels can be probed directly
in a tunneling experiment and discuss the various transport signatures. An
observation of the predicted level degeneracy would provide clear evidence for
the presence of the anomalous Josephson coupling.Comment: 10 pages, 5 figure
Universal power-law decay of electron-electron interactions due to nonlinear screening in a Josephson junction array
Josephson junctions are the most prominent nondissipative and at the same
time nonlinear elements in superconducting circuits allowing Cooper pairs to
tunnel coherently between two superconductors separated by a tunneling barrier.
Due to this, physical systems involving Josephson junctions show highly complex
behavior and interesting novel phenomena. Here, we consider an infinite
one-dimensional chain of superconducting islands where neighboring islands are
coupled by capacitances. We study the effect of Josephson junctions shunting
each island to a common ground superconductor. We treat the system in the
regime where the Josephson energy exceeds the capacitive coupling between the
islands. For the case of two offset charges on two distinct islands, we
calculate the interaction energy of these charges mediated by quantum phase
slips due to the Josephson nonlinearities. We treat the phase slips in an
instanton approximation and map the problem onto a classical partition function
of interacting particles. Using the Mayer cluster expansion, we find that the
interaction potential of the offset charges decays with an universal
inverse-square power law behavior.Comment: 10 pages, 5 figure
Spectral Properties and Local Density of States of Disordered Quantum Hall Systems with Rashba Spin-Orbit Coupling
We theoretically investigate the spectral properties and the spatial
dependence of the local density of states (LDoS) in disordered two-dimensional
electron gases (2DEG) in the quantum Hall regime, taking into account the
combined presence of electrostatic disorder, random Rashba spin-orbit in-
teraction, and finite Zeeman coupling. To this purpose, we extend a
coherent-state Green's function formalism previously proposed for spinless 2DEG
in the presence of smooth arbitrary disorder, that here incorporates the
nontrivial coupling between the orbital and spin degrees of freedom into the
electronic drift states. The formalism allows us to obtain analytical and
controlled nonperturbative expressions of the energy spectrum in arbitrary
locally flat disorder potentials with both random electric fields and Rashba
coupling. As an illustration of this theory, we derive analytical microscopic
expressions for the LDoS in different temperature regimes which can be used as
a starting point to interpret scanning tunneling spectroscopy data at high
magnetic fields. In this context, we study the spatial dependence and linewidth
of the LDoS peaks and explain an experimentally-noticed correlation between the
spatial dispersion of the spin-orbit splitting and the local extrema of the
potential landscape.Comment: 18 pages, 5 figures; typos corrected and Sec. IV A rewritten;
published versio
Probing variations of the Rashba spin-orbit coupling at the nanometer scale
The Rashba effect as an electrically tunable spin-orbit interaction is the
base for a multitude of possible applications such as spin filters, spin
transistors, and quantum computing using Majorana states in nanowires.
Moreover, this interaction can determine the spin dephasing and
antilocalization phenomena in two dimensions. However, the real space pattern
of the Rashba parameter has never been probed, albeit it critically influences,
e.g., the more robust spin transistors using the spin helix state and the
otherwise forbidden electron backscattering in topologically protected
channels. Here, we map this pattern down to nanometer length scales by
measuring the spin splitting of the lowest Landau level using scanning
tunnelling spectroscopy. We reveal strong fluctuations correlated with the
local electrostatic potential for an InSb inversion layer with a large Rashba
coefficient (~1 eV{\AA}). The novel type of Rashba field mapping enables a more
comprehensive understanding of the critical fluctuations, which might be
decisive towards robust semiconductor-based spintronic devices.Comment: A modified version will be published in Nature Physic
The pyramid scheme of the Gestapo in its fight against the German resistance movement: A Cologne case study
At some moment sooner or later in any of the big investigations and persecutions against the resistance movement conducted by the Gestapo, the German secret police of the Third Reich, they found one suspected person ready not only to reveal his own contribution to the resistance movement but to accuse other participants for their part. From the first testimony on an entire pyramid scheme of mutual allegations by the accused resistants was generated. We tried to analyze this scheme by means of methods and software developed by Social Network Analysts and were able to show how much the Gestapo benefitted from rather ordinary criminological means.At some moment sooner or later in any of the big investigations and persecutions against the resistance movement conducted by the Gestapo, the German secret police of the Third Reich, they found one suspected person ready not only to reveal his own contribution to the resistance movement but to accuse other participants for their part. From the first testimony on an entire pyramid scheme of mutual allegations by the accused resistants was generated. We tried to analyze this scheme by means of methods and software developed by Social Network Analysts and were able to show how much the Gestapo benefitted from rather ordinary criminological means
Physics case for an LHCb Upgrade II - Opportunities in flavour physics, and beyond, in the HL-LHC era
The LHCb Upgrade II will fully exploit the flavour-physics opportunities of the HL-LHC, and study additional physics topics that take advantage of the forward acceptance of the LHCb spectrometer. The LHCb Upgrade I will begin operation in 2020. Consolidation will occur, and modest enhancements of the Upgrade I detector will be installed, in Long Shutdown 3 of the LHC (2025) and these are discussed here. The main Upgrade II detector will be installed in long shutdown 4 of the LHC (2030) and will build on the strengths of the current LHCb experiment and the Upgrade I. It will operate at a luminosity up to 2×1034
cm−2s−1, ten times that of the Upgrade I detector. New detector components will improve the intrinsic performance of the experiment in certain key areas. An Expression Of Interest proposing Upgrade II was submitted in February 2017. The physics case for the Upgrade II is presented here in more depth. CP-violating phases will be measured with precisions unattainable at any other envisaged facility. The experiment will probe b → sl+l−and b → dl+l− transitions in both muon and electron decays in modes not accessible at Upgrade I. Minimal flavour violation will be tested with a precision measurement of the ratio of B(B0 → μ+μ−)/B(Bs → μ+μ−). Probing charm CP violation at the 10−5 level may result in its long sought discovery. Major advances in hadron spectroscopy will be possible, which will be powerful probes of low energy QCD. Upgrade II potentially will have the highest sensitivity of all the LHC experiments on the Higgs to charm-quark couplings. Generically, the new physics mass scale probed, for fixed couplings, will almost double compared with the pre-HL-LHC era; this extended reach for flavour physics is similar to that which would be achieved by the HE-LHC proposal for the energy frontier
LHCb upgrade software and computing : technical design report
This document reports the Research and Development activities that are carried out in the software and computing domains in view of the upgrade of the LHCb experiment. The implementation of a full software trigger implies major changes in the core software framework, in the event data model, and in the reconstruction algorithms. The increase of the data volumes for both real and simulated datasets requires a corresponding scaling of the distributed computing infrastructure. An implementation plan in both domains is presented, together with a risk assessment analysis
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