786 research outputs found
Heptagon Amplitude in the Multi-Regge Regime
As we have shown in previous work, the high energy limit of scattering
amplitudes in N=4 supersymmetric Yang-Mills theory corresponds to the infrared
limit of the 1-dimensional quantum integrable system that solves minimal area
problems in AdS5. This insight can be developed into a systematic algorithm to
compute the strong coupling limit of amplitudes in the multi-Regge regime
through the solution of auxiliary Bethe Ansatz equations. We apply this
procedure to compute the scattering amplitude for n=7 external gluons in
different multi-Regge regions at infinite 't Hooft coupling. Our formulas are
remarkably consistent with the expected form of 7-gluon Regge cut contributions
in perturbative gauge theory. A full description of the general algorithm and a
derivation of results will be given in a forthcoming paper.Comment: 14 page
Effect of chiral symmetry on chaotic scattering from Majorana zero modes
In many of the experimental systems that may host Majorana zero modes, a
so-called chiral symmetry exists that protects overlapping zero modes from
splitting up. This symmetry is operative in a superconducting nanowire that is
narrower than the spin-orbit scattering length, and at the Dirac point of a
superconductor/topological insulator heterostructure. Here we show that chiral
symmetry strongly modifies the dynamical and spectral properties of a chaotic
scatterer, even if it binds only a single zero mode. These properties are
quantified by the Wigner-Smith time-delay matrix ,
the Hermitian energy derivative of the scattering matrix, related to the
density of states by . We compute the
probability distribution of and , dependent on the number of
Majorana zero modes, in the chiral ensembles of random-matrix theory. Chiral
symmetry is essential for a significant -dependence.Comment: 5 pages, 3 figures + appendix (3 pages, 1 figure
Effect of a tunnel barrier on the scattering from a Majorana bound state in an Andreev billiard
We calculate the joint distribution of the scattering matrix and
time-delay matrix of a chaotic quantum dot coupled
by point contacts to metal electrodes. While and are statistically
independent for ballistic coupling, they become correlated for tunnel coupling.
We relate the ensemble averages of and and thereby obtain the average
density of states at the Fermi level. We apply this to a calculation of the
effect of a tunnel barrier on the Majorana resonance in a topological
superconductor. We find that the presence of a Majorana bound state is hidden
in the density of states and in the thermal conductance if even a single
scattering channel has unit tunnel probability. The electrical conductance
remains sensitive to the appearance of a Majorana bound state, and we calculate
the variation of the average conductance through a topological phase
transition.Comment: Contribution for the special issue of Physica E in memory of Markus
B\"{u}ttiker. 13 pages, 7 figure
Selective enhancement of topologically induced interface states in a dielectric resonator chain
The recent realization of topological phases in insulators and
superconductors has advanced the quest for robust quantum technologies. The
prospects to implement the underlying topological features controllably has
given incentive to explore optical platforms for analogous realizations. Here
we realize a topologically induced defect state in a chain of dielectric
microwave resonators and show that the functionality of the system can be
enhanced by supplementing topological protection with non-hermitian symmetries
that do not have an electronic counterpart. We draw on a characteristic
topological feature of the defect state, namely, that it breaks a sublattice
symmetry. This isolates the state from losses that respect parity-time
symmetry, which enhances its visibility relative to all other states both in
the frequency and in the time domain. This mode selection mechanism naturally
carries over to a wide range of topological and parity-time symmetric optical
platforms, including couplers, rectifiers and lasers.Comment: 5 pages, 4 figures, + supplementary information (3 pages, 4 figures
Significance of Ghost Orbit Bifurcations in Semiclassical Spectra
Gutzwiller's trace formula for the semiclassical density of states in a
chaotic system diverges near bifurcations of periodic orbits, where it must be
replaced with uniform approximations. It is well known that, when applying
these approximations, complex predecessors of orbits created in the bifurcation
("ghost orbits") can produce pronounced signatures in the semiclassical spectra
in the vicinity of the bifurcation. It is the purpose of this paper to
demonstrate that these ghost orbits themselves can undergo bifurcations,
resulting in complex, nongeneric bifurcation scenarios. We do so by studying an
example taken from the Diamagnetic Kepler Problem, viz. the period quadrupling
of the balloon orbit. By application of normal form theory we construct an
analytic description of the complete bifurcation scenario, which is then used
to calculate the pertinent uniform approximation. The ghost orbit bifurcation
turns out to produce signatures in the semiclassical spectrum in much the same
way as a bifurcation of real orbits would.Comment: 20 pages, 6 figures, LATEX (IOP style), submitted to J. Phys.
Classical orbit bifurcation and quantum interference in mesoscopic magnetoconductance
We study the magnetoconductance of electrons through a mesoscopic channel
with antidots. Through quantum interference effects, the conductance maxima as
functions of the magnetic field strength and the antidot radius (regulated by
the applied gate voltage) exhibit characteristic dislocations that have been
observed experimentally. Using the semiclassical periodic orbit theory, we
relate these dislocations directly to bifurcations of the leading classes of
periodic orbits.Comment: 4 pages, including 5 figures. Revised version with clarified
discussion and minor editorial change
The Bethe Roots of Regge Cuts in Strongly Coupled N=4 SYM Theory
We describe a general algorithm for the computation of the remainder function
for n-gluon scattering in multi-Regge kinematics for strongly coupled planar
N=4 super Yang-Mills theory. This regime is accessible through the infrared
physics of an auxiliary quantum integrable system describing strings in
AdS5xS5. Explicit formulas are presented for n=6 and n=7 external gluons. Our
results are consistent with expectations from perturbative gauge theory. This
paper comprises the technical details for the results announced in
arXiv:1405.3658 .Comment: 42 pages, 9 figure
Exponential sensitivity to dephasing of electrical conduction through a quantum dot
According to random-matrix theory, interference effects in the conductance of
a ballistic chaotic quantum dot should vanish
when the dephasing time
becomes small compared to the mean dwell time . Aleiner and Larkin
have predicted that the power law crosses over to an exponential suppression
when drops below the
Ehrenfest time . We report the first observation of this crossover in
a computer simulation of universal conductance fluctuations. Their theory also
predicts an exponential suppression in the
absence of dephasing -- which is not observed. We show that the effective
random-matrix theory proposed previously for quantum dots without dephasing
explains both observations.Comment: 4 pages, 4 figure
Diagnostics of entanglement dynamics in noisy and disordered spin chains via the measurement-induced steady-state entanglement transition
We utilize the concept of a measurement-induced entanglement transition to analyze the interplay and competition of processes that generate and destroy entanglement in a one-dimensional quantum spin chain evolving under a locally noisy and disordered Hamiltonian. We employ continuous measurements of variable strength to induce a transition from volume to area-law scaling of the steady-state entanglement entropy. While static background disorder systematically reduces the critical measurement strength, this critical value depends nonmonotonically on the strength of nonstatic noise. According to the extracted finite-size scaling exponents, the universality class of the transition is independent of the noise and disorder strength. We interpret the results in terms of the effect of static and nonstatic disorder on the intricate dynamics of the entanglement generation rate due to the Hamiltonian in the absence of measurement, which is fully reflected in the behavior of the critical measurement strength. Our results establish a firm connection between this entanglement growth and the steady-state behavior of the measurement-controlled systems, which therefore can serve as a tool to quantify and investigate features of transient entanglement dynamics in complex many-body systems via a steady-state phase transition
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