219 research outputs found
Spectroscopic signatures of the Larkin-Ovchinnikov state in the conductance characteristics of a normal-metal/superconductor junction
Using a discrete-lattice approach, we calculate the conductance spectra
between a normal metal and an s-wave Larkin-Ovchinnikov (LO) superconductor,
with the junction interface oriented {\em along} the direction of the
order-parameter (OP) modulation. The OP sign reversal across one single nodal
line can induce a sizable number of zero-energy Andreev bound states around the
nodal line, and a hybridized midgap-states band is formed amid a
momentum-dependent gap as a result of the periodic array of nodal lines in the
LO state. This band-in-gap structure and its anisotropic properties give rise
to distinctive features in both the point-contact and tunneling spectra as
compared with the BCS and Fulde-Ferrell cases. These spectroscopic features can
serve as distinguishing signatures of the LO state.Comment: 8 pages, 5 figures; version as publishe
Distinguishing quantum dynamics via Markovianity and Non-Markovianity
To study various quantum dynamics, it is important to develop effective
methods to detect and distinguish different quantum dynamics. A common
non-demolition approach is to couple an auxiliary system (ancilla) to the
target system, and to measure the ancilla only. By doing so, the target system
becomes an environment for the ancilla. Thus, different quantum dynamics of
target systems will correspond to different environment properties. Here, we
analytically study XX spin chains presenting different kinds of quantum
dynamics, namely localized, delocalized, and dephasing dynamics, and build
connections between Markovianity and non-Markovianity - the two most common
properties of an environment. For a qubit coupled to the XX chain, we derived
the reduced density matrix of the qubit through the projection method.
Furthermore, when dephasing noise was introduced to the XX chain, we
generalized the projection method by introducing an open-system interaction
picture - a modification of the Dirac interaction picture. By calculating the
reduced density matrix for the qubit analytically and numerically, we found
that the delocalized (localized) chain corresponds to the Markovian
(non-Markovian) bath when boundary effects are not considered, and the feature
of the chain with dephasing noise as a bath is dependent on the dephasing
strength. The three kinds of quantum dynamics can be distinguished by measuring
the qubit only.Comment: 6 pages, 8 figure
Enhanced localization in the prethermal regime of continuously measured many-body localized systems
Many-body localized systems exhibit a unique characteristic of avoiding
thermalization, primarily attributed to the presence of a local disorder
potential in the Hamiltonian. In recent years there has been an interest in
simulating these systems on quantum devices. However, actual quantum devices
are subject to unavoidable decoherence that can be modeled as coupling to a
bath or continuous measurements. The quantum Zeno effect is also known to
inhibit thermalization in a quantum system, where repeated measurements
suppress transport. In this work we study the interplay of many-body
localization and the many-body quantum Zeno effect. In a prethermal regime, we
find that the signatures of many-body localization are enhanced when the system
is coupled to a bath that contains measurements of local fermion population,
subject to the appropriate choice of system and bath parameters.Comment: 5+4 pages, 3+1 figure
Keldysh Nonlinear Sigma Model for a Free-Fermion Gas under Continuous Measurements
We analytically analyze the quantum dynamics of a -dimension free-fermion
gas subject to continuous projective measurements. By mapping the Lindblad
master equation to the functional Keldysh field theory, we observe that the
Keldysh Lindblad partition function resembles that in the Keldysh treatment of
the disordered fermionic systems. Based on this observation, we develop an
effective theory termed as the Keldysh nonlinear sigma model to describe the
low-energy physics. Two types of diffuson correlators, similar to those in the
disordered fermionic systems, are derived. In addition, up to the one-loop
level of the effective theory, we obtain a Drude-form conductivity where the
elastic scattering time is replaced by the inverse measurement strength.
According to these similarities, these two different concepts, i.e., projective
measurements and disorders, are unified in the same framework.Comment: 10 pages, updated the reference list in V
Numerical study of spin quantum Hall transitions in superconductors with broken time-reversal symmetry
We present results of numerical studies of spin quantum Hall transitions in
disordered superconductors, in which the pairing order parameter breaks
time-reversal symmetry. We focus mainly on p-wave superconductors in which one
of the spin components is conserved. The transport properties of the system are
studied by numerically diagonalizing pairing Hamiltonians on a lattice, and by
calculating the Chern and Thouless numbers of the quasiparticle states. We find
that in the presence of disorder, (spin-)current carrying states exist only at
discrete critical energies in the thermodynamic limit, and the spin-quantum
Hall transition driven by an external Zeeman field has the same critical
behavior as the usual integer quantum Hall transition of non-interacting
electrons. These critical energies merge and disappear as disorder strength
increases, in a manner similar to those in lattice models for integer quantum
Hall transition.Comment: 9 pages, 9 figure
Phonemic Adversarial Attack against Audio Recognition in Real World
Recently, adversarial attacks for audio recognition have attracted much
attention. However, most of the existing studies mainly rely on the
coarse-grain audio features at the instance level to generate adversarial
noises, which leads to expensive generation time costs and weak universal
attacking ability. Motivated by the observations that all audio speech consists
of fundamental phonemes, this paper proposes a phonemic adversarial tack (PAT)
paradigm, which attacks the fine-grain audio features at the phoneme level
commonly shared across audio instances, to generate phonemic adversarial
noises, enjoying the more general attacking ability with fast generation speed.
Specifically, for accelerating the generation, a phoneme density balanced
sampling strategy is introduced to sample quantity less but phonemic features
abundant audio instances as the training data via estimating the phoneme
density, which substantially alleviates the heavy dependency on the large
training dataset. Moreover, for promoting universal attacking ability, the
phonemic noise is optimized in an asynchronous way with a sliding window, which
enhances the phoneme diversity and thus well captures the critical fundamental
phonemic patterns. By conducting extensive experiments, we comprehensively
investigate the proposed PAT framework and demonstrate that it outperforms the
SOTA baselines by large margins (i.e., at least 11X speed up and 78% attacking
ability improvement)
Intelligent model for offshore China sea fog forecasting
Accurate and timely prediction of sea fog is very important for effectively
managing maritime and coastal economic activities. Given the intricate nature
and inherent variability of sea fog, traditional numerical and statistical
forecasting methods are often proven inadequate. This study aims to develop an
advanced sea fog forecasting method embedded in a numerical weather prediction
model using the Yangtze River Estuary (YRE) coastal area as a case study. Prior
to training our machine learning model, we employ a time-lagged correlation
analysis technique to identify key predictors and decipher the underlying
mechanisms driving sea fog occurrence. In addition, we implement ensemble
learning and a focal loss function to address the issue of imbalanced data,
thereby enhancing the predictive ability of our model. To verify the accuracy
of our method, we evaluate its performance using a comprehensive dataset
spanning one year, which encompasses both weather station observations and
historical forecasts. Remarkably, our machine learning-based approach surpasses
the predictive performance of two conventional methods, the weather research
and forecasting nonhydrostatic mesoscale model (WRF-NMM) and the algorithm
developed by the National Oceanic and Atmospheric Administration (NOAA)
Forecast Systems Laboratory (FSL). Specifically, in regard to predicting sea
fog with a visibility of less than or equal to 1 km with a lead time of 60
hours, our methodology achieves superior results by increasing the probability
of detection (POD) while simultaneously reducing the false alarm ratio (FAR).Comment: 19 pages, 9 figure
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