133 research outputs found
Dynamics of dissipative Landau-Zener transitions
A non-perturbative treatment, the Dirac-Frenkel time-dependent variation is
employed to examine dynamics of the Landau-Zener model with both diagonal and
off-diagonal qubit-bath coupling using the multiple Davydov trial states. It is
shown that steady-state transition probabilities agree with analytical
predictions at long times. Landau-Zener dynamics at intermediate times is
little affected by diagonal coupling, and is found to be determined by
off-diagonal coupling and tunneling strength between two diabatic states. We
investigate effects of bath spectral densities, coupling strengths and
interaction angles on Laudau-Zener dynamics. Thanks to the multiple Davydov
trial states, detailed boson dynamics can also be analyzed in Landau-Zener
transitions. Results presented here may help provide guiding principles to
manipulate the Laudau-Zener transitions in circuit QED architectures by tuning
off-diagonal coupling and tunneling strength
Algorithms for finding transposons in gene sequences
With the process of evolution, some genes will change their relative
positions in gene sequence. These "jumping genes" are called transposons.
Through some intuitive rules, we give a criterion to determine transposons
among gene sequences of different individuals of the same species. Then we turn
this problem into graph theory and give algorithms for different situations
with acceptable time complexities. One of these algorithms has been reported
briefly as the "iteration algorithm" in Kang et al.'s paper (in this paper,
transposon is called "core-gene-defined genome organizational framework",
cGOF). This paper provides the omitted details and discussions on general
cases.Comment: 5 pages, 2 figure
Effect of off-diagonal exciton-phonon coupling on intramolecular singlet fission
Intramolecular singlet fission (iSF) materials provide remarkable advantages
in terms of tunable electronic structures, and quantum chemistry studies have
indicated strong electronic coupling modulation by high frequency phonon modes.
In this work, we formulate a microscopic model of iSF with simultaneous
diagonal and off-diagonal coupling to high-frequency modes. A non-perturbative
treatment, the Dirac-Frenkel time-dependent variational approach is adopted
using the multiple Davydov trial states. It is shown that both diagonal and
off-diagonal coupling can aid efficient singlet fission if excitonic coupling
is weak, and fission is only facilitated by diagonal coupling if excitonic
coupling is strong. In the presence of off-diagonal coupling, it is found that
high frequency modes create additional fission channels for rapid iSF. Results
presented here may help provide guiding principles for design of efficient
singlet fission materials by directly tuning singlet-triplet interstate
coupling
Transient dynamics of a one-dimensional Holstein polaron under the influence of an external electric field
Following the Dirac-Frenkel time-dependent variational principle, transient
dynamics of a one-dimensional Holstein polaron with diagonal and off-diagonal
exciton-phonon coupling in an external electric field is studied by employing
the multi-D {\it Ansatz}, also known as a superposition of the usual
Davydov D trial states. Resultant polaron dynamics has significantly
enhanced accuracy, and is in perfect agreement with that derived from the
hierarchy equations of motion method. Starting from an initial broad wave
packet, the exciton undergoes typical Bloch oscillations. Adding weak
exciton-phonon coupling leads to a broadened exciton wave packet and a reduced
current amplitude. Using a narrow wave packet as the initial state, the bare
exciton oscillates in a symmetric breathing mode, but the symmetry is easily
broken by weak coupling to phonons, resulting in a non-zero exciton current.
For both scenarios, temporal periodicity is unchanged by exciton-phonon
coupling. In particular, at variance with the case of an infinite linear chain,
no steady state is found in a finite-sized ring within the anti-adiabatic
regime. For strong diagonal coupling, the multi- {\it Anstaz} is found
to be highly accurate, and the phonon confinement gives rise to exciton
localization and decay of the Bloch oscillations
Dissipative dynamics in a tunable Rabi dimer with periodic harmonic driving
Recent progress on qubit manipulation allows application of periodic driving
signals on qubits. In this study, a harmonic driving field is added to a Rabi
dimer to engineer photon and qubit dynamics in a circuit quantum
electrodynamics device. To model environmental effects, qubits in the Rabi
dimer are coupled to a phonon bath with a sub-Ohmic spectral density. A
non-perturbative treatment, the Dirac-Frenkel time-dependent variational
principle together with the multiple Davydov D {\it Ansatz} is employed to
explore the dynamical behavior of the tunable Rabi dimer. In the absence of the
phonon bath, the amplitude damping of the photon number oscillation is greatly
suppressed by the driving field, and photons can be created thanks to
resonances between the periodic driving field and the photon frequency. In the
presence of the phonon bath, one still can change the photon numbers in two
resonators, and indirectly alter the photon imbalance in the Rabi dimer by
directly varying the driving signal in one qubit. It is shown that qubit states
can be manipulated directly by the harmonic driving. The environment is found
to strengthen the interqubit asymmetry induced by the external driving, opening
up a new venue to engineer the qubit states
Polaron dynamics with a multitude of Davydov D trial states
We propose an extension to the Davydov D Ansatz in the dynamics study of
the Holstein molecular crystal model with diagonal and off-diagonal
exciton-phonon coupling using the Dirac-Frenkel time-dependent variational
principle. The new trial state by the name of the "multi-D Ansatz" is a
linear combination of Davydov D trial states, and its validity is carefully
examined by quantifying how faithfully it follows the Schr\"odinger equation.
Considerable improvements in accuracy have been demonstrated in comparison with
the usual Davydov trial states, i.e., the single D and D Ans\"atze.
With an increase in the number of the Davydov D trial states in the
multi-D Ansatz, deviation from the exact Schr\"odinger dynamics is
gradually diminished, leading to a numerically exact solution to the
Schr\"odinger equation.Comment: 14 pages, 15 figure
XCon: Learning with Experts for Fine-grained Category Discovery
We address the problem of generalized category discovery (GCD) in this paper,
i.e. clustering the unlabeled images leveraging the information from a set of
seen classes, where the unlabeled images could contain both seen classes and
unseen classes. The seen classes can be seen as an implicit criterion of
classes, which makes this setting different from unsupervised clustering where
the cluster criteria may be ambiguous. We mainly concern the problem of
discovering categories within a fine-grained dataset since it is one of the
most direct applications of category discovery, i.e. helping experts discover
novel concepts within an unlabeled dataset using the implicit criterion set
forth by the seen classes. State-of-the-art methods for generalized category
discovery leverage contrastive learning to learn the representations, but the
large inter-class similarity and intra-class variance pose a challenge for the
methods because the negative examples may contain irrelevant cues for
recognizing a category so the algorithms may converge to a local-minima. We
present a novel method called Expert-Contrastive Learning (XCon) to help the
model to mine useful information from the images by first partitioning the
dataset into sub-datasets using k-means clustering and then performing
contrastive learning on each of the sub-datasets to learn fine-grained
discriminative features. Experiments on fine-grained datasets show a clear
improved performance over the previous best methods, indicating the
effectiveness of our method
Polymers of Intrinsic Microporosity (PIMs) in Sensing and in Electroanalysis
Polymers of intrinsic microporosity (PIMs) provide high surface area materials (typically 1000 m2 g-1 apparent BET surface area) that are processable from organic solvents to give glassy films or composite coatings. Multi-functionality for sensing with these materials is achieved (i) based on the polymer backbone itself being fluorescent or chemically active or (ii) based on guest species (chromophores, nano-catalysts, nano-photo-catalysts, etc.) that are readily embedded into PIMs and accessible through micropores in the polymer host. The ease of forming uniform microporous films or composite films is linked to molecular rigidity and highlighted here for sensing/electroanalytical applications
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