133 research outputs found
Nonradiative lifetimes in intermediate band materials - absence of lifetime recovery
Intermediate band photovoltaics hold the promise of being highly efficient
and cost effective photovoltaic cells. Intermediate states in the band gap,
however, are known to facilitate nonradiative recombination. Much effort has
been dedicated to producing metallic intermediate bands in hopes of producing
lifetime recovery -- an increase in carrier lifetime as doping levels increase.
We show that lifetime recovery induced by the insulator-to-metal transition
will not occur, because the metallic extended states will be localised by
phonons during the recombination process. Only trivial forms of lifetime
recovery, e.g., from an overall shift in intermediate levels, are possible.
Future work in intermediate band photovoltaics must focus on optimizing subgap
optical absorption and minimizing recombination, but not via lifetime recovery.Comment: 8 page
Scaling and localization lengths of a topologically disordered system
We consider a noninteracting disordered system designed to model particle
diffusion, relaxation in glasses, and impurity bands of semiconductors.
Disorder originates in the random spatial distribution of sites. We find strong
numerical evidence that this model displays the same universal behavior as the
standard Anderson model. We use finite-size-scaling to find the localization
length as a function of energy and density, including localized states away
from the delocalization transition. Results at many energies all fit onto the
same universal scaling curve.Comment: 5+ page
Mechanistic Regimes of Vibronic Transport in a Heterodimer and the Design Principle of Incoherent Vibronic Transport in Phycobiliproteins
Following the observation of coherent oscillations in non-linear spectra of
photosynthetic pigment protein complexes, particularly phycobilliprotein such
as PC645, coherent vibronic transport has been suggested as a design principle
for novel light harvesting materials operating at room temperature. Vibronic
transport between energetically remote pigments is coherent when the presence
of a resonant vibration supports transient delocalization between the pair of
electronic excited states. Here, we establish the mechanism of vibronic
transport for a model heterodimer across a wide range of molecular parameter
values. The resulting mechanistic map demonstrates that the molecular
parameters of phycobiliproteins in fact support incoherent vibronic transport.
This result points to an important design principle: incoherent vibronic
transport is more efficient than a coherent mechanism when energetic disorder
exceeds the coupling between the donor and vibrationally excited acceptor
states. Finally, our results suggest that the role of coherent vibronic
transport in pigment protein complexes should be reevaluated
Scientific intuition inspired by machine learning-generated hypotheses
Machine learning with application to questions in the physical sciences has become a widely used tool, successfully applied to classification, regression and optimization tasks in many areas. Research focus mostly lies in improving the accuracy of the machine learning models in numerical predictions, while scientific understanding is still almost exclusively generated by human researchers analysing numerical results and drawing conclusions. In this work, we shift the focus on the insights and the knowledge obtained by the machine learning models themselves. In particular, we study how it can be extracted and used to inspire human scientists to increase their intuitions and understanding of natural systems. We apply gradient boosting in decision trees to extract human-interpretable insights from big data sets from chemistry and physics. In chemistry, we not only rediscover widely know rules of thumb but also find new interesting motifs that tell us how to control solubility and energy levels of organic molecules. At the same time, in quantum physics, we gain new understanding on experiments for quantum entanglement. The ability to go beyond numerics and to enter the realm of scientific insight and hypothesis generation opens the door to use machine learning to accelerate the discovery of conceptual understanding in some of the most challenging domains of science
Discrete single-photon quantum walks with tunable decoherence
Quantum walks have a host of applications, ranging from quantum computing to
the simulation of biological systems. We present an intrinsically stable,
deterministic implementation of discrete quantum walks with single photons in
space. The number of optical elements required scales linearly with the number
of steps. We measure walks with up to 6 steps and explore the
quantum-to-classical transition by introducing tunable decoherence. Finally, we
also investigate the effect of absorbing boundaries and show that decoherence
significantly affects the probability of absorption.Comment: Published version, 5 pages, 4 figure
A correlated-polaron electronic propagator: open electronic dynamics beyond the Born-Oppenheimer approximation
In this work we develop a theory of correlated many-electron dynamics dressed
by the presence of a finite-temperature harmonic bath. The theory is based on
the ab-initio Hamiltonian, and thus well-defined apart from any
phenomenological choice of collective basis states or electronic coupling
model. The equation-of-motion includes some bath effects non-perturbatively,
and can be used to simulate line- shapes beyond the Markovian approximation and
open electronic dynamics which are subjects of renewed recent interest. Energy
conversion and transport depend critically on the ratio of electron-electron
coupling to bath-electron coupling, which is a fitted parameter if a
phenomenological basis of many-electron states is used to develop an electronic
equation of motion. Since the present work doesn't appeal to any such basis, it
avoids this ambiguity. The new theory produces a level of detail beyond the
adiabatic Born-Oppenheimer states, but with cost scaling like the
Born-Oppenheimer approach. While developing this model we have also applied the
time-convolutionless perturbation theory to correlated molecular excitations
for the first time. Resonant response properties are given by the formalism
without phenomenological parameters. Example propagations with a developmental
code are given demonstrating the treatment of electron-correlation in
absorption spectra, vibronic structure, and decay in an open system.Comment: 25 pages 7 figure
Polynomial-time quantum algorithm for the simulation of chemical dynamics
The computational cost of exact methods for quantum simulation using
classical computers grows exponentially with system size. As a consequence,
these techniques can only be applied to small systems. By contrast, we
demonstrate that quantum computers could exactly simulate chemical reactions in
polynomial time. Our algorithm uses the split-operator approach and explicitly
simulates all electron-nuclear and inter-electronic interactions in quadratic
time. Surprisingly, this treatment is not only more accurate than the
Born-Oppenheimer approximation, but faster and more efficient as well, for all
reactions with more than about four atoms. This is the case even though the
entire electronic wavefunction is propagated on a grid with appropriately short
timesteps. Although the preparation and measurement of arbitrary states on a
quantum computer is inefficient, here we demonstrate how to prepare states of
chemical interest efficiently. We also show how to efficiently obtain
chemically relevant observables, such as state-to-state transition
probabilities and thermal reaction rates. Quantum computers using these
techniques could outperform current classical computers with one hundred
qubits.Comment: 9 pages, 3 figures. Updated version as appears in PNA
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Quantum Simulator of an Open Quantum System Using Superconducting Qubits: Exciton Transport in Photosynthetic Complexes
Open quantum system approaches are widely used in the description of physical, chemical and biological systems. A famous example is electronic excitation transfer in the initial stage of photosynthesis, where harvested energy is transferred with remarkably high efficiency to a reaction center. This transport is affected by the motion of a structured vibrational environment, which makes simulations on a classical computer very demanding. Here we propose an analog quantum simulator of complex open system dynamics with a precisely engineered quantum environment. Our setup is based on superconducting circuits, a well established technology. As an example, we demonstrate that it is feasible to simulate exciton transport in the Fenna–Matthews–Olson photosynthetic complex. Our approach allows for a controllable single-molecule simulation and the investigation of energy transfer pathways as well as non-Markovian noise-correlation effects.Chemistry and Chemical Biolog
Spin Star as Switch for Quantum Networks
Quantum state transfer is an important task in quantum information
processing. It is known that one can engineer the couplings of a
one-dimensional spin chain to achieve the goal of perfect state transfer. To
leverage the value of these spin chains, a spin star is potentially useful for
connecting different parts of a quantum network. In this work, we extend the
spin-chain engineering problem to the problems with a topology of a star
network. We show that a permanently coupled spin star can function as a network
switch for transferring quantum states selectively from one node to another by
varying the local potentials only. Together with one-dimensional chains, this
result allows applications of quantum state transfer be applied to more general
quantum networks.Comment: 10 pages, 2 figur
Generalized Kasha's Scheme for Classifying Two-Dimensional Excitonic Molecular Aggregates: Temperature Dependent Absorption Peak Frequency Shift
We propose a generalized theoretical framework for classifying
two-dimensional (2D) excitonic molecular aggregates based on an analysis of
temperature dependent spectra. In addition to the monomer-aggregate absorption
peak shift, which defines the conventional J- and H-aggregates, we incorporate
the peak shift associated with increasing temperature as a measure to
characterize the exciton band structure. First we show that there is a
one-to-one correspondence between the monomer-aggregate and the T-dependent
peak shifts for Kasha's well-established model of 1D aggregates, where
J-aggregates exhibit further redshift upon increasing temperature and
H-aggregates exhibit further blueshift. On the contrary, 2D aggregate
structures are capable of supporting the two other combinations: blueshifting
J-aggregates and redshifting H-aggregates, owing to their more complex exciton
band structures. Secondly, using spectral lineshape theory, the T-dependent
shift is associated with the relative abundance of states on each side of the
bright state. We further establish that the density of states can be connected
to the microscopic packing condition leading to these four classes of
aggregates by separately considering the short and long-range contribution to
the excitonic couplings. In particular the T-dependent shift is shown to be an
unambiguous signature for the sign of net short-range couplings: Aggregates
with net negative (positive) short-range couplings redshift (blueshift) with
increasing temperature. Lastly, comparison with experiments shows that our
theory can be utilized to quantitatively account for the observed but
previously unexplained T-dependent absorption lineshapes. Thus, our work
provides a firm ground for elucidating the structure-function relationships for
molecular aggregates and is fully compatible with existing experimental and
theoretical structure characterization tools.Comment: 29 pages, 4 figure
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