46 research outputs found
On the ubiquity of Beutler-Fano profiles: from scattering to dissipative processes
Fano models - consisting of a Hamiltonian with discrete-continuous spectrum -
are one of the basic toy models in spectroscopy. They have been succesfull in
explaining the lineshape of experiments in atomic physics and condensed matter.
These models however have largely been out of the scope of dissipative
dynamics, with ony a handful of works considering the effect of a thermal bath.
Yet in nanostructures and condensed matter systems, dissipation strongly
modulates the dynamics. In this article, we present an overview of the
theoretical works dealing with Fano interferences coupled to a thermal bath and
compare them to the scattering formalism. We provide the solution to any
discrete-continuous Hamiltonian structure within the wideband approximation
coupled to a Markovian bath. In doing so, we update the toy models that have
been available for unitary evolution since the 1960s. We find that the Fano
lineshape is preserved as long as we allow a rescaling of the parameters, and
an additional Lorentzian contribution that reflects the destruction of the
interference by dephasings. We discuss the pertinence of each approach -
dissipative and unitary - to different experimental setups: scattering,
transport and spectroscopy of dissipative systems. We finish by discussing the
current limitations of the theories due to the wideband approximation and the
memory effects of the bath.Comment: Expanded bibliography, minor typos correcte
Classification of Dark States in Multi-level Dissipative Systems
Dark states are eigenstates or steady-states of a system that are decoupled
from the radiation. Their use, along with associated techniques such as
Stimulated Raman Adiabatic Passage, has extended from atomic physics where it
is an essential cooling mechanism, to more recent versions in condensed phase
where it can increase the coherence times of qubits. These states are often
discussed in the context of unitary evolution and found with elegant methods
exploiting symmetries, or via the Bruce-Shore transformation. However, the link
with dissipative systems is not always transparent, and distinctions between
classes of CPT are not always clear. We present a detailed overview of the
arguments to find stationary dark states in dissipative systems, and examine
their dependence on the Hamiltonian parameters, their multiplicity and purity.
We find a class of dark states that depends not only on the detunings of the
lasers but also on their relative intensities. We illustrate the criteria with
the more complex physical system of the hyperfine transitions of Rb and
show how a knowledge of the dark state manifold can inform the preparation of
pure states.Comment: additional example
Continuum model for chiral induced spin selectivity in helical molecules
A minimal model is exactly solved for electron spin transport on a helix.
Electron transport is assumed to be supported by well oriented type
orbitals on base molecules forming a staircase of definite chirality. In a
tight binding interpretation, the SOC opens up an effective
coupling via interbase hopping, introducing spin coupled
transport. The resulting continuum model spectrum shows two Kramers doublet
transport channels with a gap proportional to the SOC. Each doubly degenerate
channel satisfies time reversal symmetry, nevertheless, a bias chooses a
transport direction and thus selects for spin orientation. The model predicts
which spin orientation is selected depending on chirality and bias, changes in
spin preference as a function of input Fermi level and scattering suppression
protected by the SO gap. We compute the spin current with a definite helicity
and find it to be proportional to the torsion of the chiral structure and the
non-adiabatic Aharonov- Anandan phase. To describe room temperature transport
we assume that the total transmission is the result of a product of coherent
steps limited by the coherence length
Fano-Liouville Spectral Signatures in Open Quantum Systems
The scattering amplitude from a set of discrete states coupled to a continuum
became known as the Fano profile, characteristic for its asymmetric lineshape
and originally investigated in the context of photoionization. The generality
of the model, and the proliferation of engineered nanostructures with confined
states gives immense success to the Fano lineshape, which is invoked whenever
an asymmetric lineshape is encountered. However, many of these systems do not
conform to the initial model worked out by Fano in that i) they are subject to
dissipative processes and ii) the observables are not entirely analogous to the
ones measured in the original photoionization experiments. In this letter, we
work out the full optical response of a Fano model with dissipation. We find
that the exact result for absorption, Raman, Rayleigh and fluorescence emission
is a modified Fano profile where the typical lineshape has an additional
Lorentzian contribution. Expressions to extract model parameters from a set of
relevant observables are given.Comment: corrected typo
Projection based adiabatic elimination of bipartite open quantum systems
Adiabatic elimination methods allow the reduction of the space dimension
needed to describe systems dynamics which exhibits separation of time scale.
For open quantum system, it consists in eliminating the fast part assuming it
has almost instantaneously reached its steady-state and obtaining an
approximation of the evolution of the slow part. These methods can be applied
to eliminate a linear subspace within the system Hilbert space, or
alternatively to eliminate a fast subsystems in a bipartite quantum system. In
this work, we extend an adiabatic elimination method used for removing fast
degrees of freedom within a open quantum system (Phys. Rev. A 2020, 101,042102)
to eliminate a subsystem from an open bipartite quantum system. As an
illustration, we apply our technique to a dispersively coupled two-qubit system
and in the case of the open Rabi model.Comment: 11 pages, 2 figure
Probing electronic decoherence with high-resolution attosecond photoelectron interferometry
Quantum coherence plays a fundamental role in the study and control of
ultrafast dynamics in matter. In the case of photoionization, entanglement of
the photoelectron with the ion is a well known source of decoherence when only
one of the particles is measured. Here we investigate decoherence due to
entanglement of the radial and angular degrees of freedom of the photoelectron.
We study two-photon ionization via the 2s2p autoionizing state in He using high
spectral resolution photoelectron interferometry. Combining experiment and
theory, we show that the strong dipole coupling of the 2s2p and 2p states
results in the entanglement of the angular and radial degrees of freedom. This
translates, in angle integrated measurements, into a dynamic loss of coherence
during autoionization
Optical cavity-mediated exciton dynamics in photosynthetic light harvesting 2 complexes
Strong light-matter interaction leads to the formation of hybrid polariton states and alters the photophysical dynamics of organic materials and biological systems without modifying their chemical structure. Here, we experimentally investigated a well-known photosynthetic protein, light harvesting 2 complexes (LH2) from purple bacteria under strong coupling with the light mode of a Fabry-Perot optical microcavity. Using femtosecond pump probe spectroscopy, we analyzed the polariton dynamics of the strongly coupled system and observed a significant prolongation of the excited state lifetime compared with the bare exciton, which can be explained in terms of the exciton reservoir model. Our findings indicate the potential of tuning the dynamic of the whole photosynthetic unit, which contains several light harvesting complexes and reaction centers, with the help of strong exciton-photon coupling, and opening the discussion about possible design strategies of artificial photosynthetic devices