295 research outputs found
When do weak-coupling approaches accurately capture the dynamics of complex quantum systems?
Understanding the dynamics of higher-dimensional quantum systems embedded in
a complex environment remains a significant theoretical challenge. While
several approaches yielding numerically converged solutions exist, these are
computationally expensive and often provide only limited physical insight. Here
we address the question when more intuitive and simpler to compute
weak-coupling approaches still provide adequate accuracy. We develop a simple
analytical criterion and verify its validity for the case of the much-studied
FMO dynamics as well as the canonical spin-boson model.Comment: 10 pages, 5 figures, comments are very welcome
Environmental dynamics, correlations, and the emergence of noncanonical equilibrium states in open quantum systems
Quantum systems are invariably open, evolving under surrounding influences
rather than in isolation. Standard open quantum system methods eliminate all
information on the environmental state to yield a tractable description of the
system dynamics. By incorporating a collective coordinate of the environment
into the system Hamiltonian, we circumvent this limitation. Our theory provides
straightforward access to important environmental properties that would
otherwise be obscured, allowing us to quantify the evolving system-environment
correlations. As a direct result, we show that the generation of robust
system-environment correlations that persist into equilibrium (heralded also by
the emergence of non-Gaussian environmental states) renders the canonical
system steady-state almost always incorrect. The resulting equilibrium states
deviate markedly from those predicted by standard perturbative techniques and
are instead fully characterised by thermal states of the mapped
system-collective coordinate Hamiltonian. We outline how noncanonical system
states could be investigated experimentally to study deviations from canonical
thermodynamics, with direct relevance to molecular and solid-state nanosystems.Comment: 10 pages, 4 figures, close to published versio
Nonequilibrium thermodynamics in the strong coupling and non-Markovian regime based on a reaction coordinate mapping
We propose a method to study the thermodynamic behaviour of small systems
beyond the weak coupling and Markovian approximation, which is different in
spirit from conventional approaches. The idea is to redefine the system and
environment such that the effective, redefined system is again coupled weakly
to Markovian residual baths and thus, allows to derive a consistent
thermodynamic framework for this new system-environment partition. To achieve
this goal we make use of the reaction coordinate mapping, which is a general
method in the sense that it can be applied to an arbitrary (quantum or
classical and even time-dependent) system coupled linearly to an arbitrary
number of harmonic oscillator reservoirs. The core of the method relies on an
appropriate identification of a part of the environment (the reaction
coordinate), which is subsequently included as a part of the system. We
demonstrate the power of this concept by showing that non-Markovian effects can
significantly enhance the steady state efficiency of a three-level-maser heat
engine, even in the regime of weak system-bath coupling. Furthermore, we show
for a single electron transistor coupled to vibrations that our method allows
one to justify master equations derived in a polaron transformed reference
frame.Comment: updated and improved version; 19 pages incl. 10 figures and 5 pages
appendi
Virtual excitations in the ultra-strongly-coupled spin-boson model: physical results from unphysical modes
Here we show how, in the ultra-strongly-coupled spin-boson model, apparently
unphysical "Matsubara modes" are required not only to regulate detailed
balance, but also to arrive at a correct and physical description of the
non-perturbative dynamics and steady-state. In particular, in the
zero-temperature limit, we show that neglecting the Matsubara modes results in
an erroneous emission of virtual photons from the collective ground state. To
explore this difficult-to-model regime we start by using a non-perturbative
hierarchical equations of motion (HEOM) approach, based on a partial fitting of
the bath correlation-function which takes into account the infinite sum of
Matsubara frequencies using only a biexponential function. We compare the HEOM
method to both a pseudo-mode model, and the reaction coordinate (RC) mapping,
which help explain the nature of the aberrations observed when Matsubara
frequencies are neglected. For the pseudo-mode method we present a general
proof of validity, which allows for negative Matsubara-contributions to the
decomposition of the bath correlation functions to be described by
zero-frequency Matsubara-modes with non-Hermitian coupling to the system. The
latter obey a non-Hermitian pseudo-Schr\"odinger equation, ultimately
justifying why superficially unphysical modes can give rise to physical system
behavior.Comment: 21 page
Amplified opto-mechanical transduction of virtual radiation pressure
Here we describe how, utilizing a time-dependent opto-mechanical interaction,
a mechanical probe can provide an amplified measurement of the virtual photons
dressing the quantum ground state of an ultra strongly-coupled light-matter
system. We calculate the thermal noise tolerated by this measurement scheme,
and discuss a range of experimental setups in which it could be realized.Comment: 7 + 12 pages, 1 figur
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