153 research outputs found
Gauge ambiguities imply Jaynes-Cummings physics remains valid in ultrastrong coupling QED
Ultrastrong-coupling between two-level systems and radiation is important for
both fundamental and applied quantum electrodynamics (QED). Such regimes are
identified by the breakdown of the rotating-wave approximation, which applied
to the quantum Rabi model (QRM) yields the apparently less fundamental
Jaynes-Cummings model (JCM). We show that when truncating the material system
to two levels, each gauge gives a different description whose predictions vary
significantly for ultrastrong-coupling. QRMs are obtained through specific
gauge choices, but so too is a JCM without needing the rotating-wave
approximation. Analysing a circuit QED setup, we find that this JCM provides
more accurate predictions than the QRM for the ground state, and often for the
first excited state as well. Thus, Jaynes-Cummings physics is not restricted to
light-matter coupling below the ultrastrong limit. Among the many implications
is that the system's ground state is not necessarily highly entangled, which is
usually considered a hallmark of ultrastrong-coupling.Comment: 9 pages, plus 20 page Supplementary Information. See also related
independent work arXiv:1805.05339
A master equation for strongly interacting dipoles
We consider a pair of dipoles for which direct electrostatic dipole-dipole
interactions may be significantly larger than the coupling to transverse
radiation. We derive a master equation using the Coulomb gauge, which naturally
enables us to include the inter-dipole Coulomb energy within the system
Hamiltonian rather than the interaction. In contrast, the standard master
equation for a two- dipole system, which depends entirely on well-known
gauge-invariant S-matrix elements, is usually derived using the multipolar
gauge, wherein there is no explicit inter-dipole Coulomb interaction. We show
using a generalised arbitrary-gauge light-matter Hamiltonian that this master
equation is obtained in other gauges only if the inter-dipole Coulomb
interaction is kept within the interaction Hamiltonian rather than the
unperturbed part as in our derivation. Thus, our master equation, while still
gauge-invariant, depends on different S-matrix elements, which give
separation-dependent corrections to the standard matrix elements describing
resonant energy transfer and collective decay. The two master equations
coincide in the large separation limit where static couplings are negligible.
We provide an application of our master equation by finding
separation-dependent corrections to the natural emission spectrum of the
two-dipole system.Comment: 18 pages including appendix, 8 figure
Overcoming non-Markovian dephasing in single photon sources through post-selection
We study the effects of realistic dephasing environments on a pair of
solid-state single-photon sources in the context of the Hong-Ou-Mandel dip. By
means of solutions for the Markovian or exact non-Markovian dephasing dynamics
of the sources, we show that the resulting loss of visibility depends crucially
on the timing of photon detection events. Our results demonstrate that the
effective visibility can be improved via temporal post-selection, and also that
time-resolved interference can be a useful probe of the interaction between the
emitter and its host environment.Comment: 5 pages, 2 figures, published version, title changed, references
update
Modelling exciton-phonon interactions in optically driven quantum dots
We provide a self-contained review of master equation approaches to modelling
phonon effects in optically driven self-assembled quantum dots. Coupling of the
(quasi) two-level excitonic system to phonons leads to dissipation and
dephasing, the rates of which depend on the excitation conditions, intrinsic
properties of the QD sample, and its temperature. We describe several
techniques, which include weak-coupling master equations that are perturbative
in the exciton-phonon coupling, as well as those based on the polaron
transformation that can remain valid for strong phonon interactions. We
additionally consider the role of phonons in altering the optical emission
characteristics of quantum dot devices, outlining how we must modify standard
quantum optics treatments to account for the presence of the solid-state
environment.Comment: Invited Topical Review, 26 pages, 7 figures. V2 - close to published
version, 28 pages, 9 figures. Minor changes to text, added a few new
references and two new figure
Correlation-dependent coherent to incoherent transitions in resonant energy transfer dynamics
I investigate energy transfer in a donor-acceptor pair beyond weak
system-bath coupling. I identify a transition from coherent to incoherent
dynamics with increasing temperature, due to multi-phonon effects not captured
by a standard weak-coupling treatment. The crossover temperature has a marked
dependence on the degree of spatial correlation between fluctuations
experienced at the two system sites. For strong correlations, this leads to the
possibility of coherence surviving into a high temperature regime.Comment: 5 pages, 1 figure, 2 page supplement. Published version. Figure
edited, section added on experimental relevance, reference added, various
minor changes to the text. Comments welcom
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
Model of the optical emission of a driven semiconductor quantum dot: phonon-enhanced coherent scattering and off-resonant sideband narrowing
We study the crucial role played by the solid-state environment in
determining the photon emission characteristics of a driven quantum dot. For
resonant driving, we predict a phonon-enhancement of the coherently emitted
radiation field with increasing driving strength, in stark contrast to the
conventional expectation of a rapidly decreasing fraction of coherent emission
with stronger driving. This surprising behaviour results from thermalisation of
the dot with respect to the phonon bath, and leads to a nonstandard regime of
resonance fluorescence in which significant coherent scattering and the Mollow
triplet coexist. Off-resonance, we show that despite the phonon influence,
narrowing of dot spectral sideband widths can occur in certain regimes,
consistent with an experimental trend.Comment: Published version. 5 pages, 2 figures, plus 4 page supplement. Title
changed, figure 1 revised, various edits and additions to the tex
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