71 research outputs found
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
Quantum correlations of light and matter through environmental transitions
One aspect of solid-state photonic devices that distinguishes them from their
atomic counterparts is the unavoidable interaction between system excitations
and lattice vibrations of the host material. This coupling may lead to
surprising departures in emission properties between solid-state and atomic
systems. Here we predict a striking and important example of such an effect. We
show that in solid-state cavity quantum electrodynamics, interactions with the
host vibrational environment can generate quantum cavity-emitter correlations
in regimes that are semiclassical for atomic systems. This behaviour, which can
be probed experimentally through the cavity emission properties, heralds a
failure of the semiclassical approach in the solid-state, and challenges the
notion that coupling to a thermal bath supports a more classical description of
the system. Furthermore, it does not rely on the spectral details of the host
environment under consideration and is robust to changes in temperature. It
should thus be of relevance to a wide variety of photonic devices.Comment: 8 pages, 7 figures. v2 - minor edits. v3 - more substantial edits to
the text. Title changed and new results on correlations added in Fig. 3. v4 -
close to published version, presentation clarifie
Environmental Nonadditivity and Franck-Condon physics in Nonequilibrium Quantum Systems
We show that for a quantum system coupled to both vibrational and
electromagnetic environments, enforcing additivity of their combined influences
results in non-equilibrium dynamics that does not respect the Franck-Condon
principle. We overcome this shortcoming by employing a collective coordinate
representation of the vibrational environment, which permits the derivation of
a non-additive master equation. When applied to a two-level emitter our
treatment predicts decreasing photon emission rates with increasing vibrational
coupling, consistent with Franck-Condon physics. In contrast, the additive
approximation predicts the emission rate to be completely insensitive to
vibrations. We find that non-additivity also plays a key role in the stationary
non-equilibrium model behaviour, enabling two-level population inversion under
incoherent electromagnetic excitation.Comment: 9 pages (including supplementary information), 4 figures. V2 - minor
clarifications to main text and new section in the supplemen
Few-photon Transport in Fano-resonance waveguide geometries
We present a theoretical study of Fano interference effects in few-photon
transport. Under appropriate conditions, a local defect in an optical waveguide
induces a highly asymmetric transmission lineshape, characteristic of Fano
interference. For a two-level emitter placed adjacent to such a defect, here
modeled as a partially transmitting element, we find an analytical expression
for the full time evolution of single-photon wavepackets and the emitter
excitation probability. We show how the partially transmitting element affects
the emitter lifetime and shifts the spectral position of the effective system
resonances. Using input-output formalism, we determine the single and
two-photon -matrices for both a two-level emitter and a cavity-emitter
system coupled to a waveguide with a partially transmitting element. We show
how the Fano interference effect can be exploited for the implementation of a
Hong-Ou-Mandel switch in analogy with a tunable linear or nonlinear beam
splitter.Comment: 19 pages (incl. a 6 pages long appendix), 5 figure
Quantum work statistics at strong reservoir coupling
Calculating the stochastic work done on a quantum system while strongly
coupled to a reservoir is a formidable task, requiring the calculation of the
full eigenspectrum of the combined system and reservoir. Here we show that this
issue can be circumvented by using a polaron transformation that maps the
system into a new frame where weak-coupling theory can be applied. It is shown
that the work probability distribution is invariant under this transformation,
allowing one to compute the full counting statistics of work at strong
reservoir coupling. Crucially this polaron approach reproduces the Jarzynski
fluctuation theorem, thus ensuring consistency with the laws of stochastic
thermodynamics. We apply our formalism to a system driven across the
Landau-Zener transition, where we identify clear signatures in the work
distribution arising from a non-negligible coupling to the environment. Our
results provide a new method for studying the stochastic thermodynamics of
driven quantum systems beyond Markovian, weak-coupling regimes.Comment: 15 pages, 3 figures, comments welcom
Signatures of Non-Markovianity in Cavity-QED with Color Centers in 2D Materials
Light-matter interactions of defects in two dimensional materials are
expected to be profoundly impacted by strong coupling to phonons. In this work,
we combine ab initio calculations of a defect in hBN, with a fully quantum
mechanical and numerically exact description of a cavity-defect system to
elucidate this impact. We show that even at weak light-matter coupling, the
dynamical evolution of the cavity-defect system has clear signatures of
non-markovian phonon effects, and that linear absorption spectra show the
emergence of hybridised light-matter-phonon states in regimes of strong
light-matter coupling. We emphasise that our methodology is general, and can be
applied to a wide variety of material/defect systems.Comment: 7 pages, 3 figures + 8 pages supplemen
Energy transfer in structured and unstructured environments: Master equations beyond the Born-Markov approximations
We explore excitonic energy transfer dynamics in a molecular dimer system coupled to both structured and unstructured oscillator environments. By extending the reaction coordinate master equation technique developed by Iles-Smith et al. [Phys. Rev. A 90, 032114 (2014)], we go beyond the commonly used Born-Markov approximations to incorporate system-environment correlations and the resultant non-Markovian dynamical effects. We obtain energy transfer dynamics for both underdamped and overdamped oscillator environments that are in perfect agreement with the numerical hierarchical equations of motion over a wide range of parameters. Furthermore, we show that the Zusman equations, which may be obtained in a semiclassical limit of the reaction coordinate model, are often incapable of describing the correct dynamical behaviour. This demonstrates the necessity of properly accounting for quantum correlations generated between the system and its environment when the Born-Markov approximations no longer hold. Finally, we apply the reaction coordinate formalism to the case of a structured environment comprising of both underdamped (i.e., sharply peaked) and overdamped (broad) components simultaneously. We find that though an enhancement of the dimer energy transfer rate can be obtained when compared to an unstructured environment, its magnitude is rather sensitive to both the dimer-peak resonance conditions and the relative strengths of the underdamped and overdamped contributions
Driving-induced population trapping and linewidth narrowing via the quantum Zeno effect
We investigate the suppression of spontaneous emission from a driven
three-level system embedded in an optical cavity via a manifestation of the
quantum Zeno effect. Strong resonant coupling of the lower two levels to an
external optical field results in a decrease of the exponential decay rate of
the third upper level. We show that this effect has observable consequences in
the form of emission spectra with subnatural linewidths, which should be
measurable using, for example, quantum dot--cavity systems in currently
obtainable parameter regimes. These results constitute a novel method to
control an inherently irreversible and dissipative process, and may be useful
in applications requiring the control of single photon arrival times and
wavepacket extent
Exact quantum dynamics in structured environments
Funding: DG and DK acknowledge studentship funding from EPSRC under grant no. EP/L015110//1. AS acknowledges a studentship from EPSRC under grant no. EP/L505079/1. J.I.-S. acknowledges support from the Royal Commission for the Exhibition of 1851. AN acknowledges funding from EPSRC under grant no. EP/N008154/1.The dynamics of a wide range of technologically important quantum systems are dominated by their interaction with just a few environmental modes. Such highly structured environments give rise to long-lived bath correlations that induce complex dynamics which are very difficult to simulate. These difficulties are further aggravated when spatial correlations between different parts of the system are important. By modeling the dynamics of a pair of two-level quantum systems in a common, structured, environment we show that a recently developed general purpose numerical approach, the time-evolving matrix product operator, is capable of accurate simulation under exactly these conditions. We find that tuning the separation to match the wavelength of the dominant environmental modes can drastically modify the system dynamics. To further explore this behavior, we show that the full dynamics of the bath can be calculated directly from those of the system, thus allowing us to develop intuition for the complex dynamics observed.Publisher PDFPeer reviewe
Protocol for generating multiphoton entangled states from quantum dots in the presence of nuclear spin fluctuations
Multi-photon entangled states are a crucial resource for many applications in
quantum information science. Semiconductor quantum dots offer a promising route
to generate such states by mediating photon-photon correlations via a confined
electron spin, but dephasing caused by the host nuclear spin environment
typically limits coherence (and hence entanglement) between photons to the spin
time of a few nanoseconds. We propose a protocol for the deterministic
generation of multi-photon entangled states that is inherently robust against
the dominating slow nuclear spin environment fluctuations, meaning that
coherence and entanglement is instead limited only by the much longer spin
time of microseconds. Unlike previous protocols, the present scheme
allows for the generation of very low error probability polarisation encoded
three-photon GHZ states and larger entangled states, without the need for spin
echo or nuclear spin calming techniques
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