Optical Signatures of Non-Markovian Behaviour in Open Quantum Systems

We derive an extension to the quantum regression theorem which facilitates the calculation of two-time correlation functions and emission spectra for systems undergoing non-Markovian evolution. The derivation exploits projection operator techniques, with which we obtain explicit equations of motion for the correlation functions, making only a second order expansion in the system--environment coupling strength, and invoking the Born approximation at a fixed initial time. The results are used to investigate a driven semiconductor quantum dot coupled to an acoustic phonon bath, where we find the non-Markovian nature of the dynamics has observable signatures in the form of phonon sidebands in the resonance fluorescence emission spectrum. Furthermore, we use recently developed non-Markovianity measures to demonstrate an associated flow of information from the phonon bath back into the quantum dot exciton system.Comment: Comments welcom

Strong non-linearity-induced correlations for counter-propagating photons scattering on a two-level emitter

We analytically treat the scattering of two counter-propagating photons on a two-level emitter embedded in an optical waveguide. We find that the non-linearity of the emitter can give rise to significant pulse-dependent directional correlations in the scattered photonic state, which could be quantified via a reduction in coincident clicks in a Hong-Ou-Mandel measurement setup, analogous to a linear beam splitter. Changes to the spectra and phase of the scattered photons, however, would lead to reduced interference with other photons when implemented in a larger optical circuit. We introduce suitable fidelity measures which account for these changes, and find that high values can still be achieved even when accounting for all properties of the scattered photonic state.Comment: 10 pages, 7 figure

Error distributions on large entangled states with non-Markovian dynamics

We investigate the distribution of errors on a computationally useful entangled state generated via the repeated emission from an emitter undergoing strongly non-Markovian evolution. For emitter-environment coupling of pure-dephasing form, we show that the probability that a particular patten of errors occurs has a bound of Markovian form, and thus accuracy threshold theorems based on Markovian models should be just as effective. This is the case, for example, for a charged quantum dot emitter in a moderate to strong magnetic field. Beyond the pure-dephasing assumption, though complicated error structures can arise, they can still be qualitatively bounded by a Markovian error model.Comment: 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

Comment on "General Non-Markovian Dynamics of Open Quantum Systems"

The existence of a "non-Markovian dissipationless" regime, characterized by long lived oscillations, was recently reported for a class of quantum open systems (Zhang et al, PRL, 109, 170402, (2012)). It is claimed this could happen in the strong coupling regime, a surprising result which has attracted some attention. We show that this regime exists if and only if the total Hamiltonian is unbounded from below, casting serious doubts on the usefulness of this result

Synthetically-focused surface-penetrating radar for operation from a moving vehicle

This paper reports on a research programme, started recently under DERA sponsorship, on applying synthetic focusing to an oblong horizontal stand-off array, where all the voxels in a vertical slice underneath the centre-line of the array are synthesised electronically, with the third dimension provided by the search vehicle's forward movement. This should permit a very high search rate for buried mines, in a wide range of ground conditions

The Politics of Russian Stereotypes in Joseph Conrad’s Under Western Eyes

In my project, I focus on analyzing Russian stereotypes in Joseph Conrad’s Under Western Eyes. The novel’s oversimplifications stem from centuries of Western biases, which position Russia as partially civilized and partially barbaric. Such emotionally-charged biases are reinforced by Conrad’s depictions. Although the author professes a neutral stance, his Polish background, childhood, and family politics have influenced his perception of the nation. Serving as an extension of Conrad’s critical position, the English narrator incorporates stereotypes when describing Russian men and women, and such ideas are only complicated by Natalia, whose character also embodies Western traits. My project’s revelations are significant because the novel perpetuates stereotypes that illuminate flawed Western beliefs rather than Russian authenticity

Coherent and incoherent dynamics in excitonic energy transfer: correlated fluctuations and off-resonance effects

We study the nature of the energy transfer process within a pair of coupled two-level systems (donor and acceptor) subject to interactions with the surrounding environment. Going beyond a standard weak-coupling approach, we derive a master equation within the polaron representation that allows for investigation of both weak and strong system-bath couplings, as well as reliable interpolation between these two limits. With this theory, we are then able to explore both coherent and incoherent regimes of energy transfer within the donor-acceptor pair. We elucidate how the degree of correlation in the donor and acceptor fluctuations, the donor-acceptor energy mismatch, and the range of the environment frequency distribution impact upon the energy transfer dynamics. In the resonant case (no energy mismatch) we describe in detail how a crossover from coherent to incoherent transfer dynamics occurs with increasing temperature [A. Nazir, Phys. Rev. Lett. 103, 146404 (2009)], and we also explore how fluctuation correlations are able to protect coherence in the energy transfer process. We show that a strict crossover criterion is harder to define when off-resonance, though we find qualitatively similar population dynamics to the resonant case with increasing temperature, while the amplitude of coherent population oscillations also becomes suppressed with growing site energy mismatch.Comment: 14 pages, 7 figures, builds upon PRL 103, 146404 (2009) (arXiv:0906.0592). Comments welcome. V2 - Section IV shortened to improve presentation, references updated, new Imperial College affiliation added for A. Nazir. Published versio