Decoherence and the Nature of System-Environment Correlations

We investigate system-environment correlations based on the exact dynamics of a qubit and its environment in the framework of pure decoherence (phase damping). We focus on the relation of decoherence and the build-up of system-reservoir entanglement for an arbitrary (possibly mixed) initial qubit state. In the commonly employed regime where the qubit dynamics can be described by a Markov master equation of Lindblad type, we find that for almost all qubit initial states inside the Bloch sphere, decoherence is complete while the total state is still separable - no entanglement is involved. In general, both "separable" and "entangling" decoherence occurs, depending on temperature and initial qubit state. Moreover, we find situations where classical and quantum correlations periodically alternate as a function of time in the regime of low temperatures

A plasmonic route towards the energy scaling of on-chip integrated all-photonic phase-change memories

This is the author accepted manuscript.Phase-change photonic memory devices, conventionally implemented as a thin layer of phase-change material deposited on the top of an integrated Si or SiN waveguide, have the flexibility to be applied in a widely diverse context, as a pure memory device, a logic gate, an arithmetic processing unit and for biologically inspired computing. In all such applications increasing the speed, and reducing the power consumption, of the phase-switching process is most desirable. In this work, therefore, we investigate, via simulation, a novel integrated photonic device architecture that exploits plasmonic effects to enhance the light-matter interaction. Our device comprises a dimer nanoantenna fabricated on top of a SiN waveguide and with a phase-change material deposited into the gap between the two nanoantenna halves. We observed very considerably increased device speeds and reduced energy requirements, of up to two orders of magnitude, when compared to the conventional structure.Engineering and Physical Sciences Research Council (EPSRC

System-environment correlations and Non-Markovian dynamics

We determine the total state dynamics of a dephasing open quantum system using the standard environment of harmonic oscillators. Of particular interest are random unitary approaches to the same reduced dynamics and system-environment correlations in the full model. Concentrating on a model with an at times negative dephasing rate, the issue of "non-Markovianity" will also be addressed. Crucially, given the quantum environment, the appearance of non-Markovian dynamics turns out to be accompanied by a loss of system-environment correlations. Depending on the initial purity of the qubit state, these system-environment correlations may be purely classical over the whole relevant time scale, or there may be intervals of genuine system-environment entanglement. In the latter case, we see no obvious relation between the build-up or decay of these quantum correlations and "Non-Markovianity"

New Optimization Methods for Converging Perturbative Series with a Field Cutoff

We take advantage of the fact that in lambda phi ^4 problems a large field cutoff phi_max makes perturbative series converge toward values exponentially close to the exact values, to make optimal choices of phi_max. For perturbative series terminated at even order, it is in principle possible to adjust phi_max in order to obtain the exact result. For perturbative series terminated at odd order, the error can only be minimized. It is however possible to introduce a mass shift in order to obtain the exact result. We discuss weak and strong coupling methods to determine the unknown parameters. The numerical calculations in this article have been performed with a simple integral with one variable. We give arguments indicating that the qualitative features observed should extend to quantum mechanics and quantum field theory. We found that optimization at even order is more efficient that at odd order. We compare our methods with the linear delta-expansion (LDE) (combined with the principle of minimal sensitivity) which provides an upper envelope of for the accuracy curves of various Pade and Pade-Borel approximants. Our optimization method performs better than the LDE at strong and intermediate coupling, but not at weak coupling where it appears less robust and subject to further improvements. We also show that it is possible to fix the arbitrary parameter appearing in the LDE using the strong coupling expansion, in order to get accuracies comparable to ours.Comment: 10 pages, 16 figures, uses revtex; minor typos corrected, refs. adde

Modelling phase-change integrated photonic devices

Available from E\PCOS via the link in this recordWe report the progress made on the development of a self-consistent 3-dimensional simulation framework, yielding the time and spatially resolved electric field, temperature and material phase, for integrated phase-change photonic devices. We illustrate the analysis made for a prototypical integrated phase-change photonic memory, and report the results of SET and RESET operations.Engineering and Physical Sciences Research Council (EPSRC

Behavioral modeling of integrated phase-change photonic devices for neuromorphic computing applications

This is the final version. Available from AIP Publishing via the DOI in this record. The combination of phase-change materials and integrated photonics has led to the development of new forms of all-optical devices, includingphotonic memories, arithmetic and logic processors, and synaptic and neuronal mimics. Such devices can be readily fabricated into photonicintegrated circuits, so potentially delivering large-scale all-optical arithmetic-logic units and neuromorphic processing chips. To facilitate inthe design and optimization of such large-scale systems, and to aid in the understanding of device and system performance, fast yet accuratecomputer models are needed. Here, we describe the development of a behavioral modeling tool that meets such requirements, being capableof essentially instantaneous modeling of the write, erase, and readout performance of various integrated phase-change photonic devices,including those for synaptic and neuronal mimics.Engineering and Physical Sciences Research Council (EPSRC)European Commissio

A behavioural model for integrated phase-change photonics

This is the author accepted manuscript. The final version is available from the European Phase Change and Ovonics Symposium via the link in this recordThe use of phase-change materials in integrated photonics applications has enabled the development of new types of all-optical devices, including multilevel photonic memories, arithmetic and logic processors and synaptic and neuron mimics. In order to design, optimise and understand the performance of large-scale systems, fast and accurate material and device models are needed. Here we present a behavioural model for phase-change photonic devices that can simulate the write, erase and readout operations in timespans compatible with system level performance evaluation.European Union Horizon 2020Engineering and Physical Sciences Research Council (EPSRC

Enhanced performance in plasmonic integrated phase-change memories

This is the final version.We here propose feasible strategies to improve the performance of integrated phase-change photonic memories by the use of plasmonic enhancement. Several solutions are investigated, focusing in particular on optimising the optical readout contrast (transmission modulation) that can be achieved between crystalline and amorphous states. Results show that by embedding the plasmonic nanoantenna within the body of the waveguide, or by using multiple coupled nanoantennas in series, significant improvements in optical readout contrast can be achieved, while maintaining relatively small insertion losses.European Union Horizon 202