Efficient simulation of strong system-environment interactions

Multi-component quantum systems in strong interaction with their environment are receiving increasing attention due to their importance in a variety of contexts, ranging from solid state quantum information processing to the quantum dynamics of bio-molecular aggregates. Unfortunately, these systems are difficult to simulate as the system-bath interactions cannot be treated perturbatively and standard approaches are invalid or inefficient. Here we combine the time dependent density matrix renormalization group methods with techniques from the theory of orthogonal polynomials to provide an efficient method for simulating open quantum systems, including spin-boson models and their generalisations to multi-component systems

Density Matrix Renormalization Group in the Heisenberg Picture

In some cases the state of a quantum system with a large number of subsystems can be approximated efficiently by the density matrix renormalization group, which makes use of redundancies in the description of the state. Here we show that the achievable efficiency can be much better when performing density matrix renormalization group calculations in the Heisenberg picture, as only the observable of interest but not the entire state is considered. In some non-trivial cases, this approach can even be exact for finite bond dimensions.Comment: version to appear in PRL, acronyms in title and abstract expanded, new improved numerical example

A variational description of the quantum phase transition in the sub-Ohmic spin-boson model

The sub-ohmic spin-boson model is known to possess a novel quantum phase transition at zero temperature between a localised and delocalised phase. We present here an analytical theory based on a variational ansatz for the ground state, which describes a continuous localization transition with mean-field exponents for $0<s<0.5$. Our results for the critical properties show good quantitiative agreement with previous numerical results, and we present a detailed description of all the spin observables as the system passes through the transition. Analysing the ansatz itself, we give an intuitive microscopic description of the transition in terms of the changing correlations between the system and bath, and show that it is always accompanied by a divergence of the low-frequency boson occupations. The possible relevance of this divergence for some numerical approaches to this problem is discussed and illustrated by looking at the ground state obtained using density matrix renormalisation group methods

Quantum-coherent dynamics in photosynthetic charge separation revealed by wavelet analysis

Experimental/theoretical evidence for sustained vibration-assisted electronic (vibronic) coherence in the Photosystem II Reaction Center (PSII RC) indicates that photosynthetic solar-energy conversion might be optimized through the interplay of electronic and vibrational quantum dynamics. This evidence has been obtained by investigating the primary charge separation process in the PSII RC by two-dimensional electronic spectroscopy (2DES) and Redfield modeling of the experimental data. However, while conventional Fourier transform analysis of the 2DES data allows oscillatory signatures of vibronic coherence to be identified in the frequency domain in the form of static 2D frequency maps, the real-time evolution of the coherences is lost. Here we apply for the first time wavelet analysis to the PSII RC 2DES data to obtain time-resolved 2D frequency maps. These maps allow us to demonstrate that i) coherence between the excitons initiating the two different charge separation pathways is active for more than 500 fs, and ii) coherence between exciton and charge-transfer states, the reactant and product of the charge separation reaction, respectively, is active for at least 1 ps. These findings imply that the PSII RC employs coherence i) to sample competing electron transfer pathways, and ii) to perform directed, ultrafast and efficient electron transfer.Comment: Scientific reports 201

Farm Management - Bugger the roots, where is the future?

The Farm Management discipline has long been closely aligned with agricultural economics. The question we raise is not where either discipline came from but where is Farm Management going. The impact of globalisation, the rising tide of deregulation and chain reversal mean that farm management professionals who have traditionally focused on optimisation of activities at a farm level are now commonly expected to use sociology and management science to explain economic organisation and performance on farms. They also are required to look at relationships in the value chain(s) in which the farm sits. This paper will analyse the implications of such change for Farm Management professionals.Farm Management, value chains, Farm Management,

Cost-effectiveness of insulin detemir compared with NPH insulin in people with type 2 diabetes in Denmark, Finland, Norway, and Sweden.

Abstract Objective: To assess the cost-effectiveness of insulin detemir compared with Neutral Protamine Hagedorn (NPH) insulin when initiating insulin treatment in people with type 2 diabetes mellitus (T2DM) in Denmark, Finland, Norway, and Sweden. Methods: Efficacy and safety data were derived from a 20-week multi-centre randomized controlled head-to-head clinical trial comparing insulin detemir and NPH insulin in insulin naïve people with T2DM, and short-term (1-year) cost effectiveness analyses were performed. As no significant differences in HbA(1c) were observed between the two treatment arms, the model was based on significant differences in favour of insulin detemir in frequency of hypoglycaemia (Rate-Ratio = 0.52; CI = 0.44-0.61) and weight gain (Δ = 0.9 kg). Model outcomes were measured in Quality Adjusted Life Years (QALYs) using published utility estimates. Acquisition costs for insulin and direct healthcare costs associated with non-severe hypoglycaemic events were obtained from National Health Service public sources. One-way and probabilistic sensitivity analyses were performed. Results: Based on lower incidence of non-severe hypoglycaemic events and less weight gain, the QALY gain from initiating treatment with insulin detemir compared with NPH insulin was 0.01 per patient per year. Incremental cost-effectiveness ratios for the individual countries were: Denmark, Danish Kroner 170,852 (€22,933); Finland, €28,349; Norway, Norwegian Kroner 169,789 (€21,768); and Sweden, Swedish Krona 226,622 (€25,097) per QALY gained. Possible limitations of the study are that data on hypoglycaemia and relative weight benefits from a clinical trial were combined with hypoglycaemia incidence data from observational studies. These populations may have slightly different patient characteristics. Conclusions: The lower risk of non-severe hypoglycaemia and less weight gain associated with using insulin detemir compared with NPH insulin when initiating insulin treatment in insulin naïve patients with type 2 diabetes provide economic benefits in the short-term. Based on cost/QALY threshold values, this represents good value for money in the Nordic countries. Using a short-term modelling approach may be conservative, as reduced frequency of hypoglycaemia and less weight gain may also have positive long-term health-related implications

Vibronic origin of long-lived coherence in an artificial molecular light harvester

Natural and artificial light harvesting processes have recently gained new interest. Signatures of long lasting coherence in spectroscopic signals of biological systems have been repeatedly observed, albeit their origin is a matter of ongoing debate, as it is unclear how the loss of coherence due to interaction with the noisy environments in such systems is averted. Here we report experimental and theoretical verification of coherent exciton-vibrational (vibronic) coupling as the origin of long-lasting coherence in an artificial light harvester, a molecular J-aggregate. In this macroscopically aligned tubular system, polarization controlled 2D spectroscopy delivers an uncongested and specific optical response as an ideal foundation for an in-depth theoretical description. We derive analytical expressions that show under which general conditions vibronic coupling leads to prolonged excited-state coherence

Factors underpinning improved productivity in the WA wheat industry

Wheat yields in the Western Australian wheat industry have risen by about 3.9 percent per year over a period of 20 years. This has coincided with a decline in growing season rainfall over the grain belt. However, in the last few years yields have been highly variable because of fluctuations in rainfall and because two of the three driest years in the last 70 years have occurred during the last five years. Farm managers, consultants, researchers and research funders need to understand the key drivers of improvements in productivity. Researchers from the Department of Agriculture, Western Australia have conducted intensive research on wheat varieties and management practices needed to achieve high wheat yields. These have been combined into recommendations known as High Yield Packages (HYPs) for the regions of the state. The question addressed in this paper is which elements of these packages have been important and what other factors may have contributed to the improvements in productivity.Using graphical, statistical and regression analysis of benchmark data from PlanFarm Consulting group for 1995 to 2004, supported by in-depth qualitative analysis of 40 case study farmers, and a random telephone survey of 100 farmers, key factors associated with yield increases were investigated. The evidence suggests farmers have increased their water use efficiency since 1995 and that improvements in productivity are associated with nitrogen and phosphorous use, herbicide use, higher seeding rates and management performance. There is also qualitative evidence to suggest that the widespread adoption of one pass operations or no-tillage systems has been an important complementary factor