Optimized pulse sequences for suppressing unwanted transitions in quantum systems

We investigate the nature of the pulse sequence so that unwanted transitions in quantum systems can be inhibited optimally. For this purpose we show that the sequence of pulses proposed by Uhrig [Phys. Rev. Lett. \textbf{98}, 100504 (2007)] in the context of inhibition of environmental dephasing effects is optimal. We derive exact results for inhibiting the transitions and confirm the results numerically. We posit a very significant improvement by usage of the Uhrig sequence over an equidistant sequence in decoupling a quantum system from unwanted transitions. The physics of inhibition is the destructive interference between transition amplitudes before and after each pulse.Comment: 5 figure

Entangling photons using a charged quantum dot in a microcavity

We present two novel schemes to generate photon polarization entanglement via single electron spins confined in charged quantum dots inside microcavities. One scheme is via entangled remote electron spins followed by negatively-charged exciton emissions, and another scheme is via a single electron spin followed by the spin state measurement. Both schemes are based on giant circular birefringence and giant Faraday rotation induced by a single electron spin in a microcavity. Our schemes are deterministic and can generate an arbitrary amount of multi-photon entanglement. Following similar procedures, a scheme for a photon-spin quantum interface is proposed.Comment: 4 pages, 4 figure

Effects of organic and ‘low input’ production methods on food quality and safety

The intensification of agricultural production in the last century has resulted in a significant loss of biodiversity, environmental problems and associated societal costs. The use of shorter rotations or monocropping and high levels of mineral fertilisers, pesticides and crop growth regulators may also have had negative impacts on food quality and safety. To reverse the negative environmental and biodiversity impacts of agricultural intensification, a range of different ‘low input’ farming systems have been developed and are now supported by EU and government support schemes. A range of recent reviews concluded that switching to low input, integrated or organic farming practices results in significant environmental benefits and increased biodiversity in agro-ecosystems. Some recent studies also reported higher levels of nutritionally desirable compounds (e.g. vitamins, antioxidants, mineral nutrients) in foods from organic and ‘low input’ production systems compared to food from conventional systems. The increasing demand and current price premiums achieved by foods from low input and especially organic production systems were shown to be closely linked to consumer perceptions about nutritional and health benefits of such foods. However, there are other studies reporting no significant differences in composition between low input and conventional foods, or inconsistent results. There is currently a lack of (a) factorial studies, which allow the effect of individual production system components (e.g. rotation design, fertility management, crop health management, variety choice) on food composition to be assessed and (b) dietary intervention or cohort studies which compare the effect of consuming foods from different production systems on animal and/or human health. It is therefore currently not possible to draw overall conclusions about the effect of low input production on food quality and safety. This paper will (a) describe the range of organic and other ‘low input’ standards, certification and support systems currently used, (b) summarise the currently available information on effects of organic and other low input crop production systems on the environment, biodiversity and food quality, and (c) describe the methodologies and results from subproject 2 of the EU-funded Integrated project QualityLowInputFood. This project focused on improving our knowledge about the effect of organic and low input crop and livestock production systems on food quality and safety parameters

Quantum Error Correction on Linear Nearest Neighbor Qubit Arrays

A minimal depth quantum circuit implementing 5-qubit quantum error correction in a manner optimized for a linear nearest neighbor architecture is described. The canonical decomposition is used to construct fast and simple gates that incorporate the necessary swap operations. Simulations of the circuit's performance when subjected to discrete and continuous errors are presented. The relationship between the error rate of a physical qubit and that of a logical qubit is investigated with emphasis on determining the concatenated error correction threshold.Comment: 4 pages, 5 figure

Anonymous shell companies: A global audit study and field experiment in 176 countries

To test whether firms behave consistently with international law prohibiting anonymous incorporation, we conducted a global audit study and field experiment, using data from 1639 incorporation firms in 176 countries. We requested anonymous incorporation and randomly assigned references to international law, threat of penalties, norms of appropriate behavior, or a placebo. We find a substantial number of firms willing to flout international standards and show that those in OECD countries proved significantly less compliant with rules than in developing countries or tax havens. Firms in tax havens displayed significantly greater compliance and were sensitive to experimental interventions invoking international law

Entanglement dynamics of two qubits under the influence of external kicks and Gaussian pulses

We have investigated the dynamics of entanglement between two spin-1/2 qubits that are subject to independent kick and Gaussian pulse type external magnetic fields analytically as well as numerically. Dyson time ordering effect on the dynamics is found to be important for the sequence of kicks. We show that "almost-steady" high entanglement can be created between two initially unentangled qubits by using carefully designed kick or pulse sequences

History-sensitive versus future-sensitive approaches to security in distributed systems

We consider the use of aspect-oriented techniques as a flexible way to deal with security policies in distributed systems. Recent work suggests to use aspects for analysing the future behaviour of programs and to make access control decisions based on this; this gives the flavour of dealing with information flow rather than mere access control. We show in this paper that it is beneficial to augment this approach with history-based components as is the traditional approach in reference monitor-based approaches to mandatory access control. Our developments are performed in an aspect-oriented coordination language aiming to describe the Bell-LaPadula policy as elegantly as possible. Furthermore, the resulting language has the capability of combining both history- and future-sensitive policies, providing even more flexibility and power.Comment: In Proceedings ICE 2010, arXiv:1010.530

Suppression of decoherence by bath ordering

The dynamics of two coupled spins-1/2 coupled to a spin-bath is studied as an extended model of the Tessieri-Wilkie Hamiltonian \cite{TWmodel}. The pair of spins served as an open subsystem were prepared in one of the Bell states and the bath consisted of some spins-1/2 is in a thermal equilibrium state from the very beginning. It is found that with the increasing the coupling strength of the bath spins, the bath forms a resonant antiferromagnetic order. The polarization correlation between the two spins of the subsystem and the concurrence are recovered in some extent to the isolated subsystem. This suppression of the subsystem decoherence may be used to control the quantum devices in practical applications.Comment: 32 pages, Chinese Physics (accepted