Second-order coding rates for pure-loss bosonic channels

A pure-loss bosonic channel is a simple model for communication over free-space or fiber-optic links. More generally, phase-insensitive bosonic channels model other kinds of noise, such as thermalizing or amplifying processes. Recent work has established the classical capacity of all of these channels, and furthermore, it is now known that a strong converse theorem holds for the classical capacity of these channels under a particular photon number constraint. The goal of the present paper is to initiate the study of second-order coding rates for these channels, by beginning with the simplest one, the pure-loss bosonic channel. In a second-order analysis of communication, one fixes the tolerable error probability and seeks to understand the back-off from capacity for a sufficiently large yet finite number of channel uses. We find a lower bound on the maximum achievable code size for the pure-loss bosonic channel, in terms of the known expression for its capacity and a quantity called channel dispersion. We accomplish this by proving a general "one-shot" coding theorem for channels with classical inputs and pure-state quantum outputs which reside in a separable Hilbert space. The theorem leads to an optimal second-order characterization when the channel output is finite-dimensional, and it remains an open question to determine whether the characterization is optimal for the pure-loss bosonic channel.Comment: 18 pages, 3 figures; v3: final version accepted for publication in Quantum Information Processin

On the existence of quantum representations for two dichotomic measurements

Under which conditions do outcome probabilities of measurements possess a quantum-mechanical model? This kind of problem is solved here for the case of two dichotomic von Neumann measurements which can be applied repeatedly to a quantum system with trivial dynamics. The solution uses methods from the theory of operator algebras and the theory of moment problems. The ensuing conditions reveal surprisingly simple relations between certain quantum-mechanical probabilities. It also shown that generally, none of these relations holds in general probabilistic models. This result might facilitate further experimental discrimination between quantum mechanics and other general probabilistic theories.Comment: 16+7 pages, presentation improved and minor errors correcte

Catalytic azo dye reduction in advanced anaerobic bioreactors

En un reactor anaeróbico de lecho empacado y de flujo ascendente con carbón activado (AC) biológico se obtuvieron altas velocidades de conversión del colorante azoico Acid Orange 7 a tiempos espaciales muy cortos, hasta 99% en 2.0 min. Tanto el área superficial específica como la conductividad electrónica del AC contribuyeron a las mayores velocidades de reducción. La agitación en el lecho de carbón produjo un incremento de la bioconversión del colorante. Se estableció un modelo cinético de decoloración que implica catálisis heterogénea y bioreducción. La biodegradabilidad anaeróbica de un colorante azoico en el sistema reactivo agitado pudo ser predicha a partir de su potencial de reducción. Las velocidades de decoloración fueron significativamente influenciadas por las propiedades texturales del AC y moderadamente afectadas por su química superficial. Este bioreactor catalítico parece ser una alternativa atractiva para la mejora económica de las tecnologías de tratamiento de aguas residuales textiles y de colorantes.In an anaerobic upflow packed-bed reactor with biological activated carbon (AC), high azo dye Acid Orange 7 conversion rates were achieved during very short space times up to 99% in 2.0 min. Both electron conductivity and specific surface area of AC contributed to higher reduction rates. The application of stirring in the carbon bed resulted in an increase of dye bioconversion. A decolourisation model was developed involving both heterogeneous catalysis and bioreduction. The anaerobic biodegradability of an azo dye could be predicted by its reduction potential in the stirred reactor system. The decolourisation rates were found to be significantly influenced by the textural properties of AC and moderately affected by its surface chemistry. This catalytic bioreactor system seems to be an attractive alternative for economically improving textile/dye wastewater technologies

Cloud benchmarking for maximising performance of scientific applications

This research was pursued under the EPSRC grant, EP/K015745/1, a Royal Society Industry Fellowship and an AWS Education Research grant.How can applications be deployed on the cloud to achieve maximum performance? This question is challenging to address with the availability of a wide variety of cloud Virtual Machines (VMs) with different performance capabilities. The research reported in this paper addresses the above question by proposing a six step benchmarking methodology in which a user provides a set of weights that indicate how important memory, local communication, computation and storage related operations are to an application. The user can either provide a set of four abstract weights or eight fine grain weights based on the knowledge of the application. The weights along with benchmarking data collected from the cloud are used to generate a set of two rankings - one based only on the performance of the VMs and the other takes both performance and costs into account. The rankings are validated on three case study applications using two validation techniques. The case studies on a set of experimental VMs highlight that maximum performance can be achieved by the three top ranked VMs and maximum performance in a cost-effective manner is achieved by at least one of the top three ranked VMs produced by the methodology.PostprintPeer reviewe