A Geometrical Derivation of a Family of Quantum Speed Limit Results

We derive a family of quantum speed limit results in time independent systems with pure states and a finite dimensional state space, by using a geometric method based on right invariant action functionals on SU(N). The method relates speed limits for implementing quantum gates to bounds on orthogonality times. We reproduce the known result of the Margolus-Levitin theorem, and a known generalisation of the Margolis-Levitin theorem, as special cases of our method, which produces a rich family of other similar speed limit formulas corresponding to positive homogeneous functions on su(n). We discuss the general relationship between speed limits for controlling a quantum state and a system's time evolution operator.Comment: 12 page

Retrenching the Purse: Finite Exception Logs, and Validating the Small

The Mondex Electronic Purse is an outstanding example of industrial scale formal refinement, and was the first verification to achieve ITSEC level E6 certification. A formal abstract model and a formal concrete model were developed, and a formal refinement was hand-proved between them. Nevertheless, certain requirements issues were set beyond the scope of the formal development, or handled in an unnatural manner. The retrenchment Tower Pattern is used to address one such issue in detail: the finiteness of the purse log (which records unsuccessful transactions). A retrenchment is constructed from the lowest level model of the purse system to a model in which logs are finite, and is then lifted to create two refinement developments of the purse, working at different levels of detail, and connected via retrenchments, forming the tower. The tower development is appropriately validated, vindicating the design used

Heating and Cooling of Hot Accretion Flows by Non Local Radiation

We consider non-local effects which arise when radiation emitted at one radius of an accretion disk either heats or cools gas at other radii through Compton scattering. We discuss three situations: 1. Radiation from the inner regions of an advection-dominated flow Compton cooling gas at intermediate radii and Compton heating gas at large radii. 2. Soft radiation from an outer thin accretion disk Compton cooling a hot one- or two-temperature flow on the inside. 3. Soft radiation from an inner thin accretion disk Compton cooling hot gas in a surrounding one-temperature flow. We describe how previous results are modified by these non-local interactions. We find that Compton heating or cooling of the gas by the radiation emitted in the inner regions of a hot flow is not important. Likewise, Compton cooling by the soft photons from an outer thin disk is negligible when the transition from a cold to a hot flow occurs at a radius greater than some minimum $R_{tr,min}$. However, if the hot flow terminates at $R < R_{tr,min}$, non-local cooling is so strong that the hot gas is cooled to a thin disk configuration in a runaway process. In the case of a thin disk surrounded by a hot one-temperature flow, we find that Compton cooling by soft radiation dominates over local cooling in the hot gas for \dot{M} \gsim 10^{-3} \alpha \dot{M}_{Edd}, and R \lsim 10^4 R_{Schw}. As a result, the maximum accretion rate for which an advection-dominated one-temperature solution exists, decreases by a factor of $\sim 10$, compared to the value computed under an assumption of local energy balance.Comment: LaTeX aaspp.sty, 25 pages, and 6 figures; to appear in Ap

EvoMachina : a novel evolutionary algorithm inspired by bacterial genome reorganisation

EvoMachina is a novel natural computation algorithm, inspired by recent understandings of the processes of genome reorganisation in bacteria and viruses. It has been developed as part of the EU FP7 project EvoEvo, taking inspiration from its biological experiments, and developed to support Living Technology applications. This abstract outlines the conceptual model underlying EvoMachina, its implementation, and a reference application

Local and global models of physics and computation

Classical computation is essentially local in time, yet some formulations of physics are global in time. Here I examine these differences, and suggest that certain forms of unconventional computation are needed to model physical processes and complex systems. These include certain forms of analogue computing, massively parallel field computing, and self-modifying computations

Augmenting Live Coding with Evolved Patterns

We present a new system for integrating evolutionary processes with live coding. The system is built upon an existing platform called Extramuros, which facilitates network-based collaboration on live coding performances. Our evolutionary approach uses the Tidal live coding language within this platform. The system uses a grammar to parse code patterns and create random mutations that conform to the grammar, thus guaranteeing that the resulting pattern has the correct syntax. With these mutations available, we provide a facility to integrate them during a live performance. To achieve this, we added controls to the Extramuros web client that allows coders to select patterns for submission to the Tidal interpreter. The fitness of the pattern is updated implicitly by the way the coder uses the patterns. In this way, appropriate patterns are continuously generated and selected for throughout a performance. We present examples of performances, and discuss the utility of this approach in live coding music

Formal Specification and Testing of a Management Architecture

The importance of network and distributed systems management to supply and maintain services required by users has led to a demand for management facilities. Open network management is assisted by representing the system resources to be managed as objects, and providing standard services and protocols for interrogating and manipulating these objects. This paper examines the application of formal description techniques to the specification of managed objects by presenting a case study in the specification and testing of a management architecture. We describe a formal specification of a management architecture suitable for scheduling and distributing services across nodes in a distributed system. In addition, we show how formal specifications can be used to generate conformance tests for the management architecture

The Natural Science of Computing

As unconventional computing comes of age, we believe a revolution is needed in our view of computer science

X-ray Images of Hot Accretion Flows

We consider the X-ray emission due to bremsstrahlung processes from hot, low radiative-efficiency accretion flows around supermassive and galactic black holes. We calculate surface brightness profiles and Michelson visibility functions for a range of density profiles, rho ~ r^(-3/2+p), with 0 < p < 1, to allow for the presence of outflows. We find that although the 1 keV emitting region in these flows can always extend up to 10^6 Schwarzschild radii (R_S), their surface brightness profiles and visibility functions are strongly affected by the specific density profile. The advection-dominated solutions with no outflows (p=0) lead to centrally peaked profiles with characteristic sizes of only a few tens of R_S. Solutions with strong outflows (p~1) lead to flat intensity profiles with significantly larger characteristic sizes of up to 10^6 R_S. This implies that low luminosity galactic nuclei, such as M87, may appear as extended X-ray sources when observed with current X-ray imaging instruments. We show that X-ray brightness profiles and their associated visibility functions may be powerful probes for determining the relevant mode of accretion and, in turn, the properties of hot accretion flows. We discuss the implications of our results for observations with the Chandra X-ray Observatory and the planned X-ray interferometer MAXIM.Comment: 14 pages, 4 figures, accepted by The Astrophysical Journal, minor change