Oracles and query lower bounds in generalised probabilistic theories

We investigate the connection between interference and computational power within the operationally defined framework of generalised probabilistic theories. To compare the computational abilities of different theories within this framework we show that any theory satisfying three natural physical principles possess a well-defined oracle model. Indeed, we prove a subroutine theorem for oracles in such theories which is a necessary condition for the oracle to be well-defined. The three principles are: causality (roughly, no signalling from the future), purification (each mixed state arises as the marginal of a pure state of a larger system), and strong symmetry existence of non-trivial reversible transformations). Sorkin has defined a hierarchy of conceivable interference behaviours, where the order in the hierarchy corresponds to the number of paths that have an irreducible interaction in a multi-slit experiment. Given our oracle model, we show that if a classical computer requires at least n queries to solve a learning problem, then the corresponding lower bound in theories lying at the kth level of Sorkin's hierarchy is n/k. Hence, lower bounds on the number of queries to a quantum oracle needed to solve certain problems are not optimal in the space of all generalised probabilistic theories, although it is not yet known whether the optimal bounds are achievable in general. Hence searches for higher-order interference are not only foundationally motivated, but constitute a search for a computational resource beyond that offered by quantum computation.Comment: 17+7 pages. Comments Welcome. Published in special issue "Foundational Aspects of Quantum Information" in Foundations of Physic

Classifying the computational power of stochastic physical oracles

Consider a computability and complexity theory in which theclassical set-theoretic oracle to a Turing machine is replaced bya physical process, and oracle queries return measurements ofphysical behaviour. The idea of such physical oracles is relevantto many disparate situations, but research has focussed on physicaloracles that were classic deterministic experiments whichmeasure physical quantities. In this paper, we broaden the scopeof the theory of physical oracles by tackling non-deterministicsystems. We examine examples of three types of non-determinism,namely systems that are: (1) physically nondeterministic,as in quantum phenomena; (2) physically deterministic butwhose physical theory is non-deterministic, as in statistical mechanics;and (3) physically deterministic but whose computationaltheory is non-deterministic caused by error margins. Physicaloracles that have probabilistic theories we call stochasticphysical oracles. We propose a set SPO of axioms for a basicform of stochastic oracles. We prove that Turing machinesequipped with a physical oracle satisfying the axioms SPO computeprecisely the non-uniform complexity class BPP//log* inpolynomial time. This result of BPP//log* is a computationallimit to a great range of classical and non-classical measurement,and of analogue-digital computation in polynomial time undergeneral conditions

On the possible Computational Power of the Human Mind

The aim of this paper is to address the question: Can an artificial neural network (ANN) model be used as a possible characterization of the power of the human mind? We will discuss what might be the relationship between such a model and its natural counterpart. A possible characterization of the different power capabilities of the mind is suggested in terms of the information contained (in its computational complexity) or achievable by it. Such characterization takes advantage of recent results based on natural neural networks (NNN) and the computational power of arbitrary artificial neural networks (ANN). The possible acceptance of neural networks as the model of the human mind's operation makes the aforementioned quite relevant.Comment: Complexity, Science and Society Conference, 2005, University of Liverpool, UK. 23 page