Computer science and computer engineering are disciplines that have very definitely permeated and transformed every aspect of modern society. In these fields, cutting-edge research is about new models of computation, new materials and techniques for building computer hardware and novel methods for speeding-up algorithms. But it is also about building bridges between computer science and various other scientific fields, bridges that allow scientists to both think of natural phenomena as computational procedures and to employ novel models of computation to simulate natural processes (for example, quantum walks have been used to model energy transport in photosynthetic light harvesting complexes (Hoyer et al. 2010; Caruso et al. 2010)). A convergence of scientific, technological, economic and epistemological demands is driving and integrating this research.

The spatial search problem consists of minimising the number of steps required to find a given site in a network under the restriction that only oracle queries or translations to neighbouring sites are allowed. We propose a quantum algorithm for the spatial search problem on a honeycomb lattice with N sites and torus-like boundary conditions. The search algorithm is based on a modified quantum walk on an hexagonal lattice and the general framework proposed by Ambainis, Kempe and Rivosh (Ambainis et al. 2005) is employed to show that the time complexity of this quantum search algorithm is .

In this paper we prove that any Turing machine that uses only a finite computational space for every input cannot solve an uncomputable problem even when it runs in accelerated mode. We also propose two ways to define the language accepted by an accelerated Turing machine. Accordingly, the classes of languages accepted by accelerated Turing machines are the closure under Boolean operations of the sets Σ1 and Σ2.

We pose the following question: If a physical experiment were to be completely controlled by an algorithm, what effect would the algorithm have on the physical measurements made possible by the experiment?

In a programme to study the nature of computation possible by physical systems, and by algorithms coupled with physical systems, we have begun to analyse:

1. (i) the algorithmic nature of experimental procedures; and

2. (ii) the idea of using a physical experiment as an oracle to Turing Machines.

A defining feature of quantum mechanics is that it allows systems to exist in a superposition of different eigenstates of certain observables such as position or spin. However, superpositions of other quantities such as mass or charge are not seen in nature. It is thought that this disparity is partly due to the fact that it is much easier to carry out interference experiments for certain observables than others. Here we present an interferometry scheme that should allow us to observe interference between the vacuum and a single photon or atom. We begin by presenting a scheme for a Hadamard gate that operates in the Fock state basis and then show how, by creating an interferometer from two such gates, interference between a single particle and the vacuum could indeed be observed. This would provide evidence of a superposition of different particle numbers.

We consider a variation of the Travelling Salesman Problem (TSP) in which the cities visited have non-zero spatial extent, in contrast with the classical TSP, which has destinations that are mathematical points. This new approach opens up both new analyses of the problem and new algorithms for solutions, while remaining an economic first approximation to the standard problem. We present one particular solution that, depending on the number and size of the cities, can improve existing algorithms solving the classical TSP.

It is well known that many real world networks have a power-law degree distribution (the scale-free property). However, there are no rigorous results for continuous-time quantum walks on such realistic graphs. In this paper, we analyse the space–time behaviour of continuous-time quantum walks and random walks on the threshold network model, which is a reasonable candidate model having the scale-free property. We show that the quantum walker exhibits localisation at the starting point, although the random walker tends to spread uniformly.

Pólya showed in his 1921 paper that the generating function of the return probability for a two-dimensional random walk can be written in terms of an elliptic integral. In this paper we present a similar expression for a one-dimensional quantum walk.

In this paper, we consider a discrete-time quantum walk on the N-cycle governed by the condition that at every time step of the walk, the option persists, with probability p, of exercising a projective measurement on the coin degree of freedom. For a bipartite quantum system of this kind, we prove that the von Neumann entropy of the total density operator converges to its maximum value. Thus, when influenced by decoherence, the mutual information between the two subsystems corresponding to the space of the coin and the space of the walker must eventually diminish to zero. Put plainly, any level of decoherence greater than zero forces the system to become completely ‘disentangled’ eventually.

In this paper we develop novel quantum algorithms based on Quantum Multi-Object Search (QMOS) for convex hulls and general object intersection reporting, with applications to computer graphics. These algorithms are developed and described using standard concepts from computer science by encapsulating the physics of quantum computation within black-box subroutines.