Low error measurement-free phase gates for qubus computation

We discuss the desired criteria for a two-qubit phase gate and present a method for realising such a gate for quantum computation that is measurement-free and low error. The gate is implemented between qubits via an intermediate bus mode. We take a coherent state as the bus and use cross-Kerr type interactions between the bus and the qubits. This new method is robust against parameter variations and is thus low error. It fundamentally improves on previous methods due its deterministic nature and the lack of approximations used in the geometry of the phase rotations. This interaction is applicable both to solid state and photonic qubit systems.Comment: 6 pages, 4 figures. Published versio

Knitting distributed cluster state ladders with spin chains

There has been much recent study on the application of spin chains to quantum state transfer and communication. Here we discuss the utilisation of spin chains (set up for perfect quantum state transfer) for the knitting of distributed cluster state structures, between spin qubits repeatedly injected and extracted at the ends of the chain. The cluster states emerge from the natural evolution of the system across different excitation number sectors. We discuss the decohering effects of errors in the injection and extraction process as well as the effects of fabrication and random errors.Comment: To be published in PRA. v2 includes minor corrections as well as an added discussion on refocussin

Effect of perturbations on information transfer in spin chains

Spin chains have been proposed as a reliable and convenient way of transferring information and entanglement in a quantum computational context. Nonetheless, it has to be expected that any physical implementation of these systems will be subject to several perturbative factors which could potentially diminish the transfer quality. In this paper, we investigate a number of possible fabrication defects in the spin chains themselves as well as the effect of non-synchronous or imperfect input operations, with a focus on the case of multiple excitation/qubit transfer. We consider both entangled and unentangled states, and in particular the transfer of an entangled pair of adjacent spins at one end of a chain under the mirroring rule and also the creation of entanglement resulting from injection at both end spins.Comment: Journal version fixes last typo

The quantum-classical crossover of a field mode

We explore the quantum-classical crossover in the behaviour of a quantum field mode. The quantum behaviour of a two-state system - a qubit - coupled to the field is used as a probe. Collapse and revival of the qubit inversion form the signature for quantum behaviour of the field and continuous Rabi oscillations form the signature for classical behaviour of the field. We demonstrate both limits in a single model for the full coupled system, for states with the same average field strength, and so for qubits with the same Rabi frequency.Comment: 6 pages, 3 figures (in this version the figures, text and references have all been expanded

Intrinsic quarks and heavy flavour production

A model is constructed for the diffractive production of heavy flavours in hadron-hadron interactions, based on the presence of an intrinsic heavy quark component in the hadron wavefunction. It requires three ingredients; the heavy quark content of the initial hadron, the probability that these heavy quarks are scattered, and the probability that they form heavy flavoured hadrons afterwards. The initial heavy quark distributions are calculated, using lowest order perturbative QCD, starting from the valence constituent quark distributions, and compared with deep inelastic charm production data. The valence distributions are designed to reproduce the dimensional counting rules, and, via reciprocity, to be consistent with the heavy quark fragmentation functions. The light quark-hadron scattering cross-section is parametrized by Pomeron exchange, and extended to heavy quarks using the f-dominance hypothesis for the Pomeron- quark coupling. Dynamical and kinematical factors which control the rise of these cross-sections from threshold are built in. The validity of these ideas is tested against charm photo-production data, by using a vector dominance model for the photon-hadron scattering. The probability that the scattered quarks recombine to produce heavy flavoured hadrons is assumed to be given by the overlap of the initial distribution of quarks with the distribution in a heavy hadron. We compare the predictions of our model with strangeness and charm production data, and make predictions for bottom and top production. In particular, the magnitude of the leptonic signal to be expected from the decay of top quarks produced at the CERN pp-Collider is given. We conclude that all aspects of this model are consistent with present experimental data, and that the top quark should be observed at the Collider if its mass is around 35 GeV