Overcoming non-Markovian dephasing in single photon sources through post-selection

We study the effects of realistic dephasing environments on a pair of solid-state single-photon sources in the context of the Hong-Ou-Mandel dip. By means of solutions for the Markovian or exact non-Markovian dephasing dynamics of the sources, we show that the resulting loss of visibility depends crucially on the timing of photon detection events. Our results demonstrate that the effective visibility can be improved via temporal post-selection, and also that time-resolved interference can be a useful probe of the interaction between the emitter and its host environment.Comment: 5 pages, 2 figures, published version, title changed, references update

Fault tolerant quantum computation with very high threshold for loss errors

Many proposals for fault tolerant quantum computation (FTQC) suffer detectable loss processes. Here we show that topological FTQC schemes, which are known to have high error thresholds, are also extremely robust against losses. We demonstrate that these schemes tolerate loss rates up to 24.9%, determined by bond percolation on a cubic lattice. Our numerical results show that these schemes retain good performance when loss and computational errors are simultaneously present.Comment: 4 pages, comments still very welcome. v2 is a reasonable approximation to the published versio

Privatisation in Ireland

Public enterprises in Ireland were offshoots of political nationalism. They were part of a protectionist economic policy and in 1980 employed over 90.000 staff in a total national employment figure of 1.1m. Public opinion moved away from public enterprises because of perceived high costs to both consumers and taxpayers. In the Celtic Tiger era since 1987 the share of public enterprises in total employment has fallen by almost two-thirds to 2.7 percent. Ireland has experienced major increases in GNP per head and in employment by adopting open economy policies and securing large increases in exports and in foreign direct investment. No privatised enterprise has been re-nationalised. It is public policy to retain network infrastructure such as the electricity and gas grids in public ownership while selling state companies in areas such as food, banking, telecoms, and shipping

Quantum computation via measurements on the low-temperature state of a many-body system

We consider measurement-based quantum computation using the state of a spin-lattice system in equilibrium with a thermal bath and free to evolve under its own Hamiltonian. Any single qubit measurements disturb the system from equilibrium and, with adaptive measurements performed at a finite rate, the resulting dynamics reduces the fidelity of the computation. We show that it is possible to describe the loss in fidelity by a single quantum operation on the encoded quantum state that is independent of the measurement history. To achieve this simple description, we choose a particular form of spin-boson coupling to describe the interaction with the environment, and perform measurements periodically at a natural rate determined by the energy gap of the system. We found that an optimal cooling exists, which is a trade-off between keeping the system cool enough that the resource state remains close to the ground state, but also isolated enough that the cooling does not strongly interfere with the dynamics of the computation. For a sufficiently low temperature we obtain a fault-tolerant threshold for the couplings to the environment.Comment: 9 pages, 3 figures; v2 published versio

Preparing multi-partite entanglement of photons and matter qubits

We show how to make event-ready multi-partite entanglement between qubits which may be encoded on photons or matter systems. Entangled states of matter systems, which can also act as single photon sources, can be generated using the entangling operation presented in quant-ph/0408040. We show how to entangle such sources with photon qubits, which may be encoded in the dual rail, polarization or time-bin degrees of freedom. We subsequently demonstrate how projective measurements of the matter qubits can be used to create entangled states of the photons alone. The state of the matter qubits is inherited by the generated photons. Since the entangling operation can be used to generate cluster states of matter qubits for quantum computing, our procedure enables us to create any (entangled) photonic quantum state that can be written as the outcome of a quantum computer.Comment: 10 pages, 4 figures; to appear in Journal of Optics

Comment on 'The Value of Cost Benefit Analysis of Road Projects'. Quarterly Economic Commentary Special Article, April 1985

In accepting the invitation by the editors of the Quarterly Economic Commentary to comment on Mr. Mansergh's paper it is hoped to show that his fears about cost benefit analysis and its use are unjustified. His paper proposes two important changes in cost benefit analysis. These are the treatment of labour costs as benefits and taxes as a resource rather than as a transfer payment. Labour is treated as a cost in cost benefit .. analysis because the supply price of labour is positve. Workers require a positive sum in exchange for their labour. Cost benefit analysis measures the benefits and costs to society as a whole from projects. The level of a project's benefits over its costs is the critical factor in evaluating the project. The division of the benefits between those accruing in tax revenues and elsewhere in the economy distributes the net benefits of the project. It neither increases nor reduces the level of net benefits from the, project. Transfer payments, such as taxes, are therefore excluded from cost benefit analysis. The proposed changes add two categories of benefit, labour costs and tax revenues, and delete one cost, labour. These changes would raise the rate of return on projects now rejected. They are thus inconsistent with the paper's recommendation that "a more rational result might be a reduction in the national enthusiasm for investment"..

The Naas Motorway Bypass - A Cost Benefit Analysis. Quarterly Economic Commentary Special Article, January 1984

The paper examines the Naas Motorway Bypass which cost £16m at 1983 prices. Twelve thousand vehicles a day using the bypass save over 10 minutes between 8 am and 8 pm and 6 minutes at other times. Five thousand vehicles a day using the present route through Naas also benefit by saving 4 minutes due to reduced congestion in the town. In addition to time savings, the bypass reduces accidents and fuel costs. Ninety-one per cent of the benefits accrue in time savings. The internal rate of return on the project is estimated at 20.51 per cent, assuming 2 per cent annual traffic and income growth. The sensitivity tests of the results show that even with zero growth in incomes and traffic for twenty years, a high proportion of leisure time savings with zero value and no increase in the value of fuel savings the project would have an internal rate of return which meets the test discount rate used by the Department of Finance. The environmental aspects of the bypass are positive in terms of noise and smoke and lead pollution reduction. The impact on farm severence and natural amenities on the motorway route has been mitigated by several design features of the bypass