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Aharanov-Casher-Effect Suppression of Macroscopic Flux Tunneling in a SQUID
Jonathan Friedman, Amherst College

Two recent experiments have demonstrated clearly the existence of Schrodinger cat-like macroscopic quantum coherent superpositions in superconducting rings. The superposition states are measured psuedo-spectroscopically and interpreted as a coherent tunnelling of the SQUID ring from one flux state to another. Here we study theoretically a variant of an rf-SQUID in which the Josephson junction is replaced by a Bloch transistor - two junctions separated by a small superconducting island on which the charge can be induced by an external gate voltage. When the Josephson coupling energies of the junctions are equal and the induced charge is q = e, destructive interference between tunneling paths brings the flux-tunneling rate to zero. (Destructive interference can still occur even if the two junctions are not equivalent, although the tunneling rate no longer goes precisely to zero.) We analyze the system in two limits: when the Josephson energy EJ is much larger than the island charging energy EC and vice-versa. In addition, we show that two symmetries of the system's Hamiltonian require that the tunneling rate be zero for any value of the ratio EJ/EC. We discuss the potential use of this device as a qubit.

Many atom entanglement in the micromaser
Ben Varcoe, University of Sussex

The one atom maser or micromaser uses a resonant transition between two levels of Rydberg atoms to create a steady state field in a superconducting cavity with a Q-factor of up to 4x10^10 (a photon lifetime of 0.3s). The micromaser has a unique property, in that owing to an enormous photon lifetime, the steady state field in the cavity is a product of the coherent evolution of a sequence of single atoms. This property means that the effects present in the micromaser are strongly quantum mechanical. A new generation of micromaser is under construction at Sussex University which will be optimsed to exploit these quantum effects. I will discuss the implications of this for fundamental tests of quantum mechanics and the possibility of observing long chains of entangled atoms.

Noise-assisted generation of entanglement
Susana Huelga, University of Hertfordshire

All theoretical protocols developed with the aim of minimizing the impact of noise in quantum information processing rely on the very intuitive idea of either trying to shield the system from the environmental noise or actively restore the corrupted dynamics to the ideal one. Recently we have proposed a completely different strategy where the presence of noise is used to facilitate rather than corrupt the desired dynamics. Within this framework I will shown that there are situations where dissipation can assist the generation of entanglement. Remarkably, noise does not need to be just a passive element but it can actually be the only driving force of the system. I will show that it is possible to generate entanglement just employing white noise. The amount of generated entanglement is a non-monotonic function of the intensity of the noise, being maximized for certain optimal noise intensity. This behaviour resembles the phenomenon of stochastic resonance, where the sensitivity of a nonlinear system is enhanced in the presence of noise . These results indicate that tunable noisy sources can play a constructive role in certain situations and opens a new venue for exploring efficient ways to exploit the presence of noise when the aim is to generate entanglement in a controllable way.

Two ways to make a semiconductor quantum processor
Crispin Barnes, University of Cambridge

The talk will summarize the theory and practical developments behind the production of two different semiconductor quantum processors formed from: electrons trapped in surface acoustic waves; and Sodium atoms trapped in surface ion traps. Both electrons trapped in surface acoustic waves, and Na atoms trapped on the surface of a Si MOSFET, can be forced to form a chain of coupled qubits using standard semiconductor processing techniques. These qubits can be manipulated using local magnets and electrostatic surface gates to perform an arbitrary unitary transformation on a fixed polarized initial state. The information in the qubits can then be read by a variety of means including conversion to photon polarization. I will concentrate on showing how these two processors conform to the DiVincenzo check list and outline their strengths and weaknesses.

Superconducting circuits, macroscopic quantum effects and qubits
Tim Spiller, Hewlett Packard Laboratories

A short introduction to superconductivity and its consequences is given, such as magnetic flux quantization. Superconducting circuits is discussed which can be arranged to demonstrate superposition of charge or flux states and consider (hopefully with some lively audience participation) the concept of macroscopic superposition. Finally, the application of these systems as qubits is discussed.

Taming the Wild Atom
Edward Hinds, University of Sussex

Clouds of cold atoms, collected and refrigerated by laser light, can be cooled to the lowest temperatures in the universe. This has created the new field of atom optics where cold atoms are manipulated, much as photons are controlled in traditional optics using mirrors, lenses, and waveguides. I will show some of the first movies of atom clouds being manipulated. From the viewpoint of basic science these clouds are a fantastic new tool for studying the quantum physics of gases close to absolute zero. It is now becoming possible to confine and manipulate atoms in extremely small structures, for example in tubes a millionth of a metre across. Atoms flowing in such tubes could provide the basis for a new technology similar to electronics but based on the flow and interaction of neutral atoms rather than on electricity in wires. I will describe how atom "chips" are being realised and how they might be used to construct revolutionary devices such as a quantum computer, which will be able to solve problems previously considered impossible.

What's quantum about single particle quantum information processors?
Ian Walmsley, Oxford University

We propose a measure for evaluating the resources needed to build a quantum computer, and show using this measure that all quantum computers based on unentangled particles are equivalent to classical (possibly wave-based) computers. We demonstrate the equivalence by a specific experimental example, that makes use entirely of the interference of classical optical waves. However, in light of recent theoretical proposals, it appears that these devices may be more powerful than conventional non-interference based computers. The issues of scaling associated with binary coding of the quantum register, and the need for entanglement will be raised, though not resolved.

Atomic physics implementations of quantum logic
Peter Knight, Imperial College, London

Quantum computing has been recognized as a major new development in physics, enabling us (if a quantum computer is realizable) to attack problems previously thought to be too complex for normal computation in a reasonable time. Examples of quantum algorithms involving this kind of advantage are Shors for fast factorization (in itself a threat to secure communication) and Grover©'s for data base searching. Realizations under consideration include atomic physics-based schemes as well as solid state logic. I will describe the use of trapped ions and of cavity qed, both of which have already been used to demonstrate quantum gate operation, and discuss the potential for scalable devices.

Quantum message authentication
Dr Howard Barnum, Dept of Computer Science, Bristol University

Mixed states, quantum sources, and information gain
Patrick Hayden, Centre for Quantum Computation, Oxford University

One of the fundamental features of quantum mechanics is the existence of a trade-off between information gain and state disturbance. In recent years, this trade-off has been harnessed to develop quantum cryptography, and serves as a guiding principle in quantum information theory more generally. In this talk, I will explore the manifestation of the principle in two regimes where it is not yet fully understood. First, I will discuss mixed states and state disturbance, providing a broad generalisation of the no-cloning and no-broadcasting theorems. Next, I will report on joint work that investigates the extraction of classical information during quantum noiseless coding. While the two problems appear to be only superficially related, their solutions are surprisingly similar.

Can we obtain quantum theory from reasonable axioms?
Lucien Hardy, Centre for Quantum Computation, Oxford University

Analytical and Numerical Models of Instrumental Errors in Laser Interferometer Gravitational-Wave Experiments
Ray Beausoleil, Hardcopy Technology Lab, HP Laboratories

In this talk, I will describe my part-time "extracurricular" involvement with the LIGO experiment over the past several years. I will begin with a review of some of the concepts of General Relativity, and describe some of the physical characteristics of gravitational waves. Next, I'll provide an overview of the LIGO detection scheme, and give some examples of the signals expected for some of the more spectacular sources of gravitational radiation. Finally, I will outline my own work in the LIGO trenches, which has focused on the creation of extremely detailed analytic and numerical algorithms that model the sensitivity of the coupled LIGO Fabry-Perot optical resonators to various sources of instrumental error. In particular, the minimum gravitational-wave strain which can be measured by LIGO is limited by both seismic and other mechanical disturbances that introduce translational and rotational motion in the test masses, and optical wavefront distortion caused by thermal lensing and thermoelastic surface deformation arising from minute optical power absorptions in the mirror substrates and coatings.

Trading quantum for classical bits in quantum source coding
Andreas Winter, Dept of Computer Science, Bristol University

In this talk ramifications of the quantum source coding problem, as introduced by Schumacher, are discussed: after presenting the original setting and result, I present recent work (with H. Barnum, P. Hayden and R. Jozsa) showing that in "blind" coding situations qubits cannot be saved beyond Schumacher's limit by using a free classical side channel, unless the quantum source ensembles exhibits orthogonality. Then I go on to discuss results obtained with P. Hayden and R. Jozsa in the last weeks regarding the case of "visible" coding: there it is natural to consider the function characterizing the tradeoff between classical and quantum bits. We were able to give a single-letter expression for this function. I will also discuss examples and applications of this result.

Degrees of knowledge of a quantum state
Dr Charles H Bennett, IBM Watson Laboratories

It is possible to "know" or "possess" a quantum state in infinitely many physically inequivalent ways, ranging from complete classical knowledge, through possession of of a single specimen of the state, to weaker and less compactly embodiable forms such as the ability to simulate the outcome of a single POVM measurement on the state. A less well understood hierarchy of degrees of knowledge or possession holds for unitary operations and completely positive maps.

The role of correlations in quantum computational speed-up
Vlatko Vedral, Imperial College, London

In my lecture I will first present an optical computer which will exploit the classical principle of wave interference to perform certain tasks. From this it will be immediately clear why the Fourier Transform operation plays a key role in perod estimation and database search. However, classical implementation will be seen to be inefficient as some of the resources needed (e.g. space, time and energy) start to scale exponentially with the size of the problem. Here we see the key difference between the quantum and classical computation. I then introduce the Black box model and show how to use it to provide bound on efficiency of quantum computers. The entangling capacity of the fundamental black box gate is analysed and its implications to the quantum efficiency presented. The diffrence between the classical and quantum correlations is then discussed in the view of the results presented.

Tomography and its role in quantum computation
Bill Munro, Quantum Information Processing Group, HP Laboratories

Quantum computation depends on quantum entanglement, a correlation between subsystems that cannot occur classically. A variety of theoretical measures exist for quantifying the degree entanglement in such schemes, all of which are functions of the system density matrix. How can the entanglement be measured experimentally? Using quantum tomography techniques developed for two photon entangled states, the density matrix can be reconstructed from the appropriate experimental data. In this case the state tomography gives the complete characterization of the physical system (for the relevant degree of freedom, such as spin or polarization). It gives information on both the degree of nonclassical correlation, that is entanglement, as well as the amount of decoherence in the system. In this proceedings we discuss the general state tomography procedure required to characterize a few qubit quantum computer, for any architecture.

The meaning of the interaction-free measurements
Lev Vaidman, Oxford Centre for Quantum Computation and Tel-Aviv Univ

Interaction-free measurements introduced by Elitzur and Vaidman [Found.Phys. {\bf 23}, 987 (1993)] allow finding infinitely fragile objects without destroying them. Many experiments have been successfully performed showing that indeed, the original scheme and its modifications lead to reduction of the disturbance of the observed systems. However, there is a controversy about the validity of the term "interaction-free"' for these experiments. Broad variety of such experiments are reviewed and the meaning of the interaction-free measurements is clarified.


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