Seminar Abstracts

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Molecular Electronics - A Road to 1 Terabit/cm²

^{It will be possible in the future to build computer memory and logic out of tiny organic molecules. Molecular electronic (moletronic) circuits are potentially scalable up to 1 Terabit/cm2 but many fundamental and technological problems will have to be solved before such super-densities are achieved. I shall describe major strategies involved in building the simplest moletronic application, the non-volatile memory chip. Particular attention will be paid to the operating principles of molecular wires, diodes,and switches. We are developing advanced theoretical methods to calculate current-voltage characteristics of the organic molecules. Theoretical modelling is used to help identify molecules with desired electronic properties. I shall present two original designs: for the single-molecule diode and the single-molecule switch. The computed current-voltage characteristics of the devices will be given.}

Problems facing digital and quantum cryptography

Dr Peter Shiu

Wed 15 May 2002

I shall give a rapid survey on classical and current cryptography, explaining some of the problems involved and their proposed solutions. I shall also say something about how a quantum computer might be able to solve some of these problems much more efficiently.

Physical implementations for quantum computing

Dr Timothy Spiller (Hewlett Packard Laboratories, Bristol)

Wed 16 Oct 2002

In comparison to ordinary computers (which process data classically), quantum computers can solve some problems much more quickly. Fundamental aspects of quantum physics are responsible for this advantage. This introductory talk highlights the criteria which must be met for quantum computing hardware and some of the current candidates for quantum bits---the building blocks for quantum information processors. Condensed matter approaches may help facilitate the future scaling up in qubit number. I discuss one such example in more detail, that of superconducting circuits.

Possible lattice structure-induced electronic instability in holes-doped Bi₂Sr₂CuO₆ from DFT calculations

Professor Itai Panas (Chalmers University of Technology, Sweden)

Wed 27 Nov 2002

A common origin for a two-band scenario and inter- and intra-plane charge carrier segregations, is demonstrated for holes doped Bi₂Sr₂CuO₆ by means of first principles density functional theory. Superlattice calculations are performed as electronic structure changes, caused by inhomogeneous buffer ions displacements, are monitored. An explicit understanding of the normal state, from which superconductivity emerges upon cooling, is arrived at.

Micro-mechanical systems in the quantum regime

Dr Andrew Armour (University of Nottingham)

Wed 16 Feb 2003

Recent advances in fabrication have made it possible to produce micron-sized mechanical resonators with fundamental vibrational modes of 1GHz [Nature vol 421, p 496, Jan 30th 2003]. These advances open up the possibility of demonstrating manifestly quantum behaviour in micro-mechanical systems and hence exploring the border between quantum and classical regimes. However, as well as fabricating a device which operates in the quantum regime, ways of detecting and interpreting the resulting quantum mechanical behaviour must also be devised. After reviewing the current state of play in the field, we focus on the characteristics of radio-frequency cantilevers when they are driven into superpositions of spatially separated states. A superposition of spatially separated states in a cantilever can be produced by developing an electrostatic interaction between the cantilever and a mesoscopic electronic system which is prepared in a superposition of charge states. We discuss possible ways in which the quantum coherence and decoherence of a cantilever can be detected when it is driven into a superposition of states, paying attention to both fundamental and practical constraints.

Quantum computation - from theory to experiments

Artur Ekert (University of Cambridge)

Wednesday, 30 Apr 2003, 14.00, W0.03

The theory of computation, including modern cryptography, was laid down almost seventy years ago, was implemented within a decade, became commercial within another decade, and dominated the world's economy half a century later. Quantum information technology is a fundamentally new way of harnessing nature. It is too early to say how important a way this will eventually be, but we can reasonably speculate about its impact both on computation and data security. I will review the basic concepts of quantum information science and describe experimental techniques which aim to give data processing devices new functionality.

Quantum Phenomena with vortex condensates
in Josephson array insulators

Apostol Vourdas (University of Bradford)

Wednesday, 14 May 2003, 14.00, W0.03

ac Aharonov-Casher phenomena that involve interference of vortices in Josephson array insulators, which circulate a time-dependent electric charge It +Q₀sin wt are studied. The time-averaged intensity of vortices shows peaks at the Bloch steps Ø₀=Nw. The case in which the time-dependent charge is induced by non-classical microwaves is also considered and the effect of the quantum noise of the microwaves on the interference of vortices, is studied. Weakly coupled Josephson array insulators are also considered. They act as dual Josephson junctions for vortices. A Josephson array insulating ring with such a dual Josephson junction is considered. It is the dual of a SQUID ring, for vortices. Its interaction with non-classical microwaves is studied and quantum phenomena like the exchange of energy between vortices and photons, the influence of the quantum statistics of photons on the quantum statistics of vortices, the entanglement between vortices and photons, etc, are discussed.

³He-⁴He mixtures in 2D

Dr Ján Nyéki (Royal Holloway, University of London)

Wednesday, 21 May 2003, 14.00, W0.04 - note changed venue

On a graphite substrate helium films exhibit layer-by-layer growth,  which is important for producing well characterized films with an integral number of layers. Controlled variation of the helium density within the top layer allows experimental study of  both ³He and ⁴He 2D systems ranging from gas through fluid to solid.  In addition, films on graphite can be cooled (relatively easily) to submillikelvin temperatures.
After a brief review of selected properties of pure (monoisotopic) films I will discuss the latest heat capacity experiments on 2D ³He-⁴He mixtures:  formation of excited surface-normal states of ³He in fluid films and thermodynamic evidence for ³He tunneling excitations in 2D solid mixture films.

Testing the limits of quantum mechanics:
motivation,state of play, prospects

Tony Leggett (University of Illinois at Urbana-Champaign)

Wednesday, 11 Jun 2003, 14.00, W0.03

I present the motivation for experiments which attempt to generate, and verify the existence of, quantum superpositions of two or more states which are by some reasonable criterion "macroscopically" distinct, and show that various a priori objections to this programme made in the literature are flawed. I review the extent to which such experiments currently exist in the areas of free-space molecular diffraction, magnetic biomolecules, quantum optics and Josephson devices, and sketch possible future lines of development of the programme.

2003 abstracts

Rodrigues
Knight

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