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Simon Fraser University, British Columbia, Canada

"“Highly enriched 28Si: from a new kilogram to quantum computing”"

Thursday November 29, 2012

4:00pm 203

Refreshments are served at 3:30 p.m. in Physics room 242

The kilogram is one of the few remaining fundamental units still defined by an artifact - the mass of a cylinder of Pt-Ir alloy kept in a vault in Paris. This is not satisfactory for obvious reasons, and several projects are currently seeking to redefine the kilogram. The Avogadro Project seeks to do this based on precision measurements of the lattice constant and density of highly enriched 28Si, and it has been the source of the samples we have studied over the past 10 years. Our research, which has nothing to do with the kilogram, has revealed that highly enriched 28Si is remarkably different from natural Si, in that a wide variety of optical transitions are much sharper in 28Si than in natural Si (where they are already very sharp!). This unexpected improvement in spectral resolution led to the discovery of many new phenomena, and the overturning of some widely accepted ‘facts’. In particular, it has lead to new ways of measuring and controlling nuclear spins in Si, which have direct applications to the exciting new field of quantum computing. The use of electron and nuclear spins in semiconductors as the ‘qubits’ of quantum information promises to combine these new possibilities with the scalability of semiconductor technology. I will show how the remarkable optical properties of 28Si can be used to measure the coherence times of the nuclear spins of phosphorus donors in very pure samples. These coherence times of over 180 seconds determine how long quantum information can be stored, and are by far a record for a solid state system. These times in fact rival those of trapped ions in a vacuum, and we have coined the term ‘semiconductor vacuum’ to emphasize the new and unique properties of highly enriched 28Si.