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Quantum computing is based on using the quantum mechanical behaviour of electrons to create a new way to store and process information that is faster, more powerful and more efficient than in classical computing.
It taps into the ability of these particles to be put into a correlated state in which a change applied to one particle is instantly reflected by the others. If these processes can be controlled, they could be used to create parallel processing to perform calculations that are impossible on classical computers.
“If we could harness this electron behaviour in a semiconductor, it may be a viable approach to building a quantum computer,” Manfra said. “Of course this work is just in its very early stages, and although it is very relevant to quantum computation, we are a long way off from that. Foremost at this point is the chance to glimpse unexplained physical phenomena and new particles.”
Manfra and his research team designed and built a tool which they refer to as a high-mobility GaAs MBE system, which is housed at Purdue’s Birck Nanotechnology Centre. The tool is capable of growing ultrapure semiconductor materials with atomic-layer precision.
The material is a perfectly aligned lattice of gallium and arsenic atoms that can capture electrons on a two-dimensional plane, eliminating their ability to move up and down and limiting their movement to front-to-back and side-to-side.
“We are basically capturing the electrons within microscopic wells and forcing them to interact only with each other,” continued Manfra. “The material must be very pure to accomplish this. Any impurities that made their way in would cause the electrons to scatter and ruin the fragile correlated state.”
The electrons also need to be cooled to extremely low temperatures and a magnetic field is applied to achieve the desired conditions to reach the correlated state.
Physicist Gabor Csathy is able to cool the material and electrons to 5 millikelvin - close to absolute zero or 460 degrees below zero Fahrenheit - using special equipment in his lab.
“At room temperature, electrons are known to 96
www.compoundsemiconductor.net August/September 2011
behave like billiard balls on a pool table, bouncing off of the sides and off of each other, and obey the laws of classical mechanics,” Csathy said. “As the temperature is lowered, electrons calm down and become aware of the presence of neighbouring electrons. A collective motion of the electrons is then possible, and this collective motion is described by the laws of quantum mechanics.”
The electrons do a complex dance to try to find the best arrangement for them to achieve the minimum energy level and eventually form new patterns, or ground states, he said.
Csathy, who specialises in quantum transport in semiconductors, takes the difficult measurements of the electrons’ movement. The standard metric of semiconductor quality is electron mobility measured in centimetres squared per volt-second. The group recently achieved an electron mobility measurement of 22 million centimetres squared per volt-second, which puts them among the top two to three groups in the world, he said.
Manfra and Csathy presented their work at Microsoft’s Station Q summer meeting on June 17 at the University of California, Santa Barbara. This meeting, sponsored by Microsoft Research, brings together leading researchers to discuss novel approaches to quantum computing. They also received a $700,000 grant from the Department of Energy based on their preliminary results.
The research team also includes several physicists from Purdue.
“A broad team is necessary to probe this type of physics,” Manfra concluded. “It takes a high level of expertise in materials, measurement and theory that is not often found at one institution. It is the depth of talent at Purdue and ability to easily work with researchers in other areas that made these achievements possible.”
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