Purdue professors Michael Manfra and Gabor Csathy stand next to the high-mobility gallium-arsenide molecular beam epitaxy system at the Birck Nanotechnology Center. Manfra holds a gallium-arsenide wafer on which his research team grows ultrapure gallium arsenide semiconductor crystals to observe new electron ground states that could have applications in high-speed quantum computing. Photo: Andrew Hancock
A team of Purdue
University researchers is
among a small group in the world that has successfully created ultrapure
material that captures new states of matter and could have applications in
high-speed quantum computing.
The material, gallium arsenide, is used to observe states
in which electrons no longer obey the laws of single-particle physics, but
instead are governed by their mutual interactions.
Michael Manfra, the William F. and Patty J. Miller
Associate Professor of Physics who leads the group, says the work provides new
insights into fundamental physics.
“These exotic states are beyond the standard models
of solid-state physics and are at the frontier of what we understand and what
we don’t understand,” says Manfra, who also is an associate professor of
both materials engineering and electrical and computer engineering. “They
don’t exist in most standard materials, but only under special conditions in
ultrapure gallium arsenide semiconductor crystals.”
Quantum computing is based on using the quantum mechanical
behavior of electrons to create a new way to store and process information that
is faster, more powerful and more efficient than 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 behavior in a
semiconductor, it may be a viable approach to building a quantum
computer,” Manfra says. “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 equipment
called a high-mobility gallium-arsenide molecular beam epitaxy system, or MBE,
that is housed at Purdue’s Birck
The equipment makes 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 2D plane, eliminating their ability to
move up and down and limiting their movement to front-to-back and side-to-side.
Purdue graduate student John Watson and Michael Manfra work with the high-mobility gallium-arsenide molecular beam epitaxy system at the Birck Nanotechnology Center. They are part of one of a few research teams in the world successfully creating ultrapure material that captures new states of matter. Photo: Andrew Hancock
“We are basically capturing the electrons within
microscopic wells and forcing them to interact only with each other,” he
says. “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.
Gabor Csathy, an assistant professor of physics, is able
to cool the material and electrons to 5 mK—close to absolute zero or 460 degrees
below zero Fahrenheit—using special equipment in his laboratory.
“At room temperature, electrons are known to 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 says. “As
the temperature is lowered, electrons calm down and become aware of the
presence of neighboring electrons. A collective motion of the electrons is then
possible, and this collective motion is described by the laws of quantum
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 says.
Csathy, who specializes in quantum transport in
semiconductors, takes the difficult measurements of the electrons’ movement.
The standard metric of semiconductor quality is electron mobility measured in
centimeters squared per volt-second. The group recently achieved an electron
mobility measurement of 22 million centimeters squared per volt-second, which
puts them among the top two to three groups in the world, he says.