A Rice University-led
team of physicists at seven U.S. universities has won $5 million from the Department
of Defense to build a simulator capable of tackling high-temperature superconductivity,
one of the most vexing mysteries of modern physics.
 | | This is Rice University physicist Randy Hulet. Credit: Jeff Fitlow/Rice University |
"The object is to simulate complex materials like high-temperature superconductors
using ultracold atoms in an optical lattice," said Rice's Randy Hulet,
the principal investigator on the project. "The lattice, which is created
with lasers, simulates the crystal structure of the materials, while the atoms
are stand-ins for the electrons."
The three-year grant was awarded today by the Army Research Office with funding
from the Defense Advanced Research Projects Agency (DARPA). It is the second
phase of funding awarded under DARPA's Optical Lattice Emulator (OLE) program.
In the first phase of the program, Hulet's team and others showed it was possible
to use ultracold atoms and lasers to build the type of structures needed to
simulate high-temperature superconductors and other exotic materials.
Hulet's team includes co-prinicipal investigators Han Pu, also of Rice; Carlos
Bolech of the University of Cincinnati; Jason Ho and Nandini Trivedi, both of
Ohio State University; David Ceperley and Brian DeMarco, both of the University
of Illinois, Urbana-Champaign; David Huse of Princeton University; Erich Mueller
of Cornell University; and Vincent Liu of the University of Pittsburgh.
Superconductors are materials that convey electricity freely, without any resistance.
Resistance is what causes wires to heat up as electricity moves through them,
and resistance results in billions of dollars worth of losses annually in the
U.S. power grid.
Superconductivity typically happens only at extremely cold temperatures, but
in 1986 scientists discovered that some crystalline materials become superconductors
at relatively high temperatures. Superconductivity in these materials, referred
to as the cuprate superconductors, was found to occur along the two-dimensional
planes that form their crystalline structure. Physicists still don't understand
how superconductivity emerges in the cuprates, partly because of the inherent
complexity of the real materials and partly because of the inevitable impurities
and defects present in actual samples.
"Many physicists believe that a certain model called the Hubbard model
can explain how the electrons in these materials attract one another, but it
remains controversial," said Hulet. "Even though the model is simplified,
solving it is an exponentially complex problem. It cannot be done on even the
fastest computers."
The beauty of the OLE program comes in using ultracold atoms as proxies for
electrons. Prior research in Hulet's lab and others has shown that at cold enough
temperatures, the behavior of atoms is dictated by the same quantum mechanical
rules that govern the behavior of electrons.
The other technological piece of the simulator -- the "optical lattice"
-- is created using several laser beams. The light from one beam can interact
with and cancel out light from other beams in a regular pattern. Using several
lasers, the researchers can use this effect to create two- and three-dimensional
light patterns that mimic the lattice-like atomic structure of crystals.
"It's difficult to study superconductivity in real materials, partly because
even minor defects in the crystal structure can throw off the experimental results,"
Hulet said. "But with the optical lattice, we can know, with absolute certainty,
that there are no defects. We can really probe the essence of the model."
Currently, Hulet's team is conducting two experiments that continue the proof-of-concept
work the team completed in the first phase of DARPA's OLE program. In one of
these experiments, Hulet's group is simulating a three-dimensional version of
the Hubbard model. In another, they are probing the properties of atoms confined
to one-dimensional tubes of light.
Ultimately, Hulet and his group hope to build a two-dimensional Hubbard model
that mimics the structure of the cuprate superconductors. That may sound like
a simpler task than building the 3D model, but Hulet said the technical challenges
are greater in 2D. For instance, it requires his team to chill their atoms to
temperatures that are colder than any yet achieved.
"The theoretical difficulty lies in the fact that the Hubbard model cannot
be solved analytically and our classical computers are extremely inefficient
in simulating it," said Rice physicist Han Pu, a theorist and co-principal
investigator on the project. "To tackle this problem, we need new tools,
and OLE is such a tool. An OLE is essentially a quantum computer that simulates
the Hubbard model in lab. The findings will tell us the properties of the Hubbard
model and give us crucial new insights into high-temperature superconductivity."
Posted September 23rd, 2009
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