Thursday, November 5, 2009
Physicists at Harvard
University have created a
quantum gas microscope that can be used to observe single atoms at temperatures
so low the particles follow the rules of quantum mechanics, behaving in bizarre
ways.
The work, published this week in the journal Nature,
represents the first time scientists have detected single atoms in a
crystalline structure made solely of light, called a Bose Hubbard optical lattice.
It's part of scientists' efforts to use ultracold quantum gases to understand
and develop novel quantum materials.
"Ultracold atoms in optical lattices can be used as a
model to help understand the physics behind superconductivity or quantum
magnetism, for example," says senior author Markus Greiner, an assistant
professor of physics at Harvard and an affiliate of the Harvard-MIT Center
for Ultracold Atoms. "We expect that our technique, which bridges the gap
between earlier microscopic and macroscopic approaches to the study of quantum
systems, will help in quantum simulations of condensed matter systems, and also
find applications in quantum information processing."
The quantum gas microscope developed by Greiner and his
colleagues is a high-resolution device capable of viewing single atoms -- in
this case, atoms of rubidium -- occupying individual, closely spaced lattice
sites. The rubidium atoms are cooled to just 5 billionths of a degree above
absolute zero (-273 degrees Celsius).
"At such low temperatures, atoms follow the rules of
quantum mechanics, causing them to behave in very unexpected ways,"
explains first author Waseem S. Bakr, a graduate student in Harvard's
Department of Physics. "Quantum mechanics allows atoms to quickly tunnel
around within the lattice, move around with no resistance, and even be
'delocalized' over the entire lattice. With our microscope we can individually
observe tens of thousands of atoms working together to perform these amazing
feats."
In their paper, Bakr, Greiner, and colleagues present images
of single rubidium atoms confined to an optical lattice created through
projections of a laser-generated holographic pattern. The neighboring rubidium
atoms are just 640 nanometers apart, allowing them to quickly tunnel their way
through the lattice.
Confining a quantum gas -- such as a Bose–Einstein
condensate -- in such an optically generated lattice creates a system that can
be used to model complex phenomena in condensed-matter physics, such as superfluidity.
Until now, only the bulk properties of such systems could be studied, but the
new microscope's ability to detect arrays of thousands of single atoms gives
scientists what amounts to a new workshop for tinkering with the fundamental
properties of matter, making it possible to study these simulated systems in
much more detail, and possibly also forming the basis of a single-site readout
system for quantum computation.
"There are many unsolved questions regarding quantum
materials, such as high-temperature superconductors that lose all electrical
resistance if they are cooled to moderate temperatures," Greiner says.
"We hope this ultracold atom model system can provide answers to some of
these important questions, paving the way for creating novel quantum materials
with as-yet unknown properties."
Greiner's co-authors on the Nature paper are Waseem S. Bakr,
Jonathon I. Gillen, Amy Peng, and Simon Foelling, all of Harvard's Department
of Physics and the Harvard-MIT
Center for Ultracold
Atoms. Their work was supported by the National Science Foundation, the Air
Force Office of Scientific Research, the Army Research Office, the Defense
Advanced Research Projects Agency, and the Alfred P. Sloan Foundation.
Study
abstract
Harvard University