Enlarge
This false color image shows the average density of
cesium atoms taken during multiple experimental cycles for studying
quantum criticality in the ultracold laboratory of Cheng Chin, associate
professor in physics at
UChicago. The density is lowest in the white
area on the outside, highest toward the center, where higher numbers of
atoms are blocking the incoming infrared laser light. Xibo Zhang
collected these data in connection with his recently completed doctoral
research at UChicago. (Xibo Zhang and Cheng Chin)
(PhysOrg.com) -- University of Chicago physicists have
experimentally demonstrated for the first time that atoms chilled to
temperatures near absolute zero may behave like seemingly unrelated
natural systems of vastly different scales, offering potential insights
into links between the atomic realm and deep questions of cosmology.
This ultracold state, called “
quantum
criticality,” hints at similarities between such diverse phenomena as
the gravitational dynamics of black holes or the exotic conditions that
prevailed at the birth of the universe, said Cheng Chin, associate
professor in physics at UChicago. The results could even point to ways
of simulating cosmological phenomena of the early universe by studying
systems of
atoms in states of
quantum criticality.
“Quantum criticality is the entry point for us to make connections
between our observations and other systems in nature,” said Chin, whose
team is the first to observe quantum criticality in
ultracold atoms in
optical lattices, a regular array of cells formed by multiple laser
beams that capture and localize individual atoms.
UChicago graduate student Xibo Zhang and two co-authors published their observations online Feb. 16 in
Science Express and in the March 2 issue of
Science.
Quantum criticality emerges only in the vicinity of a
quantum phase
transition. In the physics of everyday life, rather mundane phase
transitions occur when, for example, water freezes into ice in response
to a drop in
temperature.
The far more elusive and exotic quantum phase transitions occur only at
ultracold temperatures under the influence of magnetism, pressure or
other factors.
“This is a very important step in having a complete test of the
theory of quantum criticality in a system that you can characterize and
measure extremely well,” said Harvard University physics professor
Subir
Sachdev about the UChicago study.
Physicists
have extensively investigated quantum criticality in crystals,
superconductors and magnetic materials, especially as it pertains to the
motions of electrons. “Those efforts are impeded by the fact that we
can’t go in and really look at what every electron is doing and all the
various properties at will,” Sachdev said.
Sachdev’s theoretical work has revealed a deep mathematical
connection between how subatomic particles behave near a
quantum
critical point and the gravitational dynamics of black holes. A few
years hence, offshoots of the Chicago experiments could provide a
testing ground for such ideas, he said.
There are two types of critical points, which separate one phase from
another. The Chicago paper deals with the simpler of the two types, an
important milestone to tackling the more complex version, Sachdev said.
“I imagine that’s going to happen in the next year or two and that’s
what we’re all looking forward to now,” he said.
Other teams at UChicago and elsewhere have observed quantum
criticality under completely different experimental conditions. In 2010,
for example, a team led by Thomas Rosenbaum, the John T. Wilson
Distinguished Service Professor in Physics at UChicago, observed quantum
criticality in a sample of pure chromium when it was subjected to
ultrahigh pressures.
Zhang, who will receive his doctorate this month, invested nearly two
and a half years of work in the latest findings from Chin’s laboratory.
Co-authoring the study with Zhang and Chin were Chen-Lung Hung,
PhD’11,
now a postdoctoral scientist at the
California Institute of Technology,
and UChicago postdoctoral scientist Shih-Kuang Tung.
In their tabletop experiments, the Chicago scientists use sets of
crossed laser beams to trap and cool up to 20,000 cesium atoms in a
horizontal plane contained within an eight-inch cylindrical vacuum
chamber. The process transforms the atoms from a hot gas to a
superfluid, an exotic form of matter that exists only at temperatures
hundreds of degrees below zero.
“The whole experiment takes six to seven seconds and we can repeat the experiment again and again,” Zhang said.
The experimental apparatus includes a
CCD camera sensitive enough to
image the distribution of atoms in a state of quantum criticality. The
CCD camera records the intensity of laser light as it enters that vacuum
chamber containing thousands of specially configured ultracold atoms.
“What we record on the camera is essentially a shadow cast by the atoms,” Chin explained.
The UChicago scientists first looked for signs of quantum criticality
in experiments performed at ultracold temperatures from 30 to 12
nano-Kelvin, but failed to see convincing evidence. Last year they were
able to push the temperatures down to 5.8 nano-Kelvin, just billionths
of a degree above
absolute zero
(minus 459 degrees Fahrenehit). “It turns out that you need to go below
10 nano-Kelvin in order to see this phenomenon in our system,” Chin
said.
Chin’s team has been especially interested in the possibility of
using ultracold atoms to simulate the evolution of the early universe.
This ambition stems from the quantum simulation concept that Nobel
laureate
Richard Feynman proposed in 1981. Feynman maintained that if
scientists understand one quantum system well enough, they might be able
to use it to simulate the operations of another system that can be
difficult to study directly.
For some, like Harvard’s Sachdev, quantum criticality in ultracold
atoms is worthy of study as a physical system in its own right. “I want
to understand it for its own beautiful quantum properties rather than
viewing it as a simulation of something else,” he said.
More information: “Observation of Quantum Criticality
with Ultracold Atoms in Optical Lattices,” by Xibo Zhang, Chen-Lung
Hung, Shih-Kuang Tung, and Chen Chin, Science, March 2, 2012, Vol. 335,
No. 6072, pp. 1070-1072, and online Feb. in
Science Express Feb. 16.
Provided by University of Chicago (
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BEIJING, May 28, 2016 (Xinhua) -- Deputy chief engineer Zhou Yilin (3rd L) discusses with other technicians beside the quantum satellite at Shanghai Engineering Center for Microsatellites, under the Chinese Academy of Sciences, in Shanghai, east China, May 25, 2016. China plans to launch the world's first quantum satellite that can achieve secure communication in July. The satellite is dedicated to quantum science experiments. (Xinhua/Cai Yang)
