This is a false-color image of a single-crystal Si cantilever and its attached annular SRO particle. Inset: Scanning electron microscope image of the SRO “ring” with a 0.7-?m diameter hole. Credit: Raffi Budakian, Univ. of Illinois |
A new fractional vortex state observed in an
unconventional superconductor may offer the first glimpse of an exotic state of
matter predicted theoretically for more than 30 years. In a paper published in Science,
Univ. of Illinois physicists, led by Raffi
Budakian, describe their observations of a new fractional vortex state in
strontium ruthenium oxide (SRO). Such states may provide the basis for a novel
form of quantum computing in which quantum information is encoded in the topological
properties of a physical system.
“We’ve been on the trail of a state of matter
called a half-quantum vortex for more than three years,” said Budakian.
“First proposed in the 1970s to exist in superfluid helium-3, a
half-quantum vortex can be thought of as a ‘texture’ that arises from the spin
phase of the superconducting order parameter.”
Budukian’s group investigated strontium ruthenium
oxide (SRO), an unconventional superconductor that has been proposed as the
solid-state analog of the A-phase of superfluid helium-3. Using
state-of-the-art nanofabrication methods and exquisitely sensitive
cantilever-based magnetometry techniques developed by the group, the
researchers observed minute fluctuations in the magnetism of tiny rings of SRO.
“Strontium ruthenium oxide is a unique and
fascinating material, and the half-quantum vortices that have been conjectured
to exist in it are particularly interesting,” said Anthony J. Leggett, the
John D. and Catherine T. MacArthur Professor and Center for Advanced Study
Professor of Physics, who shared the 2003 Nobel Prize in Physics for his work
on superfluid helium-3. “It is believed that these half-quantum vortices
in SRO may provide the basis for topological quantum computing. If this novel
form of computing is eventually realized, this experiment will certainly be
seen as a major milestone along the road there.”
Budakian is an assistant professor of physics and
a principal investigator in the Frederick Seitz Materials Research Laboratory
at Illinois.
Five years ago, he was instrumental in pioneering a technique, magnetic
resonance force microscopy, to measure the force exerted on a micrometer-scale
silicon cantilever by the spin of a single electron in a bulk material. He and
his group have now adapted their ultrasensitive cantilever measurements to
observe the magnetic behavior of SRO.
In the experiment, the researchers first
fabricated a micron-sized ring of SRO and glued it to the tip of the silicon
cantilever. How small are these rings? Fifty of them would fit across the width
of a human hair. And the tips of the cantilevers are less than 2 ?m wide.
“We take the high-energy physics approach to
making these rings. First we smash the SRO, and then we sift through what’s
left,” said Budakian.
The researchers first pulverize the large crystals
of SRO into fragments, choose a likely micron-sized flake, and drill a hole in
it using a focused beam of gallium ions. The resulting structure, which looks
like a microscopic donut, is glued onto the sensitive silicon cantilever and
then cooled to 0.4 degrees above absolute zero.
“Positioning the SRO ring on the cantilever
is a bit like dropping one grain of sand precisely atop a slightly larger grain
of sand,” said Budakian, “only our ‘grains of sand’ are much
smaller.”
Budakian added that this technique is the first
time such tiny superconducting rings have been fabricated in SRO.
Being able to make these rings is crucial to the
experiment, according to Budakian, because the half-quantum vortex state is not
expected to be stable in larger structures.
“Once we have the ring attached to the
cantilever, we can apply static magnetic fields to change the ‘fluxoid’ state
of the ring and detect the corresponding changes in the circulating current. In
addition, we apply time-dependent magnetic fields to generate a dynamic torque
on the cantilever. By measuring the frequency change of the cantilever, we can
determine the magnetic moment produced by the currents circulating the
ring,” said Budakian.
“We’ve observed transitions between integer
fluxoid states, as well as a regime characterized by ‘half-integer’
transitions,” Budakian noted, “which could be explained by the
existence of half-quantum vortices in SRO.”
In addition to the advance in fundamental
scientific understanding that Budakian’s work provides, the experiment may be
an important step toward the realization of a so-called “topological”
quantum computer, as Leggett alluded.
Unlike a classical computer, which encodes
information as bits whose values are either 0 or 1, a quantum computer would
rely on the interaction among two-level quantum systems (e.g., the spins of
electrons, trapped ions, or currents in superconducting circuits) to encode and
process information. The massive parallelism inherent in quantal time evolution
would provide rapid solutions to problems that are currently intractable,
requiring vast amounts of time in conventional, classical machines.
For a functional quantum computer, the quantum
bits or “qubits” must be strongly coupled to each other but remain
sufficiently isolated from random environmental fluctuations, which cause the
information stored in the quantum computer to decay—a phenomenon known as
decoherence. Currently, large-scale, international projects are underway to
construct quantum computers, but decoherence remains the central problem for real-world
quantum computation.
According to Leggett, “A rather radical solution to the decoherence
problem is to encode the quantum information nonlocally; that is, in the global
topological properties of the states in question. Only a very restricted class
of physical systems is appropriate for such topological quantum computing, and
SRO may be one of them, provided that certain conditions are fulfilled in it.
One very important such condition is precisely the existence of half-quantum
vortices, as suggested by the Budakian experiment.”
