Engineers and materials scientists studying superconducting quantum
information bits (qubits)—a leading quantum computing material platform
based on the frictionless flow of paired electrons—have collected clues
hinting at the microscopic sources of qubit information loss. This loss is
one of the major obstacles in realizing quantum computers capable of
stringing together millions of qubits to run demanding computations. Such
large-scale, fault-tolerant systems could simulate complicated molecules for
drug development, accelerate the discovery of new materials for clean
energy, and perform other tasks that would be impossible or take an
impractical amount of time (millions of years) for today’s most powerful
supercomputers.
An understanding of the nature of atomic-scale defects that contribute to
qubit information loss is still largely lacking. The team helped bridge this
gap between material properties and qubit performance by using
state-of-the-art characterization capabilities at the Center for Functional
Nanomaterials (CFN) and National Synchrotron Light Source II (NSLS-II), both
U.S. Department of Energy (DOE) Office of Science User Facilities at
Brookhaven National Laboratory. Their results pinpointed structural and
surface chemistry defects in superconducting niobium qubits that may be
causing loss.
“Superconducting qubits are a promising quantum computing platform because
we can engineer their properties and make them using the same tools used to
make regular computers,” said Anjali Premkumar, a fourth-year graduate
student in the Houck Lab at Princeton University and first author on the
Communications Materials paper describing the research. “However, they have
shorter coherence times than other platforms.”
In other words, they can’t hold onto information very long before they lose
it. Though coherence times have recently improved from microseconds to
milliseconds for single qubits, these times significantly decrease when
multiple qubits are strung together.
“Qubit coherence is limited by the quality of the superconductors and the
oxides that will inevitably grow on them as the metal comes into contact
with oxygen in the air,” continued Premkumar. “But, as qubit engineers, we
haven’t characterized our materials in great depth. Here, for the first
time, we collaborated with materials experts who can carefully look at the
structure and chemistry of our materials with sophisticated tools.”
This collaboration was a “prequel” to the Co-design Center for Quantum
Advantage (C²QA), one of five National Quantum Information Science Centers
established in 2020 in support of the National Quantum Initiative. Led by
Brookhaven Lab, C²QA brings together hardware and software engineers,
physicists, materials scientists, theorists, and other experts across
national labs, universities, and industry to resolve performance issues with
quantum hardware and software. Through materials, devices, and software
co-design efforts, the C²QA team seeks to understand and ultimately control
material properties to extend coherence times, design devices to generate
more robust qubits, optimize algorithms to target specific scientific
applications, and develop error-correction solutions.
In this study, the team fabricated thin films of niobium metal through three
different sputtering techniques. In sputtering, energetic particles are
fired at a target containing the desired material; atoms are ejected from
the target material and land on a nearby substrate. Members of the Houck Lab
performed standard (direct current) sputtering, while Angstrom Engineering
applied a new form of sputtering they specialize in (high-power impulse
magnetron sputtering, or HiPIMS), where the target is struck with short
bursts of high-voltage energy. Angstrom carried out two variations of
HiPIMS: normal and with an optimized power and target-substrate geometry.
Back at Princeton, Premkumar made “transmon” qubit devices from the three
sputtered films and placed them in a dilution refrigerator. Inside this
refrigerator, temperatures can plunge to near absolute zero (minus 459.67
degrees Fahrenheit), turning qubits superconducting. In these devices,
superconducting pairs of electrons tunnel across an insulating barrier of
aluminum oxide (Josephson junction) sandwiched between superconducting
aluminum layers, which are coupled to capacitor pads of niobium on sapphire.
The qubit state changes as the electron pairs go from one side of the
barrier to the other. Transmon qubits, co-invented by Houck Lab principal
investigator and C²QA Director Andrew Houck, are a leading kind of
superconducting qubit because they are highly insensitive to fluctuations in
electric and magnetic fields in the surrounding environment; such
fluctuations can cause qubit information loss.
For each of the three device types, Premkumar measured the energy relaxation
time, a quantity related to the robustness of the qubit state.
“The energy relaxation time corresponds to how long the qubit stays in the
first excited state and encodes information before it decays to the ground
state and loses its information,” explained Ignace Jarrige, formerly a
physicist at NSLS-II and now a quantum research scientist at Amazon, who led
the Brookhaven team for this study.
Each device had different relaxation times. To understand these differences,
the team performed microscopy and spectroscopy at the CFN and NSLS-II.
NSLS-II beamline scientists determined the oxidation states of niobium
through x-ray photoemission spectroscopy with soft x-rays at the In situ and
Operando Soft X-ray Spectroscopy (IOS) beamline and hard x-rays at the
Spectroscopy Soft and Tender (SST-2) beamline. Through these spectroscopy
studies, they identified various suboxides located between the metal and the
surface oxide layer and containing a smaller amount of oxygen relative to
niobium.
“We needed the high energy resolution at NSLS-II to distinguish the five
different oxidation states of niobium and both hard and soft x-rays, which
have different energy levels, to profile these states as a function of
depth,” explained Jarrige. “Photoelectrons generated by soft x-rays only
escape from the first few nanometers of the surface, while those generated
by hard x-rays can escape from deeper in the films.”
At the NSLS-II Soft Inelastic X-ray Scattering (SIX) beamline, the team
identified spots with missing oxygen atoms through resonant inelastic x-ray
scattering (RIXS). Such oxygen vacancies are defects, which can absorb
energy from qubits.
At the CFN, the team visualized film morphology using transmission electron
microscopy and atomic force microscopy, and characterized the local chemical
makeup near the film surface through electron energy-loss spectroscopy.
“The microscope images showed grains—pieces of individual crystals with
atoms arranged in the same orientation—sized larger or smaller depending on
the sputtering technique,” explained coauthor Sooyeon Hwang, a staff
scientist in the CFN Electron Microscopy Group. “The smaller the grains, the
more grain boundaries, or interfaces where different crystal orientations
meet. According to the electron energy-loss spectra, one film had not just
oxides on the surface but also in the film itself, with oxygen diffused into
the grain boundaries.”
Their experimental findings at the CFN and NSLS-II revealed correlations
between qubit relaxation times and the number and width of grain boundaries
and concentration of suboxides near the surface.
“Grain boundaries are defects that can dissipate energy, so having too many
of them can affect electron transport and thus the ability of qubits to
perform computations,” said Premkumar. “Oxide quality is another potentially
important parameter. Suboxides are bad because electrons are not happily
paired together.”
Going forward, the team will continue their partnership to understand qubit
coherence through C²QA. One research direction is to explore whether
relaxation times can be improved by optimizing fabrication processes to
generate films with larger grain sizes (i.e., minimal grain boundaries) and
a single oxidation state. They will also explore other superconductors,
including tantalum, whose surface oxides are known to be more chemically
uniform.
“From this study, we now have a blueprint for how scientists who make qubits
and scientists who characterize them can collaborate to understand the
microscopic mechanisms limiting qubit performance,” said Premkumar. “We hope
other groups will leverage our collaborative approach to drive the field of
superconducting qubits forward.”
Reference:
Anjali Premkumar, Conan Weiland, Sooyeon Hwang, Berthold Jäck, Alexander P.
M. Place, Iradwikanari Waluyo, Adrian Hunt, Valentina Bisogni, Jonathan
Pelliciari, Andi Barbour, Mike S. Miller, Paola Russo, Fernando Camino, Kim
Kisslinger, Xiao Tong, Mark S. Hybertsen, Andrew A. Houck, Ignace Jarrige.
Microscopic relaxation channels in materials for superconducting qubits.
Communications Materials, 2021; DOI: 10.1038/s43246-021-00174-7
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Physics