Connecting the dots between material properties and qubit performance


Date:
September 30, 2021
Source:
DOE/Brookhaven National Laboratory
Summary:
Scientists studying superconducting qubits identified structural
and chemical defects that may be causing quantum information loss --
an obstacle to practical quantum computation.



FULL STORY ========================================================================== 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." This work was
supported by the DOE Office of Science, National Science Foundation
Graduate Research Fellowship, Humboldt Foundation, National Defense
Science and Engineering Graduate Fellowship, Materials Research Science
and Engineering Center, and Army Research Office. This research used
resources of the Electron Microscopy, Proximal Probes, and Theory and Computation Facilities at the CFN, a DOE Nanoscale Science Research
Center. The SST-2 beamline at NSLS-II is operated by the National
Institute of Standards and Technology.

========================================================================== Story Source: Materials provided by
DOE/Brookhaven_National_Laboratory. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Anjali Premkumar, Conan Weiland, Sooyeon Hwang, Berthold Ja"ck,
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; 2 (1) DOI: 10.1038/s43246-021-00174-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210930140720.htm

--- up 4 weeks, 8 hours, 25 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)