Precision diagnostics developed at the Department of Energy’s Sandia National Laboratories is becoming the gold standard for detecting and describing problems inside quantum computing hardware.
Two articles published today in the scientific journal Nature describe how separate research teams – including one including Sandia researchers – used a Sandia technique called gate-set tomography to develop and validate highly reliable quantum processors. Sandia has been developing gated tomography since 2012, with funding from the DOE Office of Science under the Advanced Scientific Computing Research program.
Sandia scientists collaborated with Australian researchers from the University of New South Wales in Sydney, led by Professor Andrea Morello, to publish one of today’s papers. Together they used GST to show that a sophisticated three-qubit system comprising two atomic nuclei and an electron in a silicon chip could be reliably manipulated with over 99% accuracy.
In other Nature article published today, a group led by Professor Lieven Vandersypen from the Delft University of Technology in the Netherlands used gate set tomography, implemented using Sandia software, to demonstrate the milestone of over 99% accuracy, but with a different approach, by controlling electrons trapped inside quantum dots instead of isolated atomic nuclei.
“We want researchers around the world to know they have access to a powerful, cutting-edge tool that will help them make their breakthroughs,” said Sandia scientist Robin Blume-Kohout.
Future quantum processors with many more qubits, or quantum bits, could allow users working in national security, science and industry to perform certain tasks faster than they ever could with a conventional computer. But flaws in the controls of the current system cause miscalculations. A quantum computer can correct some errors, but the more errors it has to correct, the bigger and more expensive that computer becomes to build.
So scientists need diagnostic tools to calculate how precisely they can control the single atoms and electrons that store qubits and learn how to prevent errors instead of correcting them. This increases the reliability of their system while reducing costs.
Gateset tomography is Sandia’s flagship technique for measuring the performance of qubits and quantum logic operations, also known as “gates”. It combines the results of many types of measurements to generate a detailed report describing each error occurring in the qubits. Experimental scientists like Morello can use diagnostic results to infer what they need to fix.
“The Quantum Performance Laboratory at Sandia National Labs, led by Robin Blume-Kohout, has developed the most accurate method for identifying the nature of errors occurring in a quantum computer,” Morello said.
Gateset tomography even detects an unexpected error
The Sandia team maintains free and open source GST software called pyGSTi (pronounced “pigsty”, which stands for Python Gate Set Tomography Implementation). Publicly available at http://www.pygsti.info, it has been used by both research groups publishing today in Nature.
While the Delft team used pyGSTi software without help from the Sandia team, the UNSW-Sandia collaboration used a new custom form of gated tomography developed by Sandia researchers. The new techniques allowed the team to rule out more potential error modes and focus on a few dominant error mechanisms.
But when Sandia’s team studied the GST analysis of the UNSW experimental data, they found a surprising type of error that Morello’s group hadn’t expected. The nuclear spin qubits were interacting when they should have been isolated. Fearing that this error could indicate a flaw in the qubits, the team turned to Sandia’s Andrew Baczewski, an expert in silicon qubit physics and a researcher at the Quantum Systems Accelerator, a national center for information science research. quantum, to help find its source.
“It took up a lot of my free time,” Baczewski said. “I’d go out for a walk on a Saturday morning and out of the blue something would pop into my head and I’d run home and do math for an hour.”
Eventually, Baczewski and the rest of the team tracked the error to a signal generator that was leaking microwaves into the system. This can be easily corrected in future experiments, now that the cause is known.
Blume-Kohout said, “It was really gratifying to see confirmation that GST caught even the errors no one expected.”
“Collaborating with Sandia National Laboratories has been critical in reaching the milestone of high-fidelity quantum operations in silicon,” Morello said. “The theoretical and computational methods developed at Sandia have enabled the rigorous demonstration of quantum computing with greater than 99% fidelity and provided valuable insights into the microscopic causes of residual errors. We plan to expand this strategic collaboration in the years to come.