Quantum Computer Inner Workings

Quantum Computer Inner Workings

A precision diagnostic developed at the Department of Energy’s Sandia National Laboratories is quickly becoming the gold standard for detecting and describing problems within quantum computing hardware.

Two papers published today in the scientific journal Nature describe how separate research teams, one of which included Sandia researchers, developed and validated highly reliable quantum processors using a Sandia technique called gate set tomography. Since 2012, Sandia has been developing gate set tomography with funding from the DOE Office of Science’s Advanced Scientific Computing Research program.

One of today’s papers was co-authored by Sandia scientists and Australian researchers from the University of New South Wales in Sydney, led by Professor Andrea Morello. They used GST to demonstrate that a sophisticated three-qubit system in a silicon chip comprised of two atomic nuclei and one electron could be manipulated reliably with 99%+ accuracy.

A group led by Professor Lieven Vandersypen of Delft University of Technology in the Netherlands used gate set tomography, implemented with Sandia software, to demonstrate the important milestone of 99%-plus accuracy, but with a different approach, controlling electrons trapped within quantum dots rather than isolated atomic nuclei, published in another Nature article.

The Quantum Performance Laboratory at Sandia National Labs, led by Robin Blume-Kohout, has developed the most accurate method to identify the nature of the errors occurring in a quantum computer.


“We want researchers everywhere 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 in national security, science, and industry to perform some tasks faster than a conventional computer ever could. However, flaws in current system controls result in computational errors. A quantum computer can correct some errors, but the more errors it must correct, the larger and more expensive the computer must be to be built.

As a result, scientists require diagnostic tools to determine how precisely they can control single atoms and electrons that store qubits and to learn how to prevent errors rather than correct them. This improves the system’s reliability while lowering costs.

Inner workings of quantum computers

Gate set tomography is Sandia’s flagship technique for measuring the performance of qubits and quantum logic operations, also known as “gates.” It combines results from many kinds of measurements to generate a detailed report describing every error occurring in the qubits. Experimental scientists like Morello can use the diagnostic results to deduce what they need to fix.

“The Quantum Performance Laboratory at Sandia National Labs, led by Robin Blume-Kohout, has developed the most accurate method to identify the nature of the errors occurring in a quantum computer,” Morello said.

Gate set tomography even detects unexpected error

The Sandia team maintains a free, open-source GST software called pyGSTi (pronounced “pigsty,” which stands for Python Gate Set Tomography Implementation). Publicly available at, it was used by both research groups publishing in Nature today.

While the Delft team used the pyGSTi software without assistance from the Sandia team, the UNSW-Sandia collaboration used a new, customized form of gate set tomography developed by the Sandia researchers. The new techniques allowed the team to rule out more potential error modes and focus on a few dominant error mechanisms.

However, when the Sandia team examined the GST analysis of the UNSW experimental data, they discovered a surprising type of error that Morello’s group did not expect. The nuclear-spin qubits were interacting when they should have been isolated. Concerned that this error might 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 Quantum Information Science Research Center, to help find its source.

“It came to occupy a lot of my free time,” Baczewski said. “I would be out for a walk on a Saturday morning and, out of the blue, something would occur to me and I would run home and do math for an hour.”

Baczewski and the rest of the team eventually traced the error back to a signal generator that was leaking microwaves into the system. Now that the cause has been identified, this can be easily corrected in future experiments. “It was really satisfying to see confirmation that GST even detected the errors that nobody expected,” Blume-Kohout said.

“Collaboration with Sandia National Laboratories was critical in achieving the milestone of high-fidelity quantum operations in silicon,” Morello explained. “Sandia’s theoretical and computational methods enabled the rigorous demonstration of quantum computing with greater than 99% fidelity, as well as valuable insights into the microscopic causes of residual errors. We intend to broaden this strategic collaboration in the coming years.”