Science Gazette

Quantum computers’ inner workings


Two advances were discovered and validated using gate set tomography.

A precision diagnostic developed at the Department of Energy’s Sandia National Laboratories is quickly gaining traction as the gold standard for identifying and characterizing issues with quantum computing hardware.

Separate research teams, one of which included Sandia experts, utilized a Sandia technology called gate set tomography to create and verify extremely reliable quantum computers, according to two studies published today in the scientific journal Nature. Since 2012, Sandia has been working on gate set tomography with support from the DOE Office of Science’s Advanced Scientific Computing Research program.

One of today’s articles was co-authored by Sandia scientists with Australian academics at the University of New South Wales in Sydney, lead by Professor Andrea Morello. They utilized GST to demonstrate that a complex three-qubit system including two atomic nuclei and one electron in a silicon chip could be reliably operated with 99 percent or better precision.

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

“We want researchers all across the world to know that they have access to a strong, cutting-edge instrument that will help them achieve their goals,” said Sandia scientist Robin Blume-Kohout.

Future quantum computers with many more qubits, or quantum bits, might allow users in national security, research, and business to do certain tasks quicker than ever before. Computational mistakes are caused by weaknesses in present system controls. A quantum computer can correct certain faults, but the more errors it needs correct, the bigger and more costly the computer must be to construct.

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As a result, scientists will want diagnostic tools to determine how precisely they can regulate single atoms and electrons that store qubits, as well as to understand how to avoid rather than fix mistakes. This improves the system’s dependability while also lowering expenses.

Sandia’s flagship technology for monitoring the performance of qubits and quantum logic operations, generally known as “gates,” is gate set tomography. It compiles data from a variety of sources to provide a thorough report that details every mistake that occurs in the qubits. Morello and other experimental scientists may utilize the diagnostic data to figure out what they need to change.

“The Quantum Performance Laboratory at Sandia National Laboratories, directed by Robin Blume-Kohout, has created the most precise approach for identifying the nature of quantum computer failures,” Morello stated.

Even unanticipated errors are detected using gate set tomography.

The Sandia team maintains pyGSTi, a free, open-source GST program (pronounced “pigsty,” which stands for Python Gate Set Tomography Implementation). It was utilized by both study groups publishing in Nature today and is freely accessible at

The UNSW-Sandia partnership employed a novel, tailored type of gate set tomography created by Sandia researchers, whereas the Delft team used the pyGSTi program without support from the Sandia team. The team was able to rule out more possible problem modes and concentrate on a few key fault mechanisms thanks to the new methodologies.

However, when the Sandia team looked at the GST analysis of the UNSW experimental data, they uncovered a kind of inaccuracy that Morello’s group hadn’t anticipated. When the nuclear-spin qubits should have been segregated, they were interacting. Concerned that the issue may be due to a weakness in the qubits, the team went to Sandia’s Andrew Baczewski, a silicon qubit physicist and researcher at the Quantum Systems Accelerator, a National Quantum Information Science Research Center, for assistance in determining its cause.

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“It took up a lot of my spare time,” Baczewski said. “I’d be out on a stroll on a Saturday morning when something would come to me out of nowhere, and I’d dash home and do arithmetic for an hour.”

Baczewski and the rest of the crew eventually narrowed the source of the problem to a signal generator that was leaking microwaves into the system. Now that the reason has been identified, future trials may simply correct this.

“It was incredibly satisfying to have proof that GST even caught the faults that nobody anticipated,” Blume-Kohout remarked.

“Collaboration with Sandia National Laboratories was critical in achieving the milestone of high-fidelity quantum operations on silicon,” stated Morello. “The theoretical and computational methodologies established at Sandia have allowed for a more than 99 percent fidelity demonstration of quantum computing, as well as valuable insights into the microscopic sources of residual errors. In the next years, we want to deepen this strategic partnership.”

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