Quantum Computers Check Accuracy of One Another

Quantum computers are advancing incredibly quickly, and they are one of our greatest tools for solving major computing problems. However, quantum computers are sensitive to external influences and prone to errors, which can affect their accuracy. 

Because some quantum computers can no longer be independently verified through simulation with classical computers, researchers are looking for new ways to check their accuracy. 

The research was published in the journal Physical Review X. 

Chiara Greganti is a physicist at the University of Vienna. 

“In order to take full advantage of future quantum computers for critical calculations we need a way to ensure the output is correct, even if we cannot perform the calculation in question by other means,” says Greganti.

Quantum Computers Checking Each Other

The team set out to develop and implement a cross-check procedure that enables quantum computers to check the results of a calculation of another. The devices are related but fundamentally different from one another. 

Martin Ringbauer is from the University of Innsbruck.

“We ask different quantum computers to perform different random-looking computations,” says Ringbauer. “What the quantum computers don't know is that there is a hidden connection between the computations they are doing.” 

The team can generate multiple different computations from a common source by relying on an alternative model of quantum computing built on graph structures.

“While the results may appear random and the computations are different, there are certain outputs that must agree if the devices are working correctly,” he continued.

Implementing the Method

The team implemented the method on five current quantum computers that use four different hardware technologies:

• Superconducting circuits
• Trapped ions
• Photonics
• Nuclear magnetic resonance

The method works on current hardware and doesn’t require any special requirements. The team also showed that the technique could be used to check a single device against itself. The two results only agree if they are both correct, and this is due to the computations being highly different. 

The new technique also does not require the researchers to look at the full result of the computation, which is often time consuming.

Tommaso Demarie is from Entropica Labs in Singapore.

It is enough to check how often the different devices agree for the cases where they should, which can be done even for very large quantum computers,” says Demarie.

This new technique is especially important as quantum computers are becoming increasingly available, so it can help ensure these devices are doing what they’re supposed to.

The team working on this technique consists of university researchers and quantum computing industry experts from multiple companies. 

Joe Fitzsimons is from Horizon Quantum Computing in Singapore. 

“This close collaboration of academia and industry is what makes this paper unique from a sociological perspective,” says Fitzsimons. “While there's a progressive shift with some researchers moving to companies, they keep contributing to the common effort making quantum computing reliable and useful.”