Researchers advance noise suppression for quantum computers

A team from Dartmouth College and MIT designed and implemented the first laboratory test to successfully detect and characterize a class of "non-Gaussian" complex noise processes commonly encountered in superconducting quantum computer systems.

The characterization of non-Gaussian noise in superconducting quantum bits is a crucial step in making these systems more accurate.

The joint study, published in Nature Communications, could help accelerate the realization of quantum computing systems. The experiment was based on previous theoretical research conducted in Dartmouth and published in Letters of physical examination in 2016.

"This is the first concrete step to attempt to characterize more complex types of noise processes than is commonly assumed in the quantum domain," said Lorenza Viola, a physics professor at Dartmouth, who led 2016 study as well as the theoretical component of this study. job. "As qubit consistency properties are constantly improved, it is important to detect non-Gaussian noise in order to build the most accurate quantum systems possible."

Quantum computers differ from traditional computers in going beyond the "on-off" binary sequencing favored by classical physics. Quantum computers rely on quantum bits, also called qubits, built from atomic and subatomic particles.

Essentially, qubits can be placed in a combination of "active" and "off" positions at the same time. They can also be "entangled", which means that the properties of one qubit can influence another at a distance.

The superconducting qubit systems are considered one of the main competitors in the race for the construction of evolutionary and efficient quantum computers. But, like other qubit platforms, they are very sensitive to their environment and can be affected by both external noise and internal noise.

External noise in quantum computing systems could come from control electronics or parasitic magnetic fields. Internal noise could come from other uncontrolled quantum systems such as material impurities. The ability to reduce noise is a major objective of the development of quantum computers.

"The big barrier that prevents us from having quantum computers on a large scale is now this noise problem." said Leigh Norris, a postdoctoral assistant in Dartmouth who co-wrote the study. "This research leads us to understand noise, which is a step towards its cancellation and, hopefully, with a reliable quantum computer someday."

Unwanted noise is often described in terms of simple "Gaussian" models, in which the probability distribution of random noise fluctuations creates a familiar bell-shaped Gaussian curve. Non-Gaussian noise is harder to describe and detect because it does not fall within the validity of these assumptions and may simply be less so.

When the noise statistical properties are Gaussian, a small amount of information can be used to characterize the noise, ie correlations at only two distinct moments, or equivalently, in terms of frequency domain description, the "noise" . spectrum."

Because of their high sensitivity to the environment, qubits can be used as sensors for their own noise. Building on this idea, researchers have made progress in developing techniques for identifying and reducing Gaussian noise in quantum systems, similar to the operation of noise-canceling headphones.

Although less common than Gaussian noise, the identification and removal of non-Gaussian noise is an equally important challenge for the optimal design of quantum systems.

Non-Gaussian noise is distinguished by more complex correlation models that involve several moments in time. As a result, it is necessary to have a lot more noise information to be able to identify it.

In this study, researchers were able to approximate the characteristics of non-Gaussian noise by using information on correlations at three different moments, corresponding to what is known as the "bispectrum" in the frequency domain.

"This is the first time that a detailed, frequency-resolved characterization of non-Gaussian noise can be performed in a qubit-equipped laboratory, which greatly expands the toolbox we have available for accurate noise characterization. , therefore, better and more stable qubits in quantum computers, "said Viola.

A quantum computer that can not detect non-Gaussian noise could easily be confused between the quantum signal it is supposed to process and the unwanted noise in the system. The protocols for obtaining non-Gaussian noise spectroscopy did not exist prior to the 2016 Dartmouth study.

Although the MIT experiment to validate the protocol does not immediately make large-scale quantum computers virtually viable, it is a major step in making them more accurate.

"This research started on the whiteboard and we did not know if anyone would be able to put it into practice, but despite significant conceptual and experimental challenges, the MIT team did done, "said Felix Beaudoin, a former postdoctoral fellow in Dartmouth. Viola group student who also played a pivotal role in the transition between theory and experience in the study.

"It was an absolute joy to collaborate with Lorenza Viola and her fantastic theory team at Dartmouth," said William Oliver, professor of physics at MIT. "We have been working together for several years on several projects and, while quantum computing is shifting from scientific curiosity to technical reality, I foresee the need to strengthen this interdisciplinary and interinstitutional collaboration."

According to the research team, additional years of work are still needed to perfect the detection and suppression of noise in quantum systems. In particular, future research will move from a single-sensor system to a two-sensor system to characterize noise correlations between different qubits.