MIT, Caltech scientists develop benchmark protocol for determining accuracy of quantum analog

Scientists from the Massachusetts Institute of Technology (MIT) and Caltech have developed a benchmarking protocol that can be used to determine the accuracy of quantum analog simulators by analyzing their random fluctuations, according to a news release.

These simulators are "laboratory experiments that involve super-cooling tens to hundreds of atoms and probing them with finely tuned lasers and magnets," the universities said.

The results could provide a blueprint for creating new exotic materials, practical quantum computers and smarter, more efficient electronics. 

In order for quantum simulators to provide useful information, scientists must be certain that they have a “high fidelity” and accurately represent quantum behavior. Without this, researchers could assume a quantum effect where there is none. 

There has not been, however, a reliable way to characterize the fidelity of quantum analog simulators until now. The research, published in Nature, reveals a new quantum phenomenon, in which there is a certain randomness in the quantum fluctuations of atoms, and that this random behavior exhibits a universal, predictable pattern. The team confirmed that certain random fluctuations can indeed follow a predictable statistical pattern.

The researchers used this quantum randomness as a tool to characterize the fidelity of a quantum analog simulator. They showed, through theory and experiments, that they could determine the accuracy of a quantum simulator by analyzing its random fluctuations. 

Team members developed a new benchmark protocol that can be applied to existing quantum analog simulators to gauge their fidelity, based on their pattern of quantum fluctuations. The protocol could help speed the development of new exotic materials and quantum computing systems. 

"This work would allow characterizing many existing quantum devices with very high precision,” said study co-author Soonwon Choi, assistant professor of physics at MIT. “It also suggests there are deeper theoretical structures behind the randomness in chaotic quantum systems than we have previously thought about.”

The study’s authors include MIT graduate student Daniel Mark and collaborators at Caltech, the University of Illinois at Urbana-Champaign, Harvard University and the University of California at Berkeley.

Their research was motivated by an advance in 2019 by Google, where researchers built a digital quantum computer called Sycamore that could carry out a specific computation more quickly than a classical computer. Whereas classical computers use bits that exist as either a 0 or a 1, quantum computers use qubits that can exist in a superposition of multiple states.

The Google researchers engineered a system of superconducting loops to behave as 53 qubits and demonstrated that the “computer” could carry out a specific calculation that would normally be too difficult for even the fastest supercomputer in the world to solve.

Choi and his colleagues wondered whether they could use a similar, randomized approach to gauge the fidelity of quantum analog simulators. The main obstacle was that, unlike Google’s digital quantum system, individual atoms and other qubits in analog simulators are incredibly difficult to manipulate and therefore randomly control. 

Through some theoretical modelling, however, Choi realized that the collective effect of individually manipulating qubits in Google’s system can be reproduced in an analog quantum simulator by simply letting the qubits naturally evolve. 

Researchers hypothesized that if they could develop a numerical simulation that precisely represented the dynamics and universal random fluctuations of a quantum simulator, they could compare the predicted outcomes with the simulator’s actual outcomes. 

The closer the two are, the more accurate the quantum simulator must be. To test this idea, Choi teamed with experimentalists at Caltech, who engineered a quantum analog simulator comprising 25 atoms. 

Physicists shone a laser on the experiment to collectively excite the atoms, then let the qubits naturally interact and evolve over time. They measured the state of each qubit over multiple runs, gathering 10,000 measurements in total.