By Tom Garlinghouse 

Scientists tell us that despite many technological advances, a fully functioning quantum computer is still at least several years away. However, this has not stopped researchers from developing innovative devices that can simulate quantum systems. These are called quantum simulators, and they are designed to mimic quantum processes in such a way as to allow researchers to study the dynamics of quantum phenomena. 

Now, in a paper recently published in the journal Nature, a large collaborative team that includes scientists from Google Research, Princeton University, Paul Scherrer Institute, and University of Geneva, among others, has devised a hybrid quantum simulator that combines analog and digital operations in a single component. This innovative device promises to expand our knowledge of the quantum realm by answering questions that cannot be answered by reliance on classical physics. 

“We are very excited about the prospects of using this platform to make discoveries beyond the reach of classical computers,” said Trond Andersen, a co-lead author of the paper and a senior research scientist at Google Quantum AI in Mountain View, California.

The laws of quantum mechanics differ fundamentally from those of classical physics. This dichotomy has made it difficult to study quantum physics using only classical physics. This is because quantum physics describes the behavior of matter at the atomic and subatomic scale and relies on still perplexing and counterintuitive concepts such as superposition and entanglement. 

Nowhere is this more evident than in the realm of computing. Back in the 1980s, theoretical physicist Richard Feynman famously stated that trying to use classical computers to understand the dynamics of quantum states, such as how quantum particles interact, was a fool’s errand. 

Sentiments such as Feynman’s gave rise to efforts to develop quantum computers—an ongoing area of research that continues unabated today. Quantum computers, by exploiting the laws of quantum mechanics, can perform certain computational tasks significantly more efficiently than classical computers. They are able to do this because at the heart of quantum computers are devices called qubits, which are the quantum equivalents of classical computer bits. A classical bit is encoded with information that has the value of either one or zero. By contrast, a qubit can be simultaneously a zero or a one, and this, combined with the entanglement between qubits and the phenomenon of quantum interference, increases the computer’s calculating power exponentially for certain tasks.

But while fully functioning quantum computers remain elusive, at least for now, the analog-digital hybrid developed by the researchers, with its capabilities beyond the currently operating quantum simulators, promises to expand our knowledge of quantum phenomena like never before. 

“The combination of digital and analog capabilities is what makes this platform unique and really expands the playing field for studying interacting quantum matter,” said Dmitry Abanin, a co-lead author of the paper and professor of physics at Princeton University.

An analog quantum simulator mimics the behavior of more complex quantum systems that would otherwise be difficult to study. It does this by manipulating parameters within the system so that the system as a whole continuously evolves. It is akin to simulating a physical system with a continuous evolution rather than executing a series of defined steps. By contrast, a digital simulator is a computing device that uses discrete qubits and self-programmable quantum gates (components used to manipulate qubits to perform operations) to simulate the behavior of complex quantum systems at high precision.  

“The advantages of combining digital and analog capabilities are that having the digital component allows you to prepare the states of the system under study very precisely, while analog evolution allows you to emulate what happens in nature,” added Abanin. 

To create this hybrid simulator, the researchers adapted Google’s digital quantum processor comprising 69 superconducting transmon qubits in a two-dimensional lattice. They then tweaked the qubits on this platform so that all the qubits interacted simultaneously, creating a system that mimics quantum magnetism. This created a system akin to a physical system with its own quantum evolution.  

“We then used this platform to prepare various interesting states by letting them dynamically evolve and then characterizing the resulting states they reach,” said Abanin. 

One area of research on which the researchers focused was the study of thermalization dynamics, which has long been a challenge for quantum simulators. Thermalization is the process whereby quantum states undergo what is called “thermal equilibrium,” which is simply when a quantum state reaches a level of energy stability. 

During their thermalization experiments, however, they discovered something that surprised them. 

The team explored a theory known as the Kibble-Zurek (KZ) mechanism, which was originally used to explain behavior in cosmology. It predicts that once a quantum system passes through a certain critical point during a phase transition, it will undergo inevitable defects as it settles into the new stable equilibrium. A phase transition describes a fundamental change in a system—think, for example, of water turning into ice. 

However, upon careful analysis, they found clear deviations from the predictions of the KZ theory, indicating that the dynamics of thermalization are still not fully understood and more complex than current theories allow. This demonstrates the importance of using devices such as the digital-analog simulator, which allow for direct quantum simulation. 

“The observed breakdown of the Kibble-Zurek mechanism was not expected when we performed the first measurements,” said Andersen. “Discoveries like these definitely make for the most thrilling experiments!” 

The researchers are optimistic about the platform’s ability to probe many more intriguing problems in quantum physics. 

“The platform allows for a kind of experiment that is a theorist’s dream: you can design the initial state, choose its evolution, and then measure very complex observables that are typically out of reach,” concluded Abanin. 

The study, “Thermalization and criticality on an analog-digital quantum simulator,” by T. I. Andersen, N. Astrakhantsev, A. H. Karamlou, J. Berndtsson, J. Motruk, A. Szasz, J. A. Gross,… and D. Abanin was published on February 6, 2025 in the journal Nature DOI: http://doi.org/10.1038/s41586-024-08460-3

This research was funded by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Quantum Systems Accelerator. Additional funding was provided by the German National Academy of Sciences Leopoldina under the grant number LPDS 2021-02 and by the Walter Burke Institute for Theoretical Physics at Caltech. Work in Grenoble is funded by the French National Research Agency via the JCJC project QRand (ANR-20-CE47-0005), Laboratoire d’excellence LANEF (ANR-10-LABX-51-01), from the Grenoble Nanoscience Foundation.