A recent paper published in Nature by a team from Google Quantum AI titled Thermalization and Criticality on an Analogue–Digital Quantum Simulator presents a groundbreaking approach that combines both analog and digital quantum techniques to simulate physical phenomena like the Kibble–Zurek mechanism. This dual approach allowed the researchers to identify a breakdown in the Kibble–Zurek scaling predictions, a result that had previously been thought to be accurate.
This marks the first known instance of both analog and digital processing being performed on the same quantum computer in rapid sequence. While some quantum computers, like the D-Wave quantum annealer and certain neutral atom machines, perform analog processing, they can’t run gate-based circuits without extensive modifications. On the other hand, quantum computers designed for gate-based operations can’t function in analog mode at all.
The Google team highlights the benefits of combining both modes. The digital approach provides flexibility and control, while the analog mode speeds up computations and helps reduce noise. The breakthrough enabling this combination was the development of a new calibration scheme, allowing the quantum chip’s couplers to work effectively when activated simultaneously. In their experiment, the team used digital circuits for initial state preparation, then switched to analog mode for Hamiltonian evolution, and finally returned to digital mode for measurement and state characterization.
Many quantum scientists believe that quantum advantage will first be demonstrated in simulating physical phenomena that classical computers cannot replicate. In this case, the researchers uncovered a breakdown in Kibble–Zurek scaling that had not been observed before. Initially, they thought there was a flaw in their quantum simulation, but when they verified the result using a classical simulation, they found that the quantum computer had discovered something new. While this isn’t yet considered a clear case of quantum advantage—since the classical computer could also replicate the result—the quantum simulation was the first to identify this issue.
The research was conducted using Google’s Sycamore quantum processor, but future experiments will use the more advanced Willow chip. The team is hopeful that this analog-digital approach will enable even more complex simulations, potentially leading to new scientific discoveries that can only be achieved with quantum computers, bringing them closer to realizing true quantum advantage.
For more technical details on the research, you can refer to the paper published in Nature.
February 5, 2025
