Imagine bringing a scalding hot metal bar into contact with a freezing cold one. The result is familiar: heat moves from the hot bar to the cold one until both reach the same temperature. This intuitive process, called thermal diffusion, is the system's way of seeking balance or equilibrium. The temperature difference disappears, and everything becomes uniform again.
But what if this intuitive picture could be upended? Could we engineer a system where one half stays hot and the other cold—maintaining the imbalance rather than erasing it? This surprising idea is the focus of a new study from the Ajoy Lab at UC Berkeley and Lawrence Berkeley National Laboratory in collaboration with the Max Planck Institute for the Physics of Complex Systems, recently published in Science Advances.
Instead of heating or cooling metal bars, the researchers investigate this concept by looking at a different kind of system: quantum mechanical spins. These tiny magnetic moments are carried by nuclei in solids and can be aligned in different ways. Normally, spin polarization alignment tends to spread out over time, just like heat, to balance any differences. For example, imagine a system where nuclear spins are organized in a shell-like texture, with some spins pointing "up" in one area and others pointing "down" in another. This pattern is unstable, and decades of research have shown that spin interactions cause the system to quickly smooth out any imbalances.
In other words, just as temperature gradients disappear in metal bars, so do these spin patterns in quantum systems.
In their latest research, the researchers have discovered an innovative way to create—and crucially, maintain—these delicate spin textures for much longer than previously thought possible. They show that it's possible to arrange nuclear spins inside a diamond into a shell-like pattern—intact—for several minutes, an astonishingly long lifetime given the typical timescales of spin diffusion.
Figure A: controllable “shell-like” spin texture of positive and negative polarization (shaded red and blue, respectively) is generated in a nanoscale ensemble of 13C nuclear spins surrounding a central NV electron (manipulated by an external laser, shown in green). Photo courtesy of Ajoy lab.
The breakthrough comes from a concept called a "prethermal" state. In this prethermal state, unusual behaviors are possible that wouldn't happen in a system that had reached thermal equilibrium—like maintaining a steady, structured spin pattern. To create this state, the researchers work with nuclear spins of 13C atoms embedded in diamond. They use a technique called Hamiltonian engineering, to carefully control how these spins interact. A key element in their approach is using a magnetic field gradient produced by a nitrogen-vacancy (NV) center—a special type of electron spin in the diamond. This gradient acts like a tiny antenna, which helps them finely tune the system's behavior.
The result is striking: the interaction between the NV center and the spins creates a stable, shell-like spin pattern that can span just a few nanometers and involve hundreds of carbon nuclei. Even more exciting, this structure can form naturally from various initial spin states—essentially, the system can reliably organize itself into this structured pattern, much like a system that remains "half hot and half cold" over time.
While this work is quite technical, it has broad implications. By stabilizing spin patterns at a nanoscale level, this discovery could lead to more precise quantum sensors, in turn helping to detect weak magnetic fields that are invisible to current tools. This particular capability is valuable in medical diagnostics, where quantum sensors could help detect tumors or neurodegenerative diseases (such as Alzheimer's) by identifying tiny magnetic signatures from individual molecules or cells in the body. In the environment, these sensors could detect subtle changes in magnetic fields that could help monitor pollution or natural resource deposits.
Read the full paper in Science Advances.
Harkins et al., Sci. Adv. 11, eadn9021 (2025), DOI: 10.1126/sciadv.adn9021
The study is the product of an international collaboration led by Ashok Ajoy at UC Berkeley/LBL and Marin Bukov at Max Planck Institute (MPI) for the Physics of Complex Systems in Dresden. Lead authors of the study are UC Berkeley graduate students Kieren Harkins and Noella D'Souza, and postdoctoral researcher David Marchiori; Paul Schindler from MPI Dresden; and Christoph Fleckenstein from the Karlsruhe Institute of Technology in Germany. The research was supported by the Air Force Office of Scientific Research (AFOSR), the Office of Naval Research (ONR), the U.S. Department of Energy (DOE), the CIFAR Azrieli Global Scholars Program, and the European Research Council (ERC).