Physicists at the University of California, Santa Barbara, have developed entangled spin systems in diamond that exceed classical sensing limits using quantum squeezing. This breakthrough, led by Ania Jayich and featuring work by Lillian Hughes, enables more powerful and compact quantum sensors for real-world applications. The achievement is detailed in three recent scientific papers.
The research at UC Santa Barbara focuses on laboratory-grown diamond as a key material for quantum technologies. Ania Jayich, the Bruker Endowed Chair in Science and Engineering and co-director of the NSF Quantum Foundry, leads a team studying atomic-scale imperfections known as spin qubits in diamond for advanced quantum sensing.
Lillian Hughes, who recently completed her Ph.D. and is moving to Caltech for postdoctoral work, spearheaded a major advancement. Through three co-authored papers—one published in Physical Review X in March 2025 and two in Nature in October 2025—Hughes showed that two-dimensional ensembles of quantum defects can be organized and entangled inside diamond. This marks a milestone for solid-state systems offering a measurable quantum advantage in sensing.
"We can create a configuration of nitrogen-vacancy (NV) center spins in the diamonds with control over their density and dimensionality, such that they are densely packed and depth-confined into a 2D layer," Hughes explained. "And because we can design how the defects are oriented, we can engineer them to exhibit non-zero dipolar interactions."
An NV center features a nitrogen atom replacing a carbon atom and an adjacent vacancy. Jayich noted, "The NV center defect has a few properties, one of which is a degree of freedom called a spin—a fundamentally quantum mechanical concept. In the case of the NV center, the spin is very long lived. These long-lived spin states make NV centers useful for quantum sensing."
Unlike previous experiments with single spins or non-interacting ensembles, this work leverages strongly interacting dense spin ensembles. "What's new here is that... we can actually leverage the collective behavior, which provides an extra quantum advantage, allowing us to use the phenomena of quantum entanglement to get improved signal-to-noise ratios," Jayich said.
Diamond's solid-state nature makes it easier to integrate than gas-phase atomic sensors, which require vacuum chambers and lasers. The team aims to use these sensors to probe electronic properties of materials and biological systems, such as in nuclear magnetic resonance (NMR) for detecting small magnetic fields from atoms.
To surpass the standard quantum limit set by projection noise, the researchers employ spin squeezing, which correlates quantum states to reduce uncertainty. Jayich analogized: "It's as if you were trying to measure something with a meter stick having gradations a centimeter apart... By squeezing—you effectively use quantum mechanical interactions to 'squish' that meter stick, effectively creating finer gradations."
The second Nature paper describes signal amplification to strengthen signals without adding noise. Looking forward, Jayich anticipates demonstrating quantum advantage in practical experiments soon, by enhancing squeezing or forming regular spin arrays to control positions more precisely.