The quest to create useful quantum technologies begins with a deep understanding of the strange laws that govern quantum behavior and how those principles can be applied to real materials. At the University of California, Santa Barbara, physicist Ania Jayich, Bruker Endowed Chair in Science and Engineering, Elings Chair in Quantum Science, and co-director of the NSF Quantum Foundry, leads a lab where the key material is laboratory-grown diamond.

Working at the intersection of quantum physics and materials science, Jayich and her team study how precise atomic-scale imperfections in diamond — known as spin qubits — can be engineered for advanced quantum sensing. Among the group’s standout researchers, Lillian Hughes, who recently completed her Ph.D. and is heading to Caltech for postdoctoral work, made a major breakthrough in this field.

Through three co-authored papers — one in PRX in March and two in Nature in October — Hughes demonstrated for the first time that not just individual qubits but two-dimensional ensembles of many quantum defects can be organized and entangled inside diamond. This achievement marks a milestone toward solid-state systems that deliver a measurable quantum advantage in sensing, opening a new path for the next generation of quantum devices.

Engineering Quantum Defects in Diamond

“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.” This accomplishment formed the basis of the PRX study, “A strongly interacting, two-dimensional, dipolar spin ensemble in (111)-oriented diamond.”

An NV center consists of a nitrogen atom replacing a carbon atom and an adjacent vacancy where a carbon atom is missing. “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,” said Jayich. “These long-lived spin states make NV centers useful for quantum sensing. The spin couples to the magnetic field that we’re trying to sense.”

From MRI to Quantum Sensing

The concept of using spin as a sensor dates back to the development of magnetic resonance imaging (MRI) in the 1970s. Jayich explained that MRI works by controlling the alignment and energy states of protons and detecting the signals they emit as they relax, forming an image of internal structures.

“Previous quantum-sensing experiments conducted in a solid-state system have all made use of single spins or non-interacting spin ensembles,” Jayich said. “What’s new here is that, because Lillian was able to grow and engineer these very strongly interacting dense spin ensembles, 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, providing greater sensitivity and making a better measurement possible.”

Why Diamond Matters for Quantum Sensors

The type of entanglement-assisted sensing demonstrated by Hughes has been shown before, but only in gas-phase atomic systems. “Ideally, for many target applications, your sensor should be easy to integrate and to bring close to the system under study,” Jayich said. “It is much easier to do that with a solid-state material, like diamond, than with gas-phase atomic sensors on which, for instance, GPS is based. Furthermore, atomic sensors require significant auxiliary hardware to confine and control, such as vacuum chambers and numerous lasers, making it hard to bring an atomic sensor within nanometer-scale proximity to a protein, for instance, prohibiting high-spatial-resolution imaging.”

Jayich’s team is especially focused on using diamond-based quantum sensors to study electronic properties of materials. “You can place material targets into nanometer-scale proximity of a diamond surface, thus bringing them really close to sub-surface NV centers,” Jayich explained. “So it’s very easy to integrate this type of diamond quantum sensor with a variety of interesting target systems. That’s a big reason why this platform is so exciting.”

Probing Materials and Biology with Quantum Precision

“A solid-state magnetic sensor of this kind could be very useful for probing, for instance, biological systems,” Jayich said. “Nuclear magnetic resonance [NMR] is based on detecting very small magnetic fields coming from the constituent atoms in, for example, biological systems. Such an approach is also useful if you want to understand new materials, whether electronic materials, superconducting materials, or magnetic materials that could be useful for a variety of applications.”

Overcoming Quantum Noise

Every measurement has a limit set by noise, which restricts precision. A fundamental form of this noise, called quantum projection noise, sets what’s known as the standard quantum limit — the point beyond which unentangled sensors cannot improve. If scientists can engineer specific interactions between sensors, they can surpass this boundary. One way to do this is through spin squeezing, which correlates quantum states to reduce uncertainty.

“It’s as if you were trying to measure something with a meter stick having gradations a centimeter apart; those centimeter-spaced gradations are effectively the amplitude of the noise in your measurement. You would not use such a meter stick to measure the size of an amoeba, which is much smaller than a centimeter,” Jayich said. “By squeezing — silencing the noise — you effectively use quantum mechanical interactions to ‘squish’ that meter stick, effectively creating finer gradations and allowing you to measure smaller things more precisely.”

Amplifying Quantum Signals

The team’s second Nature paper details another strategy for improving measurement: signal amplification. This approach strengthens the signal without increasing noise. In the meter stick analogy, amplifying the signal makes the amoeba appear larger so that even coarse measurement markings can capture it accurately.

Looking ahead, Jayich is confident about applying these principles in real-world systems. “I don’t think the foreseen technical challenges will prevent demonstrating a quantum advantage in a useful sensing experiment in the near future,” she said. “It’s mostly about making the signal amplification stronger or increasing the amount of squeezing. One way to do that is to control the position of the spins in the 2Dxy plane, forming a regular array.”

“There’s a materials challenge here, in that, because we can’t dictate exactly where the spins will incorporate, they incorporate in somewhat random fashion within a plane,” Jayich added. “That’s something we’re working on now, so that eventually we can have a grid of these spins, each placed a specific distance from each other. That would address an outstanding challenge to realizing practical quantum advantage in sensing.”