Optimizing Diamond as a Quantum Sensor
Diamond has long been prized for its beauty, and it holds the record as the hardest known natural material. By introducing nitrogen atoms into its crystal lattice, it can also be transformed into a remarkable quantum sensor. The associated crystal defects are known as nitrogen-vacancy (NV) centers, and they imbue such sensors with unprecedented electromagnetic-field sensitivity and excellent spatial resolution [1]. However, experimental platforms designed to exploit these sensors have so far had limited applicability because the sensing speed and resolution are difficult to simultaneously optimize. Now two research teams—one led by Shimon Kolkowitz at the University of California, Berkeley, [2] and the other by Nathalie de Leon at Princeton University [3]—have independently developed a way of manipulating and measuring more than 100 NV centers in parallel (Fig. 1). The approach expands the possibilities for using NV sensors to probe quantum phenomena, enabling measurements of nonlocal properties (such as spatial and temporal correlations) relevant to condensed-matter physics and materials science.
An NV center forms when a nitrogen atom replaces a carbon atom in the diamond crystal lattice and sits adjacent to a carbon-atom vacancy. The nitrogen atom and the vacancy together trap electrons, and the resulting negatively charged NV center has an electronic spin of 1, enabling it to serve as a three-level quantum system [4]. The energies of these levels are highly sensitive to tiny changes in magnetic and electric fields, as well as to temperature and strain. At present, NV-sensing experiments involve either an isolated NV center that can sense changes at high spatial resolution or a group of NV centers that has better sensitivity but lower resolution. Different approaches to using NV centers thus encounter a trade-off between their measurement speed, their spatial resolution, and the level of control they have over individual NV centers [2].
One example technique is called scanning-probe NV microscopy, in which a micrometer-sized diamond cantilever containing a single NV center is used to perform atomic force microscopy [1]. This strategy offers excellent spatial resolution because the resolution is set by the NV-to-sample distance, which is ultimately determined by the depth of the NV center relative to the diamond surface. However, the method is slow, and it is challenging to incorporate multiple NV sensors simultaneously.
To achieve quantum sensing with an ensemble of NV centers, a camera can be used to read out each defect’s spin state. This method provides wide-field imaging functionality to resolve a sample’s microscopic magnetic, electric, and thermal properties [5]. The large number of NV centers introduces an amplification effect that gives rise to higher field sensitivity and measurement speeds compared to a single NV center. Yet this wide-field approach has low spatial resolution—a factor that is fundamentally restricted by the optical diffraction limit.
The quantum-sensing platform developed by Kolkowitz’s and de Leon’s research teams solves this dilemma. It offers excellent spatial resolution and local controllability. But it also enables parallel manipulation and readout of multiple NV centers (Fig. 1), establishing an advanced quantum-sensing protocol for implementing multiplexed measurements.
To develop their approach, Kolkowitz and colleagues used a specialized camera that can detect single photons to read out the charge and spin states of multiple NV centers in parallel. This strategy involves converting spin information into charge information by exploiting the ultrahigh precision and fidelity of measurements using NV charge states. This process significantly boosts the optical contrast, measurement speed, and sensitivity compared with conventional methods [6].
To show the technique’s effectiveness, Kolkowitz’s team performed parallel measurements of 108 NV centers. Specifically, by using microwave pulses to prepare subgroups of NV centers in different states, the researchers made simultaneous measurements of 5778 unique correlation coefficients between the 108 NV centers. The results were in excellent agreement with the researcher’s theoretical predictions. This approach is particularly suitable for the fast screening of NV centers with specific properties, such as desired spin orientations or coherence times. It might also be useful to test for the presence of carbon-13 nuclei with strong coupling to NV centers. These atoms could be useful for quantum memory applications in virtue of their long coherence times. It could also be used to detect classical pairwise correlations between targeted NV spins.
Meanwhile, de Leon and co-workers made parallel measurements of the magnetic resonance, Rabi oscillation, and spin relaxation of about 100 NV centers. To do so, they used low-noise cameras to detect multiple NV centers simultaneously. The team also demonstrated multiplexed low-noise readout of the NV spin states through spin-to-charge conversion. This advance is crucial for improving the sensing capabilities of NV centers, opening pathways for simultaneous measurements of statistical correlations called Pearson correlations between NV centers.
De Leon and colleagues showed that noise correlations driven by a wire carrying phase-random alternating currents could be quantitatively reconstructed using their method. Specifically, the researchers measured correlations between the magnetic-field noise associated with five NV centers that were located at different positions relative to the wire, and these correlations were commensurate with their theoretical calculations. This finding suggests that the approach could be used to characterize magnetic fields and their underlying correlations over large sample areas with nanoscale precision.
These two seminal studies will undoubtedly deliver exciting opportunities at the forefront of research in quantum sensing and metrology, quantum control, and quantum information. The teams’ innovative sensing tools will pave the way for state-of-the-art magnetometry measurements of the correlations between many NV centers, providing insights into the intrinsic length scales and timescales of correlated materials [7].
The presented approach could even be extended to investigate exotic correlations in solid-state materials that display emergent phenomena, such as hydrodynamic-like electron flow in topological materials, correlated topological electronic states in unconventional superconductors, engineered 2D magnetism in twisted or stacked van der Waals superlattices, and frustrated magnetism in quantum spin liquids. The physical principles underpinning the technique are general and can be readily applied to quantum spin defects beyond NV centers, such as divacancy and silicon-vacancy centers in silicon carbide and 2D color centers embedded in van der Waals crystals [8]. Such implementations would provide diverse technical platforms for developing next-generation quantum technology.
References
- F. Casola et al., “Probing condensed matter physics with magnetometry based on nitrogen-vacancy centres in diamond,” Nat. Rev. Mater. 3, 17088 (2018).
- M. Cambria et al., “Scalable parallel measurement of individual nitrogen-vacancy centers,” Phys. Rev. X 15, 031015 (2025).
- K.-H. Cheng et al., “Massively multiplexed nanoscale magnetometry with diamond quantum sensors,” Phys. Rev. X 15, 031014 (2025).
- L. Rondin et al., “Magnetometry with nitrogen-vacancy defects in diamond,” Rep. Prog. Phys. 77, 056503 (2014).
- S. C. Scholten et al., “Widefield quantum microscopy with nitrogen-vacancy centers in diamond: Strengths, limitations, and prospects,” J. App. Phys. 130, 150902 (2021).
- B. J. Shields et al., “Efficient readout of a single spin state in diamond via spin-to-charge conversion,” Phys. Rev. Lett. 114, 136402 (2015).
- J. Rovny et al., “Nanoscale covariance magnetometry with diamond quantum sensors,” Science 378, 1301 (2022).
- A. Gottscholl et al., “Initialization and read-out of intrinsic spin defects in a van der Waals crystal at room temperature,” Nat. Mater. 19, 540 (2020).