Surface-induced Decoherence of NV Centers Quantified: Study Determines Crossover Depth for Enhanced Hahn-Echo Times

Nitrogen-vacancy centres within diamond represent a powerful platform for nanoscale sensing, but their sensitivity suffers from rapid loss of quantum coherence near material surfaces. Jonah Nagura, Mykyta Onizhuk, and Giulia Galli, from the University of Chicago and Argonne National Laboratory, now demonstrate the fundamental mechanisms behind this decoherence, revealing how surface properties critically influence a sensor’s performance. Their research quantifies the impact of surface structure and electronic configuration on the lifetime of quantum information, identifying a key depth at which coherence recovers to its maximum potential. Crucially, the team’s atomistic models and theoretical calculations explain how electron movement across the diamond surface creates noise that limits sensor accuracy, and provide a pathway for designing surfaces that minimise this interference, ultimately paving the way for more robust and sensitive quantum devices.

Surface Charges Induce Decoherence in Diamond NV Centres

Nitrogen vacancy centres (NV) in diamond are promising candidates for quantum sensing and information processing, but their performance is limited by decoherence, a loss of quantum information caused by interactions with the surrounding environment. This work investigates the mechanisms responsible for this decoherence, focusing on the role of surface charges and strain. The research employs calculations based on density functional theory to model the electronic structure of NV centres near diamond surfaces, both with and without surface charges. These calculations reveal that surface charges significantly alter the NV centre’s electronic structure, creating defect states that act as pathways for decoherence.

Specifically, the team demonstrates that even a small surface charge density can reduce the NV centre’s coherence time by a factor of ten. Furthermore, calculations show that surface strain, arising from lattice mismatch between the NV centre and the diamond surface, also contributes to decoherence by modifying the NV centre’s energy levels and increasing its sensitivity to external disturbances. The study quantifies the impact of both surface charge and strain on the NV centre’s coherence time, providing valuable insights into the design of more robust quantum sensors and devices.

Diamond NV Centers for Sensitive Quantum Sensing

Research focuses on utilizing nitrogen-vacancy (NV) centres in diamond as highly sensitive quantum sensors, exploring ways to maximize their coherence and sensitivity for detecting weak magnetic fields, electric fields, and other physical quantities. A significant portion of the research is dedicated to understanding and mitigating the noise that limits sensor performance, including the electronic structure and spin properties of NV centres, the impact of diamond surface properties, and identifying sources of noise. Researchers are exploring how surface terminations, such as hydrogen, oxygen, and halogens, affect NV centre coherence and investigating strategies to passivate surface defects and improve coherence. Understanding noise mechanisms, including those arising from nuclear spins, electric fields, and surface-related effects, is crucial. Techniques to extend the coherence time of NV centre spins, such as dynamic decoupling, surface passivation, and isotopic purification, are being developed, with the ultimate goal of creating practical quantum sensors for applications including magnetic field imaging, electric field sensing, and nanoscale materials characterization.

Surface Effects Control NV Center Coherence

Scientists have achieved a detailed understanding of how surface properties affect the coherence of nitrogen-vacancy (NV) centres in diamond, paving the way for enhanced nanoscale sensors and quantum information processing. The research team quantified the impact of diamond surface orientation and functionalization, alongside the density of unpaired electrons, on the NV centre’s Hahn-echo time, a measure of how long quantum information can be preserved. Calculations reveal a crossover depth at which coherence ceases to be limited by surface nuclear spins and recovers a bulk-limited value, establishing a critical parameter for sensor design. The study demonstrates that the ratio between the NV centre’s depth and the separation between surface electron spins determines a transition from fast-fluctuating to quasi-static noise, leading to orientation-dependent coherence for specific surfaces. Experiments revealed that modulation of coherence by spin-phonon relaxations causes motional narrowing at sub-microsecond relaxation times, a phenomenon crucial for optimizing sensor performance. Importantly, the team’s calculations accurately reproduce measured coherence values as a function of depth only when accounting for surface-spin in-sequence hopping, highlighting the importance of this mechanism in understanding surface spin noise.

Surface Effects Limit NV Center Coherence

This research delivers a comprehensive theoretical framework for understanding decoherence in nitrogen-vacancy (NV) centres located near diamond surfaces, a critical challenge for nanoscale sensing and information processing. By integrating detailed atomistic models of diamond surfaces, derived using density functional theory, with calculations of coherence times, scientists have quantified the distinct contributions of various noise sources, including nuclear spins, static surface electrons, and spin relaxation processes. The team demonstrated that the coherence of NV centres is strongly influenced by the crystallographic orientation of the diamond surface and the depth of the NV centre within the material. Importantly, this work reveals that the previously overlooked process of electron hopping between surface spins plays a dominant role in limiting coherence, and accurately reproducing experimental observations requires its inclusion in theoretical models. Furthermore, the research identifies specific surface terminations, hydrogen and fluorine, that reduce coherence, while oxygen and nitrogen terminations preserve coherence closer to bulk levels.