Surface adsorbates suppress low-frequency noise for shallow nitrogen-vacancy centers

Research Overview

Nitrogen-vacancy (NV) centers in diamond are essential for nanoscale quantum sensing, but their performance is severely limited when they are placed close to the diamond surface. It has long been assumed that surface adsorbates—such as water and hydrocarbons—are the primary culprits behind the decoherence of these shallow NV centers. This study challenges that assumption by directly comparing the spin dynamics of individual NV centers in ambient air versus an ultrahigh vacuum (UHV) environment where adsorbates are removed.

Experimental Approach

The researchers tracked the same individual NV centers as they transitioned between ambient conditions and a UHV environment (maintained at 5e-10 mbar). By performing Hahn echo measurements in both single-quantum (SQ) and double-quantum (DQ) bases, the team was able to disentangle the contributions of electric and magnetic noise. They further used dynamical decoupling sequences to reconstruct the noise spectral density and measured longitudinal relaxation (T1) to probe higher-frequency noise regimes.

Key Findings

Contrary to expectations, the removal of surface adsorbates in UHV resulted in a roughly 4-fold reduction in the Hahn echo coherence time (T2). This effect was reversible; exposing the diamond to air restored the longer coherence times. The analysis revealed that both electric and magnetic noise are enhanced in UHV, indicating that the presence of adsorbates in ambient conditions actually acts to suppress surface-induced noise. While T2 decreased, the DQ T1 measurements showed an increase in UHV, suggesting that the removal of adsorbates suppresses electric field noise specifically in the ~100 MHz frequency regime. These findings demonstrate that the diamond surface environment is complex, and adsorbates play a dual role in shaping the noise landscape.

This work provides a critical correction to the understanding of surface-induced decoherence in quantum sensors. It highlights that simply cleaning a diamond surface is not a universal solution for improving NV center performance. Instead, researchers must account for the specific role of surface charge compensation provided by adsorbates, which is vital for engineering stable and sensitive quantum interfaces for biological and condensed-matter applications.