In recent years, the nitrogen-vacancy (NV) center in diamond has emerged as a promising magnetic sensor with high spatial resolution and sensitivity under ambient conditions.  The NV center is a stable defect in the diamond lattice consisting of a substitutional nitrogen atom and a vacancy occupying adjacent lattice sites [Fig. 1(a,b)].

Nanoscale magnetic imaging and magnetic resonance spectroscopy, recently demonstrated using NV color centers, are capable of yielding unique insights into chemistry, biology and physical sciences. The sensitivity and resolution of these techniques rely heavily on the NV spin coherence properties, which empirically are much worse for shallow NV centers compared to those deep within bulk diamond.

An understanding of the origin of surface related noise is currently lacking. It is not only critical for improving NV applications in quantum sensing, quantum information processing, and photonics, but is also an outstanding problem relevant to many solid-state quantum systems. Furthermore, overcoming noise at the diamond interface is a significant obstacle to realizing hybrid quantum systems with NV centers, which are expected to play an important role in realistic devices.

For NV centers in bulk diamond, noise sources limiting coherence times have been identified with internal nuclear and electronic spin baths, and interactions with phonons. Although additional noise sources related to the diamond surface, and affecting shallow NVs, have been observed, their origin is not currently well understood. This phenomenon is general and has been observed at various semiconductor interfaces, resulting in the development of several theoretical models, which are still without significant experimental confirmation.

We used shallow implanted NV centers as nanoscale sensors to perform noise spectroscopy of the diamond surface [Fig 1(c,d)]. We performed measurements at varying conditions (surface coating, magnetic field, temperature) in order to characterize the surface-induced noise. The strength and frequency dependence of fluctuations as a function of the NV distance from the surface were investigated with nanometer precision. We directly measured the noise spectrum experienced by shallow NV centers, revealing an unexpected double-Lorentzian structure which indicates contributions from two distinct noise sources [Fig. 1(e)]. We found that the low frequency noise experienced by shallow NVs is consistent with electronic spin impurities on the surface, with a relaxation mechanism consistent with dipolar coupling between the spins. The NVs also experience previously unknown high frequency noise components (attributed to surface-modified phonons), which contribute to both decoherence and relaxation of the NV sensor.

The understanding gained from this work allows decoupling of NV centers from environmental noise, enabling higher sensitivity to be achieved. Moreover, we expect similar noise sources and spectral behavior to be relevant to a wide range of other systems, including quantum dots, superconducting qubits, and phosphorus in silicon architectures.

References

Spectroscopy of surface-induced noise using shallow spins in diamond, Romach et. al., ArXiv: 1404.3879 (2014)