Charge and Optical Stability of Color Centers in Diamond for Quantum Applications

Abstract

Certain color centers in wide-bandgap semiconductors, such as the nitrogen-vacancy (NV) and silicon-vacancy (SiV) centers in diamond, possess charge states that combine bright, spin-selective optical transitions with relatively long spin coherence times. Furthermore, these color centers exist inside a solid-state medium which enables integration in various device geometries for a wide range of promising quantum technologies. Color centers can serve as sub-nm sensors of the electric fields, magnetic fields, strain, temperature, and electron chemical potential. For certain color centers, their electronic structure can be read out through their fluorescence. Within imperfect crystals, these color centers can be indispensable diagnostic tools of the crystal quality. In high-quality crystals they can be used to detect magnetic and electric fields of interest that originate from outside the diamond, or store and process quantum information within quantum spin registers composed of electronic and nuclear spins. Furthermore these color centers are natural single photon emitters, enabling the possibility of on-demand single photon sources for secure quantum communication. Combining these two modalities, quantum processors and single photon emission, enables the creation of delocalized quantum networks. Two of the biggest limitations in practical implementations of these technologies are charge and optical frequency instabilities. Charge instability refers to undesirable ionization processes where color centers gain or lose electrons, losing quantum information and entering optically dark states in the process. Even among color centers with stable charge states there can be optical frequency instabilities, a failure of a color center to reliably emit at a fixed frequency. Both of these effects become exacerbated in realistic device environments.

In Chapter 3, we discuss measurements correlating the charge instabilities and optical frequency instabilities to the diamond's exposure to plasma used to fabricate photonic structures. We then demonstrate that photonic structures can be fabricated without exposing the diamond to plasma, while achieving reasonable color center-cavity coupling. In Chapter 4, we discuss measurements of shallow color centers formed via ion implantation and annealing to investigate the remaining sources of optical frequency instabilities. This correlated study of many color centers isolates the contributions to the frequency instabilities caused by the residual damage from ion implantation, the proximity to charge traps on the diamond surface, and the dynamic charge environment created during optical charge re-pumping. The primary contributions to the frequency instabilities are found to be the proximity to the surface, and the charge re-pumping.

In Chapter 5, we discuss the mechanism that causes the chemical treatment of the diamond surface, specifically oxidation and hydrogenation treatments, to modify the charge state of shallow color centers. We demonstrate that correlated scanning probe microscopy and optical spectroscopy are effective techniques for imaging these chemically distinct surfaces with micron-scale resolution, and the effect they have on shallow color center's charge states. We then develop a novel surface treatment utilizing focused laser light which combines optical spatial resolution and dynamic feedback for precise surface functionalization. We use the correlated scanning probe and optical spectroscopy measurements to identify the physical mechanism underlying this technique, and find that it is laser-assisted oxidation. Finally, in Chapter 6, we discuss the development of a dynamic charge re-pump scheme using above-bandgap excitation that neutralizes rather than charges defects in an effort to minimize optical frequency instabilities. We demonstrate through optical spectroscopy that the neutralization technique works on the two most commonly used optically-active color centers, the nitrogen-vacancy and silicon-vacancy center in diamond. This thesis does not claim to solve charge and optical frequency instabilities which remains a long-standing and challenging goal within the field. However the tools and techniques we develop are significant advancements in our capabilities to stabilize novel color centers, understand their underlying dynamics, and characterize their performance. We also provide insights into the behavior of color centers in wide-bandgap semiconductors where deviations from thermal equilibrium can be exceptionally long-lived. The exotic charge dynamics in realistic crystal environments complicate even basic observations, but future mastery over these exotic effects may one day lead to brand-new technologies based on color centers in wide-bandgap semiconductors like diamond.