Many of NPQC’s achievements thus far focus on quantum platforms that are based on specific flaws in a material’s structure called spin defects. A spin defect in the right crystal background can approach perfect quantum coherence, while possessing greatly improved robustness and functionality.
These imperfections can be used to make high-precision sensing platforms. Each spin defect responds to extremely subtle fluctuations in the environment; and coherent collections of defects can achieve unprecedented accuracy and precision. But understanding how coherence evolves in a system of many spins, where all the spins interact with one another, is daunting. To meet this challenge, NPQC researchers are turning to a common material that turns out to be ideal for quantum sensing: diamond.
Diamond Nitrogen Vacancy
During diamond’s formation, replacement of a carbon atom (green) with a nitrogen atom (yellow, N) and omitting another to leave a vacancy (purple, V) creates a common defect that has well-defined spin properties. Credit: NIST
In nature, each carbon atom in a diamond’s crystal structure connects to four other carbon atoms. When one carbon atom is replaced by a different atom or omitted altogether, which commonly occurs as the diamond’s crystal structure forms, the resulting defect can sometimes behave like an atomic system that has a well-defined spin – an intrinsic form of angular momentum carried by electrons or other subatomic particles. Much like these particles, certain defects in diamond can have an orientation, or polarization, that is either “spin-up” or “spin-down.”
By engineering multiple different spin defects into a diamond lattice, Norman Yao, a faculty scientist at Berkeley Lab and an assistant professor of physics at UC Berkeley, and his colleagues created a 3D system with spins dispersed throughout the volume. Within that system, the researchers developed a way to probe the “motion” of spin polarization at tiny length scales.
Schematic depicting a central pocket of excess spin (turquoise shading) in a diamond cube, which then spread out much like dye in a liquid. Credit: Berkeley Lab
Using a combination of measurement techniques, the researchers found that spin moves around in the quantum mechanical system in almost the same way that dye moves in a liquid. Learning from dyes has turned out to be a successful path toward understanding quantum coherence, as recently published in the journal Nature. Not only does the emergent behavior of spin provide a powerful classical framework for understanding quantum dynamics, but the multi-defect system provides an experimental platform for exploring how coherence works as well. Moore, the NPQC director and a member of the team who has previously studied other kinds of quantum dynamics, described the NPQC platform as “a uniquely controllable example of the interplay between disorder, long-ranged dipolar interactions between spins, and quantum coherence.”
Those spin defects’ coherence times depend heavily on their immediate surroundings. Many NPQC breakthroughs have centered on creating and mapping the strain sensitivity in the structure surrounding individual defects in diamond and other materials. Doing so can reveal how best to engineer defects that have the longest possible coherence times in 3D and 2D materials. But exactly how might the changes imposed by forces on the material itself correlate to changes in the defect’s coherence?