New materials science course offering gives senior students research experience with special diamonds used in quantum computing

Pure diamonds might be desirable for an engagement ring, but when it comes to making valuable scientific tools, scientists prefer diamonds with defects. In general, a diamond is made up of a three-dimensional net of carbon atoms called a lattice. When two carbon atoms in the lattice are replaced with a single nitrogen atom and an empty space in the lattice, this new defect is called a nitrogen-vacancy (NV) center.

NV centers can then be used for a variety of purposes, for example, as a measurement device for magnetic fields or the temperature of biological samples or as a qubit, the fundamental building blocks of quantum computers.

Recently, Dr. David Cahill, Grainger Distinguished Chair of Engineering and co-Director of the IBM-Illinois Discovery Partners Institute, has designed two new course modules allowing students to conduct hands-on experience with NV centers. The modules are part of the senior laboratory course of The Grainger College of Engineering Department of Material Science & Engineering (MatSE). As part of their degree requirements, students in MatSE must take four half-semester modules. The two novel modules are offered back-to-back for the first time this spring semester.

“The motivation for these classes was a combination of wanting something new [for MatSE course offerings], a response to growing interest in quantum systems, and my desire to think more deeply about NV centers as a measurement tool.”

First, students in the course must understand what makes NV centers so special. Their unique defect structure is key to their usefulness. When the NV center traps an extra electron, the resulting collection of electrons in the NV center has a quantum property called spin. The spin of a defect governs how it interacts with magnetic fields and light.

Physics tells us that energy comes in many forms — the light we see around us also has a certain amount of energy. An NV center is extremely useful because we can control the energy and spin of the center with light. After an NV center absorbs energy from light, it also releases energy in the form of light — this light varies according to which spin the NV center has. This is great because spin is hard to measure, but light is a lot easier to see (literally)!

Thus, scientists working with NV centers can use the diamonds as measurement devices. They can measure physical properties that change the spin of the NV centers, like magnetic fields, simply by looking at the light released by the diamonds.

“NV centers are most sensitive to magnetic fields,” says Cahill. But they are used as measurement tools in other cases as well; he says: “They can also be used as fluorescent labels in biology (by attaching them to biological samples of interest) and have been studied as single photon emitters.” The use of quantum phenomena to detect various behaviors is called quantum sensing.

The two new course modules are designed to touch on the creation and characterization of NV centers as measurement devices and their implementation — especially to measure magnetic fields and temperature. The first module is called “Quantum Materials,” while the second is called “Quantum States,” although Cahill emphasizes that the latter focuses on materials for quantum sensing in general.

NV centers are used in quantum computers, and the students in the courses use real NV centers to conduct experiments, albeit not for the task of a quantum computation. This is because the students will analyze large collections of spins, called an ensemble, together, instead of the individual spins needed for quantum computers. This technique forms the basis for quantum metrology, which uses quantum systems for measurement.

“The behavior of an ensemble of NV centers is very similar to nuclear magnetic resonance (NMR), that is used in chemical analysis and medical imaging,” says Cahill. “Here, light, in the form of optical photons, creates large collections of identical spins, called a polarization. NMR does the same thing but with a large magnetic field.”

In addition, “the students gain knowledge not only of the quantum systems but also of fundamental aspects of science like noise and error analysis,” says Jie Zhao, a MatSE graduate student who serves as the teaching assistant for the course.

The initial cohort of “Quantum Materials” MSE404, as the course is officially called, is quick to pick up on these same real-world connections. “There’s quite a bit of overlap between the concepts we cover in the class and my research,” says one undergraduate student enrolled. He was motivated by an interest in quantum physics after taking PHYS214. Another student, whose interests span from electronic and magnetic materials to machine learning, came across quantum materials through a conversation with his professor. As he prepares for a research-oriented career in industry, he values the experience that this course provides: “This is the crossroads of material science and electrical engineering.”