Electromagnetic noise poses a significant communication problem, prompting wireless carriers to invest heavily in technologies to overcome it. Despite being a nuisance, it can tell many things by studying noise. By measuring the noise in a material, physicists can learn its composition, temperature, how electrons flow and interact with one another, and how it spins to form magnets. It is generally difficult to measure how the noise changes in space or time.
Scientists at Princeton University and the University of Wisconsin-Madison have created a method to measure noise in a material by studying correlations. They can use this information to learn the spatial structure and time-varying nature of the noise. The method uses specially designed diamonds with nitrogen-vacancy centers. This method, which tracks minute variations in magnetic fields, is a significant advancement over earlier ones that averaged numerous different readings.
The highly controlled diamond structures are called nitrogen-vacancy (NV) centers. These NV centers are modifications to a diamond’s carbon atom lattice when a carbon atom is swapped out for a nitrogen atom, and there is an empty space, or vacancy, next to it in the chemical structure. A diamond with NV centers is one of the few instruments that can record changes in magnetic fields at the scale and speed required for crucial studies in quantum technology and condensed matter physics.
Even while a single NV center made it possible to monitor magnetic fields with great precision, it wasn’t until scientists figured out how to use several NV centers that they could analyze the spatial organization of noise in a material.
Nathalie de Leon, an associate professor of electrical and computer engineering at Princeton University, said, “This opens the door to understanding the properties of materials with bizarre quantum behaviors that until now have been analyzed only theoretically.”
“It’s a fundamentally new technique. It’s clear from a theoretical perspective that it would be very powerful to do this. The audience that I think is most excited about this work is condensed matter theorists; now that there’s this whole world of phenomena, they might be able to characterize differently.”
Quantum spin liquid is one such phenomenon, where electrons are constantly in flux, in contrast to the solid-state stability that characterizes a typical magnetic material when cooled to a specific temperature.
de Leon said, “The challenging thing about a quantum spin liquid is that by definition, there’s no static magnetic ordering, so you can’t just map out a magnetic field” as you would with another type of material. Until now, there’s been no way to measure these two-point magnetic field correlators directly, and people have instead been trying to find complicated proxies for that measurement.”
Scientists may determine how electrons and their spins flow throughout a material’s space and time by measuring magnetic fields simultaneously at several sites with diamond sensors. To create the novel technique, the team exposed a diamond with NV centers to calibrated laser pulses and afterward noticed two spikes in photon counts coming from a pair of NV centers, a readout of the electron spins at each center at the same instant.
Study coauthor Shimon Kolkowitz, an associate professor of physics at the University of Wisconsin-Madison, said, “One of those two spikes is a signal we’re applying, the other is a spike from the local environment, and there’s no way to tell the difference. But when we look at the correlations, the one correlated is from the signal we’re applying, and the other is not. And we can measure that, which people couldn’t measure before.”