A new technique reveals the changing shapes of magnetic noise in space and time

Electromagnetic noise is a major problem for communications, prompting wireless carriers to invest heavily in technologies to overcome it. But for a team of scientists exploring the atomic world, measuring small fluctuations in noise could hold the key to discovery.

“Noise is usually seen as a nuisance, but physicists can learn many things by studying noise,” said Natalie De Leon, assistant professor of electrical and computer engineering at Princeton University. “By measuring the noise in a material, they can learn about its composition, its temperature, how electrons flow and interact with each other, and how the windings are arranged to form a magnet. It is generally difficult to measure anything related to how noise changes in space or time.”

Using specially engineered diamonds, a team of researchers at Princeton University and the University of Wisconsin-Madison has developed a technique for measuring the noise in a material by studying correlations, and they can use this information to learn about the spatial structure and time-varying nature of it. the noise. This technique, which relies on tracking small fluctuations in magnetic fields, is a stark improvement over previous methods that averaged many separate measurements.

De Leon is a leader in the manufacture and use of highly controlled diamond structures called nitrogen vacancy (NV) centers. These NV centers are modifications of the lattice of carbon atoms in diamond where the carbon is replaced by a nitrogen atom, and next to it there is an empty space, or void, in the molecular structure. Diamonds with NV centers are one of the few instruments that can measure changes in magnetic fields at the scale and speed needed for crucial experiments in quantum technology and condensed matter physics.

While a single NV center allowed scientists to take detailed readings of magnetic fields, it was only when de Leon’s team came up with a way to harness several NV centers simultaneously that they were able to measure the spatial structure of the noise in the material. This opens the door to understanding the properties of materials with exotic quantum behaviors that have so far been analyzed only theoretically, said de Leon, senior author of a paper describing the technique published online Dec. 22 in the journal Nature. science.

“It’s a fundamentally new technology,” said de Leon. “It was clear from a theoretical perspective that it would be very powerful to be able to do that. The audience that I think is most excited about this work is the condensed matter theorists, and now that there is this whole world of phenomena that they might be able to characterize in a way different.”

One such phenomenon is the quantum spin fluid, a substance that was first explored in theories nearly 50 years ago and has been difficult to describe experimentally. In a quantum spin liquid, electrons are in a state of continuous flow, in contrast to the solid-state stability that characterizes a typical ferromagnetic material when cooled to a certain temperature.

“The thing that’s challenging with quantum spin fluid is that there’s no fixed magnetic order, so you can’t just draw a magnetic field” the way you can with another type of material, De Leon said. “So far, there is no fundamental way to directly measure two-point magnetic field bonds, and what people do instead is try to find complex approximations to this measurement.”

By measuring magnetic fields at multiple points simultaneously with diamond sensors, researchers can discover how electrons move and cycle through space and time in a material. In developing the new method, the team applied calibrated laser pulses to a diamond containing NV centers, then detected two photon spikes from a pair of NV centers — a readout of an electron spinning in each center at the same point in time. Previous techniques would have averaged these measurements, discarding valuable information and making it impossible to distinguish the internal noise of a diamond and its environment from the magnetic field signals generated by a material of interest.

said study co-author Shimon Kolkowitz, associate professor of physics at the University of Wisconsin-Madison. “But when we look at correlations, the correlative one is from the signal we’re applying and the other one isn’t. And we can measure that, something people haven’t been able to measure before.”

Kolkowitz and De Leon met as Ph.D. students at Harvard University, and have been in frequent contact ever since. Their research collaborations arose early in the COVID-19 pandemic, when lab research slowed, de Leon said, but remote collaborations became more attractive as most interactions took place over Zoom.

Jared Ruffney, lead author of the study and a postdoctoral research fellow in De Leon’s group, led the theoretical and experimental work on the new method. De Leon said Kolkowitz and his team’s contributions were crucial to designing the experiments and understanding the data. Also co-authors on the paper include Ahmed Abdallah and Laura Futamura, who conducted summer research with de Leon’s team in 2021 and 2022, respectively, as interns in the IBM and Princeton Undergraduate Quantum Research Program (QURIP), and in which de Leon participated in Founded in 2019.

The article, “Nanoheterodyne Scale with Diamond Quantum Sensors,” was posted online on December 22. science.