Caltech researchers have detected quantum “Barkhausen noise” for the first time, a phenomenon first observed in magnets by physicist Heinrich Barkhausen in 1919. The team, led by Christopher Simon and including Thomas F. Rosenbaum and Daniel Silevitch, demonstrated that this noise can be produced through quantum mechanical effects, not just traditional means.
This could have implications for creating quantum sensors and other electronic devices. The researchers used a material called lithium holmium yttrium fluoride, cooled to near absolute zero, to observe the quantum effects. The study was funded by the U.S. Department of Energy and the National Sciences and Engineering Research Council of Canada.
Quantum Effects Trigger Magnetic Avalanche
In ferromagnetic materials such as iron screws, atoms with electrons act like tiny magnets. Normally, these magnets’ orientations are aligned within one region of the material but differ from one region to the next. However, when a magnetic field is applied, the orientations of these magnets, or spins, align across the material, causing it to become fully magnetized. This process does not occur simultaneously but spreads across the material in a clumpy manner, often compared to a snow avalanche.
This avalanche effect in magnets was first demonstrated by physicist Heinrich Barkhausen in 1919. He showed that jumps in magnetism could be heard as a crackling sound, now known as Barkhausen noise. Recent research published in the Proceedings of the National Academy of Sciences (PNAS) by Caltech researchers has shown that Barkhausen noise can also be produced through quantum mechanical effects, marking the first experimental detection of quantum Barkhausen noise.
Quantum Barkhausen Noise: A New Discovery
“Barkhausen noise is the collection of the little magnets flipping in groups,” explains Christopher Simon, lead author of the paper and a postdoctoral scholar in the lab of Thomas F. Rosenbaum, a professor of physics at Caltech. “We are doing the same experiment that has been done many times, but we are doing it in a quantum material. We are seeing that the quantum effects can lead to macroscopic changes.”
Typically, these magnetic flips occur classically, through thermal activation, where the particles need to temporarily gain enough energy to jump over an energy barrier. However, the new study shows that these flips can also occur quantum mechanically through a process called quantum tunneling.
Quantum Tunneling and Co-Tunneling Effects
Quantum tunneling allows particles to jump to the other side of an energy barrier without having to pass over the barrier. “In the quantum world, the ball doesn’t have to go over a hill because the ball, or rather the particle, is actually a wave, and some of it is already on the other side of the hill,” says Simon.
The research also reveals a co-tunneling effect, where groups of tunneling electrons communicate with each other to drive the electron spins to flip in the same direction. “Classically, each one of the mini avalanches, where groups of spins flip, would happen on its own,” says co-author Daniel Silevitch, research professor of physics at Caltech. “But we found that through quantum tunneling, two avalanches happen in sync with each other. This co-tunneling effect was a surprise.”
Experimental Observations and Implications
For their experiments, the team used a pink crystalline material called lithium holmium yttrium fluoride cooled to temperatures near absolute zero. They wrapped a coil around it, applied a magnetic field, and then measured brief jumps in voltage, similar to Barkhausen’s 1919 experiment. The observed voltage spikes indicate when groups of electron spins flip their magnetic orientations, creating the Barkhausen noise.
By analyzing this noise, the researchers were able to show that a magnetic avalanche was taking place even without the presence of classical effects. Specifically, they showed that these effects were insensitive to changes in the temperature of the material. This and other analytical steps led them to conclude that quantum effects were responsible for the sweeping changes.
“We are seeing this quantum behavior in materials with up to trillions of spins. Ensembles of microscopic objects are all behaving coherently,” Rosenbaum says. “This work represents the focus of our lab: to isolate quantum mechanical effects where we can quantitively understand what is going on.”
The research represents an advance in fundamental physics that could one day have applications in creating quantum sensors and other electronic devices. It also highlights how tiny quantum effects can lead to larger-scale changes, a topic also explored in another recent PNAS paper from Rosenbaum’s lab.
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