Cornell Settled a Six-Year Quantum Debate. Neither Side Won.

Cornell physicists have settled a six-year argument about a quantum material — and in the process, overturned one of the field's most cited papers.

The team, publishing April 22 in Nature, used soundwaves to test what was really happening inside ruthenium trichloride, a material that had been declared promising evidence for Majorana fermions — particles that are their own antiparticle and theoretically useful for building stable quantum bits. Instead of Majoranas, the Cornell researchers found rotating lattice vibrations called chiral phonons, twisted into corkscrew paths by a quantum property called Hall viscosity. It is the first experimental demonstration of Hall viscosity in a quantum material.

"It's not that this is the magic material with Majorana fermions that's going to build a quantum computer," said Brad Ramshaw, associate professor of physics at Cornell who led the work. "But it's also not this story of basically fancy dirt, where the samples have impurities that are bouncing the heat one way instead of another. It's a new intrinsic effect that nobody had ever seen before."

The finding upends a 2018 paper in Nature by researchers in Japan that reported a half-integer quantized thermal Hall effect in ruthenium trichloride — a signature that, if real, would indicate Majorana fermions were carrying heat through the material. Research programs cited it. PhD theses were built on it. Funding followed.

Then other labs could not reproduce it.

"The problem is that, at the end of the day, all you're doing is flowing heat through something and measuring a change in temperature," Ramshaw said. "You don't know what's going on at the microscopic level."

Ramshaw and doctoral student Avi Shragai designed an experiment that could see the microscopic level: ultrasonic measurements tracking how phonons — lattice vibrations that carry heat as a form of soundwave — moved through the material in a magnetic field. They found the phonons did not travel straight. They twisted, like a corkscrew. This acoustic Faraday effect demonstrated the presence of Hall viscosity, which rotates phonon polarizations and deflects their heat currents.

"When we send sound pointing in one direction into the lattice, it moves like a helix and the soundwaves actually rotate their polarization," Ramshaw said. "Soundwaves don't naively couple to magnetic fields, but it turns out there's a very special property of this material, called spin orbit coupling, that lets the sound waves know left from right."

Neither camp in the original debate was right. The 2018 paper claimed Majorana fermions. A competing camp argued magnetic impurities in the sample were bouncing heat sideways. Both mechanisms were wrong. The thermal Hall effect was real — but produced by the lattice itself, via a quantum property that had been predicted theoretically but never measured in a real material.

The reproducibility failures that followed 2018 were not random noise or sample quality disputes. They may have been signal: the chiral phonon mechanism apparently only manifests under specific experimental conditions that other groups were not replicating. A research direction that absorbed six years of effort and funding has been redefined.

The Kitaev spin liquid theoretical framework that predicted Majorana behavior in this material class may need revision, or it may apply to other compounds but not ruthenium trichloride specifically. Either way, programs that cited ruthenium trichloride's Majorana behavior in their scientific rationale will need to account for this result.

For quantum computing more broadly, the implications are narrower than the 2018 excitement suggested. Microsoft's Majorana-based qubit program uses semiconductor-superconductor heterostructures, not ruthenium trichloride. But the material that was supposed to show Majoranas existed in the wild — that they were not just a theoretical abstraction — just had its best evidence reinterpreted.

Hall viscosity had been theorized as a way to detect exotic quantum phases of matter, but nobody had caught it in action before. The acoustic Faraday effect is now a working research tool.

"This technique can now be used to make new discoveries," Ramshaw said. "I mean, essentially what we have here is a very elaborate null result on someone else's bold claim. Going forward, we can use this technique to make bold claims of our own."