Physicists 'observed' Schrödinger's cat with new 'fur'

In physics, Schrödinger's cat is an allegory for two of the most impressive effects of quantum mechanics:

entanglement and superposition.Researchers from Dresden and Munich have now observed this behavior on a much larger scale than the smallest particles. Until now, it was known that materials with properties such as magnetism have so-called domains - islands in which the properties of the materials are uniform. If these were colors, they would be black and white islands. But by studying lithium holmium fluoride (LiHoF 4), physicists have discovered an entirely new phase transition in which the domains suddenly exhibit quantum mechanical properties, causing their properties to become entangled (remaining both black and white).

“Our “quantum cat” now has a new “fur”,because we have discovered a new quantum phase transition in LiHoF 4 that was not previously known to exist,” says Matthias Vojta, head of the Department of Theoretical Solid State Physics at the Technical University of Dresden.

Properties such as magnetism orsuperconductivity arise as a result of phase transitions of electrons in crystals. For phase transitions at temperatures approaching absolute zero at -273.15°C, quantum mechanical effects such as entanglement and quantum phase transitions come into play.

At very low temperatures, LiHoF 4 actslike a ferromagnet in which all magnetic moments are spontaneously directed in one direction. If a magnetic field is applied exactly vertical to the preferred magnetic direction, the magnetic moments will change direction, known as fluctuations. The higher the magnetic field strength, the stronger these fluctuations become, until, eventually, ferromagnetism disappears completely in a quantum phase transition. This leads to entanglement of adjacent magnetic moments. “If you bring a sample of LiHoF 4 to a very strong magnet, it will suddenly stop being spontaneously magnetic. This has been known for 25 years,” says Vojta.

What's new is what happens when youchange the direction of the magnetic field. “We found that the quantum phase transition continues to occur, whereas previously it was thought that even the slightest tilt of the magnetic field would immediately suppress it,” explains study co-author Christian Pfleiderer, professor of topology of correlated systems at the Technical University of Munich. However, under these conditions, it is not individual magnetic moments that undergo quantum phase transitions, but rather large magnetic regions, the so-called ferromagnetic domains.

“We used spherical samples for ourprecision measurements. This is what allowed us to accurately study the behavior of small changes in the direction of the magnetic field,” adds Andreas Wendl, who conducted the experiments as part of his doctoral dissertation.

“We have discovered a completely new type of quantumphase transitions, in which entanglement occurs on the scale of many thousands of atoms, and not just in the microcosm of a few, explains Vojta. “If you imagine the magnetic domains as a black and white pattern, the new phase transition causes the white or black areas to become infinitesimal, that is, they create a quantum pattern, and not completely dissolve.” A newly developed theoretical model successfully explains the data obtained from the experiments.

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