Whether magnets or superconductors, materials are known for their various properties. However, these properties may change spontaneously under extreme conditions. Researchers at the Technical University of Dresden (TUD) and the Technical University of Munich (TUM) have discovered a completely new type of phase transition. They demonstrated quantum entanglement involving many atoms that was previously only observed in the domain of a few atoms.The results were recently published in the scientific journal nature.
New fur for Quantum Cat
In physics, Schrödinger’s cat is an allegory for two of quantum mechanics’ most awe-inspiring effects: entanglement and superposition. Researchers in Dresden and Munich have now observed these behaviors on a larger scale than the smallest particles. So far, materials that exhibit properties such as magnetism are known to have so-called domains-islands, where the material properties are uniformly of one or different types (eg, imagine they are black or white).
Lithium holmium fluoride (LiHoF4), physicists have now discovered an entirely new type of phase transition in which domains unexpectedly behave quantum mechanically, causing their properties to become entangled (black and white at the same time). “Our quantum cat now has new fur as we discover a new quantum phase transition in LiHoF4 said Matthias Vojta, TUD Chair in Theoretical Solid State Physics.
Phase transitions and entanglement
If we look at water, we can easily observe the spontaneously changing properties of matter – at 100 degrees Celsius it evaporates into gas, at 0 degrees Celsius it freezes into ice. In both cases, these new states of matter are the result of phase transitions in which the water molecules rearrange themselves, thereby changing the properties of the matter. Properties such as magnetism or superconductivity arise as electrons undergo phase transitions in crystals. For phase transitions at temperatures close to absolute zero at -273.15 degrees Celsius, quantum mechanical effects such as entanglement and quantum phase transitions come into play.
“Despite more than 30 years of extensive research devoted to phase transitions in quantum materials, we previously thought that the phenomenon of entanglement operates only on a microscopic scale, involving only a few atoms at a time,” explains Christian Pfleiderer, Professor of Associated Systems Topology at TUM.
Quantum entanglement is a state in which entangled quantum particles exist in a shared superposition state that allows properties that normally repel each other (for example, black and white) to occur simultaneously. Usually, the laws of quantum mechanics only apply to microscopic particles. The research team from Munich and Dresden has now successfully observed the effect of quantum entanglement on a larger scale, that of thousands of atoms. For this, they chose to use the well-known compound LiHoF4.
Spherical samples for precise measurements
At very low temperatures, LiHoF4 Acts as a ferromagnet, where all magnetic moments spontaneously point in the same direction. If you then apply a magnetic field that is exactly perpendicular to the preferred magnetic direction, the magnetic moment will change direction, which is called a wave. The higher the magnetic field strength, the stronger these fluctuations become, until eventually the ferromagnetism completely disappears at the quantum phase transition. This leads to entanglement of adjacent magnetic moments. “If you lift a LiHoF4 The sample is placed into a very strong magnet and it suddenly stops being spontaneously magnetic. It’s been 25 years,” Vojta said.
What’s new is what happens when you change the direction of the magnetic field. “We found that the quantum phase transition continues to occur, whereas it was previously thought that even the smallest magnetic field tilt would suppress it immediately,” explains Pfleiderer. Under these conditions, however, it is not a single magnetic moment that undergoes these quantum phase transitions, but rather broad magnetic regions, so-called ferromagnetic domains. The magnetic domains constitute the entire island of magnetic moments pointing in the same direction.
“We use spherical samples for precise measurements. This allows us to precisely study the behavior of small changes in the direction of the magnetic field,” adds Andreas Wendl, who performed the experiments as part of his doctoral thesis.
From basic physics to applications
“We discovered an entirely new type of quantum phase transition, in which entanglement occurs on the scale of thousands of atoms, not just in miniatures of a few atoms,” Vojta explained. “If you imagine the magnetic domains as a black-and-white pattern, the new phase transition causes the white or black regions to become infinitely small, i.e. form a quantum pattern, and then dissolve completely.” A newly developed theoretical model successfully explains what happened from the experiments obtained data.
“For our analysis, we generalized existing microscopic models and took into account the feedback of large ferromagnetic domains on microscopic properties,” says Heike Eisenlohr, who performed the calculations during her Ph.D. paper.
The discovery of new quantum phase transitions has an important foundation and general frame of reference for studying quantum phenomena in materials and for new applications. “Quantum entanglement is applied and used in technologies such as quantum sensors and quantum computers,” Vojta said. Pfleiderer added: “Our work is in the field of fundamental research, but if you use material properties in a controlled way, it has a direct impact on the development of practical applications.”
Speed limits on quantum phenomena extended to macroscopic objects
Andreas Wendl et al., Emergence of mesoscale quantum phase transitions in ferromagnets, nature (2022). DOI: 10.1038/s41586-022-04995-5
Provided by the Technical University of Dresden
Citation: New fur for quantum cats: entanglement of many atoms discovered for the first time (September 2, 2022) Retrieved September 2, 2022 from https://phys.org/news/2022-09-fur-quantum- cat-entanglement-atom.html
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