entanglement of many atoms discovered for the first time – Scientific Inquirer

In physics, Schrödinger’s cat is an allegory for two of the most fascinating effects of quantum mechanics: entanglement and superposition. Researchers from Dresden and Munich have now observed this behavior on a much larger scale than the behavior of the smallest particles. Until now, it was known that materials exhibiting such properties as, for example, magnetism, have so-called domains – islands in which the properties of materials are of the same kind (imagine that they are either black or white, for example). Looking at lithium holmium fluoride (LiHoF₄), physicists have now discovered an entirely new phase transition in which domains exhibit strange quantum-mechanical features that cause their properties to become entangled (being black and white at the same time). “Our quantum cat now has new fur because we discovered a new quantum phase transition in LiHoF₄ the existence of which was not known before,” comments Matthias Vojta, Head of the Department of Theoretical Solid State Physics at TUD.

Phase transitions and entanglement

We can easily observe how the properties of matter change arbitrarily if we look at water: at 100 degrees Celsius it evaporates into a gas, at zero degrees Celsius it freezes into ice. In both cases, these new states of matter are formed as a result of a phase transition when the water molecules rearrange themselves, thus changing the characteristics of the matter. Properties such as magnetism or superconductivity arise from phase transitions of electrons in crystals. For phase transitions at temperatures approaching absolute zero at -273.15 degrees Celsius, quantum mechanical effects such as entanglement come into play and are called quantum phase transitions. “Despite the fact that there are more than 30 years of extensive research on phase transitions in quantum materials, we previously assumed that the phenomenon of entanglement only plays a role at the microscopic scale, where it involves only a few atoms at a time“, explains Christian Pfleiderer, Professor of Correlated Systems Topology at TUM.

Quantum entanglement is one of the strangest phenomena in physics, where entangled quantum particles exist in a general superposition state, allowing normally mutually exclusive properties (such as black and white) to occur simultaneously. Generally, the laws of quantum mechanics apply only to microscopic particles. Research groups from Munich and Dresden have now managed to observe the effects of quantum entanglement on a much larger scale, in thousands of atoms. To do this, they decided to work with the well-known compound LiHoF₄.

Spherical samples allow accurate measurements

At very low temperatures LiHoF₄ acts as a ferromagnet, where all the magnetic moments are spontaneously directed in the same direction. If you then apply a magnetic field exactly perpendicular to the preferred magnetic direction, the magnetic moments will change direction, which is known as fluctuations. The higher the magnetic field strength, the stronger these oscillations become, until, eventually, ferromagnetism disappears completely at the quantum phase transition. This leads to entanglement of adjacent magnetic moments. “If you hold LiHoF₄ sample to a very strong magnet, it suddenly ceases to be spontaneously magnetic. This has been known for 25 years,” Vojta summarizes.

What is new is what happens when the direction of the magnetic field changes. “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 Pfleiderer. Under these conditions, however, it is not individual magnetic moments, but fairly broad magnetic regions, so-called ferromagnetic domains, that experience these quantum phase transitions. Domains form entire islands of magnetic moments directed in one direction. “For our precise measurements, we used spherical samples. This is what allowed us to precisely 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 thesis.

From fundamental physics to applications

“We discovered a completely new type of quantum phase transition, where entanglement occurs on the scale of many thousands of atoms, not just in the microcosm of just 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 regions to become infinitesimally small, i.e., creating a quantum pattern that dissolves completely.” The newly developed theoretical model successfully explains the data obtained from the experiments. “For our analysis, we generalized existing microscopic models and also took into account the feedback of large ferromagnetic domains with microscopic properties,” explains Heike Eisenlohr, who performed the calculations as part of her doctoral thesis.

The discovery of new quantum phase transitions is important as a basis and a common frame of reference for the investigation of quantum phenomena in materials, as well as for new applications. “Quantum entanglement is applied and used in technologies such as quantum sensors and quantum computers, among others,” says Vojta. Pfleiderer adds: “Our work is in the area of ​​basic research, which, however, can have a direct impact on the development of practical applications if you use the properties of materials in a controlled way.”

IMAGE CREDIT: C. Hohmann, MCQST