Discover the entanglement of many atoms

Whether magnets or superconductors: materials are known for their diverse properties. However, these properties can change spontaneously under extreme conditions. Researchers from Technische Universität Dresden (TUD) and Technische Universität München (TUM) have discovered an entirely new type of such phase transition. They display the phenomenon of quantum entanglement involving many atoms, which was previously observed only in the domain of a few atoms. The results were recently published in the scientific journal Nature.

In physics, Schrödinger’s cat is an allegory of two of the most impressive effects of quantum mechanics: entanglement and superposition. Researchers in Dresden and Munich have now observed these behaviors on a scale much larger than that of the smallest particles. So far, it is known that materials that exhibit properties such as magnetism, for example, have so-called fields – islands in which the properties of materials are homogeneous either of one type or of a different type (imagine that they are either black where white, for example). Consider lithium holmium fluoride (LiHoF .).4), physicists have now discovered a completely new phase shift, during which the domains surprisingly exhibit quantum mechanical properties, intertwining their properties (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 whose existence was previously unknown,” comments Matthias Voeta, Head of Theoretical Solid State Physics at TUD.

Phase transitions and entanglement

We can easily notice the spontaneously changing properties of the substance if we look at water: at 100 ° C it evaporates into a gas, at 0 ° C it freezes into ice. In both cases, these new states of matter are formed as a result of a phase transition where water molecules rearrange themselves, thus changing the properties of the material. Properties such as magnetism or superconductivity appear as a result of the phase transitions of electrons in crystals. For phase transitions at temperatures approaching absolute zero at −273.15 °C, quantum mechanical effects such as entanglement play a role, referred to as quantum phase transitions. “Although there has been more than 30 years of extensive research devoted to phase transitions in quantum materials, we previously hypothesized that the phenomenon of entanglement plays a role only at the microscopic level, involving only a few atoms at a time.“, Christian Pflederer, Professor of Topology of Interconnected Systems at TUM explains.

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Quantum entanglement is one of the most amazing phenomena in physics, in which entangled quantum particles exist in a common state of superposition that usually allows for mutually exclusive properties (such as black and white) at the same time. As a rule, the laws of quantum mechanics apply only to microscopic particles. Research teams from Munich and Dresden have now succeeded in observing the effects of quantum entanglement on a much larger scale, that is, the effect of thousands of atoms. For this, they chose to work with the well-known compound LiHoF4.

Spherical samples allow accurate measurements

At very low temperature, LiHoF4 It works as a ferromagnet as all magnetic moments automatically point in the same direction. If you then apply a magnetic field exactly perpendicular to the preferred magnetic direction, the magnetic moments will change their direction, which is called fluctuations. The stronger the magnetic field, the stronger these fluctuations become, until the ferromagnetism completely disappears in the quantum phase shift. This leads to the entanglement of adjacent magnetic moments. “If you carry LiHoF4 Sample to a very strong magnet, suddenly it stops being magnetic automatically. It’s been known for 25 years,” summarizes Vojta.

What’s new is what happens when the direction of the magnetic field changes. “We discovered that a quantum phase transition continues to occur, while it was previously thought that even the smallest inclination in the magnetic field would dampen it immediately,” says Pfleiderer. However, under these conditions, it is not individual magnetic moments, but rather extended magnetic regions, called ferromagnetic fields, that undergo these quantum phase transitions. The spheres form whole islands of magnetic moments that point in the same direction. “We used spherical samples for our precise measurements. This allowed us to precisely study the behavior during small changes in the direction of the magnetic field,” adds Andreas Wendel, who conducted the experiments as part of his doctoral thesis.

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From basic physics to applications

“We discovered a completely new type of quantum phase transition where entanglement occurs on the scale of several thousand atoms rather than just a few in the microcosm,” Vojta says. “If you imagine the magnetic fields as a black and white pattern, the new phase transition causes the white or black regions to become very small, that is, they create a quantum pattern, before they completely dissolve. The newly developed theoretical model successfully explains the data obtained from the experiments. Explains Heike Eisenlohr, who performed the calculations as part of his doctoral thesis: “For our analysis, we generalized the existing microscopic models and also took into account the reactions of large magnetic fields to the microscopic properties.”

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

The research was financially supported by the German Federal and State Governments Strategy of Excellence within the Würzburg-Dresden Complexity and Topology in Quantum Matter Excellence Group (ct.qmat) and the Munich Center for Quantum Science and Technology (MCQST) Excellence Group. In addition, the work was supported by the European Research Council (ERC) through the advanced grant ExQuiSid and by the Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Centers (SFB) 1143 and TRR80.

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