The first observed interweaving of sets of books

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Editor of the website for technological innovation – 13.09.2022

Large scale quantum mechanics

As a general rule, the laws of quantum mechanics only apply to microscopic particles. It is therefore surprising that German physicists have now been able to observe the effects of quantum entanglement (or quantum entanglement) on a much larger scale, involving not one, but thousands of atoms.

Quantum entanglement is one of the most surprising phenomena in physics, with quantum particles taking on a common superposition state that allows generally mutually exclusive properties (eg black and white) to appear simultaneously – this is what gives all the “power” to qubits, the objects of quantum computers.

The most famous allegory to describe these two quantum effects – entanglement and superposition – is Schroedinger’s cat, which is alive and dead at the same time, until a particle is measured, which causes it to “collapse” into a single state.

Researchers have now observed all of these behaviors occurring on a much larger scale than with individual particles.

Materials that exhibit properties like magnetism, for example, have regions known as domains—islands where the material’s properties are homogenous of one or different kinds (think black or white, for example).

When they looked at lithium chromium fluoride (LiHoF₄), however, physicists have discovered a completely new phase transition, in which the domains, which are large, surprisingly exhibit quantum mechanical characteristics, resulting in their intertwined properties (as if they are simultaneously black and white). “Our quantum cat now has a new coat because we discovered a new quantum phase transition in LiHoF₄ this was not known before,” commented Professor Matthias Vojta from the Technical University of Dresden.

Experimental setup and LiHoF sample₄.
[Imagem: Andreas Wendl et al. – 10.1038/s41586-022-04995-5]

Phase transitions and interlacing

We can easily observe the phase change properties of substances by looking at water: at 100°C it evaporates into a gas, at 0°C it turns into solid ice. In both cases, these states of matter arise as a result of a phase transition in which the water molecules are reorganized, thus changing the properties of matter. Likewise, properties such as magnetism or superconductivity arise as a result of the passage of electrons through phase transitions in crystals.

As we approach absolute zero (-273.15 C), however, quantum-mechanical effects such as entanglement come into play, so physicists speak of quantum phase transitions.

what is happening with the LiHoF₄: Close to absolute zero, it is ferromagnetic, in which all magnetic moments spontaneously point in the same direction. If you apply a magnetic field exactly perpendicular to this preferred magnetic direction, the magnetic moments will change direction, into what is known as “hovering”.

The higher the strength of the magnetic field, the stronger these fluctuations will be, up to the limit where ferromagnetism completely disappears – a quantum phase transition. And this phase transition leads to the intertwining of adjacent magnetic moments. “If you hold a LiHoF sample₄ near a very strong hand, it suddenly spontaneously ceases to be magnetic. This has been known for 25 years,” Vojta describes.

That’s where the innovation the team discovered comes in, the change that happens when you change the direction of the magnetic field. “We found that the quantum phase transition continues to occur, whereas it was previously believed that even the slightest tilt of the magnetic field would immediately suppress it,” explained Christian Pfleiderer, co-author of the study.

Under these conditions, however, it is not individual magnetic moments but large magnetic regions, so-called ferromagnetic domains, that undergo these quantum phase transitions. Domains form entire islands of magnetic moments directed in the same direction.

quantum mechanics acting on a very large number of particles, greatly expanding the debate on the boundary between the quantum world and the classical world.

From fundamental physics to application

The discovery of new quantum phase transitions is important as a foundation and a general frame of reference for the investigation of quantum phenomena in materials, as well as for new applications.

“Quantum entanglement is, among other things, applied and used in technologies such as quantum sensors and quantum computers,” said Vojta.

“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 the material in a controlled way,” added Pfleiderer.

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