Quantum Materials: Research Team Finds Entanglement of Many Atoms

Brief

Researchers from the Technical Universities of Dresden and Munich announce that they have detected entanglement and superposition on a considerably wider scale—in the hundreds of atoms. Previously, these phenomena were considered to only occur on the tiniest particles.

The discoveries could have an impact on quantum computers and quantum sensing.

"Our work is in the domain of fundamental research, but if you employ the properties of the materials in a regulated way, it may have a direct influence on the development of practical applications." — Christian Pfleiderer, TUM professor and expert in the topology of linked systems.

RESEARCH NEWS RELEASE

Schroedinger's cat is a metaphor in physics for entanglement and superposition, two of the most astounding aspects of quantum mechanics. Now, scientists from Dresden and Munich have seen these phenomena on a scale that is far bigger than that of the tiniest particles. Materials that exhibit characteristics like, for instance, magnetism have traditionally been thought to have "domains"—islands where the characteristics of the material are uniformly either of one sort or another (imagine them being either black or white, for example). The scientists have now identified a brand-new phase transition in lithium holmium fluoride (LiHoF4) during which the domains surprise display quantum mechanical characteristics, causing their properties to become entangled (being black and white at the same time).

According to Matthias Vojta, chair of theoretical solid state physics at TUD, "our quantum cat now has a new fur since we've uncovered a novel quantum phase transition in LiHoF4 which has not before been recognized to exist."

Phase transitions and entanglement

If we look at water, we can clearly see how a substance may spontaneously change its characteristics. At 100 degrees Celsius, water vaporizes into a gas, and at 0 degrees Celsius, it freezes into ice. In both instances, the formation of these new states of matter results from a phase transition in which the water molecules rearrange themselves, altering the properties of the matter. When electrons go through phase transitions in crystals, properties like magnetism or superconductivity appear. Quantum phase transitions are used to describe phase changes that occur at temperatures close to absolute zero, or -273.15 degrees Celsius, where quantum mechanical processes like entanglement are at play.

Despite more than 30 years of intensive research on phase transitions in quantum materials, according to Christian Pfleiderer, professor of the topology of correlated systems at the TUM, "we had previously assumed that the phenomenon of entanglement played a role only on a microscopic scale, where it involves only a few atoms at a time."

One of physics' most astounding phenomena, quantum entanglement permits normally mutually exclusive qualities (like black and white) to appear concurrently by putting the entangled quantum particles in a shared superposition state. The laws of quantum physics often only hold for small particles. Now that the consequences of quantum entanglement have been seen on a much greater scale—that of thousands of atoms—the research teams from Dresden and Munich have succeeded in doing so. They have opted to work with the well-known substance LiHoF4 for this.

Spherical samples enable precision measurements

LiHoF4 behaves as a ferromagnet, where all magnetic moments spontaneously point in the same direction, at very low temperatures. The magnetic moments will then undergo variations if a magnetic field is subsequently applied precisely vertically to the desired magnetic direction. These oscillations intensify with increasing magnetic field strength, eventually making ferromagnetism totally evaporate at a quantum phase transition. This causes magnetic moments in close proximity to become entangled.

"A LiHoF4 sample stops being magnetic spontaneously if you hold it up to an extremely powerful magnet. Vojta claims that this has been known for 25 years.

What occurs when you reverse the magnetic field's polarity is novel.

Pfleiderer says, "We found that the quantum phase transition persists when it was thought that even the slightest tilt of the magnetic field would rapidly inhibit it. However, under these circumstances, these quantum phase transitions occur in large magnetic regions, or so-called ferromagnetic domains, rather than individual magnetic moments. The domains are complete islands of magnetic moments that are oriented in the same way. "For our precise measurements, we employed spherical samples. According to Andreas Wendl, who carried out the tests as part of his Ph.D. dissertation, "it is what allowed us to carefully investigate the behavior upon slight changes in the direction of the magnetic field.

From fundamental physics to applications

According to Vojta, "We have found a whole new class of quantum phase transitions where entanglement occurs on the size of many thousands of atoms rather than simply in the microcosm of only a few." The new phase transition causes either the white or the black portions to become infinitesimally tiny, i.e. create a quantum pattern, before dissolving altogether, if you think of the magnetic domains as a black-and-white pattern. The experimental findings are satisfactorily explained by a recently established theoretical model. Heike Eisenlohr, who carried out the calculations as part of her Ph.D. thesis, explains, "For our research, we expanded previous microscopic models and additionally took into consideration the feedback of the huge ferromagnetic domains to the microscopic features.

Novel applications and a basis for studying quantum processes in materials have both been made possible by the discovery of new quantum phase transitions.

According to Vojta, "Quantum entanglement is utilized and exploited in a variety of technologies, including quantum sensors and quantum computers."

Pfleiderer continues, "Our work is in the domain of fundamental research, but if you employ the properties of the materials in a regulated way, it may have a direct influence on the development of practical applications."

Source: EurekAlert