Engineering 101

A First—Entanglement of Many Atoms Discovered

Schroedinger’s cat needs no introduction – it’s either there for you or not. Researchers from Dresden and Munich discovered an entirely new phase transition. The domains exhibit quantum mechanical features, so their properties become entangled (being black and white simultaneously). “Our quantum cat now has a new fur because we’ve discovered a new quantum phase transition in LiHoF4 which has not previously been known to exist,” said Matthias Vojta, Chair of Theoretical Solid State Physics at TUD.

For phase transitions at temperatures approaching the absolute zero at -273.15 degrees Celsius, quantum mechanical effects such as entanglement come into play, and one speaks of quantum phase transitions. The entangled quantum particles exist in a shared superposition state that allows for usually mutually exclusive properties (black and white) to co-occur. However, the laws of quantum mechanics only apply to microscopic particles. The research teams succeeded in observing the effects of quantum entanglement on a much larger scale, that of thousands of atoms. For this, they have chosen to work with the well-known compound LiHoF4.

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 exactly vertically to the preferred magnetic direction, the magnetic moments will change direction or fluctuate. The higher the magnetic field strength, the stronger these fluctuations become until, eventually, the ferromagnetism disappears completely at a quantum phase transition. This leads to the entanglement of neighboring magnetic moments.

What is new is what happens when you change the direction of the magnetic field. “We discovered that the quantum phase transition continues to occur, whereas it had previously been believed that even the smallest tilt of the magnetic field would immediately suppress it,” explains Pfleiderer. Under these conditions, however, it is not individual magnetic moments but rather extensive magnetic areas, so-called ferromagnetic domains, that undergo these quantum phase transitions. The domains constitute entire islands of magnetic moments pointing in the same direction.

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

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