Quantum entanglement of many atoms observed for the first time

For the first time, researchers have observed the quantum effect called “entanglement” on a large scale involving many atoms.

One of the fundamental effects of quantum mechanics is the “principle of superposition”. Superposition tells us that unless directly observed, the physical properties of a particle cannot be determined. Instead, the particle occupies a “superposition” of all possible states, each with its own associated probability.

The famous thought experiment known as Schrödinger’s cat seeks to explain this phenomenon in slightly morbid terms.

Schrödinger encourages us to imagine a cat in a box. Also inside the box is a poisonous substance in a vial which, if broken, would release the poison killing the cat. Above the flask is a hammer which is electronically connected to a switch which is set to turn off if a Geiger counter measures the radioactive decay of a radioactive substance also inside the tin.

The substance may or may not undergo radioactive decay. It is a probabilistic event. Is the cat dead or alive? The observer does not know because he cannot be seen.

Schrödinger’s thought experiment leads us to conclude that the poor feline is in a “superposition” – it is simultaneously dead and living.

When the superpositions extend over several particles, their physical states can be linked. This is called “quantum entanglement”. Einstein called it “frightening action at a distance” because, once entangled, a change in one particle will affect the other, regardless of their distance.

Such entanglement is very difficult to observe. This requires cooling microscopic objects to absolute zero degrees – the coldest temperature possible.

Read more: Physicists cool particles to less than a billionth of a degree above absolute zero to probe quantum magnetism

Materials are made up of many atoms. The macroscopic properties of material, for example magnetism, are produced by the microscopic properties and arrangements of atoms.

These macroscopic properties show up in “domains” – pockets in the material where its qualities are homogeneous of one type or different type (like the cat being dead or alive).

The transition between two different qualities, due to microscopic changes, is called a “phase transition” – like liquid water freezing at 0°C to become ice or boiling at 100°C to become vapour.

Physicists examining lithium-holmium fluoride (LiHoF4), discovered an entirely new phase transition, where the domains surprisingly exhibit quantum mechanical features, resulting in the entanglement of their properties.

Their findings are published in Nature.

“Our quantum cat now has new fur because we discovered a new quantum phase transition in LiHoF4 whose existence was not previously known,” says co-author Matthias Vojta, a physicist at Dresden University of Technology in Germany.

Magnetism and superconductivity are properties that arise when electrons undergo a phase transition in crystals. But when temperatures approach absolute zero, quantum effects like entanglement come into play.

“Even though there are more than 30 years of extensive research devoted to phase transitions in quantum materials, we had previously assumed that the phenomenon of entanglement only plays a role at the microscopic scale, where it does not involved only a few atoms at a time.“, explains co-author Christian Pfleiderer, of the Munich University of Technology (TUM), also in Germany.

At very low temperature, LiHoF4 becomes a ferromagnetic, which means that all of its atoms spontaneously align their poles and cause the whole slab of material to become magnetic.

But, in the presence of a sufficiently powerful external magnet, this ferromagnetism disappears completely. “If you are holding a LiHoF4 sample to a very powerful magnet, it suddenly ceases to be magnetic spontaneously. It’s been known for 25 years,” says Vojta.

But physicists have discovered something new when the direction of the external magnetic field is changed.

“We discovered that the quantum phase transition continues to occur, whereas it was previously thought that even the smallest tilt of the magnetic field would immediately suppress it,” says Pfleiderer.

Entire ferromagnetic domains undergo quantum phase transitions when the direction of the magnetic field is changed. Entire islands of magnetic moments point in the same direction.

“We used spherical samples for our precision measurements. This has allowed us to precisely study the behavior during small changes in the direction of the magnetic field,” adds first author Andreas Wendl from TUM.

“We discovered a whole new kind of quantum phase transition where entanglement takes place on the scale of many thousands of atoms instead of just a few in the microcosm,” says Vojta.

The team believe the discovery is important for research into quantum phenomena in materials, as well as for new technological applications. “Quantum entanglement is applied and used in technologies such as quantum sensors and quantum computers, among others,” says Vojta.

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