Quantum entanglement of many atoms observed for the first time

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

One of the fundamental effects of quantum mechanics is the “superposition principle.” Superposition tells us that, unless directly observed, the physical properties of a particle  can’t 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 mildly morbid terms.

Schrödinger encourages us to imagine a cat in a box. Also inside the box is a toxic substance in a flask 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 go off if a Geiger counter measures radioactive decay from a radioactive substance also within the box.

The substance may or may not undergo radioactive decay. It is a probabilistic event. Is the cat dead or alive? The observer doesn’t know because it can’t be seen.

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

When superpositions are extended over multiple particles, their physical states can be linked. This is called “quantum entanglement.” Einstein called it “spooky action at a distance” because, once entangled, a change to one particle will affect the other no matter how much they are separated.

Such entanglement is very hard to observe. It requires cooling microscopic objects down to within a degree of absolute zero – the coldest possible temperature.

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 lots of atoms. The macroscopic properties of the material, for example magnetism, are produced by the microscopic properties and arrangements of the atoms.

These macroscopic properties come about in “domains” – pockets in the material where its qualities are homogeneously of one or a different kind (like the cat being dead or alive).

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

Physicists looking at lithium holmium fluoride (LiHoF₄), discovered a completely new phase transition, where domains surprisingly exhibit quantum mechanical features, resulting in their properties becoming entangled.

Their findings are published in Nature.

“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,” says co-author Matthias Vojta, a physicist at Germany’s Dresden University of Technology.

Magnetism and superconductivity are properties which emerge 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 dedicated to phase transitions in quantum materials, 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,” explains co-author Christian Pfleiderer, from the Munich University of Technology (TUM), also in Germany.

At very low temperatures, LiHoF₄ becomes a ferromagnet meaning all its atomsspontaneously align their poles and cause the whole slab of material to become magnetic.

But, in the presence of a strong enough external magnet, that ferromagnetism disappears completely.  “If you hold up a LiHoF₄ sample to a very strong magnet, it suddenly ceases to be spontaneously magnetic. This has been known for 25 years,” explains Vojta.

But the physicists 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 had previously been believed that even the smallest tilt of the magnetic field would immediately suppress it,” explains Pfleiderer.

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

“We have used spherical samples for our precision measurements. That is what enabled us to precisely study the behaviour upon small changes in the direction of the magnetic field,” adds first author Andreas Wendl from TUM.

“We have discovered an entirely new type of quantum phase transition where entanglement takes place on the scale of many thousands of atoms instead of just in the microcosm of only a few,” explains Vojta.

The team believes this 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 like quantum sensors and quantum computers, amongst other things,” says Vojta.