Quantum entanglement can be measured in solids for the first time

We finally have a way to measure the quantum entanglement of solids, which could lead to advances in both quantum technology and fundamental physics.

When it comes to quantum entanglement – ​​an inextricable link between quantum particles that keeps their behaviors correlated, even when they are extremely far apart – researchers have limited experimental tools. They can determine whether two particles are entangled using, for example, a procedure called Bell’s test and deliberately create entanglement between multiple objects in quantum computers.

But it’s more difficult to determine whether a piece of material is filled with entangled particles. This is particularly important for developing new and better quantum computing and quantum communication devices, which require entanglement.

Allen Scheie of Los Alamos National Laboratory in New Mexico and his colleagues spent more than half a decade developing a technique to do just that – and now it works.

“We have established that it works 100 percent and we are now establishing the procedures to be able to do it with different materials,” says Scheie.

The team’s method involves bombarding a sample of a material with neutrons, which are then collected on a detector. Since the 1950s, researchers have known that analyzing the properties of these neutrons can reveal the arrangement and behavior of quantum particles inside the material. Scheie and his colleagues used them to calculate quantum Fisher information (QFI), a number that indicates the minimum number of quantum particles in the material that must be entangled to have affected neutrons in the way detected.

The researchers tested their method on several magnetic materials, including a well-studied crystal made from potassium, copper and fluorine. University of Missouri team member Pontus Laurell says that in this case the results could be directly compared to a computer simulation of the crystal’s quantum innards to verify the new method. “It was a remarkably close agreement between the experimental and theoretical curves.”

Laurell says other researchers have previously studied QFI and similar numbers as possible “experimental witnesses” for entanglement, but his team is the first to establish a clear, reliable and generally applicable way to measure it. Much of the work has gone into getting the details right, which has now allowed researchers to try all sorts of materials, including those that could eventually be used to build new devices.

Notably, the team’s method works even if a good mathematical model for the material already exists, and it is effective even when the samples are imperfect. “That’s what’s interesting. You can measure Fisher’s quantum information no matter what,” says Scheie. He presented his work at the American Physical Society’s World Physics Summit in Denver, Colorado, on March 17.

In a month, the researchers will take their method to the next level by measuring the QFI of a material as it approaches a phase transition – the quantum equivalent of the point where water turns to ice. Theoretical models often collapse at this point or predict that entanglement will explode, so there is a chance for a true quantum discovery, Scheie explains.