Quantum entanglement can be measured in solids for the first time

We finally have a way to measure 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 behaviours correlated, even when they are extremely far apart – researchers have limited experimental tools. They can determine if two particles are entangled by using a procedure called the Bell test, for example, and purposely create entanglement between several objects within quantum computers.

But finding out whether a piece of some material is full of entangled particles is more challenging. This is especially important for developing new and better devices for quantum computing and quantum communication, which require entanglement.

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

“We’ve established that it works, 100 per cent, and now we’re establishing the procedures you need to go through to be able to do it in different materials,” says Scheie.

The team’s method involves pelting a sample of a material with neutrons, which are then collected on a detector. Since the 1950s, researchers have known that analysing these neutrons’ properties can reveal the arrangement and behaviour 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 within the material that must be entangled to have affected the neutrons in the detected way.

The researchers tested their method on several magnetic materials, including a well-studied crystal made from potassium, copper and fluorine. Team member Pontus Laurell at the University of Missouri says that in this case, the findings could be directly compared to a computer simulation of the crystal’s quantum innards in order 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 “entanglement witnesses”, but his team is the first to establish a clear, reliable and generally applicable way to measure it. Much of the work has been in getting the details just right, which has now opened the door for the researchers to try all sorts of materials, including those that could eventually be used to build new devices.

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

In a month’s time, 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 becomes ice. Theoretical models often break down at this point or predict that entanglement will go through the roof, so there is a chance for a real quantum discovery, says Scheie.