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—the inextricable connection between quantum particles that keeps their behavior correlated even when they are extremely far apart—researchers have limited experimental tools. They can determine whether two particles are entangled using a procedure called the Bell test, for example, and deliberately create entanglement between several objects in quantum computers.

But finding out if a piece of material is full of entangled particles is more challenging. This is especially important for the development of new and better devices for quantum computing and quantum communication that 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 found that it works 100 percent, and now we’re putting in place procedures that you have to go through to do it in different materials,” says Scheie.

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

The researchers tested their method on several magnetic materials, including a well-studied crystal made of 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 interior to validate the new method. “It was a remarkably close match between the experimental and theoretical curves.”

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

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

In a month, the researchers will take their method to the next level by measuring the material’s QFI 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’s a chance for a real quantum discovery, Scheie says.

topics:

• materials/
• quantum physics