Novel quantum entanglement lets researchers spy on atomic nuclei

The finding was made possible using the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory in New York, which is capable of colliding gold ions at near light-speed. It led to the discovery of a new kind of quantum entanglement.

The term quantum entanglement describes an invisible link that connects distant objects; no matter how far away they are in space, they affect each other. That means if two particles are entangled on a quantum level, by measuring the quantum state of one of the particles, you can immediately know the quantum state of the other, wherever it may be. For example, using a coin analogy, if one particle is "heads," scientists instantly discern that the other particle is "tails," no matter where in the universe it is.

Theoretical physicist Albert Einstein once dismissed the phenomenon of quantum entanglement as "spooky action at a distance," but Daniel Brandenburg, co-author of the study and a professor of physics at The Ohio State University, said that learning more about this codependent relationship is fundamental to understanding the mysteries of the world around us.

"Entanglement is one of the defining characteristics that makes quantum mechanics so different from the kind of physics that normally happens around us," he said.

The study of how photons and electrons interact with and affect matter, quantum mechanics is the foundation on which many technologies -- such as quantum computing and quantum chemistry -- are built. Despite these advancements, scientists previously believed that only particles of the same kind were capable of quantum interference: Photons could only interfere with photons and neutrons with neutrons. That is, until now.

This new study, published in the journal Science Advances, describes how a team of researchers -- called the STAR Collaboration -- used the RHIC to uncover a form of quantum entanglement that shows that particles of all different kinds are able to interact with one another, leading to interference in a variety of different patterns.

"We've gotten different kinds of particles to interfere for the first time, even though previously people thought that it wasn't possible in quantum mechanics," said Brandenburg. Using the collider like a large 3D digital camera, researchers used light to track the particles that escaped from the center of the machine once the atoms collided, taking high-resolution, two-dimensional images much like how a PET scan can be used to image and measure changes in the human body.

This method allowed researchers to map the arrangement of gluons -- gluelike particles that act as a binding force for quarks, the particles within the protons and neutrons inside atomic nuclei. These interactions produced a subatomic particle called a pion that, by measuring the velocity and angles at which light struck the collider, researchers were able to essentially use as a microscope to see inside atomic nuclei in a way like never before.

"By playing these quantum mechanical tricks, we can get to a precision which shouldn't be possible otherwise," Brandenburg said. "This precision allowed us to actually see, within an individual gold nucleus, where the protons and the neutrons reside."

This novel result was in part made thanks to a discovery Brandenburg made about two years ago, called the Breit-Wheeler process, which details how light can be turned into matter and antimatter. Building on the physics of this previous discovery, the team was able to view inside the nucleus on a scale of a tenth to a hundredth the size of an individual proton.

"That's mind-blowingly small," said Brandenburg.

The findings could eventually help advance research in several fields, from quantum computing to astrophysics, he said.

Brandenburg, whose interest in nuclear physics originally began in astronomy, notes that because all matter is connected, investigating the inner workings of atomic nuclei could also allow astrophysicists to discern aspects like a star's stability, its size, density, and even how it formed. "By doing this work here on Earth, we're helping to actually understand better the things that are far out in the universe," Brandenburg said.

Going forward, the team hopes to extend its work by mapping the depths of other kinds of quantum objects.

"One of the big questions in our field is how do we understand the properties of this fundamental building block of matter," he said. "With the discovery of this new type of entanglement, we can start to test these ideas for the first time."

This work was supported by the Office of Nuclear Physics within the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, the National Natural Science Foundation of China, and others.