Physicists confirm that quantum entanglement persists between top quarks, the heaviest known fundamental particles

An experiment by a group of physicists led by University of Rochester physics professor Regina Demina produced a significant result related to quantum entanglement, an effect that Albert Einstein called “spooky action at a distance.”

Entanglement refers to the coordinated behavior of tiny particles that have interacted with each other but then drifted apart. Measuring properties—such as the position or momentum or spin—of one of the separated pairs of particles immediately changes the results of the other particle, no matter how far the other particle has moved from its twin. In fact, the state of one entangled particle, or qubit, is inseparable from another.

Quantum entanglement has been observed between stable particles such as photons or electrons.

But Demina and her group broke new ground in discovering for the first time the entanglement that persists between unstable top quarks and their antimatter partners at distances greater than can be covered by information carried at the speed of light. Specifically, the researchers observed the spin correlation between the particles.

Thus, the particles demonstrated what Einstein described as “spooky action at a distance.”

A ‘new path’ for quantum exploration

The discovery was reported by the Compact Muon Solenoid (CMS) Collaboration at the European Center for Nuclear Research, or CERN, where the experiment was conducted.

“The confirmation of quantum entanglement between the heaviest fundamental particles, the top quarks, has opened a new way to explore the quantum nature of our world at energies far beyond what is available,” the report said.

CERN, located near Geneva, Switzerland, is the world’s largest particle physics laboratory. Producing top quarks requires the very high energies available at the Large Hadron Collider (LHC), which allows scientists to send high-energy particles spinning around a 17-mile underground orbit at speeds close to the speed of light.

The phenomenon of entanglement has become the foundation of the burgeoning field of quantum information science, which has broad implications in areas such as cryptography and quantum computing.

Top quarks, each as heavy as an atom of gold, can only be produced at accelerators such as the LHC and are therefore unlikely to be used to build a quantum computer. But studies like those conducted by Demina and her group can shed light on how long entanglement persists, whether it is passed on to “daughter” particles or decay products, and what, if anything, ultimately breaks the entanglement.

Theorists believe that the universe was in an entangled state after the initial phase of rapid expansion. The new result observed by Demina and her researchers could help scientists understand what led to the loss of quantum coupling in our world.

Top quarks in long-range quantum relations

Demina made a video for CMS social media channels to explain her group’s result. She used the analogy of an indecisive king of a distant land whom she called “King Top”.

King Top receives word that his country is under attack, so he sends messengers to tell all the people of his country to prepare to defend themselves. But then, Demina explains in the video, he changes his mind and sends messengers to order the people to stand down.

“He keeps flailing like this and no one knows what his decision will be in the next moment,” says Demina.

No one, Demina continues to explain, except for the leader of one village in this kingdom, who is known as “Anti-Top.”

“They know each other’s state of mind at every moment,” says Demina.

Demina’s research group consists of herself, graduate student Alan Herrera, and postdoctoral fellow Otto Hindrichs.

As a graduate student, Demina was on the team that discovered the top quark in 1995. Later, as a faculty member at Rochester, Demina co-led a team of scientists from across the United States that built a tracking device that played a key role in the discovery of the Higgs boson in 2012— an elementary particle that helps explain the origin of matter in the universe.

Rochester researchers have a long history at CERN as part of the CMS Collaboration, which brings together physicists from around the world. Recently, another Rochester team achieved a major milestone in measuring the electroweak mixing angle, a key part of the Standard Model of particle physics that explains how the building blocks of matter interact.