Physicists have observed quantum entanglement between quarks — the fundamental building blocks of protons and neutrons — for the very first time. This discovery, made at CERN’s Large Hadron Collider (LHC) near Geneva, Switzerland, marks a new frontier in the study of quantum mechanics and opens doors to exploring how quantum information behaves in high-energy environments.

For decades, quantum entanglement has been a well-studied phenomenon. It describes a state where two particles, no matter how far apart, become so deeply linked that their fates are intertwined. For example, two entangled electrons remain connected so that when the spin of one is measured and found to be “up,” the other, even light-years away, will simultaneously have a spin of “down.”

However, until now, entanglement has mainly been observed in low-energy environments — places where quiet conditions make delicate measurements easier. Detecting it in the chaotic aftermath of proton collisions at the LHC, where energy levels soar, is orders of magnitude more challenging.

Cracking the High-Energy Barrier

The experiment at CERN took place on the ATLAS detector. Here physicists sifted through data from roughly one million pairs of top and anti-top quarks These are the heaviest known elementary particles and their antimatter counterparts. These quarks exist only briefly before decaying into other particles, living just 10^-25 seconds (zero point 25 decimals seconds). Despite their ephemeral nature, scientists found clear evidence of quantum entanglement. The results were published in the journal Nature in September.

Giulia Negro, a physicist at Purdue University who worked on a parallel analysis with CERN’s CMS detector, expressed the excitement of the discovery. “It is really interesting because it’s the first time you can study entanglement at the highest possible energies obtained with the LHC,” she told Nature.

While it may not come as a shock to physicists that quarks can be entangled — after all, quantum mechanics, the theory governing the universe’s smallest particles, predicts this — the discovery is very significant.

“You don’t really expect to break quantum mechanics, right?” said Juan Aguilar-Saavedra, a theoretical physicist from Madrid. “Having an expected result must not prevent you from measuring things that are important.”

Why Top Quarks?

The idea to measure entanglement in top quarks emerged from a casual conversation between two physicists, Yoav Afik and Juan Muñoz de Nova, several years ago. They wondered if it was possible to detect quantum entanglement in the LHC’s high-energy environment. Their brainstorming eventually led to a method for measuring the spins of top quarks and determining whether those spins were entangled.

One reason top quarks were ideal for this study is their short lifespan. Unlike lighter quarks, which quickly combine with others to form larger particles like protons and neutrons, top quarks decay so quickly that they retain their spin information. This allowed researchers to work backward from the decay products to infer the properties of the original quarks.

The teams at ATLAS and CMS compared their experimental data with theoretical models, finding that both easily met the mathematical threshold for entanglement.

Implications for the Future

This observation of entangled quarks could reshape how scientists approach high-energy physics. It might even pave the way for more rigorous tests of quantum mechanics. For example, researchers may use the elusive Higgs boson to perform a Bell test — a gold-standard experiment for probing entanglement.

The authors hope this success will change how physicists view the potential of particle colliders. Entanglement, after all, has long been confirmed in low-energy systems. But now that the phenomenon has been proven in the high-energy chaos of the LHC, there’s a whole new realm of quantum phenomena waiting to be explored.

This breakthrough signals an exciting new chapter in the study of quantum entanglement and might just be the beginning of a deeper understanding of the universe’s most fundamental particles.