An experiment hints at quantum entanglement inside protons

Protons are complicated. The subatomic particles are themselves composed of smaller particles called quarks and gluons. Now, data from the Large Hadron Collider hint that protons’ constituents don’t behave independently. Instead, they are tethered by quantum links known as entanglement, three physicists report in a paper published April 26 at arXiv.org.

Quantum entanglement has previously been probed on scales much larger than a proton. In experiments, entangled particles seem to instantaneously influence one another, sometimes even when separated by distances as large as thousands of kilometers (SN: 8/5/17, p. 14). Although scientists suspected that entanglement occurs within a proton, signs of that phenomenon hadn’t been experimentally demonstrated inside the particle, which is about a trillionth of a millimeter across.

“The idea is, this is a quantum mechanical particle which, if you look inside it, … it’s itself entangled,” says theoretical physicist Piet Mulders of Vrije Universiteit Amsterdam, who was not involved with the research.

In the new study, the team analyzed collisions of protons, which had been accelerated to high speeds and slammed together at the Large Hadron Collider in Geneva. Using data from the CMS experiment there, the researchers studied the entropy resulting from entanglement within the proton. Entropy is a property that depends on the number of possible states a system can take on, on a microscopic level. An analogy is a deck of cards: A shuffled deck has multiple ways that it could be ordered, whereas an ordered deck has only one, so the scrambled cards have higher entropy.

If entanglement exists within a proton, there will be additional entropy as a result of those linkages. That entropy can be teased out by counting the number of particles produced in each collision. The amount of entropy the researchers found agreed with that expected assuming the quarks and gluons were entangled, the physicists report in their paper, which is now awaiting peer review before publication in a journal.

The indication of entanglement is not yet definitive, says theoretical physicist Stefan Floerchinger of Heidelberg University in Germany, who was not involved in the study. To conclusively confirm entanglement, strict tests are needed to rule out other possible explanations. Instead, he says, the researchers’ study is “more of a door opener,” and could lead to further research that could clarify the internal physics of protons.

One puzzle that future work could tackle is why quarks are always confined within larger particles, and are never seen on their own. That confinement is “the ultimate example of entanglement,” says theoretical physicist Dmitri Kharzeev of Stony Brook University in New York, a coauthor of the study. Quarks “simply cannot exist as isolated states,” he says, and are always connected with their companions.

Although this property of quarks is well-known, there’s no fundamental mathematical explanation for it. Kharzeev hopes that studying quantum entanglement in protons could help explain the conundrum.