Brookhaven Researchers Find Maximal Quantum Entanglement Inside Protons

Researchers from Brookhaven National Laboratory and their collaborators have discovered strong evidence that quarks and gluons inside protons are maximally entangled. This finding suggests that the internal structure of the proton operates as a complex, dynamic system, fundamentally altering the scientific understanding of how these subatomic particles interact.

By applying quantum information science to data from the Hadron-Electron Ring Accelerator (HERA) in Hamburg, the team found that entanglement occurs over distances of less than one quadrillionth of a meter. The study, published in the journal Reports on Progress in Physics (ROPP), validated these findings by comparing experimental collision data with entropy calculations. According to theorist Dmitri Kharzeev of Brookhaven Lab and Stony Brook University, this maximal entanglement emerges from strong interactions that generate a high volume of gluons and quark-antiquark pairs.

A Shift in Perspective

Traditionally, nuclear physics viewed protons primarily as collections of individual quarks and gluons, focusing on the properties of single particles. This new research shifts the focus toward "emergent behavior," where the collective state of the system—specifically its entanglement entropy—determines the outcome of high-energy collisions. This perspective suggests that the overall quantum state of the proton is more critical to the result of a "smashup" than the intermediate complexity of the individual particles involved.

Implications for Nuclear Physics

This discovery provides physicists with a new analytical tool to study the strong-force interactions that keep quarks and gluons confined within the proton. By treating the proton as an entangled system, scientists can better map the forces that govern the most basic building blocks of matter. "For decades, we’ve had a traditional view of the proton as a collection of quarks and gluons," said Zhoudunming (Kong) Tu, a physicist at Brookhaven Lab. "Now, with evidence that quarks and gluons are entangled, this picture has changed. We have a much more complicated, dynamic system."

The Road to the EIC

The findings lay the essential groundwork for a new generation of experiments. Scientists are now looking toward the Electron-Ion Collider (EIC), scheduled for the 2030s, to further investigate these phenomena. Future research at the EIC will focus on how the broader nuclear environment affects proton entanglement and the process of decoherence, where quantum states break down. These upcoming experiments aim to determine if the entanglement observed in isolated protons persists or changes when they are embedded within larger atomic nuclei.