Spooky Action: Scientists Map Out Quantum Entanglement in Protons

Dmitri Kharzeev

Scientists at the U.S. Department of Energy's Brookhaven National Laboratory (BNL) and collaborators - including Stony Brook University Distinguished Professor of Physics and Astronomy Dmitri Kharzeev - have a new way to use data from high-energy particle smashups to peer inside protons.

Their approach uses quantum information science to map out how particle tracks streaming from electron-proton collisions are influenced by quantum entanglement inside the proton.

The results reveal that quarks and gluons, the fundamental building blocks that make up a proton's structure, are subject to so-called quantum entanglement. This quirky phenomenon, famously described by Albert Einstein as "spooky action at a distance," holds that particles can know one another's state - for example, their spin direction - even when they are separated by a great distance. In this case, entanglement occurs over incredibly short distances - less than one quadrillionth of a meter inside individual protons - and the sharing of information extends over the entire group of quarks and gluons in that proton.

The team's latest paper, just published in the journal Reports on Progress in Physics (ROPP), summarizes the group's six-year research effort. It maps out precisely how entanglement affects the distribution of stable particles that emerge at various angles from the particle smashups after quarks and gluons liberated in the collisions coalesce to form these new composite particles.

Data from past proton-electron collisions provide strong evidence of entanglement among the proton's sea of quarks (spheres) and gluons (squiggles). (Valerie Lentz/Brookhaven National Laboratory)

This new view of entanglement among quarks and gluons adds a layer of complexity to an evolving picture of protons' inner structure. It may also offer insight into other areas of science where entanglement plays a role.

For this study, the scientists used the language and equations of quantum information science to predict how entanglement should impact particles streaming from electron-proton collisions. This approach, published in 2017, was developed by Kharzeev, a theoretical physicist and joint appointee with BNL, and co-author on the paper.

"For a maximally entangled state of quarks and gluons, there is a simple relation that allows us to predict the entropy of particles produced in a high energy collision," Kharzeev said. "In our paper, we tested this relation using experimental data."

"For decades, we've had a traditional view of the proton as a collection of quarks and gluons and we've been focused on understanding so-called single-particle properties, including how quarks and gluons are distributed inside the proton," said physicist Zhoudunming (Kong) Tu, a co-author on the paper and collaborator on this exploration since joining Brookhaven Lab in 2018. "Now, with evidence that quarks and gluons are entangled, this picture has changed. We have a much more complicated, dynamic system."

Read the full story at the BNL website.

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