Physicists have long suspected that the building blocks of protons experience quantum entanglement. Now, researchers have the first direct evidence — after using a trick to infer the entropy of subatomic particles.
Scientists have peered inside protons and discovered that quarks and gluons, their fundamental building blocks, undergo quantum entanglement .
Entangled particles are connected to each other so that a change in one instantly causes a change in the other, even if they are separated by vast distances. Albert Einstein famously dismissed the idea as “spooky action at a distance,” but subsequent experiments have proven that the bizarre locality-breaking effect is real.
Physicists have observed entanglement between quarks before , but they’ve never found evidence that they exist in a connected quantum state inside protons.
Now, a team of researchers has discovered entanglement between quarks and gluons inside protons at a distance of one quadrillionth of a meter — allowing the particles to share information through the proton. The researchers published their findings on December 2, 2024, in the journal Reports on Progress in Physics.
“For decades, we have had a traditional view of the proton as a collection of quarks and gluons, and we have focused on understanding the so-called single-particle properties, including how the quarks and gluons are distributed within the proton,” study co-author Zhoudunming Tu, a physicist at Brookhaven National Laboratory in Upton, New York, said in a statement. “Now, with evidence that quarks and gluons are entangled, this picture has changed. We have a much more complicated and dynamic system.”
‘Spooky action’ on the smallest scale
Experimental proof of quantum entanglement first emerged in the 1970s, but many aspects of the phenomenon remain relatively unexplored — including the entangled interactions between quarks. This is largely because subatomic particles do not exist on their own, and instead coalesce into various combinations of particles known as hadrons. For example, baryons, like protons and neutrons, are combinations of three quarks tightly bound by strong-force-carrying gluons.
When individual quarks are ripped out of hadrons, the energy used to extract them renders them unstable, turning them into branching jets of particles in a process called hadronization. This makes the task of sifting through the trillions of particle decay products to reconstruct their original state incredibly difficult.
But that’s exactly what the researchers did. To probe the inner workings of protons, the scientists mined data collected by the Large Hadron Collider (LHC) and Hadron-Electron Ring Accelerator (HERA) experiments.
Then they applied a principle from quantum information science that says a system’s entropy (a measure of how many energy states a system can be organized into, often incorrectly called “disorder”) increases with its entanglement—making the distribution of particle jets appear messier.
By comparing the particle jets with calculations of their entropy, the physicists found that the quarks and gluons inside the colliding protons existed in a state of maximum entanglement, each sharing as much information as possible.
“Entropy is usually associated with uncertainty about some information, while entanglement leads to the ‘sharing’ of information between the two entangled parties,” the researchers wrote. So these two can be related to each other in quantum mechanics,” Tu told Live Science in an email. “We used the predicted entropy (with entanglement assumed) to check what the data says, and we found great agreement.”
The scientists say their discovery could help gain more insights into fundamental particles — how quarks and gluons remain confined inside protons. The research also raised more questions about how entanglement changes when protons are trapped inside atomic nuclei.
“Since nuclei are made of protons and neutrons, it is natural to ask what entanglement would do to the structure of nuclei,” Tu said. “We plan to use the electron-ion collider (EIC) to study this. This will happen in 10 years. Before that, some types of collisions, so-called ultraperipheral collisions in heavy-ion collisions, may also work.”
