Researchers have examined protons and uncovered that quarks and gluons, their essential components, undergo quantum entanglement.
Entangled particles are linked, such that an alteration to one promptly results in a change in the other, regardless of the immense distances separating them. Albert Einstein notably rejected the concept as “spooky action at a distance,” but subsequent experiments verified that this strange, locality-defying phenomenon is indeed real.
Physicists have previously detected entanglement among quarks but had never presented proof that they exist in a quantum-connected configuration within protons.
Now, a group of scientists has found entanglement between quarks and gluons inside protons across a distance of one quadrillionth of a meter — permitting the particles to share information within the proton. The researchers published their results on Dec. 2, 2024, in the journal Reports on Progress in Physics.
“For many years, we’ve held a conventional perspective of the proton as an assemblage of quarks and gluons, concentrating on understanding the so-called single-particle characteristics, including how quarks and gluons are arranged within the proton,” remarked study co-author Zhoudunming Tu, a physicist at Brookhaven National Laboratory in Upton, New York, stated in an announcement. “Now that we have evidence confirming quark and gluon entanglement, this understanding has shifted. We are facing a much more intricate, dynamic system.”
‘Spooky action’ at the smallest dimension
Experimental validation of quantum entanglement initially arose in the 1970s, yet numerous facets of the phenomenon are still relatively underexplored — particularly the entangled interactions among quarks. This is largely due to the fact that subatomic particles do not exist independently; they combine into various configurations known as hadrons. For instance, baryons such as protons and neutrons consist of three tightly bound quarks, held together by strong-force-carrying gluons.
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When distinct quarks are extracted from hadrons, the energy applied to remove them renders them unstable, converting them into branching jets of particles in a process termed hadronization. This renders the challenge of analyzing the trillions of particle decay products to reconstruct their original state exceedingly complex.
However, this is exactly what the researchers accomplished. To investigate the inner workings of protons, the scientists exploited data gathered by the Large Hadron Collider (LHC) and Hadron-Electron Ring Accelerator (HERA) particle collider experiments.
Subsequently, they employed a principle from quantum information theory asserting that a system’s entropy (a measure of how many energy states a system can be configured into, often mistakenly referred to as “disorder”) increases with entanglement — leading the distribution of particle sprays to appear more chaotic.
By comparing the particle sprays to estimations of their entropy, the physicists uncovered that the quarks and gluons inside the colliding protons existed in a maximally entangled state, each sharing the maximum possible information.
“Entropy typically correlates with uncertainty about some information, whereas entanglement leads to information ‘sharing’ between the two entangled entities. Therefore, these two can be interconnected within quantum mechanics,” Tu conveyed to Live Science in an email. “We utilize the predicted entropy (with entanglement presumed) to validate against what the data reveals, and we found significant similarity.”
The scientists assert that their findings could enhance understanding of fundamental particles — including how quarks and gluons remain confined within protons. The investigation has also raised additional inquiries regarding how entanglement evolves when protons are situated within atomic nuclei.
“Given that nuclei consist of protons and neutrons, it is logical to inquire how entanglement affects nuclear structure,” Tu stated. “We intend to employ the electron-ion collider (EIC) to explore this. This will occur in around ten years. Before that, certain collision types, referred to as ultra-peripheral collisions in heavy-ion collisions, might also be effective.”