September 25, 2024
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“Spooky action at a distance” observed for the first time with quarks
Physicists report the first observation of quantum entanglement of quarks, the heaviest fundamental particles, in the Large Hadron Collider.
The ATLAS detector, part of the Large Hadron Collider, is located 100 metres underground and measures various properties of high-energy particles.
For the first time, scientists have observed quantum entanglement between quarks — a state in which particles blend together, lose their individuality, and can no longer be described separately. The feat, achieved at the CERN particle physics laboratory near Geneva, Switzerland, could open the door to further exploration of the quantum information of high-energy particles.
While entanglement has been measured for decades in particles such as electrons and photons, it is a delicate phenomenon and is easiest to measure in low-energy or “quiet” environments, such as the cryogenic refrigerators that house quantum computers. Particle collisions, such as those between protons at CERN’s Large Hadron Collider, are relatively noisy and energetic, making measuring entanglement from debris about as difficult as picking out a whisper at a rock concert.
To observe entanglement at the LHC, physicists working with the ATLAS detector analyzed about one million pairs of top quarks and antiquarks, the heaviest known fundamental particles. They found statistically overwhelming evidence of entanglement, which they announced last September and detailed today. Nature. Physicists working on the LHC’s other main detector, CMS, also confirmed their observation of entanglement in a report posted to the preprint server arXiv in June.
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“This is very exciting because it’s the first time we’ve been able to study entanglement at the highest energies we can get at the LHC,” said Julia Negro, a particle physicist at Purdue University in West Lafayette, Indiana, who worked on the CMS analysis.
Scientists had never doubted that the pair of top quarks was entangled—the Standard Model of particle physics, our best current theory of fundamental particles and the forces by which they interact, is built on quantum mechanics, which describes entanglement—but the latest measurement is still valuable, the researchers say.
“We don’t really expect to break quantum mechanics, right?,” says theoretical physicist Juan Aguilar Saavedra of the Madrid Institute of Theoretical Physics. “Just because we get the results we expect, doesn’t stop us from measuring something important.”
Temporary top
During a coffee break a few years ago, experimental physicist Yoav Afik, now at the University of Chicago in Illinois, and condensed matter physicist Juan Muñoz de Nova, now at the Complutense University of Madrid, wondered whether entanglement could be observed at a collider. Their conversation resulted in a paper that paved the way for measuring entanglement using the top quark.
The number of top and anti-top quark pairs produced after proton collisions is 10−25 After a few seconds, it disintegrates into longer-lived particles.
Previous work had shown that during their short lifetimes, top quarks can potentially acquire correlated “spin,” a quantum property similar to angular momentum. Afik and Muñoz de Nova thought they could extend this measurement to show that the top quarks’ spins are truly entangled, rather than just somewhat correlated. They isTo indicate the degree of correlation, is If is less than -1/3, the top quark will be entangled.
One reason Affick and Muñoz de Nova’s proposal ultimately succeeded is the short lifetime of the top quark. “You just can’t do this with lighter quarks,” says James Howarth, an experimental physicist at the University of Glasgow, UK, who worked with Affick and Muñoz de Nova on the ATLAS analysis. Quarks really don’t like being separated, so they can only split apart for a mere 10−24 After a few seconds, they start to mix with each other and form hadrons, like protons and neutrons. But because the top quark decays so quickly, it doesn’t have time to “hadronize” and lose its spin information through mixing, Howarth says. Instead, all of that information “gets transferred to the decay particles,” he adds. This meant that researchers could measure properties of the decay products to work backwards and infer properties, such as the spin, of the parent top quark.
After experimentally measuring the top quark’s spin, the team compared their results with theoretical predictions, but their models for how the top quark is created and decayed did not match the detector’s measurements.
The ATLAS and CMS researchers tackled the uncertainties in different ways: for example, the CMS team found that adding a hypothetical bound top quark and anti-top quark called “toponium” to their analysis led to better agreement between theory and experiment.
In the end, both experiments easily reached the −1/3 entanglement limit, and ATLAS is was -0.537 and CMS was -0.480.
Topping Off
Successfully observing top quark entanglement could improve researchers’ understanding of the physics of the top quark and pave the way for future high-energy entanglement tests. Other particles, such as the Higgs boson, could also be used to perform the Bell test, an even more rigorous probe of entanglement.
Afik says the top quark experiment might change physicists’ thinking. “At first, it was a bit difficult to convince the physicist community that this was worth their time,” he says. After all, entanglement is the basis of quantum mechanics and has been tested many times.
But the fact that entanglement at high energies hasn’t been rigorously studied is reason enough for Afik and other enthusiasts of the phenomenon: “People are starting to realize that we now have the ability to do these tests at hadron colliders and other types of colliders,” Howarth says.
This article is reprinted with permission. First Edition September 18, 2024.
