The Large Hadron Collider (LHC) is best known for smashing particles together at high speed, but now this huge machine has been used for a more delicate experiment – investigating quantum entanglement. The results, demonstrating entanglement between two subatomic particles called quarks, are the first of new efforts to test whether strange quantum phenomena can occur at extremely high energy levels, and could reveal new insights into the fundamental nature of reality.
Entanglement is a strange quantum effect where objects or particles are linked – so that measuring a property of one object reveals that of the other. Researchers have studied entanglement in some fundamental particles, such as electrons and photons, but others are more difficult. In particular, the indivisible building blocks of protons and neutrons, called quarks, are almost always bound up together and so tricky to study by themselves.
Now, at CERN, the particle physics laboratory in Switzerland that hosts the LHC, and his colleagues have measured entanglement between pairs of quarks. “This is directly testing whether quantum mechanics still behaves the same way when you’re at 10 trillion times higher energy than you would be in all other experiments” says Howarth. “It looks like the answer is, mostly, yes.”
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To do this, the team studied top quarks, a variety that is around 175 times heavier than a proton. The researchers produced the top quarks by accelerating two beams of protons to high energies and pointing them so that they come very close to meeting, without actually colliding. The excess energy produces pairs of top quarks as the beams encounter each other, which then rapidly decay into a soup of other high energy particles.
Whether the top quarks are entangled or not affects where their child particles go, so the researchers were able to reconstruct their quantum nature by observing the directions of the particle shrapnel, . “It’s the top quarks that are entangled, and then we are measuring the decay particles that carry the signature of that entanglement,” says Howarth.
This is the first time quantum entanglement has been measured at such high energies, and the team found an unexpected wrinkle: a slight disagreement with the level of entanglement predicted by the standard model of particle physics, our current best understanding of how particles interact. “This is probably highlighting the fact that when we go to these very extreme regions, there’s a little bit of subtlety missing in the theory, but it’s probably not any kind of bizarre new physics,” says Howarth.
“This a particularly interesting new measurement because it’s opening up the LHC to doing a different type of physics that it hasn’t traditionally been associated with,” says at the University of Oxford, who wasn’t involved in the work.
Future experiments that measure entanglement between other fundamental heavy particles, like Higgs, W and Z bosons, could also help probe where the standard model doesn’t work, as could more advanced quantum tests. “We would be surprised to find that quantum theory didn’t work in this area, but nature has thrown plenty of surprises at us in the past,” says Barr. “Until you go away and actually make the measurements, you don’t know what the results are going to be.”
