CERN’s ATLAS experiment has measured the quantum entanglement of top quarks, the most massive fundamental particles, at the highest energy yet, 12 orders of magnitudes (one trillion times) higher than standard entanglement experiments.
Quantum entanglement is a property that we do not experience at our level of reality. It is very much in the domain of the smallest players in the universe. When particles are entangled, they can only be understood as part of a single state and not as individuals anymore. So if something affects one, the other will be instantaneously affected, even if it was on the other side of the universe. This might seem a violation of special relativity and the finiteness of the speed of light but it isn’t, although Einstein was very bothered by it and called it “spooky action at a distance.”
But it's not spooky, it's actually a very useful quality that underpins the future of quantum computers and communications, though certainly not at the energy achieved at CERN. Some of the top quarks created in particle collisions form in pairs that are entangled and it is these that the ATLAS experiment has been studying, using data collected between 2015 and 2018.
Top quarks are very weird. They are fundamental particles, like a heavier cousin of the up quark found inside protons and neutrons, the particles that make up all the atoms in existence. They are so heavy that they have the same mass as a caffeine molecule. But you wouldn’t get a kick from the top quark. It is unstable and decays too quickly. It disappears in 5×10−25 seconds.
That time frame is incredibly small. That interval is to one second like one second is to 100 million times the age of the universe. That is so short that it can’t decay into particles made of quarks (the so-called hadrons) and allows physicists to test indirectly many properties of the standard model of particle physics, such as the mass of the Higgs boson.
All of those tests are done by looking at the decay products, the particles that are created in the aftermath of the top-quark pairs coming into existence. The team managed to measure a degree of entanglement that could not be explained if the quarks were not entangled, with a precision that exceeded the golden standard for particle physics.
The results were presented at the ATLAS conference on September 28.