On the smallest level, the universe operates in such a bizarre way that even Albert Einstein had a difficult time making sense of it. An example of the strangeness in the quantum realm—one that has no equivalent in the world as we experience it—is the phenomenon of quantum entanglement.
In our classical world, if Iman flips a coin in Indonesia and Olaf flips a coin in Norway, both Iman and Olaf have a 50% chance of landing on “heads” and a 50% chance of landing on “tails.” The result of one coin flip will have no effect on the result of the other.
In the quantum world, that wouldn’t necessarily be true. Two particles can become entangled, which in this case, would mean the coins would be connected by the outcome of their flips. If one landed on heads, the other would land on tails, and vice versa. So if Iman saw her coin land on heads, she would immediately know that Olaf had gotten tails.
This seemed impossible to Einstein. The problem was: For one entangled particle to affect another, even if separated by a great distance, it appeared that information—perhaps the result of a coin flip, or the resolution of a particle’s quantum state—would need to travel between them faster than the speed of light.
But quantum entanglement, which Einstein famously described as “spooky action at a distance,” is now an accepted fact of nature.
Quantum entanglement is the most distinctive signature of quantum mechanics, says Juan R. Muñoz de Nova, a condensed-matter physicist at the Complutense University of Madrid. “It contradicts the intuitions we have on a daily basis,” he says. “That is why entanglement is so intrinsic to quantum mechanics.”
This phenomenon has been observed by researchers around the world, and the 2022 Nobel Prize in physics was awarded to three scientists for experimentally advancing our understanding of it. Scientists have detected quantum entanglement through experiments involving macroscopic diamonds and ultracold gases.
In September 2023, the ATLAS collaboration made another advancement when they unveiled the highest-energy measurement of quantum entanglement ever, using top quarks produced in the Large Hadron Collider at CERN. Interestingly, the measurement turned out a bit differently than expected.
When worlds collide
At the LHC, scientists accelerate protons to relativistic speeds at high energies, bringing them into collision with one another about 40 million times a second. The energy of the collisions can generate new particles, allowing experiments like ATLAS to study their properties.
When particles are created in collisions at the LHC, some of them naturally become entangled. For instance: A characteristic of a particle that can be measured is its spin; two particles produced in the same collision can become entangled so that they have opposite spin.
One of the types of particles that scientists study at the LHC are top quarks, which are heavier than any other known fundamental particle. Due to their large mass, top quarks decay extremely quickly after they are produced. This quick decay prevents a process that quarks typically go through called hadronization.
Hadronization causes quarks to grab other quarks and combine into more stable quark bundles like protons and neutrons. The fact that top quarks can’t hadronize is important, because once a quark hadronizes, it is a lot more challenging to measure its spin.
Instead of hadronizing, top quarks decay, transferring their energy into less massive, more stable particles. They pass their spins on to their decay products, which ATLAS researchers can detect and measure. By reconstructing the spin correlations between top-quark pairs, the scientists can confirm whether they were entangled.
The ATLAS collaboration faced many challenges as they brought this science from theory to experimentation.
The biggest issue was accounting for neutrinos. Top quarks decay into leptons—some leptons, like electrons, are easy for the ATLAS detector to spot. Others, however, are practically invisible. Neutrinos, which can pass through the entire planet without ever interacting, easily escape the ATLAS detector without leaving a trace.
Their presence can be inferred by the amount of input and output energy from the collision. If energy has gone missing, scientists can attribute it to neutrinos. But it’s not a perfect solution, as a decaying top quark pair can produce more than one neutrino, and there’s only one measurement of the collision’s total energy. “We use some clever, quite complicated bits of math, and some experimental hacks to correct for this,” says James Howarth, a physics lecturer at the University of Glasgow and a coordinator of the analysis.
Scientists placed strict requirements on the collisions to carefully reconstruct the motions of top quark pairs. This problem took the most time to solve since reconstruction is crucial for detecting entanglement.
A second issue lay in the detector itself. When measuring entanglement, scientists paid close attention to the way decay products were distributed after a collision. But they needed to do so quickly because imperfections in the detector can affect the experiment. “The detector actually kind of distorts the shape of the distribution that we’re interested in,” says Yoav Afik, a physicist at the University of Chicago and a coordinator of the analysis.
To counteract the distortion, ATLAS collaborators used simulations to model what the collision looked like without the detector’s effects.
A third issue: To measure entanglement experimentally, scientists needed a way to test alternative hypotheses using modeled data. The way the simulations were constructed, testing different scenarios was tricky.
With help from de Nova, who collaborated as a theorist during the analysis, researchers developed a weighted method in the model that allowed them to change the distribution of the decay products. In doing so, they could test varying hypotheses that would ultimately indicate whether entanglement existed in the system.
An unexpected finding
After a grueling three years of testing and analyzing, the ATLAS collaboration came to a rewarding yet puzzling result. “What we found is that the data shows us stronger entanglement than expected by the simulations,” Afik says.
It could be that the physicists have found an intrinsic difference between prediction and reality that points to a misunderstanding in our theory of quantum entanglement. Or, it could be a sign that something needs to be tweaked in the analysis. Howarth reasons that the simulations may not fully describe the region in which the collisions are taking place, so the modeling could be incomplete.
Whether or not the measurement eventually lines up with predictions, it still stands as the highest-energy detection of quantum entanglement and the first measurement of entanglement between a pair of quarks.
“We never observed entanglement in such a high-energy relativistic system as the LHC,” says de Nova, “so this is like opening a whole new world to quantum information.”