One of the most intriguing and unexpected phenomena in physics is quantum entanglement—a mysterious connection between objects that can exist even when separated by great distances.
While entanglement is most famously observed in particles like photons at low energies, a groundbreaking experiment at the Large Hadron Collider (LHC) in Geneva has now detected entanglement in pairs of top quarks, the heaviest known particles.
The ATLAS collaboration, the team behind this discovery, recently published these results in Nature.
What is Entanglement?
In everyday life, we consider objects either "separate" or "connected." For example, two balls a kilometer apart are clearly separate, while two balls tied by a string are physically connected. Entanglement, however, defies this logic.
In an entangled state, two objects remain deeply connected, even when there's no physical link between them. Measuring one of the objects instantaneously reveals information about the other, regardless of the distance between them.
This mind-boggling phenomenon has been shown to work with photons, even when they're on opposite sides of a city. In this scenario, the two particles behave as if they are part of a single, unified system, despite having no apparent connection.
This concept may sound familiar to fans of the sci-fi series The Three-Body Problem by Liu Cixin. In the show, aliens use a tiny supercomputer entangled with its twin on their homeworld, allowing them to manipulate and communicate with it across vast distances.
While this is a creative exaggeration, real-world quantum entanglement doesn’t enable faster-than-light communication. According to quantum physics, entanglement doesn’t violate the cosmic speed limit, and so far, experiments confirm this limitation.
The Reality of Entanglement
Although faster-than-light communication remains a dream of science fiction, entanglement is very real. It was first demonstrated in photons in the 1980s through pioneering experiments.
Today, you can even buy devices that generate entangled photon pairs. Scientists and engineers are now harnessing entanglement for revolutionary technologies, like quantum computing.
Since those early experiments, entanglement has been observed in various systems, including atoms, subatomic particles, and even small vibrating objects—though typically at low energies. The latest discovery, however, involves high-energy particles: top quarks.
Enter the Top Quark
Matter is composed of molecules, which in turn consist of atoms. At the atomic level, electrons orbit a dense nucleus of protons and neutrons. This structure was well understood by the early 20th century. By the 1970s, physicists discovered that protons and neutrons are made of even smaller particles called quarks.
There are six types of quarks: the "up" and "down" quarks that form protons and neutrons, and four heavier varieties. Among these, the fifth quark—called the "bottom" or "beauty" quark—was once thought to be incredibly heavy, weighing about four-and-a-half times more than a proton.
However, the sixth and final quark, the "top" quark, dwarfs them all, with a mass 184 times that of a proton, making it slightly heavier than a tungsten atom.
Why the top quark is so massive remains a mystery. Its large mass makes it a prime subject of study at the LHC, as scientists suspect it could hold clues to unexplored physics. Some theorize that the top quark may interact with forces beyond the four fundamental forces we know, or it could be connected to new, undiscovered physics.
In fact, much of the work done at the ATLAS experiment in Geneva—and at institutions like mine—is focused on understanding the top quark's unique properties.
Entanglement and Top Quarks: A New Frontier
So, what does the detection of entanglement in top quarks mean? Does it suggest they are special? Not necessarily. According to quantum mechanics, entanglement is a common phenomenon that can happen to a wide range of systems.
However, entanglement is notoriously delicate, and experiments are often conducted at extremely low temperatures to avoid disturbing the quantum state.
What makes this discovery so exciting is that top quarks are being observed at very high energies, a context where entanglement had never been confirmed before.
The immense mass of the top quark creates a unique opportunity to study entanglement under extreme conditions. In fact, this measurement would not have been feasible with the other, lighter quarks. However, this doesn’t mean top quarks will soon power practical technologies—after all, the LHC is not exactly portable!
Still, entanglement involving top quarks opens up a new avenue of experimentation, allowing physicists to probe the fundamental nature of reality. While we don’t yet know all the implications, this is an exciting step forward in understanding both entanglement and the universe’s heaviest particles.
