High-Energy Entanglement: Observing Quantum Links Between Top Quarks

High-Energy Entanglement: Observing Quantum Links Between Top Quarks

The Quantum Frontier at 13 TeV

For nearly a century, quantum mechanics has been the undisputed ruler of the microscopic world. Its predictions—superposition, wave-particle duality, and the "spooky action at a distance" known as entanglement—have been verified in countless experiments. However, these experiments have traditionally been confined to the quiet, pristine environments of low-energy laboratories. We typically observe entanglement in fragile photons, cold atoms, or superconducting circuits, carefully shielded from the noise of the outside world.

But in September 2024, the ATLAS and CMS collaborations at CERN announced a breakthrough that shattered this delicate image of quantum phenomena. Published in Nature, their results confirmed the observation of quantum entanglement between top quarks—the heaviest fundamental particles known to science—produced at the Large Hadron Collider (LHC).

This discovery does not merely add another particle to the list of entangled species. It represents a seismic shift in physics. By observing quantum links at energy scales of 13 teraelectronvolts (TeV)—roughly twelve orders of magnitude higher than typical quantum experiments—physicists have proven that the bizarre laws of quantum mechanics hold firm even in the chaotic, infernal conditions of the highest-energy particle collisions ever created by humanity. We have moved from the quiet optical table to the heart of a volcano, and found that the "spooky action" is just as potent there.

The Heavyweight Champion: Why Top Quarks?

To understand why this observation is so significant, one must first understand the protagonist: the top quark.

Quarks are the building blocks of protons and neutrons, but they are notoriously shy. In nature, quarks are subject to "confinement," a rule of the strong nuclear force that forbids them from existing in isolation. As soon as a quark is created, it typically "hadronizes" within distinct fractions of a second, snapping together with other quarks to form composite particles like protons, pions, or kaons. This hadronization process is messy; it scrambles the quantum information carried by the original quark, effectively erasing its spin state before we can measure it.

The top quark is the exception. It is a monster of the subatomic world, with a mass of approximately 173 GeV—about as heavy as an entire gold atom condensed into a single point. Because of this immense mass, the top quark is incredibly unstable. It lives for only about $10^{-25}$ seconds.

This fleeting lifespan is its greatest asset. The top quark decays so quickly that it dies before the strong force has time to grab it and force it into hadronization. It is the only quark that decays as a "bare" quark. When it decays, it transfers its quantum properties—specifically its spin—directly to its decay products (typically a W boson and a bottom quark, which eventually produce leptons like electrons or muons).

By catching these decay products, physicists can look back in time and reconstruct the exact spin state of the top quark at the moment of its death. This unique property makes the top quark the perfect laboratory for high-energy quantum mechanics. It is a probe that allows us to see the raw, naked quantum state of a particle created in a high-energy collision, without the blurring effects of hadronization.

The Experiment: Smashing Protons

The stage for this discovery was the Large Hadron Collider (LHC) at CERN, a 27-kilometer ring buried beneath the border of France and Switzerland. Inside this ring, beams of protons are accelerated to near light speed and smashed together billions of times per second.

When two protons collide at 13 TeV, the energy density is so high that it ripples the quantum vacuum, occasionally spitting out a top quark and its antimatter counterpart, a top antiquark ($t\bar{t}$). These pairs are born from the intense gluon interactions inside the collision. According to the Standard Model of particle physics, when a top-antitop pair is produced at the "threshold"—meaning they have just enough energy to be created and move slowly relative to each other—they should be in a highly correlated quantum state. Specifically, their spins should be entangled.

If the top quark has its spin pointing "up," the antitop quark doesn't just have a random spin; its spin is instantaneously linked to its partner. They form a singlet state, a combined quantum system where the individual identities of the particles are lost in favor of a shared wavefunction.

The Smoking Gun: Measuring the "D"

Detecting this entanglement is far more difficult than creating it. The ATLAS and CMS detectors are colossal machines, standing several stories tall and packed with millions of sensors. They act as giant 3D cameras, tracking the debris of the collisions.

To prove entanglement, the teams looked for specific collision events where the top and antitop quarks decayed into leptons (electrons or muons). The direction of these leptons is the "smoking gun." Because the top quark transfers its spin to the lepton, the angle at which the lepton flies off is directly related to the top quark's spin axis.

The collaborations measured a specific observable known as $D$, a parameter derived from the angular separation between the charged leptons from the top and antitop decays.

In a classical world, where the particles are merely correlated like a pair of gloves (one left, one right) but not entangled, the value of $D$ is bounded.
In a quantum world, where the particles are entangled, the correlations are stronger. The "spooky" link enforces a tighter relationship between the angles of the leptons than classical physics allows.

Mathematically, for the specific phase space investigated (where the top quarks are produced with low relative momentum), a value of $D$ lower than $-1/3$ (in certain units/conventions) indicates entanglement.

The results were unequivocal.

ATLAS measured the observable $D$ to be $-0.537 \pm 0.020$.
CMS performed a similar analysis, measuring $D$ values that also shattered the classical limit.
Both experiments achieved a statistical significance greater than 5 sigma ($5\sigma$).

In particle physics, $5\sigma$ is the gold standard. It means there is less than a 1 in 3.5 million chance that the data is a statistical fluke. It is a discovery.

Diverging Paths: Particle vs. Parton

While both ATLAS and CMS confirmed the same fundamental reality, they approached the analysis with slightly different philosophies, highlighting the rigor of the scientific process.

The ATLAS collaboration chose to report their results at the "particle level." They measured the angles of the detected leptons (electrons and muons) directly after correcting for detector inefficiencies. This approach is incredibly robust because it relies minimally on theoretical models of what happened "under the hood." They essentially said, "Here is what the detector saw, and it proves entanglement."

The CMS collaboration took the analysis a step further to the "parton level." They used complex simulations to unwind the decay process mathematically, effectively reconstructing the properties of the top quarks themselves before they decayed. This allowed them to directly quantify the entanglement of the top quarks, rather than just their decay products. CMS also conducted a dedicated study to rule out "toponium"—a hypothetical bound state where the top and antitop orbit each other. They demonstrated that even if toponium exists, the observed entanglement is real and significant.

Why High Energy Matters

A skeptic might ask: "We already knew entanglement existed. We won the Nobel Prize for it in 2022. Why does it matter if we see it at 13 TeV?"

The answer lies in the universality of physics. Until now, our confirmation of quantum mechanics has been somewhat bifurcated. We had the "low energy" regime of quantum optics and cold atoms, where quantum effects are king. And we had the "high energy" regime of particle colliders, which we often treat with a mix of quantum field theory and relativistic kinematics.

Observing entanglement at the LHC bridges these worlds. It proves that quantum coherence can survive in the most violent environment we can create. It confirms that the "qubits" of the universe are not just delicate photons in a freezer, but also massive, short-lived particles forged in the fires of nuclear interaction.

Furthermore, this measurement opens a new window for New Physics. The Standard Model predicts the exact degree of entanglement we should see. If there were unknown forces acting on the top quark—say, a new heavy boson or a connection to Dark Matter—it could perturb this delicate quantum correlation. The fact that the measured $D$ value aligns with the Standard Model is a triumph, but as precision improves, any deviation could be the first sign of physics beyond our current understanding.

Beyond Entanglement: The Bell Inequality

The observation of entanglement is just the beginning. The physics community is already eyeing the next prize: the violation of Bell's Inequalities at high energy.

Entanglement is a necessary condition for violating Bell's inequalities, but it is not sufficient. Bell's theorem provides a strict mathematical test to distinguish between quantum mechanics and any theory based on "local hidden variables" (the idea that particles have secret, pre-determined states).

While the current ATLAS and CMS results confirm entanglement with $>5\sigma$, definitively proving the violation of Bell's inequalities requires an even stricter measurement strategy. The top quarks must be selected in a very specific kinematic region (often where they are "boosted" or moving fast) to ensure the angles are measured in a way that corresponds to the Bell test.

Theoretical proposals suggest that with the data from the upcoming High-Luminosity LHC (HL-LHC), we will be able to observe Bell violation with top quarks. This would be a historic achievement: a "loophole-free" test of local realism performed with the heaviest particles in the universe, at energies that would have made Einstein's head spin.

Quantum Information Science at the TeV Scale

This discovery has birthed a new subfield: Quantum Information at High Energy Physics (QI in HEP).

Physicists are now viewing the LHC not just as a particle crasher, but as a Quantum Computer. Every time protons collide, they perform a quantum calculation. The input is the proton state; the "gate" is the interaction (QCD); and the output is the entangled top quark pair.

By studying these events, researchers are beginning to apply concepts from quantum computing—like "Quantum Discord," "Quantum Steering," and "Quantum Magic" (a measure of how hard a state is to simulate)—to particle physics. The CMS collaboration has already begun analyzing "magic" in top quark pairs. This convergence of fields could lead to new techniques for analyzing collider data, potentially revealing secrets that traditional kinematic analysis has missed.

Conclusion: A New Era

The observation of quantum entanglement between top quarks is more than just a measurement; it is a statement. It declares that the strange laws of the quantum world are absolute. They do not fade away as we scale up in mass or energy. From the single photon to the massive top quark, the universe is fundamentally connected in ways that defy classical intuition.

As the LHC prepares for its high-luminosity upgrade, we stand on the precipice of a new era. We are no longer just hunting for new particles; we are probing the structure of reality itself, testing the limits of quantum mechanics in the roar of the collider. The top quark, once just a signature of high-energy decay, has become our most valuable witness in the trial of quantum reality.