CERN Detects Quantum Entanglement in Particles Born From The Higgs Boson

Something spooky has been spotted in CERN's Large Hadron Collider.

In the cascade of debris that sprays out from high-energy collisions in the world's most powerful particle collider, physicists have found strong evidence of a phenomenon Albert Einstein famously called "spooky action at a distance".

It's not the first time quantum entanglement has been detected at the LHC – but this time, it's somewhere new.

Now, scientists have found the first strong evidence of entanglement between a pair of Z bosons produced in the decay of a Higgs boson – their spins so inextricably linked that one cannot be fully described without the other.

"The spins of the two Z bosons are extremely entangled, considerably more so than in the top-antitop case that was measured previously," said physicist Juan Antonio Aguilar-Saavedra of the Institute of Theoretical Physics in Spain, translated from a Spanish-language video.

Quantum entanglement is one of the strangest things two particles can do.

It happens when their properties become so closely linked that they can no longer be described independently of one another, even when separated by vast distances.

When you measure something about one of the particles, you instantly know something about the other. It was this deeply counterintuitive feature of quantum mechanics that prompted Einstein's "spooky" description.

Scientists have been producing and measuring entangled particles such as atoms and photons for decades, more recently hoping to exploit them for quantum communications and quantum computing. 

In 2024, the ATLAS Collaboration reported the first observation of quantum entanglement between pairs of top quarks produced at the LHC. 

Z bosons do things a little differently.

Unlike top quarks, which have two possible spin states, Z bosons have three – a distinction that, in the language of quantum information, makes a Z boson a qutrit, rather than a two-state qubit.

"And that makes another important difference, because this is the first time entanglement has been measured with elementary particles that are qutrits," Aguilar-Saavedra explained.

The Higgs boson – created by smashing protons together at high speeds – is a particularly useful pathway to a pair of entangled Z bosons.

Because the Higgs has no spin of its own, the two Z bosons can't simply emerge with any old combination of spins. Their spins have to fit together in ways that preserve the zero spin of the particle they came from.

There are several ways this can happen – and quantum mechanics allows those possibilities to exist simultaneously, as a shared state between the two Z bosons. It is this shared state that can leave them entangled.

There's a catch, though. A Higgs boson has a mass of around 125 GeV, while a Z boson weighs in at around 91 GeV. There simply isn't enough energy available for the Higgs to produce two ordinary Z bosons at once.

So at least one of them has to be virtual.

Virtual particles are strange even by quantum-physics standards. They emerge fleetingly during collider interactions, but can't be observed as free particles in the way their "real" counterparts can.

That raises an intriguing question: if one of the Z bosons in the pair is virtual, can it still participate in something as fundamentally quantum as entanglement? The new result suggests that it can.

Of course, there's another fly in the collider ointment – a Z boson lasts for only about 3 x 10^-25 seconds before it decays. Not even the "real" Z hangs around long enough for its spin to be measured directly.

But the Z bosons leave clues behind.

In the decays the collaboration studied, each Z produces two charged particles called leptons - either electrons or their heavier cousins, muons. That gives physicists four particles whose paths through the detector they can measure.

And that's the critical point – because the directions those four particles travel contain information about the spins of the Z bosons that produced them.

The researchers could work backward from those directions, like rewinding a tape, to reconstruct the spins of the Z boson decays that birthed them – a process that has a vast difficulty gulf between "said" and "done".

That's partially because the particular chain of decays – a Higgs into two Zs into four leptons – is incredibly rare. Even with years of particle-collision data, the researchers only had about 400 events to work from.

But they were enough.

With the reconstructed spins of their Z bosons, the researchers looked for the signature of entanglement – a pattern that can be identified in the ATLAS data.

The result strongly suggested entanglement.

In their most sensitive test, the team found that the data favored the entangled state over the non-entangled alternative with a statistical significance of 4.7 sigma – tantalizingly close to the 5-sigma threshold particle physicists traditionally demand before declaring a discovery.

While not quite meeting that bar, the result does constitute strong evidence that the two Z bosons really were entangled.

But the experiment also raises another, more intriguing question about the virtual Z itself.

Physicists don't entirely agree on what it means to say a virtual particle "exists". Unlike ordinary particles, virtual particles can't be directly observed; some physicists regard them primarily as mathematical ingredients used to describe interactions.

Related: The Large Hadron Collider May Already Have Detected Hints of Dark Matter

Yet here, a virtual Z appears to behave in a strikingly particle-like way – it carries spin information that can become entangled with another Z.

"One can ask: do virtual particles exist, or are they a construct of our minds that we use to calculate things?" Aguilar-Saavedra said.

He cautioned that the result doesn't settle that philosophical question, but said it could "shed a little light on whether virtual particles are particles or not."

"If it walks like a duck and quacks like a duck, then what we're looking at must be a duck," he said.

"In this case, a virtual duck."

The results have been published in Physical Review Letters.