ATLAS finds strong evidence of entanglement between two Z bosons from a Higgs decay

The bosons themselves never reach the detector. Each one decays first, and ATLAS records the charged leptons left behind ^[2][8]. Every Z boson goes to a pair of electrons or muons, so a single Higgs decay leaves four leptons to reconstruct ^[6][17]. The angles at which those leptons come out are the surviving trace of the parent spins, and it was from those angles that the team inferred the spins and tested them for entanglement ^[8].

The paper is in Physical Review Letters, with Oxford physicists among the authors ^[5]. The phys.org account calls the outcome strong evidence and one of the highest-energy confirmations of entanglement recorded, and it does not give a statistical significance or the number of collisions analysed ^[3][4][18]. That same account describes entanglement as one particle affecting another ^[19]. The analysis produced a correlation between two spins, each of them reconstructed after both bosons had already decayed ^[8].

"Using particle colliders allows us to test quantum mechanics at energies a trillion times higher and over distances smaller than the size of the nucleus. This probes some of the extreme conditions where quantum mechanics might break down, which would have profound consequences for the foundations of science," said Alan Barr, a professor in Oxford's Department of Physics and a co-author of the study ^[10].

Barr was among the first to propose that colliders could be used for this, having worked on the construction of the LHC ^[12]. His ideas fed a 2023 ATLAS analysis that found entanglement between pairs of top quarks, the heaviest known elementary particle ^[13].

"We're used to thinking of entanglement as something delicate, seen in laboratory experiments with single photons. Finding it alive and well among particles as heavy and short-lived as Z bosons, created in some of the most violent collisions we can produce on Earth, shows just how fundamental and robust this quantum effect really is," Barr said ^[11].

Entanglement had already been shown in photons, electrons and trapped ions ^[14], all systems an experimenter prepares and controls. This pair arrived as the decay products of a Higgs boson, itself made by smashing protons together at 99.99% of the speed of light ^[6][7]. At 13 trillion electron volts, the two spins were entangled ^[3][7].