Entanglement discovered in Higgs boson decays

Working together with other collaborators from the ATLAS experiment at CERN, the Oxford ATLAS group has observed quantum entanglement in Higgs boson decays.

The Z boson in the standard model is a spin-1 particle. It has three possible spin states, making it a qutrit in quantum information language. The pair of Z bosons that are sometimes emitted during a Higgs boson decays have to be anti-correlated, since the Higgs boson has no spin. This means that the system of the two Z bosons should be a nearly perfect maximally entangled "Bell state".

The Z bosons then reveal their spin as they themselves decay a tiny fraction of a second alter. Weak bosons like the Z have different interactions with left-handed and right-handed fermions. This means that we are able to reconstruct the spin density matrix of each Z from the electrons and muons that eventually hitting the ATLAS detector. By measuring the different angles at which we observe electrons and muons we can then check the spin properties of the Z boson pair, and see if they really do show entanglement as predicted in the Standard Model.

Using data from the ATLAS experiment at the LHC we used four-lepton events to measure elements of the Z-boson pair spin density matrix. We also checked a hypothesis test against a non-entangled (separable) state.

In the hypothesis test entanglement was observed at 4.7 sigma, providing strong evidence of entanglement in this Z-boson pair system.

This is particularly interesting as it shows that entanglement can be observed even in the most short-lived "virtual" particle states.