51ÁÔÆæÈë¿Ú

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 produced in a Higgs boson decay have correlated spin states, since the Higgs boson has no spin. This means that the system of the two Z bosons is expected to be in a nearly maximally entangled state.

The Z bosons then reveal their spin as they themselves decay a tiny fraction of a second later. Weak bosons such as 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 the Z-boson pair from the electrons and muons that eventually hit the ATLAS detector. By measuring the different angles at which we observe these electrons and muons, we can check the spin properties of the Z-boson pair and see whether they show entanglement as predicted by 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 performed a hypothesis test against a non-entangled (separable) state.

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

This is particularly interesting because it shows that entanglement can be observed even in extremely short-lived, "virtual" particle states.