Fig. 1: The density matrices ρA and ρA,Q entering in the definitions (1) and (2) of the entanglement asymmetries.
From: Entanglement asymmetry as a probe of symmetry breaking

In the eigenbasis of the subsystem charge QA, ρA generically displays off-diagonal elements. Under a projective measurement of QA, we get ρA,Q, where the off-diagonal blocks are annihilated. The difference \({{\Delta }}{S}_{A}^{(n)}\) between the entanglement entropies of these matrices is our probe of symmetry breaking.