A "perfect tensor" is defined here : https://ncatlab.org/nlab/show/perfect+tensor very abstractly as "its tensor/hom-adjuncts V⊗k⟶V⊗n−k for k≤n/2 are isometries". The only example I'm aware of is the one used in the construction of the HaPPY code : https://ncatlab.org/nlab/show/HaPPY+code What are other examples?