Quadratic tensors as a unification of Clifford, Gaussian, and free-fermion physics

Certain families of quantum mechanical models can be described and solved efficiently on a classical computer, including qubit or qudit Clifford circuits and stabilizer codes, free-boson or free-fermion models, and certain rotor and GKP codes. We show that all of these families can be described as instances of the same algebraic structure, namely quadratic functions over abelian groups, or more generally over (super) Hopf algebras. Different kinds of degrees of freedom correspond to different "elementary" abelian groups or Hopf algebras: mathbb{Z}₂ for qubits, mathbb{Z}_d for qudits, mathbb{R} for continuous variables, both mathbb{Z} and mathbb{R}/mathbb{Z} for rotors, and a super Hopf algebra mathcal F for fermionic modes. Objects such as states, operators, superoperators, or projection-operator valued measures, etc, are tensors. For the solvable models above, these tensors are quadratic tensors based on quadratic functions. Quadratic tensors with n degrees of freedom are fully specified by only O\(n²\) coefficients. Tensor networks of quadratic tensors can be contracted efficiently on the level of these coefficients, using an operation reminiscent of the Schur complement. Our formalism naturally includes models with mixed degrees of freedom, such as qudits of different dimensions. We also use quadratic functions to define generalized stabilizer codes and Clifford gates for arbitrary abelian groups. Finally, we give a generalization from quadratic (or 2nd order) to ith order tensors, which are specified by O\(nⁱ\) coefficients but cannot be contracted efficiently in general.