Tensor Networks and Classical Optimization

Though Cliffords and matchgates are both examples of classically simulable circuits, they are considered simulable for different reasons. While the simulability of Clifford conjugated matchgate circuits for single qubit outputs has been briefly considered, the simulability of Clifford and matchgate hybrid circuits has not been generalized up to this point. In this paper we resolve this, studying simulability of marginals as well as Pauli expectation values of Clifford and matchgate hybrid circuits. We describe a hierarchy of Clifford circuits, and find that as we consider more general Cliffords, we lose some amount of simulability of bitstring outputs. Most importantly, we show that the known simulability of Pauli expectation values of Clifford circuits acting on product states can be generalized to Clifford circuits acting after any matchgate circuit. We conclude with some general discussion about the relationship between Cliffords and matchgates, and argue that we can understand stabilizer states as the vacuum states of particular fermion-to-qubit encodings.