def _instruction_to_superop(cls, instruction):
        """Convert a QuantumCircuit or Instruction to a SuperOp."""
        # Convert circuit to an instruction
        if isinstance(instruction, QuantumCircuit):
            instruction = instruction.to_instruction()
        # Initialize an identity superoperator of the correct size
        # of the circuit
        op = SuperOp(np.eye(4 ** instruction.num_qubits))
        op._append_instruction(instruction)
        return op