I want to know if a quantum computing model exists where you can prepare two different states on n qubits and interfere (i.e. add) them together. For example, for two qubits, consider the states (|00> + |01>)/√2 and -(|00> + |10>)/√2 (both pretty easy to prepare in most quantum computing models). Adding these two states would give (|01> - |10>)√2. Is there a model where any two states that can be prepared can also be added together?