Quantum state elimination measurements tell us what states a quantum system does not have. This is different from state discrimination, where one tries to determine what the state of a quantum system is, rather than what it is not. Apart from being of fundamental interest, quantum state elimination may find uses in quantum communication and quantum cryptography. We consider unambiguous quantum state elimination for two or more qubits, where each qubit can be in one of two possible states. Optimal measurements for eliminating one and two states out of four two-qubit states are given. We also prove that if we want to maximise the average number of eliminated overall N-qubit states, then individual measurements on each qubit are optimal.
Crickmore, J., Puthoor, I. V., Ricketti, B., Croke, S., Hillery, M., & Andersson, A. E. E. (2020). Unambiguous quantum state elimination for qubit sequences. Physical Review, 2(1), . https://doi.org/10.1103/PhysRevResearch.2.013256