# If a qubit is in $|1\rangle$ state, is it possible to prove that it is in $|1\rangle$ state?

Measuring is clearly not the answer here. Is there some trick to prove it is $$|1\rangle$$ state?

• @glS You only have one qubit, not the way it is produced. One shot experiment. Feb 24, 2019 at 17:22
• I mean to say that, If it's measured and the answer is 1, we can't know whether it was in |1> state originally, for sure. Feb 24, 2019 at 18:08

Say, the qubit was in some state $$\alpha |0\rangle + \beta |1\rangle$$. Now, if by any way we are able to prove that $$\alpha =0$$ (or $$\beta =0$$, for that matter), with a single shot experiment, then in a sense we are able to 'know' the quantum state of the qubit and hence we can clone it. This is impossible.