# quantum measurement destroys information

I start to become less optimistic on quantum computing these days. It is not because of the noise problem that everyone is talking about. I feel that the noise (decoherence) problem may get solved in the future. My concern is another problem about quantum computing –measurement! After you program builds a quantum state, which contains a lot of information, through many sophisticated operations. You want to know some information from the quantum state using a measurement. All you get is a n-bit string (n is # of qubits). The worst is that the measurement destroys the quantum state. Non-cloning theorem also prevents you from copying the quantum state. You do not have a second chance. You have to rebuild the quantum state from scratch.

This really sucks. It is not a technical problem but the way how nature works. With such a problem, how can quantum computing become useful in the future? All I think of doing is to somehow change the quantum state so that it has high probability on one of the eigenstates so that you can find out what you are looking for? But at the end, you have very little information left.

Could some people please comment on the measurement problem? I searched many papers and got an impression that a solution does not seem to exist.