What postulate of quantum mechanics tells us how to extract information from quantum systems?

The postulate of quantum mechanics that tells us how to extract information from quantum systems is known as the measurement postulate. According to this postulate, when a measurement is performed on a quantum system, the system will collapse into one of its eigenstates, and the outcome of the measurement will correspond to the eigenvalue associated with that eigenstate. In other words, the measurement process allows us to obtain information about the quantum system by observing its properties.