Summary
Projection Measurement Traditionally, a measurement, in the sense of Von Neumann, is a series of projection operators. By performing spectral decomposition on the self-adjoint operator corresponding to the observable, denoted as $ O = \sum_i \lambda_i |\varphi_i\rangle\langle\varphi_i| $, we obtain these projection operators $ |\varphi_i\rangle\langle\varphi_i| $. This part is well-known to students who have studied elementary quantum mechanics. Besides the Von Neumann measurement, there is a mo...