Few postulates of Quantum mechanics are stated below:

**Postulate 1 (describes the state of quantum mechanical system):** At a given time the state of quantum mechanical system is defined by specifying a ket belonging to state space .

**Postulate 2 (description of physical quantity):** Every measurable quantity A is described by an observable acting in the state space .

**Postulate 3 (Measurement of physical quantity):** The only possible result of a measurement of a physical quantity A is one of the Eigen values of observable .

**Postulate 4 (Principle of spectral decomposition) Statement 1 (Discrete non-degenerate spectrum):** When a physical quantity A is measured on a quantum mechanical system in the normalized state , the probability of obtaining the non degenerate Eigen values of the observable is here and is normalized.

**Postulate 4 Statement 2 (Discrete degenerate spectrum):** When a physical quantity A is measured on a quantum mechanics system in the normalized state , the probability of obtaining the degenerate Eigen value of the observable is

where and is degeneracy of .

**Postulate 5 (Reduction of wave packet):** If the measurement of a physical quantity A on a quantum mechanical system in the state , gives the result , the state immediately after the measurement is given by the normalized projection of onto the Eigen space associated with that is $latex \frac{P_n|\psi>}{\sqrt{<\psi|P_n|\psi>}} $

**Postulate 6 (Time evolution of quantum mechanical state):** The time evolution of state vector is governed by the Schrodinger equation.

here is Hamiltonian of system.