From the infinitely small to the infinitely big covering over sixty spatial orders of magnitude
quantum theory is used as much to describe the still largely mysterious vibrations of the
microscopic strings that could be the basic constituents to explain the fluctuations of the Radiation reaching us from the depths of outer space.
Serge Haroche

Quantum theory consists of two equal important parts:
Density Probability of the Schrodinger Equation
The Von Neumann Measurement Postulate

The evolution of the wave function described by the Schrödinger equation has two key Features:
It is a reversible process, so one can always undo the process in order to return to the initial state from the final state.
It is a deterministic evolution in which the final state is uniquely determined by the initial state if the Hamiltonian is known.
Note from the Author

Table of Contents
Light in Contact with Matter
Quantum Mechanics
Density Probability of the Schrödinger equation
The Von Neumann Measurement Postulate
Probability/Phase Space
The Preparation and Reconstruction of a Cat