- Introduction

The two preceding quotations of Schro¨dinger and Pauli from the

first half of the 20th century might suggest the opposite, but it is in

fact research on individual quantum objects and on semiconductors

which has seen tremendous progress in the last decades. Single

quantum systems are not anymore only a theoretical concept that is

applied in Gedanken experiments, but single atoms, ions, photons, or

individual solid state quantum systems are frequently used in many

real-world experiments.