Haroche and Wineland showed with real experiments of photons that it is possible for a quantum system to exist in two states at the same time. Originally, this was proposed by the Austrian physicist Erwin Schroedinger (Nobel Prize in Physics, 1933) within the quantum theory framework. The fact that a photon or an atom can exist in two states at the same time can have practical applications in i.e. the quantum computer. Today's computers store information in bits, which can have the value of 0 or 1 only. In quantum computing, a bit can exist in 0 and 1 at the same time (qubit), allowing for parallel computations at astronomical speeds.
Serge Haroche was also interested in macroscopic phenomena as well and was first who measured directly van der Waals interactions between sodium atoms, which is discussed in this paper. This experiment provided the first direct and precise check of the Lennard-Jones formula.