In January 1964 he started on a PhD program at Utrecht University in The Netherlands, and finished his degree in summer 1968 in experimental low-energy neutron physics. Subsequently he joined the nuclear physics group at Eindhoven University of Technology in The Netherlands. In January 1972 Spilling joined the Norwegian Defence Research Establishment (NDRE) at Kjeller in Norway, and in 1974 started working on computer networks under the leadership of Yngvar Lundh at NDRE in collaboration with the DARPA-funded research program on packet switching and Internet technology in the US.