When did Nazi Germany start its heavy water experiments?

Nazi Germany began its 'heavy water' experiments (or, more precisely, production) in mid-1940 after its invasion and conquest of Norway, which was at the time the world's only source of this key ingredient for the production of a nuclear weapon. Production at this facility ceased several years later as a result of several Allied bombing raids and the subsequent attempted transfer to Germany of the heavy-water supply, which was lost in yet another Allied-inspired sabotage operation.