Stephenson - the BBC's controller of drama commissioning - told The Hollywood Reporter that the sci-fi drama's anniversary celebrations will "unite all the fans across the world".
"Doctor Who is an international brand, and this year marks its 50th anniversary," he said. "That is a really exciting moment for the BBC at home and abroad.
"We haven't revealed yet what we are doing, but it will be an absolutely enormous, very, very ambitious event."
It was announced last night (February 11) that the Who 50th special will be filmed in 3D, with showrunner Steven Moffat promising "a whole new dimension of adventure for the Doctor to explore".
Rumours continue to circulate that the special episode will see former Doctor Who stars return to the series, though Colin Baker - who played the Sixth Doctor between 1984 and 1986 - denied the latest reports last month.
Doctor Who will return to BBC One with new episodes from March 30.
> Doctor Who Steven Moffat on 50th: 'I'm aware of the expectation'
Watch a trailer for Doctor Who's new 2013 episodes below: