With Charles Bennett, Ethan Bernstein, and Gilles Brassard, he showed that quantum computers cannot solve black-box search problems faster than  in the number of elements to be searched.
He demonstrated with Charles Bennett, Ethan Bernstein, and Gilles Brassard that quantum computers are limited in their ability to solve black-box search problems compared to classical computers, as they can only achieve a speed advantage proportional to the number of elements being searched.