  Heisenberg's intuition was that there should be a tradeoff between measuring
a particle's position with greater precision and disturbing its momentum.
Recent formulations of this idea have focused on the question of how well two
complementary observables can be jointly measured. Here, we provide an
alternative approach based on how enhancing the predictability of one
observable necessarily disturbs a complementary one. Our
measurement-disturbance relation refers to a clear operational scenario and is
expressed by entropic quantities with clear statistical meaning. We show that
our relation is perfectly tight for all measurement strengths in an existing
experimental setup involving qubit measurements.
