  The de Haas-van Alphen effect was observed in the underdoped cuprate
YBa$_2$Cu$_3$O$_{6.5}$ via a torque technique in pulsed magnetic fields up to
59 T. Above an irreversibility field of $\sim$30 T, the magnetization exhibits
clear quantum oscillations with a single frequency of 540 T and a cyclotron
mass of 1.76 times the free electron mass, in excellent agreement with
previously observed Shubnikov-de Haas oscillations. The oscillations obey the
standard Lifshitz-Kosevich formula of Fermi-liquid theory. This thermodynamic
observation of quantum oscillations confirms the existence of a well-defined,
close and coherent, Fermi surface in the pseudogap phase of cuprates.
