  One of the leading challenges of condensed matter physics in the past few
decades in an understanding of the high-temperature copper-oxide
superconductors. While the d-wave character of the superconducting state is
well understood, the normal state in the underdoped regime has eluded
understanding. Here we review the past few years of quantum oscillation
measurements performed in the underdoped cuprates that have culminated in an
understanding of the normal ground state of these materials. A nodal electron
pocket created by charge order is found to characterise the normal ground state
in YBa2Cu3O6+x and is likely universal to a majority of the cuprate
superconductors. An open question remains regarding the origin of the
suppression of the antinodal density of states at the Fermi energy in the
underdoped normal state, either from mainly charge correlations, or more
likely, from mainly pairing and / or magnetic correlations that precede charge
order.
