  The observation of a reconstructed Fermi surface via quantum oscillations in
hole-doped cuprates opened a path towards identifying broken symmetry states in
the pseudogap regime. However, such an identification has remained inconclusive
due to the multi-frequency quantum oscillation spectra and complications
accounting for bilayer effects in most studies. We overcome these impediments
with high resolution measurements on the structurally simpler cuprate
HgBa2CuO4+d (Hg1201), which features one CuO2 plane per unit cell. We find only
a single oscillatory component with no signatures of magnetic breakdown
tunneling to additional orbits. Therefore, the Fermi surface comprises a single
quasi-two-dimensional pocket. Quantitative modeling of these results indicates
that biaxial charge-density-wave within each CuO2 plane is responsible for the
reconstruction, and rules out criss-crossed charge stripes between layers as a
viable alternative in Hg1201. Lastly, we determine that the characteristic gap
between reconstructed pockets is a significant fraction of the pseudogap
energy.
