  The pseudogap state is one of the most enigmatic characteristics in the
anomalous normal state properties of the high temperature cuprate
superconductors. A central issue is to reveal whether there is a symmetry
breaking and which symmetries are broken across the pseudogap transition. By
performing high resolution laser-based angle-resolved photoemission
measurements on the optimally-doped Bi2Sr1.6La0.4CuO6+{\delta} superconductor,
we report the observations of the particle-hole symmetry conservation in both
the superconducting state and the pseudogap state along the entire Fermi
surface. These results provide key insights in understanding the nature of the
pseudogap and its relation with high temperature superconductivity.
