  One of the most puzzling features of high-temperature cuprate superconductors
is the pseudogap state, which appears above the temperature at which
superconductivity is destroyed. There remain fundamental questions regarding
its nature and its relation to superconductivity. But to address these
questions, we must first determine whether the pseudogap and superconducting
states share a common property: particle-hole symmetry. We introduce a new
technique to test particle-hole symmetry by using laser pulses to manipulate
and measure the chemical potential on picosecond time scales. The results
strongly suggest that the asymmetry in the density of states is inverted in the
pseudogap state, implying a particle-hole asymmetric gap. Independent of
interpretation, these results can test theoretical predictions of the density
of states in cuprates.
