  The strongly correlated two-dimensional electron liquid within
SmTiO$_3$/SrTiO$_3$/SmTiO$_3$ quantum well structures exhibits a pseudogap
phase when the quantum well width is sufficiently narrow. Using low-temperature
transport and optical experiments that drive the quantum-well system out of
equilibrium, we find evidence of mobile, long-lived, negatively-charged
quasiparticles, consistent with the idea that the pseudogap phase arises from
strongly bound electron pairs.
