  Indirect excitons in coupled quantum wells have long radiative lifetimes and
form a cold quasi-two-dimensional population suitable for studying collective
quantum effects. Here we report the observation of the exciton Mott transition
from an insulating (excitons) to a conducting (ionized electron-hole pairs)
phase, which occurs gradually as a function of carrier density and temperature.
The transition is inferred from spectral and time-resolved photoluminescence
measurements around a carrier density of $2\times10^{10} \mathrm{cm}^{-2}$ and
temperatures of $12-16 \mathrm{K}$. An externally applied electric field is
employed to tune the dynamics of the transition via the quantum-confined Stark
effect. Our results provide evidence of a gradual nature of the exciton Mott
transition.
