  We have studied the fractional and integer quantum Hall (QH) effects in a
high-mobility double-layer two-dimensional electron system. We have compared
the "stability" of the QH state in balanced and unbalanced double quantum
wells. The behavior of the n=1 QH state is found to be strikingly different
from all others. It is anomalously stable, though all other states decay, as
the electron density is made unbalanced between the two quantum wells. We
interpret the peculiar features of the nu=1 state as the consequences of the
interlayer quantum coherence developed spontaneously on the basis of the
composite-boson picture.
