  Here we report on a transparent method to characterize individual layers in a
double-layer electron system which forms in a wide quantum well and to
determine their electron densities. The technique relies on the simultaneous
measurement of the capacitances between the electron system and gates located
on either side of the well. Modifications to the electron wave function due to
the population of the second subband and appearance of an additional electron
layer can be detected. The magnetic field dependence of these capacitances is
dominated by quantum corrections caused by the occupation of Landau levels in
the nearest electron layer. The technique should be equally applicable to other
implementations of a double layer electron system.
