  We introduce a design of electrically isolated floating bilayer GaAs quantum
wells (QW) in which application of a large gating voltage controllably and
highly reproducibly induces charges that remain trapped in the bilayer after
removal of the gating voltage. At smaller gate voltages, the bilayer is fully
electrically isolated from external electrodes by thick insulating barriers.
This design permits full control of the total and differential densities of two
coupled 2D electron systems. The floating bilayer design provides a unique
approach for studying systems inaccessible by simple transport measurements. It
also provides the ability to measure the charge transfer between the layers,
even when the in-plane resistivities of the 2D systems diverge. We measure the
capacitance and inter-layer tunneling spectra of the QW bilayer with
independent control of the top and bottom layer electron densities. Our
measurements display strongly enhanced inter-layer tunneling current at the
total filling factor of 1, a signature of exciton condensation of a strongly
interlayer-correlated bilayer system. With fully tunable densities of
individual layers, the floating bilayer QW system provides a versatile platform
to access previously unavailable information on the quantum phases in electron
bilayer systems.
