  Two dimensional electrons in a magnetic field can form new states of matter
characterized by topological properties and strong electronic correlations as
displayed in the integer and fractional quantum Hall states. In these states
the electron liquid displays several spectacular characteristics which manifest
themselves in transport experiments with the quantization of the Hall
resistance and a vanishing longitudinal conductivity or in thermodynamic
equilibrium when the electron fluid becomes incompressible. Several experiments
have reported that dissipation-less transport can be achieved even at weak,
non-quantizing magnetic fields when the electrons absorb photons at specific
energies related to their cyclotron frequency. Compressibility measurements on
electrons on liquid helium demonstrate the formation of an incompressible
electronic state under these resonant excitation conditions.
