  When the kinetic energy of a collection of interacting two-dimensional (2D)
electrons is quenched at very high magnetic fields so that the Coulomb
repulsion dominates, the electrons are expected to condense into an ordered
array, forming a quantum Wigner crystal (WC). Although this exotic state has
long been suspected in high-mobility 2D electron systems at very low Landau
level fillings ($\nu<<1$), its direct observation has been elusive. Here we
present a new technique and experimental results that directly probe the
magnetic-field-induced WC. We measure the magneto-resistance of a bilayer
electron system with unequal layer densities at high magnetic fields. One layer
has a very low density and is in the WC regime ($\nu<<1$), while the other
("probe") layer is near $\nu=1/2$ and hosts a sea of composite fermions,
quasi-particles formed by attaching two flux-quanta to each interacting
electron. The composite fermions feel the periodic electric potential of the WC
in the other layer and exhibit magneto-resistance maxima whenever their
cyclotron orbit encircles certain integer number of the WC lattice points. The
positions of the maxima reveal that the WC has a triangular lattice and yield a
direct measure of its lattice constant. Our results provide a striking example
of how one can probe an exotic many-body state of 2D electrons using equally
exotic quasi-particles of another many-body state.
