  We propose a three-pulse coherent ultrafast optical technique that is
particularly sensitive to two-exciton correlations. Two Liouville-space
pathways for the density matrix contribute to this signal which reveals double
quantum coherences when displayed as a two-dimensional correlation plot.
Two-exciton couplings spread the cross peaks along both axes, creating a
characteristic highly resolved pattern. This level of detail is not available
from conventional one-dimensional four-wave mixing or other two-dimensional
correlation spectroscopy signals such as the photo echo, in which two-exciton
couplings show up along a single axis and are highly congested.
