  We employ two-dimensional (2D) coherent, nonlinear spectroscopy to
investigate couplings within individual InAs quantum dots (QD) and QD
molecules. Swapping pulse ordering in a two-beam sequence permits to
distinguish between rephasing and non-rephasing four-wave mixing (FWM)
configurations. We emphasize the non-rephasing case, allowing to monitor
two-photon coherence dynamics. Respective Fourier transform yields a double
quantum 2D FWM map, which is corroborated with its single quantum counterpart,
originating from the rephasing sequence. We introduce referencing of the FWM
phase with the one carried by the driving pulses, overcoming the necessity of
its active-stabilization, as required in 2D spectroscopy. Combining single and
double quantum 2D FWM, provides a pertinent tool in detecting and ascertaining
coherent coupling mechanisms between individual quantum systems, as exemplified
experimentally.
