  Single-molecule fluorescence spectroscopy is a powerful method that avoids
ensemble averaging, but its temporal resolution is limited by the fluorescence
lifetime to nanoseconds at most. At the ensemble level, two-dimensional
spectroscopy provides insight into ultrafast femtosecond processes such as
energy transfer and line broadening, even beyond the Fourier limit, by
correlating pump and probe spectra. Here, we combine these two techniques and
demonstrate 2D spectroscopy of individual molecules at room temperature using
the example of dibenzoterrylene (DBT) in a polymer matrix. We excite the
molecule in a confocal microscope with a phase-modulated train of femtosecond
pulses and detect the emitted fluorescence with single-photon counting
detectors. Using a phase sensitive detection scheme, we were able to measure
the nonlinear 2D spectra of most of the DBT molecules we studied. Our method is
applicable to a wide range of single emitters and opens new avenues for
understanding energy transfer in single quantum objects on ultrafast time
scales.
