  The ability to characterize the complete quantum state of light is essential
for both fundamental and applied science. For single photons the quantum state
is provided by the mode that it occupies. The spectral temporal mode structure
of light has recently emerged as an essential means for quantum information
science. Here we experimentally demonstrate a self-referencing technique to
completely determine the pulse-mode structure of single photons by means of
spectral shearing interferometry. We detail the calibration and resolution of
the measurement and discuss challenges and critical requirements for future
advances of this method.
