  Direct measurements on the temporal envelope of quantum light are a
challenging task and not many examples are known since most classical pulse
characterisation methods do not work on the single photon level. Knowledge of
both spectrum and timing can however give insights on properties that cannot be
determined by the spectrum alone. While temporal measurements on single photons
on timescales of tens of picoseconds are possible with superconducting photon
detectors and picosecond measurements have been performed using streak cameras,
there are no commercial single photon sensitive devices with femtosecond
resolution available. While time-domain sampling using sum-frequency generation
has been already exploited for such measurement, inefficient conversion has
necessitated long integration times to build the temporal profile. We
demonstrate a highly efficient waveguided sum-frequency generation process in
Lithium Niobate to measure the temporal envelope of single photons with
femtosecond resolution with short enough acquisition time to provide a
live-view of the measurement. We demonstrate the measurement technique and
combine it with spectral measurements using a dispersive fiber time-of-flight
spectrometer to determine upper and lower bounds for the spectral purity of
heralded single photons. The approach complements the joint spectral intensity
measurements as a measure on the purity can be given without knowledge of the
spectral phase.
