Happy to say that, after a while in the pipes, the result of a collaboration between the IQO group at UPaderborn and the Bayer group at TU Dortmund, Streak-camera measurements of single photons at telecom wavelength, has been published in Applied Physics Letters.
In this experiment, we took photons from a downconversion source and upconverted them in a long nonlinear waveguide. For the current discussion, this process does two important things. Firstly, since the group velocity of the green light in the upconversion waveguide is much slower than the infrared light, the output pulse is stretched in time, from the sub-picosecond scale to about 20 picoseconds. Secondly, it converts the wavelength from the telecom to the visible regime, where efficient, low-noise, single-photon-sensitive streak cameras exist to measure the shape of the photon in time.
In the picture below, you can see the image built up on the streak camera, which essentially deflects photons onto a camera fast enough to build up an image of the temporal profile. By looking at the spread on the camera, we can back out the length of the pulse in time. To emphasize, this signal is one arm of a weakly pumped downconversion, showing that this technique works for characterizing quantum-ready sources.
For slightly complicated reasons that I’m super happy to blab about for days, the upconversion process used is actually not great for this application; the group-velocity mismatch is aligned such that the upconversion is mode-selective. Long story short: the shape of the signal in time is determined nearly entirely by the upconversion, not the input shape. Still, the signal is still very much a quantum one, and we outline some potential techniques that could be used to extend tailored upconversion schemes to upconversion streak-camera imaging.