  We present a technique to completely characterise the spectral-temporal wave
function of a broadband photon pair using spectral-shearing interferometry. Our
method is fully self-referencing and is generalisable across a wide range of
wavelengths. To demonstrate, we generate an entangled photon pair with
controllable time-frequency entanglement and perform a full reconstruction.
These results allow previously unobservable features of quantum states to be
measured and utilised for the first time. We foresee this will unlock new
opportunities in high-dimensional quantum technology, enabling the development
of novel ways of generating and interacting with quantum systems.
