Photon correlations in both time and frequency

While quantum mechanics precludes the perfect knowledge of so-called "conjugate" variables, such as time and frequency, we discuss the importance of compromising to retain a fair knowledge of their combined values. In the case of light, we show how time and frequency photon correlations allow us to identify a new type of photon emission, which can be used to design a new type of quantum source where we can choose the distribution in time and energy of the emitted photons.