  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.
