  We observed a strong non-linearity in the system of quasiparticles of a
superconducting aluminum resonator, due to the Cooper-pair breaking from the
absorbed readout power. We observed both negative and positive feedback
effects, controlled by the detuning of the readout frequency, which are able to
alter the relaxation time of quasiparticles by a factor greater than 10. We
estimate that the (70+/-5) % of the total non-linearity of the device is due to
quasiparticles.
