  Quantum criticality has attracted considerable attention both theoretically
and experimentally as a way to describe part of the phase diagram of strongly
correlated systems. A scale-invariant fluctuation spectrum at a quantum
critical point implies the absence of any intrinsic scale. Any experimental
probe may therefore create an out-of-equilibrium setting; the system would be
in a non-linear response regime, which violates the fluctuation-dissipation
theorem. Here, we study this violation and related out-of equilibrium phenomena
in a single electron transistor with ferromagnetic leads, which can be tuned
through a quantum phase transition. We review the breakdown of the
fluctuation-dissipation theorem and study the universal behavior of the
fluctuation dissipation relation of various correlators in the quantum critical
regime. In particular, we explore the concept of effective temperature as a
means to extend the fluctuation-dissipation theorem into the non-linear regime.
