  Ultrafast optical control of quantum systems is an emerging field of physics.
In particular, the possibility of light-driven superconductivity with
ultrashort laser pulses has attracted much of attention. To identify
non-equilibrium superconductivity, it is necessary to measure fingerprints of
superconductivity on ultrafast timescales. Recently non-linear THz third
harmonic generation (THG) was shown to directly probe the collective degrees of
freedoms of the superconducting condensate including particularly the Higgs
mode. Here we extend this idea to light-driven non-equilibrium states in
superconducting La2-xSrxCuO4 establishing a protocol to access the transient
superconducting (SC) order-parameter fluctuations. We perform an optical
pump-THz-THG drive experiment and use a two-dimensional spectroscopy approach
to disentangle the driven third-harmonic response of optically excited
quasiparticles and the pure condensate response. In this way, 2D spectroscopy
separately probes both the ultrafast pair breaking dynamics and transient
pairing amplitude of the condensate.
