  The dynamics of an order parameter's amplitude and phase determines the
collective behaviour of novel states emerged in complex materials. Time- and
momentum-resolved pump-probe spectroscopy, by virtue of its ability to measure
material properties at atomic and electronic time scales and create excited
states not accessible by the conventional means can decouple entangled degrees
of freedom by visualizing their corresponding dynamics in the time domain.
Here, combining time-resolved femotosecond optical and resonant x-ray
diffraction measurements on striped La1.75Sr0.25NiO4, we reveal unforeseen
photo-induced phase fluctuations of the charge order parameter. Such
fluctuations preserve long-range order without creating topological defects,
unlike thermal phase fluctuations near the critical temperature in
equilibrium10. Importantly, relaxation of the phase fluctuations are found to
be an order of magnitude slower than that of the order parameter's amplitude
fluctuations, and thus limit charge order recovery. This discovery of new
aspect to phase fluctuation provides more holistic view for the importance of
phase in ordering phenomena of quantum matter.
