  In strong-field physics experiments with intense lasers, it is of paramount
importance to single-shot diagnose the temporal contrast between laser pulse
peak and its noise pedestal. This allows fast optimization of pulse contrast
and meaningful comparison with theory for each pulse shot, and it can help new
outcomes from clean laser-plasma interactions. Thus far, high contrast ratios
up to ~10^10, required by present petawatt (PW) class lasers, have been
accessible in both generation and single-shot characterization. However,
ultrahigh contrast ~10^13, required by the planned 200-PW lasers, challenges
intense laser technology and remains an open question. This paper reports on
the first demonstration of such an ultrahigh-contrast measurement by adapting
single-shot cross-correlator (SSCC). We introduce an ultrafast method that
enables to determine the SSCC detection limit. Our strategy mimics the test
laser having known ultrahigh contrast in the measurement frame of time-to-space
mapping. The ultimate contrast-measurement limit of 10^13 is achieved, which
corresponds to the highest pulse intensity set by SSCC damage threshold and the
lowest noise pedestal set by single-photon detection. As a consequence, photon
noise in the detection is observed and increases as the noise pedestal reduces.
The demonstrated measurement ability at the photon noise limit is applied to a
high-contrast laser system based on second-harmonic generation and optical
parametric chirped-pulse amplification, suggesting accessible of ultrahigh
contrast pulses.
