  Reaching light intensities above $10^{25}$ W/cm$^{2}$ and up to the Schwinger
limit ($10^{29}$ W/cm$^{2}$) would enable testing decades-old fundamental
predictions of Quantum Electrodynamics. A promising yet challenging approach to
achieve such extreme fields consists in reflecting a high-power femtosecond
laser pulse off a curved relativistic mirror. This enhances the intensity of
the reflected beam by simultaneously compressing it in time down to the
attosecond range, and focusing it to sub-micron focal spots. Here we show that
such curved relativistic mirrors can be produced when an ultra-intense laser
pulse ionizes a solid target and creates a dense plasma that specularly
reflects the incident light. This is evidenced by measuring for the first time
the temporal and spatial effects induced on the reflected beam by this
so-called 'plasma mirror'. The all-optical measurement technique demonstrated
here will be instrumental for the use of relativistic plasma mirrors with the
emerging generation of Petawatt lasers, which constitutes a viable experimental
path to the Schwinger limit.
