  The availability of ever stronger, laser-generated electromagnetic fields
underpins continuing progress in the study and application of nonlinear
phenomena in basic physical systems, ranging from molecules and atoms to
relativistic plasmas and quantum electrodynamics. This raises the question: how
far will we be able to go with future lasers? One exciting prospect is the
attainment of field strengths approaching the Schwinger critical field
$E_\text{cr}$ in the laboratory frame, such that the field invariant $E^2 -
c^2B^2 > E_\text{cr}^2$ is reached. The feasibility of doing so has been
questioned, on the basis that cascade generation of dense electron-positron
plasma would inevitably lead to absorption or screening of the incident light.
Here we discuss the potential for future lasers to overcome such obstacles, by
combining the concept of multiple colliding laser pulses with that of frequency
upshifting via a tailored laser-plasma interaction. This compresses the
electromagnetic field energy into a region of nanometer size and attosecond
duration, which increases the field magnitude at fixed power but also
suppresses pair cascades. Our results indicate that 10-PW-class laser
facilities could be capable of reaching $E_\text{cr}$. Such a scenario opens up
prospects for experimental investigation of phenomena previously considered to
occur only in the most extreme environments in the Universe.
