  Field-emission of electrons underlies major advances in science and
technology, ranging from imaging the atomic-scale structure of matter to signal
processing at ever-higher frequencies. The advancement of these applications to
their ultimate limits of temporal resolution and frequency calls for techniques
that can confine and probe the field emission on the sub-femtosecond time
scale. We used intense, sub-cycle transients to induce optical field emission
of electron pulses from tungsten nanotips and a weak replica of the same
transient to directly probe the emission dynamics in real-time. Access into the
temporal profile of the emerging electron pulses, including the duration $\tau$
= (53 as $\pm$ 5 as) and chirp, and the direct probing of nanoscale
near-fields, open new prospects for research and applications at the interface
of attosecond physics and nanooptics.
