  The need for ever-faster information processing requires extremely small
devices operating at frequencies approaching the terahertz and petahertz
regimes. For the diagnostics of such devices, researchers need a spatiotemporal
tool that surpasses the device under test in speed and spatial resolution and
therefore, such a tool cannot be provided by electronics itself. Here we show
how ultrafast electron beam probe with terahertz-compressed electron pulses can
directly sense local electro-magnetic fields in electronic devices with
femtosecond, micrometre and millivolt resolution under normal operation
conditions. We analyse the dynamical response of a coplanar waveguide circuit
and reveal the impulse response, signal reflections, attenuation and waveguide
dispersion directly in the time domain. The demonstrated measurement bandwidth
reaches 10 THz and the sensitivity to electric fields is tens of millivolts or
-20 dBm. Femtosecond time resolution and the potential to directly integrate
our technique into existing electron-beam inspection devices in semiconductor
industry makes our femtosecond electron beam probe a promising tool for
research and development of next-generation electronics at unprecedented speed
and size.
