  We demonstrate an ultrafast voltage sampling technique using a stream of
electron wavepackets. Electrons are emitted from a single-electron pump and
travel through electron waveguides towards a detector potential barrier. Our
electrons sample an instantaneous voltage on the gate upon arrival at the
detector barrier. Fast sampling is achieved by minimising the duration that the
electrons interact with the barrier, which can be made as small as a few
picoseconds. The value of the instantaneous voltage can be determined by
varying the gate voltage to match the barrier height to the electron energy,
which is used as a stable reference. The test waveform can be reconstructed by
shifting the electron arrival time against it. We argue that this method has
scope to increase the bandwidth of voltage sampling to 100 GHz and beyond.
