Compression of electron bunch and system timing jitter. (a) Measured (red square) and simulated (black line) electron bunch length as a function of the applied THz field in the compression mode. The field strength of the experimental result is obtained by fitting the measured compression results with the simulated values. (b) Simulated bunch length along the propagation direction with different longitudinal THz field strength. (c) Measured timing jitter between the zero crossing of the longitudinal THz electric field and the laser pulses. The measured timing jitter of about 3.8 fs RMS deviations reveals the excellent longer-term stability of the setup without any active stabilization.