Abstract
We have developed an ultra-accurate THz frequency counter based on a photocarrier frequency comb in a photoconductive antenna using a femtosecond-pulse mode-locked laser. Comparison of two THz frequencies that were determined using two independent counters showed a fractional frequency uncertainty of 1.4×10-17 at 0.3 THz and an instability of 4.1×10-14/τ up to an averaging time of 10000 s, which are the highest precision results reported to the best of the authors' knowledge. The THz counter developed here can be used for precise spectroscopy of the ultra-cold molecules proposed to detect the time variation of physical constants.