Appl. Phys. Express 6, 102202 (2013)

A highly accurate terahertz frequency counter has been developed by scientists in Japan. The counter could become a vital component of a future clock for frequency standards in the terahertz spectral region. It could also be used to perform precise spectroscopy of ultracold molecules. Hiroyuki Ito and co-workers from the National Institute of Information and Communications Technology in Tokyo, Japan, built the counter by creating a terahertz frequency comb using a photoconductive antenna and a femtosecond mode-locked ytterbium fibre laser. To test the precision of the system, the team made two identical systems and then performed a frequency comparison between them. This revealed a fractional frequency uncertainty of 1.4 × 10−17 at 0.3 THz and an instability of 4.1 × 10−14 τ−1 for an averaging time of 10,000 s.