Dual-comb spectroscopy based on terahertz (THz) quantum cascade lasers (QCLs) can be a powerful tool for molecular fingerprint characterization. However, sensitive, fast detectors in the THz frequency range are usually required. Now, Markus Rösch and colleagues from ETH Zurich have developed an on-chip THz dual-comb set-up that does not require a fast detector. Two THz QCLs were processed next to each other by dry-etching techniques on the same chip and connected individually to two current sources. The two lasers had the same width (50 μm) and cavity length (1.9 mm), and were spaced 450 μm apart. A small amount of light from one laser perturbed the other laser. The dynamics of the second laser thus act as an efficient ultrafast detector. A multi-heterodyne spectrum was detected for up to 30 modes corresponding to a total bandwidth of 630 GHz. The Swiss scientists further used their device to measure the spacing of the modes of the comb, showing that they are equally spaced to an accuracy of 10−12 at a frequency of 2.5 THz.