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The advancement of technologies for the precise interrogation of moleculesoffers exciting possibilities for new studies in the realms of precisionspectroscopy and quantum technologies. Experiments in these domains oftenaddress molecular vibrations in the mid-infrared (MIR) spectral region, thusgenerating the need for spectrally pure and accurate MIR laser sources.Quantum-cascade lasers (QCLs) have emerged as flexible sources of coherentradiation available over a wide range of MIR frequencies. Here, we demonstratea robust approach for the simultaneous linewidth narrowing, frequencystabilisation, and absolute frequency referencing of MIR QCLs all of which areprerequisites for precise spectroscopic experiments. Following upconversion ofits radiation to the visible domain, we implement a phase lock of the QCL to alinewidth-narrowed optical frequency comb which is referenced to a remoteSI-traceable primary frequency standard via a fibre link for absolute frequencycalibration. To achieve a reliable frequency counting of the beat note betweenthe QCL and the OFC, we employ redundant tracking oscillators and demonstrate afrequency instability of $5\times10^{-13}$ at 1 s and $2\times10^{-14}$ at 1000s integration time, limited by the accuracy of our remote reference.

Frequency stabilisation and SI tracing of mid-infrared quantum-cascade lasers for precision molecular spectroscopy