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\bibitem{KhosropanahP._etal2009} Khosropanah P., Baryshev A., Zhang W., Jellema W., Hovenier J.N., Gao G.R., et al. (2009). Phase-locking of a 2.7-THz quantum cascade laser to a microwave reference. \textit{Optics Letters}, \textit{34}, 2958--2960.

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