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The presence of ultralight dark matter particles could lead to a fundamental constant oscillation according to the models [See refs. in 1]. This oscillation could be detected using atomic and molecular spectroscopy techniques [2, 3]. In this work, we present a Cs Doppler-free spectroscopy apparatus for searching for the fundamental constant oscillation in a range of 20 kHz-125 MHz, which corresponds to the particle mass range 8x10-11 to 3x10-7 eV. Using a graphics card to perform a power spectral density calculation in parallel to the data acquisition process allows having a continuous, 100% duty cycle measurement with an ultimate sensitivity and frequency resolution better than 1 Hz. This setup gives us three order of magnitude better constraints on the dark matter to the electron mass and the fine-structure constant coupling than in the earlier work [2]. [1] M. G. Kozlov, D. Budker, Annalen Der Physik 2019, 531, 1800254. [2] D. Antypas et al., Phys. Rev. Lett. 123, 2019 [3] R. Oswald et al., arXiv:2111.06883
