We present improved constraints on the coupling of ultralight bosonic dark matter to photons based on long-term measurements of two optical frequency ratios. In these optical clock comparisons, we relate the frequency of the ${{}^{2}S}_{1/2}(F=0)\leftrightarrow {{}^{2}F}_{7/2}(F=3)$ electric-octupole (E3) transition in ${{}^{171}\mathrm{Yb}}^{+}$ to that of the ${{}^{2}S}_{1/2}(F=0)\leftrightarrow {{}^{2}D}_{3/2}(F=2)$ electric-quadrupole (E2) transition of the same ion, and to that of the ${{}^{1}S}_0\leftrightarrow {{}^{3}P}_0$ transition in ${}^{87}\mathrm{Sr}$. Measurements of the first frequency ratio ${\nu }_{\mathrm{E}3}/{\nu }_{\mathrm{E}2}$ are performed via interleaved interrogation of both transitions in a single ion. The comparison of the single-ion clock based on the E3 transition with a strontium optical lattice clock yields the second frequency ratio ${\nu }_{\mathrm{E}3}/{\nu }_{\mathrm{Sr}}$. By constraining oscillations of the fine-structure constant $\alpha$ with these measurement results, we improve existing bounds on the scalar coupling $d_e$ of ultralight dark matter to photons for dark matter masses in the range of about $({10}^{-24}–{10}^{-17})\mathrm{eV}/c^2$. These results constitute an improvement by more than an order of magnitude over previous investigations for most of this range. We also use the repeated measurements of ${\nu }_{\mathrm{E}3}/{\nu }_{\mathrm{E}2}$ to improve existing limits on a linear temporal drift of $\alpha$ and its coupling to gravity.

• Received 9 January 2023
• Revised 31 March 2023
• Accepted 24 April 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.253001

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Published by the American Physical Society