Improved Limits on the Coupling of Ultralight Bosonic Dark Matter to Photons from Optical Atomic Clock Comparisons

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 ²S1/2(F=0)↔²F7/2(F=3) electric-octupole (E3) transition in ¹⁷¹Yb⁺ to that of the ²S1/2(F=0)↔²D3/2(F=2) electric-quadrupole (E2) transition of the same ion, and to that of the ¹S₀↔³P₀ transition in ⁸⁷Sr. Measurements of the first frequency ratio νE3/νE2 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 νE3/νSr. By constraining oscillations of the fine-structure constant α with these measurement results, we improve existing bounds on the scalar coupling dₑ of ultralight dark matter to photons for dark matter masses in the range of about (10⁻²⁴-10⁻¹⁷) eV/c². 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 νE3/νE2 to improve existing limits on a linear temporal drift of α and its coupling to gravity.