Limit on the Temporal Variation of the Fine-Structure Constant Using Atomic Dysprosium

Over 8 months, we monitored transition frequencies between nearly degenerate, opposite-parity levels in two isotopes of atomic dysprosium (Dy). These frequencies are sensitive to variation of the fine-structure constant ($\alpha$) due to relativistic corrections of opposite sign for the opposite-parity levels. In this unique system, in contrast to atomic-clock comparisons, the difference of the electronic energies of the opposite-parity levels can be monitored directly utilizing a rf electric-dipole transition between them. Our measurements show that the frequency variation of the 3.1-MHz transition in ${}^{163}\mathrm{Dy}$ and the 235-MHz transition in ${}^{162}\mathrm{Dy}$ are $9.0\pm 6.7\mathrm{Hz}/\mathrm{yr}$ and $-0.6\pm 6.5\mathrm{Hz}/\mathrm{yr}$, respectively. These results provide a rate of fractional variation of $\alpha$ of $(-2.7\pm 2.6)\times {10}^{-15}{\mathrm{yr}}^{-1}$ ($1\sigma$) without assumptions on constancy of other fundamental constants, indicating absence of significant variation at the present level of sensitivity.