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

Over a period of eight months, we have monitored transition frequencies between nearly degenerate, opposite-parity levels in two isotopes of atomic dysprosium (Dy), which are highly sensitive to temporal variation of the fine-structure constant ($\alpha$). In this system, the electronic energies of the opposite-parity levels can be compared utilizing a direct, radio-frequency (rf) electric-dipole transition between them. Our measurements show that the frequency variation of the 3.1-MHz transition in $^{163}$Dy and the 235-MHz transition in $^{162}$Dy are 9.6$\pm$6.7 Hz/yr and -0.8$\pm$6.6 Hz/yr, respectively. These results provide an upper limit on the magnitude of fractional variation of $\alpha$ without any assumptions on the constancy of other fundamental constants at 5.5$\times 10^{-15}$ yr$^{-1}$ (1 $\sigma$).