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 (α) 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 ¹⁶³Dy and the 235-MHz transition in ¹⁶²Dy are 9.0±6.7  Hz/yr and -0.6±6.5  Hz/yr, respectively. These results provide a rate of fractional variation of α of (-2.7±2.6)×10^-15 yr^-1 (1σ) without assumptions on constancy of other fundamental constants, indicating absence of significant variation at the present level of sensitivity.