Precise absolute frequency measurement of three-electron $\bf^{6}$Li atom D-line transitions

Abstract

We achieve the photon shot-noise limited optical spectroscopy of $^{6}$Li cold atoms by developing a heterodyne detection technique, under the condition of extremely low optical power density (below 10$^{-3}$ saturation intensity) of probe beam. We establish a transport chain of frequency stability based on an optical frequency comb, and achieve absolute frequencies measurement of D$_{1}$ lines with an uncertainty of about 1 kHz, which is one order of magnitude more accurate than previous measurements. The 2P fine structure splitting of $^{6}$Li and the hyperfine splitting of 2P$_{1/2}$ are 10052.7804(18) and 26.1031(14) MHz, respectively, in agreement with recent theoretical calculations. Our results could provide a benchmark to test the theory at the higher precision and help to resolve large discrepancies among previous experiments.