Hyperfine constant $A$ is a key parameter of the hyperfine structure and underpins precision spectroscopy and metrology. In this Letter, we develop a magnetic-field-calibration-free method for determining the ground-state hyperfine constant $A$ in an ultracold ${}^{40}\mathrm{K}$ Fermi gas by utilizing a pair of magnetically insensitive (“clock”) transitions. This overcomes the stringent magnetic-field calibration requirements of conventional methods. We measure the frequencies of these two magnetically insensitive transitions with Hz-level resolution over a range of magnetic fields and obtain the ground-state hyperfine constant $A=-h\times 285.730536(2)\mathrm{MHz}$, corresponding to an absolute uncertainty of about 2 Hz. Our value reduces the uncertainty by nearly 3 orders of magnitude compared with previous determinations, providing a substantially improved reference for high-precision spectroscopy and metrology with ${}^{40}\mathrm{K}$.

Close

PrevNext

View figure in article

• Atomic, optical & lattice clocks
• Coherent control
• Decoherence in quantum gases
• Fermi gases