Heading-Error-Free Optical Atomic Magnetometry in the Earth-Field Range.

We demonstrate an alignment-based ^{87}Rb magnetometer that is immune to nonlinear Zeeman (NLZ) splitting, addressing an important problem in alkali-metal atomic magnetometry. In our scheme, there is a single magnetic resonance peak and well-separated hyperfine transition frequencies, making the magnetometer insensitive or even immune to NLZ-related heading errors. It is shown that the magnetometer can be implemented for practical measurements in geomagnetic environments, and the photon-shot-noise-limited sensitivity reaches 9 fT/sqrt[Hz] at 5 µT and remains at tens of fT/sqrt[Hz] at 50 µT at room temperature.