Magnetometry and frequency references with coherent dark states

Abstract

The 30 kHz linewidth and extremely steep dispersion of a coherent dark resonance in a caesium vapor cell allows measurements of magnetic fields to about 10 nT in 0.1 s integration time, limited by stray fields. The narrow resonance should make possible a compact frequency standard. In a preliminary experiment the authors used a coherent dark resonance in a 2 cm long caesium vapor cell to measure magnetic fields with about 10-5 uncertainty. Two diode lasers (852 nm wavelength) were phase-locked onto each other with a 9.2 GHz frequency offset, corresponding to the frequency ground state separation of the two caesium levels. The difference frequency is derived from a stable tunable rf source. Absorption spectrum of the caesium coherent population trapping resonance for linear laser polarizations is illustrated; it is split into seven components by a magnetic field parallel to the laser polarization direction. The width of the individual lines is only 130 kHz due to power broadening. The steep slope of the resonance could be used to lock the frequency of the 9.2 GHz rf source. This would provide a compact frequency reference.