Experimental demonstration of the sensitivity of an optically pumped quantum magnetometer

The sensitivity σ of an optically pumped quantum magnetometer is measured directly by comparing the output frequencies of ⁸⁵Rb and ⁸⁷Rb magnetometers integrated in a single two-isotope cell. The result σ₈₇=59.6±6 fT Hz^−1/2 agrees well with the value σ ₈₇ ^light =53.7±2 fT Hz^−1/2 obtained indirectly by measuring 87 the ratio of the resonance steepness to the shot noise of light.