Optically pumped vector magnetometer using a strong bias magnetic field

We present an approach allowing an optically pumped magnetometer (OPM) to be operated within the Earth’s magnetic field as a vector magnetometer, the sensitive axis of which can be freely defined. This approach enables the measurement of any vector component of the Earth’s magnetic field with the same sensitivity. The OPM is realized by a microfabricated cesium-vapor cell with nitrogen buffer gas, which is immersed in a constant homogeneous bias field of approximately <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mn>730</a:mn><a:mspace width="0.2em"/><a:mtext fontfamily="times">μ</a:mtext><a:mrow><a:mi mathvariant="normal">T</a:mi></a:mrow></a:math>. Since this bias field is about one order of magnitude stronger than the Earth’s magnetic field, it defines the sensitive axis of the OPM. The bias field is generated by solid-state magnets and has been designed to exhibit a very low relative inhomogeneity (<g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><g:mo>&lt;</g:mo><g:mo>±</g:mo><g:msup><g:mn>10</g:mn><g:mrow><g:mo>−</g:mo><g:mn>4</g:mn></g:mrow></g:msup></g:math> in relative units) within the vapor-cell dimensions as well as a point of vanishing temperature dependence at around <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><j:msup><j:mn>40</j:mn><j:mo>∘</j:mo></j:msup><j:mrow><j:mi mathvariant="normal">C</j:mi></j:mrow></j:math>. The OPM utilizes the light-narrowing effect, which enables effective suppression of spin-exchange relaxation even in such a large magnetic field amplitude. Based on this implementation, we demonstrate a white-noise floor of below <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><n:mn>60</n:mn><n:mspace width="0.2em"/><n:mrow><n:mi>fT</n:mi><n:mo>/</n:mo><n:msqrt><n:mi>Hz</n:mi></n:msqrt></n:mrow></n:math> in the frequency interval between <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><r:mrow><r:mn>100</r:mn><r:mspace width="0.1em"/><r:mi>Hz</r:mi></r:mrow></r:math> and 600 Hz and a 3-dB sensor bandwidth of <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><v:mo>&gt;</v:mo><v:mn>5</v:mn><v:mspace width="0.2em"/><v:mi>kHz</v:mi></v:math>. Our approach enables unshielded ultrasensitive vectorial measurement capabilities that are relevant in many important applications.