Zero-field atomic magnetometer to extract longitudinal magnetic field

Zero-field atomic ensembles enable the development of ultrahigh-sensitivity magnetometry. However, their intrinsic insensitivity to longitudinal magnetic fields along the pumping direction poses a fundamental limitation, restricting precise weak-field quantification and complex system decoupling. Herein, we propose a novel zero-field weak parametric modulation scheme capable of ultrahigh-sensitivity extraction of longitudinal magnetic fields. Transverse modulation is employed to manifest the electron spin polarization projection induced by the longitudinal fields. Based on the perturbation model established, first-order parametric resonance associated with the longitudinal magnetic field yields significant signal enhancement under low-frequency and weak modulation conditions, while simultaneously suppressing spin-exchange relaxation caused by magnetic field modulation. We experimentally achieved a longitudinal measurement sensitivity of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>16</a:mn><a:mspace width="0.28em"/><a:mi>fT</a:mi><a:mo>/</a:mo><a:msup><a:mrow><a:mi>Hz</a:mi></a:mrow><a:mrow><a:mn>1</a:mn><a:mo>/</a:mo><a:mn>2</a:mn></a:mrow></a:msup></a:mrow></a:math>, more than an order of magnitude improved over traditional schemes, with a bandwidth of 70 Hz. Benefiting from low spin relaxation, the noise floor of the transverse magnetic field measurement is preserved at 4 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>fT</c:mi><c:mo>/</c:mo><c:msup><c:mrow><c:mi>Hz</c:mi></c:mrow><c:mrow><c:mn>1</c:mn><c:mo>/</c:mo><c:mn>2</c:mn></c:mrow></c:msup></c:mrow></c:math>. The proposed scheme pushes the boundaries of existing weak magnetic field measurement, providing critical capabilities for state-of-the-art precision experiments including new physical quantity detection.