A Spin-Polarisation Magnetometer: Computed Sensitivity of a Dense Polarised Alkali Ensemble

A dense, highly polarised alkali vapour is a magnetic sensor whose fundamental noise limits lie far below what practical instruments achieve, and the gap between the two is where magnetometer design actually happens. This paper specifies a spin-polarisation magnetometer on such an ensemble, 7.20e+14 atoms at a polarisation of 0.85 with a coherence time of twelve milliseconds, and computes each limit separately: spin projection noise at 0.0019 femtotesla per root hertz, photon shot noise at 0.0031 femtotesla per root hertz, and a technical floor of 0.55 femtotesla per root hertz that dominates the total of 0.55 femtotesla per root hertz. The measurement bandwidth is 13 hertz and the operating range is plus or minus five nanotesla around a compensated zero, requiring a shielding and compensation factor of 1e+04 against the ambient geomagnetic field. SIMULATED, NOT MEASURED: the figures are computed from the stated assumptions and no instrument has been operated.