Ultra-high Sensitivity Atomic Magnetometers

Abstract

Alkali-metal magnetometers can provide a precise measurement of magnetic fields, including their operation in Earth’s ambient environments. In this thesis, we present developments of all-optical magnetometers with quantum-noise-limited sensitivity for a wide range of magnetic fields. The quantum sensitivity for magnetic fields is intrinsically limited by the spin-projection noise, showing the standard quantum limit δB ∝ 1/√NT2 with the number of atoms N and spin coherence relaxation time T2. This spin-noise-limited sensitivity can be reached with high optical density on resonance of the atomic vapor, which is crucial for quantum-non-demolition measurements of the collective atomic spin. We have developed a 87Rb scalar magnetometer with a multipass cell to increase the effective optical path length and the optical density on resonance. By obtaining high optical rotation, we demonstrate a magnetic sensitivity of 7 fT/√Hz in the geomagnetic fields, which agrees well with the quantum noise limit. The scalar sensor also exhibits high field accuracy and linearity of 0.1 nT level up to Earth’s field around 50 µT. This is achieved via correction of heading errors which cause false shifts of the field measurement depending on the sensor orientation with respect to the field. The described scalar magnetometers are miniaturized sensors with compact multipass vapor cells and therefore provide portability. Additionally, we demonstrate a 87Rb vector magnetometer which measures vector field components. Its intrinsic sensitivity is determined by the population relaxation time T1, with the limit of δB ∝ 1/√NT1. The population relaxation time T1 is usually much longer than the spin coherence relaxation time T2 at finite fields because it is robust against the spin-exchange relaxation. It can therefore highly surpass the usual magnetometer sensitivity which is limited by T2 as δB ∝ 1/√NT2.