Optical Magnetometry - Measuring Tiny Magnetic Fields Using Light and Atoms

While many animals possess magnetic field sensing organs, humans do not. Therefore, we build magnetometers, devices that measure magnetic fields. A simple compass can be referred to as a magnetometer. However, magnetic fields emanated by the human brain or a fetal heart are billions of times smaller than the earth's magnetic field, hence far more sophisticated devices are required. State-of-the-art magnetometers imprint weak magnetic signatures on a laser beam, causing detectable fluctuations in beam intensity. This imprinting is accomplished by exploiting a fascinating quantum property of atoms: Not only does each atom behave like a tiny spinning current, hence like a tiny magnet which can behave like a tiny compass, the atom also sensitively interacts with light. In a sample of vapor, all these “atomic compasses” can be aligned to form a single macroscopic-sized compass, a process known as “spin-polarization”. When  a small external magnetic field is applied, this compass is deflected, which affects the absorption of a probe light field passing through the sample. This is optical magnetometry. 

Our experiment uses a vapor of 87Rb, which is spin-polarized by a laser at 795nm. A probe laser at 780nm measures the change in spin-polarization relative to an applied magnetic field.  For accurate measurements we suppress stray magnetic fields by enclosing our magnetometer in mu-metal shields and a three-dimensional helmholtz coil-system. However, there exists residual magnetic noise arising from atomic collisions and electrical devices. My goal is to minimize this residual noise. Finally, we wish to push the sensitivity of our magnetometer to its quantum limit: Even the most stable probe laser has inherent "quantum fluctuations" which prevents the detection of ultraweak magnetic fields. By exploiting quantum entanglement, we seek to prepare a probe light beam in which these quantum fluctuations have been "squeezed" out.  

• Further enhance sensitivity of magnetometer to the quantum regime 

• Record and Measure photon shot noise

[1] A. Fabricant et al, "How to Build a Magnetometer With Thermal Atomic Vapor: A Tutorial," New J. Phys. 25 025001 (2023).

[2] D. Budker, M. Romalis, "Optical Magnetometry," Nature Phys 3 227-234 (2007).