Potassium Vapour Magnetometers – A Short Summary (Part 1)

By Dr. Ivan Hrvoic, President, GEM Systems, Inc.

Introduction

This paper is intended to provide a short overview for professionals and students who are interested in learning more about potassium magnetometers and their differences from other types of magnetometers available today. Key topics include:

• Physical overview of quantum magnetometers
• Optical pumping of alkali vapours
• Broad line Cesium, Rubidium and Helium versus narrow line Potassium
• Standard and super-resolution K-sensors and systems
• Future directions

This paper is based on more than 10 years of research and development into the topic by GEM Systems, Inc. as well as other published results from the scientific community.

Physical Overview of Quantum Magnetometers

Some subatomic particles, in particular electronic and nuclei of some elements process spin; rotation and they have an accompanying mechanical moment.

P= I h/2π              I = spin number, half integer              h = Plank’s constant

Since the particles with spin have charge, they also possess a magnetic moment related to the mechanical by

µ = γ pγ = gyromagnetic constant

In an applied magnetic field, such as the Earth’s, magnetic moments can only assume discrete orientations governed by the spin number, I. Coupling energy between the particle and the magnetic field is

E = -µ H = µ H cos this is a scalar product of the two vectors

There can be only 2 I + 1 permitted states. For I = ½ for electrons and protons there are only 2 allowed energy states, permitted angles between the two vectors are + 450 . In an assembly of spins the distribution of populations of the two levels is regulated by

e-E/kT = e µH/kT

so the higher energy level is less populated. The result is a slight paramagnetism of the assembly of particles with spin due to spin and the magnetic moment.

Individual spinning particles precess around the magnetic field with the angular frequency

ωo = γH

where γ is a gyromagnetic constant. Since there are many particles spinning incoherently, there is no macroscopic effort of it i.e. the magnetization due to spins appear static.

However, if one applies a rotating magnetic field of the angular frequency ωo in the plane perpendicular to the magnetic field, the vector of magnetization will be deflected off the direction of magnetic field and will precess around it with the same frequency.

Precessing or rotating magnetization will induce a voltage in a coil suitably wound around the assembly of spins. Frequency of the detected voltage is proportional to the applied field to a great precision.

In a weak magnetic field such as Earth’s, the induced voltage is far too small for direct detection. Instead various means are applied in order to polarize the sensor spin assembly i.e. to increase the macroscopic magnetization due to spins.

There are three principally different groups of quantum magnetometers:

• Proton magnetometers use strong DC magnetic fields to increase protons magnetization.
• Overhauser effect is based on a mixture of electrons and protons. Electrons are manipulated to transfer their polarization to protons.
• Some alkali metals and He 3 and 4 can be “optically pumped” to increase the magnetization due to their electron spins.

We will be concerned only with the third group.