# Open-Source Magnetometer for Characterizing Magnetic Fields in Ultracold Experiments
## 1 Introduction  
Magnetic fields are ubiquitous in ultracold experiments. They are generated either by the flow of current through an arrangement of conductors (called coils) [1; 2; 3] or by permanent magnets [4; 5]. Their role can vary from enabling optical trapping in magneto-optical traps [6], control of interactions with Feshbach resonances [7] and assisting with slowing of atomic velocities in Zeeman slowers [8] to setting a quantization axis and nullifying stray magnetic fields [9]. When constructing a new experimental setup or upgrading an existing one with an additional source of magnetic field, one relies on simulations to guide the final design. This usually provides a fairly good estimation of what magnetic fields and their gradients one should expect within the region of interest, such as over the extent of a laser-cooled cloud of atoms. In practice, simulations rarely deviate from measurements by more than several percent [10], but this can be said only in hindsight, after the magnetic field source has been characterized. Typical coils used in ultracold experiments are home-built and the winding process uses equipment constructed ad hoc to guide and secure a relatively thick (often 1-5 mm in diameter) copper wire to assure well-organized layers. In the end, however, windings rarely form a nicely organized stack as assumed in simulations. Mounting coils in the setup might also introduce tilts and cause deviations from the simulated arrangement. Permanent magnets, on the other hand, are harder to simulate reliably without any prior knowledge of the field generated by each individual magnet, and it is essential to actually measure the magnetic field produced by the final assembly. In ultracold experiments, magnetic fields need to be changed, often quickly (\(\sim\)1 us), during various stages of the experimental sequence. This flexibility can be easily achievedwith coils, which makes them ubiquitous in many experimental setups. Even in a very simple single-chamber setup used to create a magneto-optical trap, at least four pairs of coils are present: one pair to create a quadrupole potential plus three perpendicularly oriented pairs to provide a uniform magnetic field for stray fields compensation and setting the quantization axis. This number can easily grow for dual-species experiments with an independent source for each atom type [11], containing multi-stage Zeeman slowers [8] or using an Ioffe-Pritchard-type magnetic trap [12]. The prevalence of coils in modern setups would suggest that there is abundance of commercial or home-made tools to characterize magnetic fields. Commercial gaussmeters are widely available, but they cannot be used right out of the box as they are usually handheld (like, e.g., Hirst GM07 used in this work), which requires the user to build some sort of a rig that would enable sliding the sensor in and out of the region one wants to characterize while noting the position of each magnetic field readout. In practice, such measurements can be time-consuming, and, to partially compensate for that, the spatial resolution or the measurement range are often decreased. Somewhat surprisingly, there are very few reports on home-built magnetometers that would provide functionality and ease of implementation matching our design. What can be found is mentioned in passing in various master's or doctoral theses [10; 13], and even then these solutions seem to offer a rather limited measurement range, with virtually no discussion of the performance. As such, the designs are virtually impossible to reproduce by others, and this forces many researchers to construct their own tools. Some of the solutions found in the literature use microcontrollers like Arduino to read out data from a Hall sensor, but they often offer very little in terms of functionality.