Optical magnetometry in the atmosphere

Budker: This is a rare case when I can give you a precise date (April 7, 2008) and circumstance. Our group was developing all-optical magnetometry techniques using alkali atoms confined in vapor cells. In all-optical magnetometers, the only fields applied to the atoms apart from the magnetic field to be measured are appropriately modulated laser beams. I was chairing the Physics Colloquium Committee, and on that day was hosting a speaker, Professor Peter Milonni of Los Alamos, who was developing a theory of laser guide stars (LGS).

To produce such a guide star, one shines a laser beam into the sky and excites sodium atoms in a layer of the upper atmosphere (the mesosphere) some 90 to 100 km above the surface. The excited atoms fluoresce like a lightbulb in the sky, the LGS. Astronomers use LGS to measure the atmospheric perturbations experienced by light coming from astronomical objects, which can be compensated in real time with adaptive optics in the telescope. 

As Peter discussed LGS, I wondered if we modulate the LGS laser beam the same way we do it in all-optical vapor-cell magnetometers, the brightness of the LGS should have a resonance as a function of modulation frequency, measuring which one can determine the magnetic field in the mesosphere!

After the “Eureka moment,” we discussed the idea with my colleagues, and decided: this is so obvious and straightforward that there are really only two possibilities:

1.    There is something wrong with the idea;

2.     Someone must have thought about this already. But who?

It was with great trepidation, I called LGS pioneer William Happer at Princeton. His answer was another one of the most exciting moments of my life. He had indeed thought of this himself. Yet, he never published the idea and, though he attempted to arrange for testing it on the sky, for one reason or another, he was not able to do this.

From there, the path was clear. We started working out the detailed theory of LGS magnetometry, while also looking for ways to do sky experiments. Then, yet another “small miracle” happened: we got in contact with Domenico Bonaccini Calia and Ronald Holzlöhner of the European Southern Observatory (ESO). They had heard about the theoretical expertise in laser-atom interactions developed by my then PhD student Simon Rochester and wanted to apply Simon’s codes to optimize the LGS performance. We gladly agreed to collaborate with them on LGS optimization, in exchange for them helping us “launch” LGS magnetometry.

Kaenders: TOPTICA also was drawn into this field by Domenico and his colleagues from ESO. They presented to us the quest for developing a high-power yellow laser resonant to the sodium resonance line. We were lucky enough to be able to apply a trick learned in the cold-atom lab. Instead of using a broadband source matched to the Doppler-broadened line profile as it was the standard at the time, we together started to work on narrowband lasers, and to employ the so-called optical re- or back-pumper trick. After providing first prototypes of these lasers, then finally full- fledged solutions for the Very Large Telescopes in Chile were produced and in operation since 2016.

Budker: Together with two postdocs in our group, James Higbie and Brian Patton, we published a joint LGS- magnetometry proposal paper in 2011 and began a preparation for on-sky experiments. After some initial difficulties, the experiments championed by my PhD student Felipe Pedreros Bustos finally succeeded (at La Palma, Figs. 2,3), and we published our experimental results in a 2018 Nature Communication paper. In the meantime, the LGS magnetometry idea was picked up by groups in US, China, and Europe.