Atomic spectroscopy is a sensitive probe of the interactions amongst the constituents of an atom as well as between the atom and its environment. Historically, this has lead to numerous fundamental discoveries as well as practical technologies. For example, the electric quadrupole moment of the deuteron and the Lamb shift were discovered via spectroscopic measurements in molecular deuterium and atomic hydrogen, respectively. Likewise, state-of-the-art accelerometers, clocks, and magnetometers are based on spectroscopic measurements in a variety of atomic species. However, these technologies only become practical if they can be deployed outside of the laboratory setting, where robustness and portability are necessities.

In the paper “Hyperfine Paschen-Back regime realized in a Rb nanocell”, Sargsyan et al. use permanent magnets to apply a large magnetic field to a sample of rubidium atoms confined in a nanometrically thin cell and then probe the D1 atomic transition with a laser. The nanocell geometry is similar to a crepe, i.e. relatively wide in two-dimensions but very thin along one axis. Here, “relative wide” means about one centimeter and “very thin” means one wavelength of the light that is resonant with the D1 transition in rubidium. The magnetic field is applied along the thin axis of the nanocell and is large enough to decouple the electronic angular momentum from that of the nucleus, i.e. they are working in the “hyperfine Paschen-Back regime.” This requires a magnetic field about 1000 times larger than the typical field at the surface of the earth. The probe laser is collinear with the magnetic field and is circularly polarized.

The nanocell is operated at a temperature near 150 C, which would generally yield a blur of overlapping atomic resonances due to Doppler broadening. However, probing along the thin axis of the nanocell generates velocity-selective optical pumping resonances with sub-Doppler linewidths. Additionally, working in the high-field regime reduces the number of observed transitions and isolates the individual transitions from each other due to the Zeeman Effect. Thus, the author’s experimental approach significantly cleans up the atomic spectrum and its analysis. Ultimately, they measure the transmission of the laser beam through the nanocell as a function of the laser frequency and applied magnetic field and find good agreement with the theoretical predictions.

The use of (i) a nanocell (rather than laser cooling) to mitigate the deleterious effects of Doppler broadening and (ii) permanent magnets (rather than high-current-carrying electromagnets) to generate large magnetic fields gives this device the potential to be both robust and portable. The thin dimension of the nanocell would allow this device to be used for mapping inhomogeneous magnetic fields with submicron spatial resolution. The authors also suggest their device could serve as a magnetic-field-tunable frequency reference. Similar devices comprising atomic species other than rubidium are possible.

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