Microwave spectroscopy of nanofiber-trapped cesium atoms

Hybrid quantum systems are promising candidates for realizing a quantum network. A particularly interesting approach in this context relies on optical-nanofiber-based atom-photon interfaces [1]. Using a two-color evanescent field, cold neutral atoms are trapped about 200nm away from the nanofiber surface. This small distance allows evanescent coupling between the trapped atoms and additional fiber-guided light fields. The control of the atoms' internal degrees of freedom has already been demonstrated extensively in the nanofiber system, including the preparation of Zeeman states [2, 3] and coherent dark states [4]. However, for complete control, one should also be able to manipulate the external degrees of freedom at the quantum level. In order to characterize the motional states of the trapped atoms, we record microwave spectra of the atomic hyperfine ground state manifold. As the trapping potential is spin-state-dependent, we can drive microwave transitions between different motional states [5]. In this case, motional sidebands appear in the spectra as additional resonances which are separated by integer multiples of the trap frequency from the microwave carrier transition. We use microwave spectroscopy as a diagnostic tool in order to precisely infer the trap frequency and the temperature of the atoms in the nanofiber-based trap. Furthermore, we can cool the atoms by repeatedly driving microwave transitions to lower vibrational states. Once atoms are cooled, their coherence time is increased as the influence of position-dependent light shifts is reduced. Moreover, thanks to the improved localization, the coupling of the atoms to nanofiber-guided optical fields is even more precisely controlled. This is beneficial for realizing nanofiber-based quantum memories, which have the potential to become an important building block in future quantum networks.