DAALI achieves one of its first milestones
In a recent paper published in PRL, researchers from the Vienna Center for Quantum Science and Technology and the Humboldt University of Berlin, and partners of DAALI, have demonstrated the trapping of a single rubidium atom at nanoscale distances from the surface of a whispering-gallery-mode microresonator.
Breakthrough in Trapping Atoms in WGM Resonators
Achieving strong, coherent, and robust interactions between single atoms and single photons remains a long-standing goal in the field of quantum optics, and it would open the door to diverse quantum technological applications. While researchers initially focused on cavities made of macroscopic mirrors, in recent years, they have also interfaced atoms with cavity modes of nano- or microscopic dielectric structures, aiming to exploit the increased interaction strength that arises from light confined to small volumes. Whispering-gallery-mode (WGM) micro-resonators serve as a promising example of such cavities, where total internal reflection forces light to circulate around the circumference of a dielectric material. WGM resonators exhibit exceptional properties, like high Q-factors and efficient transfer of light into and out of the device.
Different bottle resonators made from an Er-doped figure. The resonator glows in green whenever there is light circulating in the resonator. In this way, one can nicely see the ring-type structure of the resonator modes. The photograph illustrates the case where the research team excites four different resonator modes. Note that the resonator used in the mentioned experiment does not exhibit this feature, as it was constructed from a standard (non-Er doped) fiber.
Despite the promise of WGM resonators for quantum optics, researchers have encountered limitations in experiments until recently due to the untrapped nature of the atoms. As a consequence, a single atom coupled to the resonator only for a short transient time and with varying coupling strength, as it freely traversed the evanescent cavity field. This short, varying coupling presents a challenge in realizing more complex, lengthy manipulations necessary for many applications.
Breakthrough in Trapping Atoms in WGM Resonators
Now, in a recent paper published in PRL, researchers Elisa Will, Luke Masters, Arno Rauschenbeutel, and Michael Scheucher, led by Jürgen Volz, from the Vienna Center for Quantum Science and Technology and Humboldt University of Berlin, and partners of the project DAALI, have been able to overcome this obstacle. In particular, they have demonstrated the trapping of a single rubidium atom at a distance of 200nm from the surface of a WGM resonator.
In their experiment, the team initially launched a cold atomic cloud toward the resonator. Then, upon an atom coupling to the resonator field, they were able to rapidly detect it via a change in the optical transmission of the resonator. Immediately afterwards, they activated a deep standing-wave optical dipole trap to capture the atom. Furthermore, they demonstrated that the atom remained trapped for an average of 2 milliseconds. Consequently, with the atom trapped, the team observed a vacuum Rabi splitting in the cavity transmission spectrum, ultimately revealing the desired strong, coherent coupling between the atom and the cavity field.
Atom-light interactions at the nanoscale: a bright future for quantum photonics.
Moreover, the results of the experiment clearly show that long-lived coherent interfaces between single atoms and WGM resonators are indeed possible. Therefore, these results will undoubtedly help to establish a path towards the realization of more complex quantum controlled photonic circuits. In addition, more generally, they allow for the exploration of exotic effects involving atom-light interactions at the nanoscale.
Schematic setup for trapping a single atom at a distance of 200 nm from a whispering-gallery-mode microresonator. The incident and reflected trap-laser beam form a standing-wave optical dipole trap, which captures the atom. The team investigates the coherent atom-resonator interaction by measuring the resonator transmission through an optical nanofiber (coupling fiber).
Reference: Coupling a Single Trapped Atom to a Whispering-Gallery-Mode Microresonator, Elisa Will, Luke Masters, Arno Rauschenbeutel, Michael Scheucher, and Jürgen Volz, Phys. Rev. Lett. 126, 233602, 2021
Caption -fig. above: Different bottle resonators made from an Er-doped figure. The resonator glows in green whenever there is light circulating in the resonator. In this way, one can see nicely see the ring-type structure of the resonator modes and the photograph shows the case where the team of researchers can excite four different resonator modes (take into account that the resonator used in the mentioned experiment does not show this as it was made from a standard (non-Er doped) fiber.
