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.
Achieving strong, coherent, and robust interactions between single atoms and single photons is a long-standing goal in the field of quantum optics, and would open the door to diverse quantum technological applications. While early efforts involved cavities made of macroscopic mirrors, in recent years it has also become possible to interface atoms with cavity modes of nano- or microscopic dielectric structures, with the aim to exploit the increased interaction strength that arises from light confined to small volumes. Whispering-gallery-mode (WGM) micro-resonators are a promising example of such cavities, wherein light is forced to circulate around the circumference of a dielectric material due to total internal reflection. WGM resonators can have 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 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.
Despite the promise of WGM resonators for quantum optics, experiments until recently have been hampered by the fact that the atoms were untrapped. As a result, a single atom only coupled to the resonator for a short transient time, and with a varying coupling strength, as the atom freely traversed through the evanescent cavity field. This short, varying coupling makes it challenging to realize more complex, lengthy manipulations as required for many applications.
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, a cold atomic cloud is launched toward the resonator. Should an atom become coupled to the resonator field, it can be rapidly detected via the change in the optical transmission of the resonator, upon which a deep standing-wave optical dipole trap is turned on to trap the atom. The atom was shown to be trapped for an average time of 2 milliseconds. With the atom trapped, the team was then able to observe a so-called vacuum Rabi splitting in the cavity transmission spectrum, which reveals the desired strong, coherent coupling of the atom to the cavity field.
The results of the experiment show that long-lived coherent interfaces between single atoms and WGM resonators is possible. These results will help to establish a path towards the realization of more complex quantum controlled photonic circuits, and more generally 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 atom is trapped in a standing-wave optical dipole trap that is formed by the incident and the reflected trap-laser beam. The coherent atom-resonator interaction is investigated by measuring the resonator transmission via 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.
