The Nanofiber experiment investigates the feasibility of bringing trapped ions into the evanescent field of an optical nanofiber as a possible ion-photon interface for quantum networks. Because such a system requires ion-fiber separations on the order of the wavelength of the light propagating down the fiber, ion heating and displacement from electric field fluctuations occurring at the nanofiber's surface present substantial challenges. We are currently exploring these limiting interactions and investigating methods to mitigate their effects.
A nanofiber in an ion trap
Being able to transfer quantum information between separate registers of quantum-bits is a requirement for scalable quantum computers. In the world of trapped-ion-based quantum information processing, ions act as stationary quantum-bits in isolated registers while photons, with their ability to reliably transmit information via standard optical fibers, act as traveling quantum-bits between remote registers. Yet, the ideal ion-photon interface remains an open question.
|Illustration of a nanofiber integrated into a macroscopic linear Paul trap. In this system, the fiber is oriented perpendicular to the trapping axis to allow easy and unperturbed ion-loading away from the fiber.
Optical nanofibers, with waist diameters smaller than optical wavelengths, can support an evanescent field that extends significantly outside the fiber. Extensive work with coupling neutral atoms to this evanescent field has already been demonstrated by the group of Arno Rauschenbeutel [1,2] but so far, no such experiments have been carried out with trapped ions. An ion-nanofiber system could be a novel approach to not only an ion-photon interface, but also open the door to a host of quantum optics experiments.
In collaboration with Arno Rauschenbeutel, optical nanofibers are fabricated at the Atom Institute in Vienna and are then integrated into a linear Paul trap in Innsbruck. A removable end-cap electrode allows for easy insertion of the nanofiber which is oriented perpendicular to the trap axis. In addition, the fiber is mounted onto a 2-dimensional translational stage capable of nanometer position resolution.
|Left: Realization of a tapered optical fiber integrated into a macroscopic linear Paul trap. The bottom end-cap is removable to allow insertion of the nanofiber. The fiber is injected with 635 nm light. Right: Cross sectional view of the linear Paul trap and nanofiber with the bottom end-cap removed. The blade separation of the trap is approximately 1 mm. The high intensity scattered light from the 500 nm diameter nanofiber is due to surface contaminants.
The loading of a calcium ion is quick and efficient when the nanofiber is initially positioned far away from the ion, but a displacement of the ion occurs at closer distance. This displacement depends on the sign and magnitude of the charges on the nanofiber. In this setup, the ion is effectively used as a field probe to measure these charges.
|Left: A linear string of six Ca+ ions split by the presence of a positively charged nanofiber, 500 nm in diameter. The ion-fiber separation is approximately 150 µm, with the fiber out of focus, roughly 100 µm above the image plane of the ions. The light scattered by the fiber is from the 395 nm laser beam used for imaging and cooling the Ca+ ions. Right: An ion is trapped roughly 26 µm from a nanofiber 180 nm in diameter. The fiber is in the image plane of the ion, with most of its scattered light blocked from the CCD camera using spatial filtering. The various large spots seen vertically on the fiber are particles adsorbed at the surface of the fiber, acting as point-like scatterers.
The Nanofiber experiment is currently investigating obstacles that stand in the way of bringing an ion closer to the nanofiber. These results will be applicable to trapped-ion experiments where small ion-surface distances become a concern, in particular experiments that use microfabricated traps such as the CryoTrap and 2D Arrays experiments in our group.
- Ben Ames (PhD student)
- Rainer Blatt (group leader)
 G. Sague et al., Cold-Atom Physics Using Ultrathin Optical Fibers: Light-Induced Dipole Forces and Surface Interactions, Phys. Rev. Lett. 99, 163602 (2007)
 E. Vetsch et al., Optical Interface Created by Laser-Cooled Atoms Trapped in the Evanescent Field Surrounding an Optical Nanofiber, Phys. Rev. Lett. 104, 203603 (2010)