Over the last decade, quantum information experiments with trapped ions have demonstrated essential steps towards quantum computing and quantum simulation ^[1] .  Large fields are required to achieve strong coupling to the ions via dipolar interactions. To accomplish this coupling, we are integrating transmission line microresonators into the 2D trap structures already implemented at the Center for Ultracold Atoms at MIT ^{[2] [3]} . The resonators are superconducting niobium to minimize loss and maximize quality factors ^{[4] [5]} . Using the large fields locally generated from the accumulation of microwave photons in the resonator, we hope to demonstrate interaction with the trapped ions first by heating them and eventually to control the rotational states of the ions by coherently coupling to them.

We designed integrated resonator-ion traps and fabricated them on niobium on A-plane sapphire using optical lithography. We deposited the 200-nm niobium layers on the sapphire with an in-house DC magnetron sputtering system.  Tests showed that the niobium achieved superconductivity at the expected temperature of 9.2 K.  Once we completed the material stack, we patterned the front niobium layer in PR1-2000A resist using the Heidelberg uPG-101 pattern generator, developed it, and then reactive-ion etched it with CF₄ and O₂ to transfer the pattern into the niobium.

Current results from our fabricated resonators show first-resonance quality factors of at least 10⁴ at 3.23 GHz at device temperatures of 3-4 K.  We have recently successfully demonstrated on-chip superconductivity while trapping crystallized ions, bringing us closer to our goal of coupling microwave photons to the ions.  In the coming months, we hope to integrate the high quality factor resonators into the trap design and use them incoherently heat the trapped ion cloud.  We also intend to shift our process from niobium to niobium nitride, allowing us a higher temperature threshold in which we may still be superconducting.

1. A. André, D. DeMille, J. M. Doyle, et al. “A coherent all-electrical interface between polar molecules and mesoscopic superconducting resonators,” Nature Physics, vol. 2, no. 9, pp. 636-642, 2006
2. J. Labaziewcz, Y. Ge, P. Antohi, D. Leibrandt, K. Brown, and I. L. Chuang, “Suppression of heating rates in cryogenic surface-electrode ion traps,” Phys. Rev. Lett. vol. 100, p. 130001, 2008
3. D. Stick, W. Hensinger, S. Olmschenk, et al. “Ion trap in a semiconductor chip,” Nature Physics, vol. 2, no. 1, pp. 36-39, 2005
4. M. Göppl, A. Fragner, M. Baur, R. Bianchetti, S. Filipp, J. Fink, et al. “Coplanar waveguide resonators for circuit quantum electrodynamics,” Journal of Applied Physics, vol. 104, no. 11, pp. 3904, 2008.
5. J. Gao, “The physics of superconducting microwave resonators,” PhD thesis, California Institute of Technology, Pasadena, 2008.