Project goals

By placing a trapped ion within an optical cavity, we can generate a coherent interaction between the ion and the cavity field. One can think of this interaction as an atom-photon interface, allowing us to transfer quantum information between light and matter. We hope to use such an interface both to explore fundamental quantum optics and to couple trapped ions to each other via a cavity-based quantum network.

Specific goals for the project include:

• Atom-photon entanglement and quantum state mapping;
• Demonstration of cavity-mediated entangling protocols;
• Implementation of a small quantum simulator;
• A new generation of micro-structured surface ion traps with integrated optical cavities.

Experimental apparatus and results

Previous ion-cavity experiments in Innsbruck coupled a cavity mode to the ⁴⁰Ca⁺ quadrupole transition, which has a 1 s lifetime. It was observed that a single ion could act as a highly sensitive probe of the cavity standing wave field [1] and that the ion's position in the cavity modifies its spontaneous emission rate [2], as predicted by cavity quantum electrodynamics.

Our current apparatus is designed to couple up to three ions to a cavity. The atom-cavity interaction takes place on either the 4P_1/2-3D_3/2 or the 4P_3/2-3D_5/2 ⁴⁰Ca⁺ transition, with a lifetimes of around 90 ns. This choice of transition allows us to operate in an intermediate coupling regime in which the rate of coherent atom-cavity coupling is similar to the incoherent rates of cavity and effective atomic decay. The apparatus consists of a linear Paul trap within an optical cavity under ultra-high vacuum. The cavity is 2 cm in length, with finesse of 7 x 10⁴ at 866 nm, and can be translated in all directions with respect to the trap. We detect cavity photons on a pair of avalanche photodiodes, and we detect atomic fluorescence from the side of the cavity on a photomultiplier tube.

Current apparatus for the ion-cavity experiment. Within a vacuum chamber, we trap calcium ions between two high-finesse mirrors that form an optical cavity.

By coupling the cavity to the 4P_1/2-3D_3/2 transition of ⁴⁰Ca⁺ and driving the 4S_1/2-4P_1/2 transition with a laser, we can generate vacuum-stimulated Raman transitions between the S and D states of trapped calcium ions. We have recorded spectra of these transitions in the cavity output field and have used them to map out the standing-wave structure of the atom-field coupling [3]. Recently, we have realized an efficient single-photon source by driving Raman transitions in a pulsed scheme, generating one photon within the cavity in 88±17% of all attempts [4].

Fiber cavity development

In parallel with the efforts described above, we are working toward a strongly coupled, fiber-based cavity which can be integrated with surface ion traps under development in our research group. This work is in collaboration with J. Reichel at ENS, Paris, who has developed these fiber cavities for atom-chip experiments with Bose Einstein condensates [5].

Development of next-generation fiber-based cavities.

Project members

• Helena G. Barros (graduate student)
• Birgit Brandstätter (graduate student)
• Tracy E. Northup (postdoc)
• Andreas Stute (graduate student)
• Piet O. Schmidt (principal investigator)
• Rainer Blatt (group leader)

CQED team (P. O. Schmidt missing).

References

[1] "Coupling a single atomic quantum bit to a high finesse optical cavity," A. B. Mundt, A. Kreuter, C. Becher, D. Leibfried, J. Eschner, F. Schmidt-Kaler, R. Blatt, Phys. Rev. Lett. 89, 103001 (2002).

[2] "Spontaneous emission lifetime of a single trapped Ca⁺ ion in a high finesse cavity," A. Kreuter, C. Becher, G. P. T. Lancaster, A. B. Mundt, C. Russo, H. Häffner, C. Roos, J. Eschner, F. Schmidt-Kaler, R. Blatt, Phys. Rev. Lett. 92, 203002 (2004).

[3] "Raman spectroscopy of a single ion coupled to a high-finesse cavity," C. Russo, H. G. Barros, A. Stute, F. Dubin, E. S. Phillips, T. Monz, T. E. Northup, C. Becher, T. Salzburger, H. Ritsch, P. O. Schmidt, R. Blatt, Appl. Phys. B 95, 205 (2009).

[4] "Deterministic single-photon source from a single ion," H. G. Barros, A. Stute, T. E. Northup, C. Russo, P. O. Schmidt, R. Blatt, New J. Phys. 11, 103004 (2009).

[5] "Strong atom-field coupling for Bose-Einstein condensates on a chip," Y. Colombe, T. Steinmetz, G. Dubois, F. Linke, D. Hunger, J. Reichel, Nature 450, 272 (2007).