In our experiments, we investigate interfaces between single trapped ions and single photons in the framework of cavity quantum electrodynamics (CQED). CQED describes the interaction between quantized matter (e.g., ions) and single quanta of light (photons) with boundary conditions on the light mode provided by an optical resonator. By integrating an ion trap with the cavity, it is possible to investigate what happens when you leave an ion in the company of a single photon.
A light–matter interface
Our project addresses the following questions:
- How can we build a quantum network  for quantum information processing?
- What is the interplay between coherent ion–photon interactions and dissipation introduced by the environment?
- How can we use ions in a cavity to simulate other quantum systems ?
Our current setup consists of a linear Paul trap for trapping 40Ca+-ions inside an optical cavity. Using a cavity-mediated Raman process, we can access a regime in which the rate of coherent atom–cavity coupling is similar to the rates of decoherent processes in the system, namely cavity and atomic decay. By translating the cavity with respect to the ions and changing the ion–ion separation, we can control the individual coupling of ions to the cavity mode. The ions’ electronic state is detected using fluorescence measurements with a photomultiplier tube or a camera, while cavity photons are detected on avalanche photodiodes.
Current setup with ion trap (vertical) and cavity (horizontal).
The lenses outside of the cavity are used for coupling into and out of the cavity.
Fiber cavity apparatus
We are currently developing a second experimental apparatus, the fiber-cavity setup, consisting of a miniaturized cavity-QED interface.
Ion trap integrated with a fiber cavity.
It features a linear trap designed to integrate a fiber-based Fabry-Pérot cavity with minimum disturbance of the ion.
Fabry-Pérot cavity built from two fiber tips. The cavity mode is indicated by the red structure.
The picture of the fiber tips was inserted into the picture of the ion trap.
The fibers forming the cavities are machined by CO2-laser ablation at the ENS-Paris in collaboration with J. Reichel. They are then coated with a low-loss dielectric multilayer stack for high reflectivity at 854 nm in order to couple to the P3/2-D5/2 transition of 40Ca+. With a 400-600 μm long cavity of finesse 40,000, we expect to reach cavity parameters (g,κ,γ)=2π × (20,5,11.5) MHz , where g represents the atom-cavity coupling rate, κ is the decay rate of the cavity field, and γ is the spontaneous emission rate of the ion. These parameters would allow us to access the regime of strong coupling between a single ion and an optical cavity, where coherent processes are dominant.
CQED experiments in Innsbruck started in the early 2000s with a cavity resonant with the quadrupole transition S1/2-D5/2 in 40Ca+, which has a natural linewidth of 1 Hz. The following experiments were done in this setup:
- Single ions were used as sensitive probes of the cavity standing wave  (2002).
- The spontaneous emission of states in the 3D5/2-manifold was shown to be enhanced by the coupling of the ion to the cavity vacuum field  (2004).
In order to reach a more favorable regime, we have built a new experiment in which the cavity is resonant with a dipole transition.
- The coupling of a single ion to the cavity was investigated using Raman spectroscopy  (2009).
- A single ion inside the cavity was used as a deterministic source of single photons  (2009).
- A single-ion laser with tunable photon statistics was demonstrated (2010).
- Building blocks for a quantum interface  were demonstrated, including spectroscopy and coherent manipulation of a calcium qubit in the cavity (2012).
Two quantum-network protocols were implemented using a single ion:
- Tunable entanglement  between a single ion and a single photon was shown (2012).
- The qubit state of an ion was coherently mapped  onto the polarization of a single photon (2013).
In an ion trap, it is also possible to couple more than one ion to the cavity mode.
- We heralded entanglement  of two ions coupled to the cavity mode by detecting photons with orthogonal polarization (2013).
- The collective emission of two ions into the cavity mode was tuned  from subradiance to superradiance (2015).
Rainer Blatt, Tracy Northup, Dario Fioretto, Konstantin Friebe, Moonjoo Lee, Florian Ong, Klemens Schüppert, Markus Teller
From left to right: Tracy, Markus, Konstantin, Klemens, Dario, Florian, Moonjoo
Former members: Bernardo Casabone, Birgit Brandstätter, Andreas Stute, Andrew McClung, Diana Habicher, Helena G. Barros, Piet Schmidt, Carlos Russo, François Dubin, Eoin Philips, Thomas Monz, Christian Maurer, Christoph Becher
 Kimble, H. (2008). The quantum internet. Nature, 453(7198), 1023-1030.
 Barrett, S., Hammerer, K., Harrison, S., Northup, T. E., & Osborne, T. J. (2013). Simulating Quantum Fields with Cavity QED. Physical review letters, 110(9), 090501.
 Brandstätter, Birgit, Andrew McClung, Klemens Schüppert, Bernardo Casabone, Konstantin Friebe, Andreas Stute, Piet O. Schmidt et al. "Integrated fiber-mirror ion trap for strong ion-cavity coupling." Review of Scientific Instruments 84, no. 12 (2013): 123104.
 Mundt, A. B., Kreuter, A., Becher, C., Leibfried, D., Eschner, J., Schmidt-Kaler, F., & Blatt, R. (2002). Coupling a single atomic quantum bit to a high finesse optical cavity. Physical review letters, 89(10), 103001.
 Kreuter, A., C. Becher, G. P. T. Lancaster, A. B. Mundt, C. Russo, H. Häffner, C. Roos, J. Eschner, F. Schmidt-Kaler, and R. Blatt. "Spontaneous emission lifetime of a single trapped Ca+ ion in a high finesse cavity." Physical review letters 92, no. 20 (2004): 203002.
 Russo, C., H. G. Barros, A. Stute, F. Dubin, E. S. Phillips, T. Monz, T. E. Northup et al. "Raman spectroscopy of a single ion coupled to a high-finesse cavity." Applied Physics B 95, no. 2 (2009): 205-212.
 Barros, H. G., Stute, A., Northup, T. E., Russo, C., Schmidt, P. O., & Blatt, R. (2009). Deterministic single-photon source from a single ion. New Journal of Physics, 11(10), 103004.
 Stute, A., B. Casabone, B. Brandstätter, D. Habicher, H. G. Barros, P. O. Schmidt, T. E. Northup, and R. Blatt. "Toward an ion–photon quantum interface in an optical cavity." Applied Physics B 107, no. 4 (2012): 1145-1157.
 Stute, A., B. Casabone, P. Schindler, T. Monz, P. O. Schmidt, B. Brandstätter, T. E. Northup, and R. Blatt. "Tunable ion-photon entanglement in an optical cavity." Nature 485, no. 7399 (2012): 482-485.
 Stute, A., B. Casabone, B. Brandstätter, K. Friebe, T. E. Northup, and R. Blatt. "Quantum-state transfer from an ion to a photon." Nature photonics 7, no. 3 (2013): 219-222.
 Casabone, B., A. Stute, K. Friebe, B. Brandstätter, K. Schüppert, R. Blatt, and T. E. Northup. "Heralded entanglement of two ions in an optical cavity." Physical Review Letters 111, no. 10 (2013): 100505.
 Casabone, B., K. Friebe, B. Brandstätter, K. Schüppert, R. Blatt, and T. E. Northup. "Enhanced Quantum Interface with Collective Ion-Cavity Coupling." Physical Review Letters 114, no. 2 (2015): 023602.