Cavity QED

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 [1] 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 [2]?


Experimental apparatus

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.

FC IontrapFiberAssembly

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.

FC TrapFibers sketch

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 [3], 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.


Key results

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:

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 [6] (2009).
  • A single ion inside the cavity was used as a deterministic source of single photons [7] (2009).
  • A single-ion laser with tunable photon statistics was demonstrated (2010).
  • Building blocks for a quantum interface [8] 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:

In an ion trap, it is also possible to couple more than one ion to the cavity mode.


Project members

Rainer Blatt, Tracy Northup, Lorenzo Dania, Dario Fioretto, Pierre Jobez, Konstantin Friebe, Moonjoo Lee, Florian Kranzl, Pau Mestres, Florian Ong, Klemens Schüppert, Markus Teller

Back: Saile (Nockspitze)
Middlle: Moonjoo, Konstantin, Florian O., Markus, Lorenzo, Klemens
Front: Florian K., Pierre, Dario, Pau, Tracy

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



Funding for this project is provided by

  • the Austrian Science Fund (FWF) through the SFB FoQuS: Foundations and Applications of Quantum Science, Project F 4019; through the Elise Richter Program, Project V 252; and through the Lise Meitner Program, Project M 1964; 
  • the Army Research Laboratory's Center for Distributed Quantum Information, via the project SciNet: Scalable Ion-Trap Quantum Network, Cooperative Agreement No. W911NF15-2-0060;
  • the European Union's Horizon 2020 research program through the Marie Skłodowska-Curie Actions, Grant No. 656195.



[1] H. J. Kimble, "The quantum internet," Nature 453, 1023 (2008).
[2] S. Barrett, K. Hammerer, S. Harrison, T. E. Northup, and T. J. Osborne, "Simulating Quantum Fields with Cavity QED," Physical Review Letters, 110, 090501 (2013).
[3] B. Brandstätter, A. McClung, K. Schüppert, B. Casabone, K. Friebe, A. Stute, P. O. Schmidt, C. Deutsch, J. Reichel, R. Blatt, and T. E. Northup, "Integrated fiber-mirror ion trap for strong ion-cavity coupling," Review of Scientific Instruments 84, 123104 (2013).
[4] A. B. Mundt, A. Kreuter, C. Becher, D. Leibfried, J. Eschner, F. Schmidt-Kaler, and R. Blatt, R, "Coupling a single atomic quantum bit to a high finesse optical cavity," Physical Review Letters, 89, 103001 (2002).
[5] A. Kreuter, 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, 203002 (2004).
[6] 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, and R. Blatt, "Raman spectroscopy of a single ion coupled to a high-finesse cavity," Applied Physics B 95, 205 (2009).
[7] H. G. Barros, A. Stute, T. E. Northup, C. Russo, P. O. Schmidt, and R. Blatt, "Deterministic single-photon source from a single ion," New Journal of Physics 11, 103004 (2009).
[8] A. Stute, 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, 1145 (2012).
[9] A. Stute, 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, 482 (2012).
[10] A. Stute, B. Casabone, B. Brandstätter, K. Friebe, T. E. Northup, and R. Blatt, "Quantum-state transfer from an ion to a photon," Nature Photonics 7, 219 (2013).
[11] B. Casabone, 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, 100505 (2013).
[12] B. Casabone, K. Friebe, B. Brandstätter, K. Schüppert, R. Blatt, and T. E. Northup, "Enhanced quantum interface with collective ion-cavity coupling," Physical Review Letters 114, 023602 (2015).