The interaction of single photons with single atoms is a fundamental process in quantum optics which manifests the quantum mechanical nature of light and matter. As an experimentally controlled process it enables the implementation of quantum technological applications, such as quantum information processing and quantum metrology. In the last decades, substantial progress has been attained to experimentally control and manipulate single atoms. For example, single or multiple ions can be stored in an ion trap and their quantum states can be controlled with great precision. At the same time, experimental techniques to generate and control single photons and entangled photon pairs have been widely developed. However, only modest progress has been attained in controlling the interaction of single atoms with single photons. The aim of this thesis project was to experimentally study the absorption of single photons and entangled photons pairs by single trapped 40Ca+ ions. To this end, a source of polarization-entangled photon pairs based on the process of spontaneous parametric down-conversion (SPDC) was designed and constructed. Generating photon pairs whose interaction with an atom can be controlled imposed stringent requirements on the bandwidth, tunability, stability and brightness of the source, making its setup much more complex than usual SPDC sources. In a parallel project, a set of two separate ion traps were built, capable of storing a single or a set of a few 40Ca+ ions.

The first part of the thesis documents the design, construction, operation and characterization of the entangled photon pair source. The characterization proves all the necessary requirements for the photons to interact with either of two transitions, D3/2↔P3/2 and D5/2↔P3/2 at 850 and 854 nm respectively, in 40Ca+. One of the crucial requirements was to make the source bandwidth match the bandwidth of these transitions (22MHz). This was not straightforward since the typical emission bandwidths of the SPDC process is several orders of magnitude higher (105MHz). The ion trap setup is also briefly described. One of its crucial features is the inclusion of two high numerical-aperture objectives very close to the ion trap, allowing to enhance the interaction efficiency of the SPDC photons with the ion, and to efficiently collect the fluorescence light they emit. In a second part of the thesis, a concise description of the schemes and techniques used to detect the absorption of single photons by a single trapped ion is given, including a theoretical treatment of the methods used to analyze the data.

The core part of the thesis describes the methods used to couple the entangled photon source to one of the ion traps, and explains the results obtained with the combined setup. First it shows how the source is configured to produce heralded single photons. It then presents experiments in which the absorption by a single ion of photons produced by the source is detected, and how these absorption events are time-correlated with the detection of the heralding photon. These experiments are performed under several conditions, with a growing level of control over the interaction process. Finally, two proof-of-principle experiments are presented. In the first one, the ion is pumped to polarization-sensitive Zeeman substates and the polarization dependence of the correlated absorption rate is observed. This is a first step toward more advanced experiments in which the polarization entanglement of the photon pairs is transferred to the entanglement of one photon with one atom or to two distant atoms. A second experiment performs heralded single photon spectroscopy on a single ion, showing the expected frequency dependence of the absorption process.

Finally, methods to extend these experiments to implement entanglement transfer from photons to atoms are outlined. In particular, it is shown how the efficiency of the source can be improved to make these experiments feasible.


Friday, October 22, 11:00. ICFO Auditorium

Thesis Director: Prof. Juergen Eschner