SINgle PHOtoN InterActions (SINPHONIA)

Understanding the nature of atom-light interaction has intrigued famous physicists from Newton, via Maxwell to Planck and Einstein in the past. Today, technical progress in isolating single quantum systems from their environment has enabled us to investigate atom-light interaction in its purest nature and to test the established quantum electro-dynamics theory beyond previous limits. As a possible application in the context of quantum information, efficient coupling between atoms and photons could serve to interconnect distant computational units in a quantum network. This project aims at enhancing the magnitude of single-photon single-atom interaction using an ion trap apparatus to carry out fundamental investigations on quantum electro-dynamics and entanglement generation. Using high- numerical aperture (HNA) optical elements and sideband cooling the ions will be major technical resources that will be implemented. Such technical improvements are currently under way and with the personnel applied for within this project they will allow us to push the sensitivity of the experiment to yet unprecedented limits and enable us to observe new phenomena. Specifically, within this project we will explore three distinct areas: A) We will investigate the absorption of weak coherent light by a single ion. The addition of HNA optics will allow us to test current theories that predict up to perfect reflection from a single ion. With the two available traps, the emitted single photons of a single laser- cooled ion can be sent to another single ion, for the first time realizing cascaded quantum systems in a precisely well controlled way. B) A HNA mirror will set a stringent boundary condition to the electromagnetic field interacting with the atom. This allows control of the internal excitation as well as manipulation of the wave packet motion of a single ion by careful control of the back-reflected light from the external mirror. Together, these techniques will enable us to induce and eventually measure tiny level shifts appearing from the presence of the boundary condition which will become accessible using the entanglement of two ions and quantum information based metrology. C) The technological advances and the superior control will allow us to demonstrate teleportation of atoms between traps and to investigate quantum communication procedures between two distant traps.

Goal of the Sinphonia-project was to investigate the fundamental interaction of single photons with single atoms, and to investigate the realization of interfaces for transmitting quantum information. For this, single ions were trapped in a Paul trap under ultrahigh vacuum conditions and they were optically cooled to a temperature near absolute zero. Specially developed objectives enabled focusing of light on single ions and thus an optimized interaction between light and ions. Conversely, these optical elements allowed for the detection of resonance fluorescence with high collection efficiency. In a first experiment, we investigated how the presence of a single atom can be observed in the transmission of a laser beam. After focusing the light beam on the ion, we measured intensity and polarization of the transmitted light and observed an absorption of 1.5% as well as a polarization rotation in transmission. This single-atom-detection by means of the Faraday effect and the observation of quantum jumps in transmission had never been seen before. In a second experiment with two trapped ions we showed for the first time that entanglement of the two atoms can already be achieved by observing a single emitted photon only. So far, protocols needed the coincident detection of a photon from each atom. Thus, the rate to create entanglement depended quadratically on the collection efficiency of the objectives used. With the protocol employed in this project the emitted fluorescence light was collected in such a way that the detection of a single photon does not reveal any information as to which atom actually emitted the light. This in turn leaves the atoms in an entangled state and the rate at which entanglement is created depends only linearly on the collection efficiency. With this method it will be possible to create entanglement across large distances, a requirement for the transfer of quantum information.All experiments in this project depend crucially on the collection efficiency of the objectives used, and so does the quantum-optical control of the atomic states (for example using self-interference) of the ions. Therefore, within this project we designed a new apparatus, which will allow us to increase the collection efficiency and the capability to show self-interference of the emitted photons by employing a hemispherical mirror. In the project, we developed methods to produce a precision hemispherical mirror where the surface shows a mean deviation from the ideal form by only 11 nm. The observed deviation from the ideal form is so small that we envision a new kind of experiments with such mirrors, which for example allow the suppression of light emission from a single ion placed at the center of the hemispherical mirror.