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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Generation of a light mode optimized for efficient free-space photon-atom coupling

Andrea Golla, Bénoît Chalopin, Robert Maiwald, Alessandro S. Villar, Irina Harder, Markus Sondermann, Gerd Leuchs

Published May 1, 2011• 0 citations
Publisher

Abstract

Today the interaction of light and matter on the single quantum level is a wide research field. Strong coupling between a single photon and a single atom has already been achieved with optical cavities. Here, we treat strong interaction in free space, i.e. strong interaction whilst maintaining the free space density of states of the electromagnetic field. We pursue the coupling of a properly designed single photon wave packet to a single ion aiming at an excitation probability close to unity. For this purpose a linear dipole transition of an ion localized in the focal point of a deep parabolic mirror is addressed by a collimated laser beam focused onto the ion from nearly the full solid angle. In order to reach the highest possible coupling strength, the incident vectorial light field must be matched to the radiation pattern of the corresponding atomic transition both in the spatial and in the temporal domain. Here the generation of such an incident light mode is presented.

DOI
10.1109/cleoe.2011.5943420

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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•May 1, 2011•0 citations
Today the interaction of light and matter on the single quantum level is a wide research field. Strong coupling between a single photon and a single atom has already been achieved with optical cavities. Here, we treat strong interaction in free space, i.e. strong interaction whilst maintaining the free space density of states of the electromagnetic field. We pursue the coupling of a properly designed single photon wave packet to a single ion aiming at an excitation probability close to unity. For this purpose a linear dipole transition of an ion localized in the focal point of a deep parabolic mirror is addressed by a collimated laser beam focused onto the ion from nearly the full solid angle. In order to reach the highest possible coupling strength, the incident vectorial light field must be matched to the radiation pattern of the corresponding atomic transition both in the spatial and in the temporal domain. Here the generation of such an incident light mode is presented.
We demonstrate the generation of an optical dipole wave suitable for the process of efficiently coupling single quanta of light and matter in free space. We employ a parabolic mirror for the conversion of a transverse beam mode to a focused dipole wave and show the required spatial and temporal shaping of the mode incident onto the mirror. The results include a proof of principle correction of the parabolic mirror's aberrations. For the application of exciting an atom with a single photon pulse, we demonstrate the creation of a suitable temporal pulse envelope. We infer coupling strengths of 89% and success probabilities of up to 87% for the application of exciting a single atom for the current experimental parameters.
This thesis describes an experiment to optimize the interaction of light and a single ion in free space. The key to efficient coupling consists in mode-matching the incident electromagnetic field to the radiative properties of the ion. For perfect coupling, the incident mode has to match the emission pattern of the ion. In free space, this requires illumination of the ion with an incoming dipole wave. In the present work, such a situation is realized experimentally. To this end, a radially polarized doughnut mode is focused with a deep parabolic mirror while the ion is localized in the focal point of the parabolic mirror by means of a trap of special geometry. This thesis reports first on the implementation of the two basic components of the experiment: the ion trapping in the focal point of the parabolic mirror and the generation of the required doughnut mode. Then, the coupling efficiency of the combined system is analyzed by means of saturation measurements. Characterization of our setup reveals aberrations of the parabolic mirror which prevent perfect coupling. However, the measured coupling efficiency of (7.2 ± 0.2)% is to date among the highest values reported for coupling experiments with a single atom in free space. A special aim of our group is the full excitation of an ion by a single photon. This requires, in addition to the above mentioned spatial mode matching, temporal shaping of the incident light. Perfect excitation can be achieved if the single-photon wavepacket shows an exponentially rising envelope with a time constant matching the lifetime of the excited state of the ion. The present work demonstrates the power of temporal mode matching using a model system: an empty optical cavity. Both, light inside a cavity and atomic excitation, decay exponentially. Thus, there is an analogy between atomic excitation and field energy inside a cavity. The task of exciting an ion translates to the task of coupling a classical light pulse into a cavity. This thesis confirms experimentally that almost the entire energy of a rising exponential pulse can be coupled into a cavity, provided the pulse time constant matches the cavity lifetime.
The efficient interaction between single photons and single matter objects in free space is of key importance for quantum technologies. An experimental setup for testing this possibility involves single two-level ion trapped at the focus of a parabolic metallic mirror. We study the conditions for the setup, under which the assumption about the free-space mode structure of the radiation field in the vicinity of the atom is justified. In our analysis we apply vectorial properties of light by including the polarization degree of freedom. We look for possible changes in the spontaneous emission rate of the atom resulting from the presence of the parabolic boundary conditions.
Rapid advances in quantum technology have made possible the control of quantum states of elementary material quantum systems, such as atoms or molecules, and of the electromagnetic radiation field resulting from spontaneous photon emission of their unstable excited states to such a level of precision that subtle quantum electrodynamical phenomena have become observable experimentally. Recent developments in the area of quantum information processing demonstrate that characteristic quantum electrodynamical effects can even be exploited for practical purposes provided the relevant electromagnetic field modes are controlled by appropriate cavities. A central problem in this context is the realization of an ideal transfer of quantum information between a state of a material quantum system and a quantum state of the electromagnetic radiation field which contains a small number or even a single photon only. Despite much recent work in cavity quantum electrodynamics, which has explored this problem successfully in cases in which the number of accessible electromagnetic field modes is strongly restricted by a cavity, currently still much less is known about ideal quantum state transfer between matter and few-photon quantum states of the electromagnetic field in the extreme opposite case of free space. In this contribution we review recent work which explores characteristic quantum electrodynamical phenomena governing the interaction of a material quantum system with the electromagnetic field in extreme many-mode cases which are close to cavity-free situations. For this purpose the simplest possible quantum electrodynamical situation, namely the interaction of a single material two-level system with the wave packet of a single optical photon, is investigated in free space and in situations in which a large cavity modifies the mode structure of the electromagnetic field modes. It is demonstrated that perfect excitation of an atom by a properly shaped single-photon wave packet is possible even in free space. In particular, it is demonstrated that perfect excitation of an atom can be achieved by a single optical photon which is prepared initially in an appropriate superposition of plane-wave modes, provided this photon is focused subsequently onto the absorbing atom by a parabolic cavity.
Our research aims at the free-space interaction of a single ion with a surrounding light field. A particularly important ingredient is the illumination of the ion from almost the full solid angle. Conventional ion traps are not built with wide open optical access as their primary design goal and are thus restricting the solid angle accessible to incoming light. By combining a reflective optical element such as a parabolic mirror with a specially designed wide open ion trap we aim to overcome this limitation.