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Conference on Lasers and Electro-Optics

A Single Ion Interacting with Single Spontaneous Parametric Down-Conversion Photons

Nicolás Piro, Felix Rohde, Carsten Schuck, Marc Almendros, Jan Huwer, Markus Hennrich, Albrecht Haase, Morgan W. Mitchell, François Dubin, Jürgen Eschner

Published Jan 1, 2009• 0 citations
Publisher

Abstract

We present a tunable, frequency-stabilized, narrow-bandwidth source of frequency-degenerate, entangled photon pairs, which can address the two D-P transitions in 40Ca+ ions. We also demonstrate single ion-single photon interaction by the detection of quantum jumps.
DOI
10.1364/iqec.2009.imj2

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•Jan 1, 2010•2 citations
We present the development and operation of a tunable, frequency-stabilized, narrow-bandwidth source of entangled photon pairs, which can be tuned to the two D–P transitions in Ca+ ions at 850 and 854 nm. The source is based on spontaneous parametric down-conversion in periodically poled KTiOPO4 (PPKTP) followed by tunable optical filters. Its output bandwidth of 22 MHz coincides with the absorption bandwidth of the calcium ions. Its spectral power density is 1.0 generated pairs (s MHz mW)−1. Here, we report details of the setup which was first described in Haase et al (2009 Opt. Lett. 34 55).
We present the design and set-up of a narrow-bandwidth source of entangled photon pairs, which will allow us to address the transitions D3/2-P3/2 and D5/2-P3/2 in 40Ca+ ions at 850 and 854 nm wavelength, respectively. The source has been designed to emit photons with a high spectral power density into the absorption window of the Calcium ions, which has a bandwidth of ~20 MHz. The design has been chosen such that the produced photon rate and the output bandwidth can be varied independently, to be adapted to the experimental needs. Its application will be an experiment that studies methods of probabilistic entanglement of distantly trapped single atoms.
We use a single trapped 40 Ca + ion as a resonant, polarization-sensitive absorber to detect and characterize the entanglement of tunable narrowband photon pairs from a spontaneous parametric down-conversion source. Single-photon absorption is marked by a quantum jump in the ion and heralded by coincident detection of the partner photon. For three polarization basis settings of the absorption and detection of the herald, we find maximum coincidences always for orthogonal polarizations. The polarization entanglement is further evidenced by tomographic reconstruction of the biphoton quantum state with an overlap fidelity of 93% with the Bell singlet state. This is an essential step toward a single-ion based quantum memory for photonic entanglement.
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 sin~le photons. The aim of this thesis project was to experimentally study he absorption of single photons and entangled photons pairs by single trapped DCa· 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 4DCa· ions. he 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, D312 P312 and D5i2 P312 at 850 and 854 nm respectively, in 4DCa· 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 (1 05MHz). 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 heoretical treatment of the methods used to analyze the data. he 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 entanglemen 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.