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arXiv:0905.3725 · Physical Review Letters

Bandwidth-Tunable Single-Photon Source in an Ion-Trap Quantum Network

Marc Almendros, Jan Huwer, Nicolás Piro, Felix Rohde, Carsten Schuck, Markus Hennrich, François Dubin, Jürgen Eschner

Published Nov 16, 2009• 39 citations
arXivHTMLPDF

Abstract

We report a tunable single-photon source based on a single trapped ion. Employing spontaneous Raman scattering and in-vacuum optics with large numerical aperture, single photons are efficiently created with controlled temporal shape and coherence time. These can be varied between 70 ns and 1.6 micros, as characterized by operating two sources simultaneously in two remote ion traps which reveals mutual and individual coherence through two-photon interference.

DOI
10.1103/physrevlett.103.213601

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•Jan 1, 2009•3 citations
We report a tunable single-photon source based on a single trapped ion. Employing spontaneous Raman scattering and in-vacuum optics with large numerical aperture, single photons are efficiently created with controlled temporal shape and coherence time. These can be varied between 70 ns and 1.6 micros, as characterized by operating two sources simultaneously in two remote ion traps which reveals mutual and individual coherence through two-photon interference.
Experiments in quantum optics are motivated by fundamental questions about the quantum mechanical description of matter and light as well as by applications in quantum technologies such as quantum information processing. Exploiting the quantum character of both matter and light has turned out to be extremely rewarding for novel technological applications in the emerging fields of quantum communication and computation. Here we consider the implementation of quantum information technology with trapped ions and single photons, focusing on two key issues: the entanglement of distant atoms and the controlled interaction of a single photon with a single atom. This thesis describes the construction of a double ion trap apparatus including the necessary frequency-stabilized laser systems, and first experiments to investigate the generation of distant atomic entanglement, the implementation of a simple quantum network and controlled single-atom single-photon interaction. The experimental setup comprises two ion traps at a distance of one meter, which are independent but identical in construction. We introduce novel realizations of a photoionization method for calcium atoms, a laser frequency stabilization scheme and a radio frequency pulse sequencer. Linear Paul traps are used to load individual or strings of calcium ions. High-numerical aperture objectives mounted on sub-micrometer positioning systems allow us to address and manipulate a single ion with strongly focused light fields and collect the emitted fluorescence photons from a large solid angle. With these bidirectional single-atom single-photon interfaces the essential building blocks of a quantum network with atoms and photons are available. One of the main objectives we pursue with the double-trap apparatus is the generation of entanglement between ions in different traps, which is one of the key tasks in a quantum network. We compare single- and two-photon schemes for probabilistic entanglement creation and analyze how the performance of the schemes depends on photon detection efficiency. As a first step towards the experimental realization of such schemes we measure the second order correlation function of fluorescence photons emitted from two distant trapped ions. We observe high-contrast quantum (Hong-Ou-Mandel) interference, proving the indistinguishability of photons emitted from different ions, which is a prerequisite for the considered entanglement schemes. To investigate the interaction of single photons and a single atom in free space we combine the ion trap apparatus with a tunable, narrowband spontaneous parametric down-conversion source. The high-numerical aperture objectives allow us to efficiently couple bandwidth-tunable photons to a single trapped ion. Such atom-photon interaction at the single-particle level is the most fundamental process in quantum optics and lies literally at the focus of many experimental implementations of quantum information processing protocols and of studies of the (quantum) limits of matter -light interaction, which were so far restricted to weak coherent light fields. Here we observe individual atomic absorption events of one photon of an entangled pair by employing a quantum jump detection scheme. To characterize and control the interaction process we modify the spectral properties of the down conversion photons and perform single-photon spectroscopy of a single ion. To go beyond experiments with cohe rent fields we study temporal correlations between the absorption of photons from the spontaneous parametric down conversion source by a single ion and the detection of the corresponding partner photon. For the applied scheme we observe promising indications that the quantum correlations of photons produced in spontaneous parametric down conversion are preserved in the resonant interaction with a single atom. The implementation of more efficient atom-light coupling and entanglement transfer schemes with our apparatus is outlined.
The generation of ultra-narrowband, pure and storable single photons with widely tunable wave shape is an enabling step toward hybrid quantum networks requiring interconnection of remote disparate quantum systems. It allows interaction of quantum light with several material systems, including photonic quantum memories, single trapped ions and opto-mechanical systems. Previous approaches have offered a limited tuning range of the photon duration of at most one order of magnitude. Here we report on a heralded single photon source with controllable emission time based on a cold atomic ensemble, which can generate photons with temporal durations varying over three orders of magnitude up to 10 μs without a significant change of the readout efficiency. We prove the nonclassicality of the emitted photons, show that they are emitted in a pure state, and demonstrate that ultra-long photons with nonstandard wave shape can be generated, which are ideally suited for several quantum information tasks.
Irhe field of quantum information processing (QIP) has evolved rapidly in the last decade. Strings of single rapped ions have proven to be one of the most promising candidates for the implementation of quantum information processing tasks such as universal gate operations or eleportation. Besides the challenge of scaling up these quantum processors to a larger number of ions, another important problem is to interconnect different processors on a macroscopic scale and integrate them into a quantum network. Irhis thesis describes the construction of a new experiment dedicated to the study of quantum communication and quantum networking with laser cooled single ions and first experiments carried out with it. Two linear Paul traps for 4DCa+ have been set up at a distance of one meter. Two-photon interference of the fluorescence light of two remotely trapped single ions could be observed with a contrast of 80%. This proves the suitability of the setup for the creation of remote entanglement, an important building block for quantum networking. Irhe first part of the thesis describes the experimental setup that ~as constructed from scratch. It consists of a newly designed trap apparatus and the laser setup. The apparatus was designed to ulfill the requirement of interferometric stability, which is a pre requisite for one remote entanglement protocol. Another important feature are two high numerical aperture in-vacuum lenses hat optimize the collection efficiency of photons scattered by he ions. In order to frequency-stabilize the laser systems, a ransfer stabilization scheme referenced to a transition in atomic ~s has been developed. The stabilization chain comprises low-cost open transfer cavities and offers very good long term stability. Irhe second part of the thesis presents the experimental results starting with the calibration of the two traps through excitation spectra. After that, the realization of two-photon interference rom single ions under continuous excitation is presented. The model analysis of the data usinq eiqht level Bloch equations reveals the individual coherence properties of the interfering ~ingle photons. The coherence time of the fluorescence light does not exceed 20- ns, indicating that incoherent scattering is ~ominant. fA,n analysis of the efficiency of methods for remote entanglement relying on two-photon and single-photon interference, focussing on he role of the detection efficiency is discussed. It is revealed hat Single-photon schemes are more efficient if the detection ~fficiency is low (like in present experiments). The created setup is suitable for the realization of both types of protocols, based pn single- and two-photon interference. Jhe conditioned dynamics of photon emission of a single ion are studied using correlation functions. The engineering of second ~rder correlation functions used as a tool which relies on conditional dynamics, might have interesting applications in probabilistic quantum information processing. pingle-atom single-photon interfaces are an important application in quantum networks. As opposed to the emission of single photons by single atoms, the absorption of Single photons by single atoms cannot be controlled in a similar way. With the future goal of realizing a heralded absorption of a single photon by a single atom, one of the ion traps was linked to a Spontaneous Parametric Down Conversion source. Quantum jump spectroscopy on a single ion using single photons from this source is shown. Finally the reported two-photon interference could be extended also to pulsed excitation. This marks an important technological step towards remote entanglement.