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Optics and Spectroscopy

Towards high-fidelity interference of photons emitted by two remotely trapped Rb-87 atoms

Wenjamin Rosenfeld, Julian Hofmann, Norbert Ortegel, Michael Krug, Florian Henkel, Christian Kurtsiefer, M. Weber, Harald Weinfurter

Published Oct 1, 2011• 5 citations
Publisher

Abstract

We investigate the requirements for achieving high-fidelity interference of photon pairs emitted by two independently trapped Rb-87 atoms at remote locations. For this purpose, one has to ensure indistin-guishability of the single photon wave packets. The crucial parameters here are the synchronization of the two setups, the frequency matching of the photons, as well as their spatial mode overlap. We show that in our experiment these parameters can be controlled to a high degree. This is a first step towards entangling two trapped atoms over a large distance.

DOI
10.1134/s0030400x11110233

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We investigate the requirements for achieving high-fidelity interference of photon pairs emitted by two independently trapped Rb-87 atoms at remote locations. For this purpose, one has to ensure indistin-guishability of the single photon wave packets. The crucial parameters here are the synchronization of the two setups, the frequency matching of the photons, as well as their spatial mode overlap. We show that in our experiment these parameters can be controlled to a high degree. This is a first step towards entangling two trapped atoms over a large distance.
We report the observation of entanglement between a single trapped atom and a single photon at a wavelength suitable for low-loss communication over large distances, thereby achieving a crucial step towards long range quantum networks. To verify the entanglement, we introduce a single atom state analysis. This technique is used for full state tomography of the atom-photon qubit pair. The detection efficiency and the entanglement fidelity are high enough to allow in a next step the generation of entangled atoms at large distances, ready for a final loophole-free Bell experiment.
An experiment is performed where a single rubidium atom trapped within a high-finesse optical cavity emits two independently triggered entangled photons. The entanglement is mediated by the atom and is characterized both by a Bell inequality violation of S=2.5, as well as full quantum-state tomography, resulting in a fidelity exceeding F=90%. The combination of cavity-QED and trapped atom techniques makes our protocol inherently deterministic--an essential step for the generation of scalable entanglement between the nodes of a distributed quantum network.
This thesis reports on the generation of entanglement between two single atoms, located in different laboratories at a distance of 21 m. The entanglement is established in the conceptually most fundamental way - by exchange of a single photon. To this end, we efficiently create high-fidelity entanglement between one of the atoms and a photon, and then faithfully map the entanglement onto the other atom. Both the photon emission and absorption process are facilitated by employing high-finesse optical resonators that strongly enhance the coupling between light and matter.
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.
The coherent control of single-photon emitters as, e.g., single ions or atoms, is a crucial element for mapping quantum information between light and matter. The possibility of generating entanglement between a photon and the emitter system provides an interface between matter-based quantum memories and photonic quantum communication channels, which is the essential resource for quantum repeaters and other future quantum information applications. To generate entangled atom-photon states, in our experiment, we store a single 87 Rb atom in an optical dipole trap. The single-atom/single-photon character is confirmed by the observation of photon antibunching in the detected fluorescence light. The spectral properties of single photons emitted by the atom allowed us to determine the mean kinetic energy of the atom corresponding to 105 μK. We describe a single-atom state analysis method which allowed us to characterize the entanglement between the atom and a single photon emitted in the spontaneous decay. We obtain an entanglement fidelity of 89% that clearly shows the high degree of entanglement in our system and potential for further applications in quantum communication.
Remote entanglement of matter qubits plays an important role in quantum networks and quantum repeater. Collective excitations generated via Rydberg blockade in an atomic ensemble-known as Rydberg superatoms-are promising candidates due to collectively enhanced atom-photon coupling and Rydberg-enabled nonlinearity. Here, we experimentally realize remote entanglement between two Rydberg superatoms via single-photon interference. The two setups are separated by 3 m and linked with two 20-m fibers, operated with independent control lasers. We verify that the entanglement generated is free from high-order excitation noise that is a key drawback in the traditional Duan-Lukin-Cirac-Zoller scheme. This Letter paves the way for entangling long-distance separated Rydberg superatoms at a high rate, as the success probability of a single-photon-based entangling scheme scales with the square root of the channel transmittance.