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

Long-distance atom-photon entanglement

Wenjamin Rosenfeld, Fredrik Hocke, Florian Henkel, Michael Krug, Norbert Ortegel, Julian Hofmann, Markus Weber, Harald Weinfurter

Published Jun 1, 2009• 0 citations
Publisher

Abstract

Atoms and photons are well-suited candidates for quantum information processing and quantum communication. While atoms can be used for storage and manipulation, photons are ideal carriers for the transfer of quantum information. Our group pursues the goal of creating a basic node of a quantum communication network by entangling two independently trapped87Rb atoms over a large distance. Besides its prospects in quantum communication (e.g. quantum repeater), this would also allow a loophole-free test of Bell's inequality which can help to conclusively answer the long-standing question about the completeness of the quantum theory. The key ingredient is atom-photon entanglement which will enable us to entangle two atoms by entanglement swapping. We plan to achieve a distance of 300m between two independent single atom traps.

DOI
10.1109/cleoe-eqec.2009.5192724

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Atoms and photons are well-suited candidates for quantum information processing and quantum communication. While atoms can be used for storage and manipulation, photons are ideal carriers for the transfer of quantum information. Our group pursues the goal of creating a basic node of a quantum communication network by entangling two independently trapped87Rb atoms over a large distance. Besides its prospects in quantum communication (e.g. quantum repeater), this would also allow a loophole-free test of Bell's inequality which can help to conclusively answer the long-standing question about the completeness of the quantum theory. The key ingredient is atom-photon entanglement which will enable us to entangle two atoms by entanglement swapping. We plan to achieve a distance of 300m between two independent single atom traps.
Distributed quantum information processing requires a reliable quantum memory and a faithful carrier of quantum information. Atomic qubits have very long coherence times and are thus excellent candidates for quantum information storage, whereas photons are ideal for the transport of quantum information as they can travel long distances with a minimum of decoherence. We discuss the theoretical and experimental combination of these two systems and their use for not only quantum information transfer but also scalable quantum computation architectures.
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.
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In 1935 Einstein, Podolsky and Rosen used the assumption of local realism to conclude in a Gedankenexperiment with two entangled particles that quantum mechanics is not complete. For this reason EPR motivated an extension of quantum mechanics by so-called local hidden variables. Based on this idea in 1964 Bell constructed a mathematical inequality whereby experimental tests could distinguish between quantum mechanics and local-realistic theories. Many experiments have since been done that are consistent with quantum mechanics, disproving the concept of local realism. But all these tests suffered from loopholes allowing a local-realistic explanation of the experimental observations by exploiting either the low detector efficiency or the fact that the detected particles were not observed space-like separated. In this context, of special interest is entanglement between different quantum objects like atoms and photons, because it allows one to entangle distant atoms by the interference of photons. The resulting space-like separation together with the almost perfect detection efficiency of the atoms allows a first loophole-free test of Bell's inequality. The primary goal of the present thesis is the experimental realization of entanglement between a single localized atom and a single spontaneously emitted photon at a wavelength suitable for the transport over long distances. In the experiment a single optically trapped Rb87 atom is excited to a state which has two selected decay channels. In the following spontaneous decay a photon is emitted coherently with equal probability into both decay channels. This accounts for perfect correlations between the polarization state of the emitted photon and the Zeeman state of the atom after spontaneous decay. Because these decay channels are spectrally and in all other degrees of freedom indistinguishable, the spin state of the atom is entangled with the polarization state of the photon. To verify entanglement, appropriate correlation measurements in complementary bases of the photon polarization and the internal quantum state of the atom are performed. It is shown, that the generated atom-photon state yields an entanglement fidelity of 0.82. The experimental results of this work mark an important step towards the generation of entanglement between space-like separated atoms for a first loophole-free test of Bell's inequality. Furthermore entanglement between a single atom and a single photon is an important tool for new quantum communication and information applications, e.g. the remote state preparation of a single atom over large distances.
Quantum information technology has been developing rapidly in last two decades. Very recently, quite a few optical quantum key distribution networks have been reported, enabling preliminary practical applications in secure information transfer. Meanwhile, a number of significant progresses based on quantum network have also been made in the research of linear optical quantum computing, simulation and metrology. However, on the way towards large-scale optical quantum network serious problems occur. On the one hand, the distance of fiber-based quantum communication is limited, due to intrinsic fiber loss and decreasing of entanglement quality caused by the noisy environment. On the other hand, the probabilistic feature of single and entangled photon sources would also cause an exponentially increasing overhead for large-scale quantum network. To solve the above problems, quantum repeaters and/or transmission of optical quantum bits over free space channel can be efficiently exploited for future wide-area realization of quantum communication. In addition, memory built-in quantum repeaters would also enable scalable linear optical quantum computing, simulation and high precision measurement. In this talk I will present some recent experiments from our group, including eight-photon entanglement, topological quantum error-correction, quantum repeater and efficient and long-lived quantum memory, and entanglement distribution and quantum teleportation over 100km-scale free-space quantum channels. These experiments show the promising future possibility towards scalable quantum network with photons and atoms.