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

Towards a basic quantum repeater link over 400m with heralded entanglement of 87Rb-atoms

Kai Redeker, Daniel Burchardt, Norbert Ortegel, Julian Hofmann, Michael Krug, Markus Weber, Wenjamin Rosenfeld, Harald Weinfurter

Published May 1, 2013• 0 citations
Publisher

Abstract

This paper presents progress towards creating entanglement between two single trapped87Rb-atoms over a distance of 400m. The scheme starts by entangling each atom with a photon. In the next step the two photons are projected onto an entangled state via interferometric Bell-state analysis. The detection of the two photons heralds entanglement of the two atoms as was recently verified at a distance of 20m. The next step is increasing the physical distance of the atom traps to 400m. Furthermore a detection scheme consisting of state dependent ionization and subsequent detection of the ionization fragments will be implemented. This fast and efficient state measurement scheme will allow the measurement of the state of an atomic qubit in less than 1μs.

DOI
10.1109/cleoe-iqec.2013.6801717

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This paper presents progress towards creating entanglement between two single trapped87Rb-atoms over a distance of 400m. The scheme starts by entangling each atom with a photon. In the next step the two photons are projected onto an entangled state via interferometric Bell-state analysis. The detection of the two photons heralds entanglement of the two atoms as was recently verified at a distance of 20m. The next step is increasing the physical distance of the atom traps to 400m. Furthermore a detection scheme consisting of state dependent ionization and subsequent detection of the ionization fragments will be implemented. This fast and efficient state measurement scheme will allow the measurement of the state of an atomic qubit in less than 1μs.
Summary form only given. Here we present our experiment using heralded entanglement between two 87Rb-atoms separated by 400 m violating Bell's inequality [1]. First each atom is entangled with a photon, which is coupled into single-mode fiber. The interference of the two photons at a beam splitter enables one to project the state onto a Bell state, which leaves the atoms in an entangled state [2]. For space-like separation, the state measurements on the two atoms together with the random choice of the measurement basis, have to be performed in a time shorter than 1.33μs. The atomic state readout itself is based on a highly efficient state-selective ionization and subsequent detection of the ionization fragments by channel-electron-multipliers (CEM) [3]. Altogether the time for the state readout is below 1μs and the locality condition is fulfilled. Evaluation of 5000 events for each of two entangled atom-atom states shows a significant violation of Bell's inequality with S-parameters of 2.240 ± 0.047 and 2.204 ± 0.047.
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.
Abstract Quantum networks promise to provide the infrastructure for many disruptive applications, such as efficient long-distance quantum communication and distributed quantum computing1, 2. Central to these networks is the ability to distribute entanglement between distant nodes using photonic channels. Initially developed for quantum teleportation3, 4 and loophole-free tests of Bell’s inequality5, 6, recently, entanglement distribution has also been achieved over telecom fibres and analysed retrospectively7, 8. Yet, to fully use entanglement over long-distance quantum network links it is mandatory to know it is available at the nodes before the entangled state decays. Here we demonstrate heralded entanglement between two independently trapped single rubidium atoms generated over fibre links with a length up to 33 km. For this, we generate atom–photon entanglement in two nodes located in buildings 400 m line-of-sight apart and to overcome high-attenuation losses in the fibres convert the photons to telecom wavelength using polarization-preserving quantum frequency conversion9. The long fibres guide the photons to a Bell-state measurement setup in which a successful photonic projection measurement heralds the entanglement of the atoms10. Our results show the feasibility of entanglement distribution over telecom fibre links useful, for example, for device-independent quantum key distribution11–13 and quantum repeater protocols. The presented work represents an important step towards the realization of large-scale quantum network links.
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.
We report the observation of entanglement between a single trapped atom and a single photon at remote locations. The degree of coherence of the entangled atom-photon pair is verified via appropriate local correlation measurements, after communicating the photon via an optical fiber link of 300 m length to a receiver 3.5 m apart. In addition, we measured the temporal evolution of the atomic density matrix after projecting the atom via a state measurement of the photon onto several well-defined spin states. We find that the state of the single atom dephases on a time scale of 150 micros, which represents an important step towards long-distance quantum networking with individual neutral atoms.
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