Entanglement is the essential feature of quantum mechanics. Notably, observers of two or more entangled particles will find correlations in their measurement results that cannot be explained by classical statistics. To make it a useful resource, particularly for scalable long-distance quantum communication, the heralded generation of entanglement between distant massive quantum systems is necessary. We report on the creation and analysis of heralded entanglement between spins of two single rubidium-87 atoms trapped independently 20 meters apart. Our results illustrate the viability of an integral resource for quantum information science, as well as for fundamental tests of quantum mechanics.

An experimental test of Bell's inequality allows ruling out any local-realistic description of nature by measuring correlations between distant systems. While such tests are conceptually simple, there are strict requirements concerning the detection efficiency of the involved measurements, as well as the enforcement of spacelike separation between the measurement events. Only very recently could both loopholes be closed simultaneously. Here we present a statistically significant, event-ready Bell test based on combining heralded entanglement of atoms separated by 398 m with fast and efficient measurements of the atomic spin states closing essential loopholes. We obtain a violation with S ¼ 2.221 AE 0.033 (compared to the maximal value of 2 achievable with models based on local hidden variables) which allows us to refute the hypothesis of local realism with a significance level P < 2.57 × 10

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Quantum teleportation allows to transfer the quantum state of a particle to a remote location without sending the particle itself. This makes it a versatile tool for the distribution of quantum states over long distances or the remote preparation of quantum memories as they are necessary in various scenarios of quantum information processing and quantum communication. In this context atomic systems are often discussed as promising candidates for the storage and manipulation of qubit states whereas photonic qubit states are easy to prepare and to distribute. Hybrid systems of entangled atom-photon pairs can serve as an interface between those different carriers of quantum information. We utilize this property to perform teleportation of the polarization state of an attenuated laser pulse onto a single atom over a distance of 20 m.Our experimental sequence consists of two steps. First the spin state of the single atom is entangled with the polarization state of a single photon in a spontaneous emission process [1]. The single photon is then sent to a second laboratory 20 m away where an attenuated laser pulse with a well defined polarization state |Ψ is prepared. In a second step a Bell-state measurement on the joint polarization state of the single photon from the atom and the attenuated laser pulse is performed (Fig. 1a). This is done via interference of the photon and the pulse at a fiber beam-splitter and subsequent coincidence detection of photons at the outputs of the beam-splitter (Fig. 1b). This teleports the input state |Ψ onto the atom -up to a unitary tranformation depending on the outcome of the Bell-state measurement. (a) atom PBS APD single-mode fiber fiber BS 20 m BSM coherent pulse atom coherent pulse atom click click BS BS (b) Fig. 1 a) Scheme for teleportation of a photonic input state |Ψ onto a single atom. b) Experimental setup: A 20m fiber link guides the single photon from the atom to the Bell state measurement were it interferes with a coherent pulse carrying the input state |Ψ .For a high fidelity of the Bell-state projection the wave functions of the single photon and the coherent pulse need to be indistinguishable in all degrees of freedom except for their polarization. In particular, the temporal shape of the pulse was matched to that of the single photon using an arbitrary waveform generator and an AOM in a closed-loop feedback control.We performed this experiment with input states |Ψ = |H , |+45 • and |R and evaluated the fidelity to find the atom in the correspondig output state. For |+45 • and |R the fraction of coherent pulses containing two or more photons can cause the same signal as expected from a successful Bell-state projection which reduces the overall fidelity to 0.81 and 0.84, respectively. However, for |H the fidelty is practically only limited by the precision of the atomic state readout and reaches a value of 0.93.

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