High-fidelity transmission of entanglement over a high-loss free-space channel

Abstract

Quantum entanglement enables tasks not possible in classical physics. Many quantum communication protocols [1] require the distribution of entangled states between distant parties. Here we experimentally demonstrate the successful transmission of an entangled photon pair over a 144 km free-space link. The received entangled states have excellent, noise-limited fidelity, even though they are exposed to extreme attenuation dominated by turbulent atmospheric effects. The total channel loss of 64 dB corresponds to the estimated attenuation regime for a two-photon satellite quantum communication scenario. We confirm that the received two-photon states are still highly entangled by violating the CHSH inequality by more than 5 standard deviations. From a fundamental point of view, our results show that the photons are subject to virtually no decoherence during their 0.5 ms long flight through air, which is encouraging for future world-wide quantum communication scenarios.Entanglement is at the heart of many peculiarities encountered in quantum mechanics and has allowed many ground-breaking tests on the fundamentals of nature. Entangled photons are ideal tools to investigate the laws of quantum mechanics over long distances and timescales since they are not subject to decoherence. Furthermore photons can be easily generated, manipulated and transmitted over large distances via optical fibres or free-space links. Since the maximal distance for the distribution of quantum entanglement in optical fibres is limited to the order [2, 3, 4, 5] of ∼ 100 km, the most promising option for testing quantum entanglement on a global scale is currently free-space transmission, ultimately using satellites and ground stations [6].In recent years, various free-space quantum communication experiments with weak coherent laser pulses [7,8,9,10,11] and entangled photons [12,13,14,15] have been performed on ever larger distance scales and with increasing bit rates. The to-date most advanced test bed for free-space distribution of entanglement is a 144 km free-space link between two Canary Islands, where the successful transmission of one photon of an entangled pair was recently achieved [16]. In the present experiment we demonstrate a fundamentally more interesting scenario by sending both photons of an entangled pair over this free-space channel. By violating a Clauser-HorneShimony-Holt (CHSH) Bell inequality [17] we find that entanglement is highly stable over these long time spans -the photon-pair flight time of ∼ 0.5 ms is the longest lifetime of photonic Bell states reported so far, almost twice as long as the previous high [4,5] of ∼ 250 µs.The achieved noise-limited fidelity paves the way for free-space implementations of quantum communication protocols that require the transmission of two photons, e.g. quantum dense coding [18], entanglement purification [19], quantum teleportation [20] and quantum key distribution without a shared reference frame [21]. From a technological perspective, the overall two-photon loss bridged in our exper...

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