Loophole-free Bell inequality violation with superconducting circuits.

Superposition, entanglement and non-locality constitute fundamental features of quantum physics. The fact that quantum physics does not follow the principle of local causality 1-3 can be experimentally demonstrated in Bell tests 4 performed on pairs of spatially separated, entangled quantum systems. Although Bell tests, which are widely regarded as a litmus test of quantum physics, have been explored using a broad range of quantum systems over the past 50 years, only relatively recently have experiments free of so-called loopholes 5 succeeded. Such experiments have been performed with spins in nitrogen-vacancy centres 6 , optical photons 7-9 and neutral atoms 10 . Here we demonstrate a loophole-free violation of Bell's inequality with superconducting circuits, which are a prime contender for realizing quantum computing technology 11 . To evaluate a Clauser-Horne-Shimony-Holt-type Bell inequality 4 , we deterministically entangle a pair of qubits 12 and perform fast and high-fidelity measurements 13 along randomly chosen bases on the qubits connected through a cryogenic link 14 spanning a distance of 30 metres. Evaluating more than 1 million experimental trials, we find an average S value of 2.0747 ± 0.0033, violating Bell's inequality with a P value smaller than 10 -108 . Our work demonstrates that non-locality is a viable new resource in quantum information technology realized with superconducting circuits with potential applications in quantum communication, quantum computing and fundamental physics 15 .