ETH Zurich researchers demonstrated a loophole-free Bell test using superconducting circuits connected through a roughly 30-metre cryogenic link. The experiment tested correlations between entangled quantum systems while arranging the measurements so that a signal travelling at the speed of light could not pass between them in time to explain the result. Andreas Wallraff's group carried out the work on ETH's Zurich campus.
More than a million measurements violated Bell's inequality, extending this form of test to superconducting circuits used in quantum-computing hardware. The apparatus required a long vacuum and cooling system linking the two experimental stations while preserving fragile quantum states. Beyond testing quantum theory, the work demonstrated techniques relevant to connecting superconducting quantum devices over distance. The measured correlations provided a stringent test of quantum behaviour in these engineered electrical circuits.
