IQI Weekly Seminar

For more than 80 years, the counterintuitive predictions of quantum theory have stimulated debate about the nature of reality. In his seminal work, John Bell proved that no theory of nature that obeys locality and realism can reproduce all the predictions of quantum theory. In any local realist theory the correlations between distant measurements satisfy an inequality that can be violated according to quantum theory if the measurements are performed on entangled particles. This provided a recipe for experimental tests of the fundamental principles underlying the laws of nature. In the past decades, numerous ingenious Bell inequality tests have been reported. However, because of experimental limitations, all experiments required additional assumptions to obtain a contradiction with local realism, resulting in loopholes. Here we report on a Bell experiment that is free of any such additional assumption. We employ an event-ready scheme that enables the generation of high-fidelity entanglement between distant electron spins. Efficient spin readout avoids the fair sampling assumption (detection loophole), while the use of fast random basis selection and spin readout combined with a spatial separation of 1.3 km ensure the required locality conditions. We perform 245 trials testing the CHSH-Bell inequality S ≤ 2 and find S = 2.42 ± 0.20. A null hypothesis test yields a probability of at most p = 0.039 that a local-realist model for space-like separated sites produces data with a violation at least as large as we observe, even when allowing for memory in the devices. This result rejects large classes of local realist theories, and paves the way for implementing device-independent quantum-secure communication and randomness certification.