Loophole-free test of local realism via Hardy's violation

TL;DR

This paper reports a loophole-free experimental test of Hardy's paradox using photonic entanglement, achieving high detection efficiency and fidelity, and strongly violating local realism with a large number of trials.

Contribution

The authors demonstrate a loophole-free violation of Hardy's paradox with high detection efficiency and fidelity, advancing experimental tests of local realism.

Findings

Strong violation of Hardy's paradox observed

Detection efficiency of 82.2% achieved

Results can be explained by local realism with probability less than 10^-16348

Abstract

Bell's theorem states that quantum mechanical description on physical quantity cannot be fully explained by local realistic theories, and lays solid basis for various quantum information applications. Hardy's paradox is celebrated to be the simplest form of Bell's theorem concerning its "All versus Nothing" way to test local realism. However, due to experimental imperfections, existing tests of Hardy's paradox require additional assumptions of experimental systems, which constitute potential loopholes for faithfully testing local realistic theories. Here, we experimentally demonstrate Hardy's nonlocality through a photonic entanglement source. By achieving a detection efficiency of $82.2\%$, a quantum state fidelity of $99.10\%$ and applying high speed quantum random number generators for measurement setting switching, the experiment is implemented in a loophole-free manner. During $6$ hours of running, a strong violation of $P_{\text{Hardy}}=4.646\times 10^{-4}$ up to $5$ standard deviations is observed with $4.32\times 10^{9}$ trials. A null hypothesis test shows that the results can be explained by local realistic theories with an upper bound probability of $10^{-16348}$. These testing results present affirmative evidence against local realism, and provide an advancing benchmark for quantum information applications based on Hardy's paradox.