Belltesten

Belltesten, or Bell tests, are experiments designed to test Bell inequalities derived by physicist John Bell in 1964. They probe whether the predictions of quantum mechanics for entangled particles can be explained by local realism, a view combining locality with predetermined properties (hidden variables). In a typical Bell test, two spatially separated measurement stations independently choose measurement settings (for example, polarizer angles for photons or spin directions for electrons) and record outcomes. The choices are made quickly and randomly to ensure independence (closing the freedom-of-choice loophole). The correlations of measurement outcomes are then analyzed to compute a Bell parameter, such as CHSH, and compared with the bound predicted by local hidden-variable theories.

Bell tests have shown violations of Bell inequalities that agree with quantum mechanical predictions, indicating that local realism cannot fully explain quantum correlations. Early experiments in the 1970s-80s by Alain Aspect and colleagues demonstrated violations but left loopholes. Subsequent experiments improved detector efficiency and spacelike separation to address the locality and detection loopholes. In the 2010s, multiple groups performed loophole-free Bell tests, providing stronger empirical support for quantum mechanics over local realism.

Bell tests have implications for foundational questions in quantum theory and practical applications in quantum information science, including device-independent quantum cryptography and randomness generation, where the observed quantum correlations certify security without relying on detailed models of the devices.