The Bell test experiments are a series of experiments designed to test the predictions of quantum mechanics against certain assumptions made in Einstein's local realism, which aimed to challenge the concept of quantum entanglement. The most famous version of the Bell test is known as the Bell's inequality test, which was formulated by physicist John Bell in the 1960s.
In Einstein's local realism, it was proposed that physical properties of particles exist independently of any measurement, and that there is a hidden reality underlying quantum mechanics. This idea suggests that there should be a limit to the correlations between measurements on entangled particles, known as Bell's inequalities.
However, Bell's inequality test, based on the work of Bell and later experiments inspired by it, has consistently shown results that violate these inequalities. This violation implies that the predictions of quantum mechanics are in conflict with the assumptions of local realism and that quantum entanglement does indeed exist.
The basic setup of a Bell test experiment involves creating pairs of entangled particles, such as photons or electrons, and measuring certain properties (e.g., spin) of these particles at different angles or orientations. By comparing the measured correlations between the particles, researchers can evaluate whether the observed correlations match the predictions of quantum mechanics or violate Bell's inequalities.
The experimental results consistently show correlations that violate Bell's inequalities, indicating that the observed correlations between the entangled particles cannot be explained by local hidden variables. This implies that the entangled particles are truly interconnected, regardless of the distance between them, which is often referred to as "spooky action at a distance."
The Bell test experiments have been conducted using various setups and particle systems, including photons and electron pairs. Notable experiments include those performed by John Clauser, Alain Aspect, and others in the 1970s and subsequent decades.
The results of these experiments have provided strong empirical evidence supporting the concept of quantum entanglement and challenging Einstein's local realism. While the experiments themselves cannot definitively prove that quantum entanglement is the ultimate reality, they demonstrate that local realism is inconsistent with the observed phenomena, paving the way for a better understanding of quantum mechanics and its implications for the nature of reality.