Experimental Tests of Bell's Theorem
Numerous experiments have been conducted to test Bell's inequality. The overwhelming majority have confirmed the predictions of quantum mechanics and violated Bell's inequalities.
Typical Experimental Setup
- Source: Produces pairs of entangled particles (e.g., photons from parametric down-conversion, or atoms).
- Detectors: Alice and Bob each have a detector that can measure a property of their particle along a chosen axis. The settings (axes) can be changed.
- Random Setting Choice: Ideally, the settings and are chosen randomly and fast enough to prevent any light-speed communication between the choice of setting at one station and the measurement at the other (closing the "locality loophole").
- High Detection Efficiency: Detectors should be efficient enough to ensure that the detected pairs are a fair sample of all emitted pairs (closing the "detection loophole" or "fair sampling loophole").
Key Experiments
- Freedman and Clauser (1972): One of the first experiments, using entangled photons from calcium atom decay. Their results were consistent with quantum mechanics and violated Bell's inequality.
- Aspect, Grangier, and Roger (1981, 1982): Alain Aspect and his team performed a series of increasingly sophisticated experiments. Their 1982 experiment used time-varying polarizers, changing settings while photons were in flight, addressing a potential loophole related to the settings being fixed.
- Weihs et al. (1998): Gregor Weihs and Anton Zeilinger's group performed an experiment with random setting choices and spacelike separation of measurements, significantly strengthening the closure of the locality loophole.
- Hensen et al. (2015): Bas Hensen and Ronald Hanson's group at Delft University performed the first "loophole-free" Bell test, simultaneously closing both the locality and detection loopholes using entangled electron spins in diamond nitrogen-vacancy centers separated by 1.3 kilometers.
These experiments, and many others, have consistently shown violations of Bell's inequalities, supporting the predictions of quantum mechanics over local realism.