Final answer:
The Next Generation Bell tests closed the detection loophole, which related to the issue of measuring devices failing to detect all entangled particles, thus ensuring a more accurate demonstration of quantum entanglement.
Step-by-step explanation:
The Next Generation Bell tests were designed to address and close the detection loophole. The Bell tests, based on the Bell's theorem, are experimental tests that aim to prove the existence of quantum entanglement and refute the principle of local realism, an aspect of classical physics which insists that the results of measurements should be determined by pre-existing conditions (or 'hidden variables') in the immediate surroundings of the test, rather than being influenced by events happening elsewhere in the universe.
The detection loophole occurs when measuring devices fail to detect all the entangled particles, introducing the possibility that the sample detected is not representative of the entire system. This can potentially skew the results of the test and compromise its validity. By improving the efficiency of detection and ensuring that a sufficient percentage of particles are detected, the Next Generation Bell tests successfully closed this loophole, thereby reinforcing the validity of quantum mechanical predictions over local realism.