Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [179]
The reason for employing photons rather than electrons is that they are easier to produce in the laboratory, especially since the experiment would involve numerous pairs of particles being measured. It was 1972 before Clauser and Freedman were ready to put Bell's inequality to the test. They heated calcium atoms until they acquired enough energy for an electron to jump from the ground state to a higher energy level. As the electron fell back down to the ground state, it did so in two stages and emitted a pair of entangled photons, one green and the other blue. The photons were sent in opposite directions until detectors simultaneously measured their polarisations. The two detectors were initially oriented at 22.5 degrees relative to each other for the first set of measurements, and then realigned at 67.5 degrees for the second set. Clauser and Freedman found, after 200hours of measurements, that the level of photon correlations violated Bell's inequality.
It was a result in favour of Bohr's non-local Copenhagen interpretation of quantum mechanics with its 'spooky action at a distance', and against the local reality backed by Einstein. But there were serious reservations as to the validity of the outcome. Between 1972 and 1977 different teams of experimenters conducted nine separate tests of Bell's inequality. It was violated in only seven.48 Given these mixed results, there were misgivings concerning the accuracy of the experiments. One problem was the inefficiency of the detectors that resulted in only a small fraction of the total number of pairs generated being measured. No one knew precisely what effect this had on the level of correlations. There were other loopholes that needed to be closed before it could be conclusively shown for whom Bell's theorem tolled.
As Clauser and others were busy planning and executing their experiments, a French physics graduate was doing voluntary work in Africa and spending his spare time reading up on quantum mechanics. It was while working his way through an influential French textbook on the subject that Alain Aspect first became fascinated by the EPR thought experiment. After reading Bell's seminal papers, he began thinking about subjecting Bell's inequality to a rigorous test. In 1974, after three years in Cameroon, Aspect returned to France.
The 27-year-old set about making his African dream come true in a basement laboratory at the Institut d'Optique Théoretique et Appliquée, Université Paris-Sud in Orsay. 'Do you have a permanent position?' Bell asked, when Aspect went to see him in Geneva.49 Aspect explained that he was just a graduate student aiming for a doctorate. 'You must be a very courageous graduate student', replied Bell.50 He was concerned that the young Frenchman could be damaging his future prospects by attempting to conduct such a difficult experiment.
It took longer than he imagined at the outset, but in 1981 and 1982 Aspect and his collaborators used the latest technological innovations, including lasers and computers, to perform not one but three delicate experiments to test Bell's inequality. Like Clauser, Aspect measured the correlation of the polarisation of entangled pairs of photons moving in opposite directions after being simultaneously emitted from individual calcium atoms. However, the rate at which photon pairs were created and measured was many times higher. His experiments revealed, said Aspect, 'the strongest violation of Bell's inequalities ever achieved, and excellent agreement with quantum mechanics'.51
Bell was one of the examiners when Aspect received his doctorate in 1983, but some doubts remained concerning the results. Since the nature of quantum reality