Quantum mechanics and reality
Associate Professor Margaret Reid
Centre for Cold Atoms and Ultrafast Spectroscopy, Swinburne University of Technology, Melbourne
Einstein was not happy with quantum mechanics. He believed it correct, but incomplete. In 1935, Einstein, Podolsky and Rosen (EPR) formulated a strong argument in favour of this viewpoint. The modern version of their famous “paradox”, developed by Bohm and Bell, is based on the singlet state of two spin-half particles, and reveals the incompatibility of quantum mechanics with “local reality”. “Local reality” is essentially the idea that values of measurement are predetermined, and that there is no “spooky action-at-a-distance”. While Einstein would have argued in favour of local realism, recent experiments performed by Zeilinger and co-workers have confirmed with no loopholes that quantum mechanics correctly describes the correlations of an equivalent singlet state for photons, and that local realism fails in this example.
Where does that leave us? Schrodinger responded to 1935 EPR’s paper, by constructing his famous “cat paradox”, in which quantum mechanics seemingly predicts failure of macroscopic reality. Penrose and Diosi among other theorists have challenged quantum mechanics in the regime of superpositions involving massive objects. Yet, mesoscopic Schrodinger cat states of several atoms have now been generated in the laboratory of Wineland, and the decoherence of photonic cat states observed in the laboratory of Haroche.
After a review, I will discuss the possibility of new tests of quantum mechanics. These include testing mesoscopic reality with cold atoms using Leggett-Garg inequalities, weak values and measurements, and a new sort of photonic Schrodinger cat that can be shown to be clearly not “dead or alive”.