Speaker: Jonathan Halliwell (Imperial College, London)
Title: Comparing conditions for macrorealism: Leggett-Garg inequalities vs no-signalling in time
Macrorealism is the view that a macroscopic system evolving in time possesses definite properties which can be determined without disturbing the future or past state.
I discuss two different types of conditions which were proposed to test macrorealism in the context of a system described by a single dichotomic variable Q. The Leggett-Garg (LG) inequalities, the most commonly-studied test, are only necessary conditions for macrorealism, but I show that when the four three-time LG inequalities are augmented with a certain set of two-time inequalities also of the LG form, Fine's theorem applies and these augmented conditions are then both necessary and sufficient. A comparison is carried out with a very different set of necessary and sufficient conditions for macrorealism, namely the no-signaling in time (NSIT) conditions proposed by Brukner, Clemente, Kofler and others, which ensure that all probabilities for Q at one and two times are independent of whether earlier or intermediate measurements are made in a given run, and do not involve (but imply) the LG inequalities. I argue that tests based on the LG inequalities have the form of very weak classicality conditions and can be satisfied, in quantum mechanics, in the face of moderate interference effects, but those based on NSIT conditions have the form of much stronger coherence witness conditions, satisfied only for zero interference. The two tests differ in their implementation of non-invasive measurability so are testing different notions of macrorealism. The augmented LG tests are indirect, entailing a combination of the results of different experiments with only compatible quantities measured in each experimental run, in close analogy with Bell tests, and are primarily tests for macrorealism per se. By contrast the NSIT tests entail sequential measurements of incompatible quantities and are primarily tests for non-invasiveness.
Based on the two papers J.J.Halliwell, Phys.Rev. A93, 022123 (2016); A96, 012121 (2017).