Consistent quantum measurements

• The quantum measurement problems are discussed using consistent histories.
• A Stern–Gerlach spin measurement illustrates the problems and their solutions.
• An appropriate framework solves the problem of definite output pointer states.
• The second problem is to relate measurement outcomes to measured properties.
• It is resolved by choosing a suitable family of consistent histories.

In response to recent criticisms by Okon and Sudarsky, various aspects of the consistent histories (CH) resolution of the quantum measurement problem(s) are discussed using a simple Stern‐Gerlach device, and compared with the alternative approaches to the measurement problem provided by spontaneous localization (GRW), Bohmian mechanics, many worlds, and standard (textbook) quantum mechanics. Among these CH is unique in solving the second measurement problem: inferring from the measurement outcome a property of the measured system at a time before the measurement took place, as is done routinely by experimental physicists. The main respect in which CH differs from other quantum interpretations is in allowing multiple stochastic descriptions of a given measurement situation, from which one (or more) can be selected on the basis of its utility. This requires abandoning a principle (termed unicity), central to classical physics, that at any instant of time there is only a single correct description of the world.