Non-Ideal Measurements and Physical Possibility in Quantum Mechanics

M Suarez, Marco Del Seta

Matthew Colleti

According to the Heisenberg uncertainty principle, if we measure one variable describing a physical system, we inevitably introduce noise into some other variable describing the same system. In practice, the situation is worse than that: measuring one variable usually introduces random noise into the very quantity we are measuring. It is possible in principle to make measurements, known as quantum non-demolition (or QND) measurements, which do not have this latter effect. A QND measurement may approach the ideal quantum measurement commonly encountered in quantum mechanics textbooks. Philippe Grangier and his co-workers at the Université Paris-Sud, at Orsay in France, have recently attempted to measure the intensity of a beam of light in this way (Phys. Rev. Len. (1991) 66 1418). Their experiment has shown that it is indeed possible to make a measurement which at least partially preserves the quantum mechanical state of the detected beam.

C. Piron

Debashis Saha, Zhen-Peng Xu, Adán Cabello

A fundamental prediction of quantum mechanics is that observables can be measured through ideal measurements, that is, measurements that give the same outcome when repeated and do not disturb any observable that is jointly measurable. In terms of quantum instruments, such an ideal measurement is described by projective effects together with the associated Luders state-update rule. Here, we show that quantum mechanics allows for self-testing any ideal measurement. Using solely the output statistics of bipartite Bell tests in which a non-demolition measurement is sometimes added, it is possible to certify in a device-independent manner whether the added measurement is ideal.

Mauricio Suárez

Olímpia Lombardi, Mario Castagnino

The aim of this paper is to introduce a new member of the family of the modal interpretations of quantum mechanics. In this modal-Hamiltonian interpretation, the Hamiltonian of the quantum system plays a decisive role in the property-ascription rule that selects the definite-valued observables whose possible values become actual. We show that this interpretation is effective for solving the measurement problem, both in its ideal and its non-ideal versions, and we argue for the physical relevance of the property-ascription rule by applying it to well-known physical situations. Moreover, we explain how this interpretation supplies a description of the elemental categories of the ontology referred to by the theory, where quantum systems turn out to be bundles of possible properties.