Operational Characterization of Simultaneous Measurements in Quantum Mechanics

Quantum mechanics predicts the joint probability distribution of the outcomes of simultaneous measurements of commuting observables, but, in the state of the art, has lacked the operational definition of simultaneous measurements. The question is answered as to when the consecutive applications of measuring apparatuses give a simultaneous measurement of their observables. For this purpose, all the possible state reductions caused by measurements of an observable is also characterized by their operations.Comment: 9 pages, REVTe

Controlling Quantum State Reduction

Every measurement leaves the object in a family of states indexed by the possible outcomes. This family, called the posterior states, is usually a family of the eigenstates of the measured observable, but it can be an arbitrary family of states by controlling the object-apparatus interaction. A potentially realizable object-apparatus interaction measures position in such a way that the posterior states are the translations of an arbitrary wave function. In particular, position can be measured without perturbing the object in a momentum eigenstate.Comment: 9 pages, REVTe

Perfect correlations between noncommuting observables

The problem as to when two noncommuting observables are considered to have the same value arises commonly, but shows a nontrivial difficulty. Here, an answer is given by establishing the notion of perfect correlations between noncommuting observables, and applied to obtain a criterion for precise measurements of a given observable in a given state.Comment: 7 pages, Revtex4, v3: added a section on conventional models of measurement

Quantum Reality and Measurement: A Quantum Logical Approach

The recently established universal uncertainty principle revealed that two nowhere commuting observables can be measured simultaneously in some state, whereas they have no joint probability distribution in any state. Thus, one measuring apparatus can simultaneously measure two observables that have no simultaneous reality. In order to reconcile this discrepancy, an approach based on quantum logic is proposed to establish the relation between quantum reality and measurement. We provide a language speaking of values of observables independent of measurement based on quantum logic and we construct in this language the state-dependent notions of joint determinateness, value identity, and simultaneous measurability. This naturally provides a contextual interpretation, in which we can safely claim such a statement that one measuring apparatus measures one observable in one context and simultaneously it measures another nowhere commuting observable in another incompatible context.Comment: 16 pages, Latex. Presented at the Conference "Quantum Theory: Reconsideration of Foundations, 5 (QTRF5)," Vaxjo, Sweden, 15 June 2009. To appear in Foundations of Physics

An Operational Approach to Quantum State Reduction

An operational approach to quantum state reduction, the state change of the measured system caused by a measurement of an observable conditional upon the outcome of measurement, is founded without assuming the projection postulate in any stages of the measuring process. Whereas the conventional formula assumes that the probe measurement satisfies the projection postulate, a new formula for determining the state reduction shows that the state reduction does not depend on how the probe observable is measured, or in particular does not depend on whether the probe measurement satisfies the projection postulate or not, contrary to the longstanding attempts in showing how the macroscopic nature of probe detection provokes state reduction.Comment: 17 pages, LaTeX, no figures, to appear in Ann. Phys. (N.Y.