Uncertainties in Successive Measurements

TL;DR

This paper investigates how quantum measurements affect subsequent uncertainties, revealing limitations of Ozawa's inequality and providing sharper bounds for successive measurement uncertainties in quantum systems.

Contribution

The paper re-examines uncertainties in successive quantum measurements, showing Ozawa's inequality is ineffective and deriving sharper lower bounds for these uncertainties.

Findings

Ozawa's inequality cannot be saturated in the cases studied

Finite detector resolution significantly impacts successive measurement uncertainties

New sharper bounds improve understanding of quantum measurement uncertainties

Abstract

When you measure an observable, A, in Quantum Mechanics, the state of the system changes. This, in turn, affects the quantum-mechanical uncertainty in some non-commuting observable, B. The standard Uncertainty Relation puts a lower bound on the uncertainty of B in the initial state. What is relevant for a subsequent measurement of B, however, is the uncertainty of B in the post-measurement state. We re-examine this problem, both in the case where A has a pure point spectrum and in the case where A has a continuous spectrum. In the latter case, the need to include a finite detector resolution, as part of what it means to measure such an observable, has dramatic implications for the result of successive measurements. Ozawa proposed an inequality satisfied in the case of successive measurements. Among our results, we show that his inequality is ineffective (can never come close to being saturated). For the cases of interest, we compute a sharper lower bound.