Allowed region of the mean values of angular momentum observables and their uncertainty relations

TL;DR

This paper characterizes the allowed regions of expectation values for angular momentum observables, derives tight uncertainty relations, and explores their dependence on quantum number, including the multi-qubit limit.

Contribution

It provides a detailed analysis of the numerical range of angular momentum operators and establishes new tight uncertainty relations based on these regions.

Findings

Allowed regions depend on the angular momentum quantum number.

Tight uncertainty relations are derived for various functions of angular momentum.

In the large quantum number limit, results connect to multi-qubit systems via the de Finetti theorem.

Abstract

The expectation values of operators drawn from a single quantum state cannot be outside of a particular region, called their allowed region or the joint numerical range of the operators. Basically, the allowed region is an image of the state space under the Born rule. The maximum-eigenvalue-states---of every linear combination of the operators of interest---are sufficient to generate boundary of the allowed region. In this way, we obtain the numerical range of certain Hermitian operators (observables) that are functions of the angular momentum operators. Especially, we consider here three kinds of functions---combinations of powers of the ladder operators, powers of the angular momentum operators and their anticommutators---and discover the allowed regions of different shapes. By defining some specific concave (and convex) functions on the joint numerical range, we also achieve tight uncertainty (and certainty) relations for the observables. Overall, we demonstrate how the numerical range and uncertainty relations change as the angular momentum quantum number grows. Finally, we apply the quantum de Finetti theorem by taking a multi-qubit system and attain the allowed regions and tight uncertainty relations in the limit where the quantum number goes to infinity.