Joint numerical ranges, quantum maps, and joint numerical shadows

TL;DR

This paper introduces the concept of joint numerical shadows for hermitian matrices, explores their relationship with quantum maps, and demonstrates how quantum dynamics can influence numerical ranges.

Contribution

It defines joint numerical shadows for multiple matrices, relates them to quantum maps, and analyzes how quantum dynamics affect numerical ranges under certain conditions.

Findings

Joint numerical shadows are supported on the joint numerical range.

Quantum maps with identity resolution shrink numerical ranges.

The joint numerical range and shadow coincide with classical numerical concepts for complex matrices.

Abstract

We associate with k hermitian N\times N matrices a probability measure on R^k. It is supported on the joint numerical range of the k-tuple of matrices. We call this measure the joint numerical shadow of these matrices. Let k=2. A pair of hermitian N\times N matrices defines a complex N\times N matrix. The joint numerical range and the joint numerical shadow of the pair of hermitian matrices coincide with the numerical range and the numerical shadow, respectively, of this complex matrix. We study relationships between the dynamics of quantum maps on the set of quantum states, on one hand, and the numerical ranges, on the other hand. In particular, we show that under the identity resolution assumption on Kraus operators defining the quantum map, the dynamics shrinks numerical ranges.