Unified Framework for Direct and Complete Characterization of an Unknown Kraus Operator and Density Matrix Using a Single Input State

TL;DR

This paper introduces a unified method for fully characterizing unknown quantum operations and states using only one input state, overcoming practical limitations in experimental quantum systems.

Contribution

It presents a novel framework that enables complete quantum characterization with a single input state, applicable to Kraus operators, observables, unitaries, and density matrices.

Findings

Allows characterization with a single input state

No constraints on system-probe coupling strength

Applicable to various quantum operators and states

Abstract

Characterization of quantum measurements and dynamical processes is typically performed using pure state preparations. However, in realistic experimental settings, the preparation of pure states is often infeasible due to noise and system constraints. In this work, we present a unified framework that enables the direct and complete characterization of an unknown Kraus operator using only a single input state. The same framework also supports the characterization of unknown observable, unitary operator, and density matrix. Remarkably, all these tasks are accomplished using a single input state, a set of projector-based unitary evolution operators, and the measurement of a single observable. Importantly, our approach imposes no constraints on the strength of the coupling between the system and a probe.