Pauli Tomography: complete characterization of a single qubit device

TL;DR

This paper presents Pauli Tomography, a method to fully characterize a quantum device by applying a linear quantum operation to one half of a maximally entangled pair and performing joint Pauli-quorum tomography on the output. By mapping the operation to a Choi-like state, the technique reconstructs the device's transfer matrix from experimental data, demonstrated here on single-qubit polarization qubits using SPDC-generated entanglement. The authors show complete reconstruction of the device unitary for test waveplates, with results agreeing with theory and quantified uncertainties. They argue the approach generalizes to multi-qubit systems and could enable efficient quantum process tomography for complex devices like CNOT gates, offering a practical, entanglement-enabled framework for quantum measurement and information processing.

Abstract

The marriage of Quantum Physics and Information Technology, originally motivated by the need for miniaturization, has recently opened the way to the realization of radically new information-processing devices, with the possibility of guaranteed secure cryptographic communications, and tremendous speedups of some complex computational tasks. Among the many problems posed by the new information technology there is the need of characterizing the new quantum devices, making a complete identification and characterization of their functioning. As we will see, quantum mechanics provides us with a powerful tool to achieve the task easily and efficiently: this tools is the so called quantum entanglement, the basis of the quantum parallelism of the future computers. We present here the first full experimental quantum characterization of a single-qubit device. The new method, we may refer to as ''quantum radiography'', uses a Pauli Quantum Tomography at the output of the device, and needs only a single entangled state at the input, which works on the test channel as all possible input states in quantum parallel. The method can be easily extended to any n-qubits device.