Quantum Process Tomography of a Universal Entangling Gate Implemented with Josephson Phase Qubits

TL;DR

This paper demonstrates quantum process tomography on a superconducting two-qubit entangling gate, providing a complete characterization of its performance and decoherence mechanisms, marking a first in solid-state two-qubit systems.

Contribution

It is the first to perform quantum process tomography on a two-qubit superconducting gate, enabling detailed analysis of gate fidelity and decoherence in solid-state quantum computing.

Findings

Achieved a gate fidelity measurement using QPT

Demonstrated a high on/off coupling ratio of 300

Provided insights into decoherence mechanisms affecting the gate

Abstract

Quantum logic gates must perform properly when operating on their standard input basis states, as well as when operating on complex superpositions of these states. Experiments using superconducting qubits have validated the truth table for particular implementations of e.g. the controlled-NOT gate [1,2], but have not fully characterized gate operation for arbitrary superpositions of input states. Here we demonstrate the use of quantum process tomography (QPT) [3,4] to fully characterize the performance of a universal entangling gate between two superconducting quantum bits. Process tomography permits complete gate analysis, but requires precise preparation of arbitrary input states, control over the subsequent qubit interaction, and simultaneous single-shot measurement of the output states. We use QPT to measure the fidelity of the entangling gate and to quantify the decoherence mechanisms affecting the gate performance. In addition to demonstrating a promising fidelity, our entangling gate has a on/off ratio of 300, a level of adjustable coupling that will become a requirement for future high-fidelity devices. This is the first solid-state demonstration of QPT in a two-qubit system, as solid-state process tomography has previously only been demonstrated with single qubits [5,6].