High-fidelity software-defined quantum logic on a superconducting qudit

TL;DR

This paper demonstrates a high-fidelity, software-defined quantum gate on a superconducting qudit, showcasing a flexible control method that bypasses traditional primitive gate sets with an average fidelity of 99.4%.

Contribution

It introduces a novel optimal control approach to implement a nontrivial quantum gate without relying on primitive gates, enhancing quantum control flexibility.

Findings

Achieved a 99.4% fidelity for the SWAP gate.

Developed a procedure for optimal control calibration and characterization.

Showcased a generalizable method for quantum gate implementation.

Abstract

Nearly all modern solid-state quantum processors approach quantum computation with a set of discrete qubit operations (gates) that can achieve universal quantum control with only a handful of primitive gates. In principle, this approach is highly flexible, allowing full control over the qubits' Hilbert space without necessitating the development of specific control protocols for each application. However, current error rates on quantum hardware place harsh limits on the number of primitive gates that can be concatenated together (with compounding error rates) and remain viable. Here, we report our efforts at implementing a software-defined $0\leftrightarrow2$ SWAP gate that does not rely on a primitive gate set and achieves an average gate fidelity of $99.4\%$. Our work represents an alternative, fully generalizable route towards achieving nontrivial quantum control through the use of optimal control techniques. We describe our procedure for computing optimal control solutions, calibrating the quantum and classical hardware chain, and characterizing the fidelity of the optimal control gate.