Optimization of Two-Qubit Gates in Tunable-Coupler Architectures Using Single Flux Quantum Control

TL;DR

This paper introduces a gradient-based optimization method for high-fidelity two-qubit gates in tunable-coupler architectures using single flux quantum pulses, achieving fidelities above 99.9%.

Contribution

It presents a novel continuous embedding scheme for optimizing discrete SFQ pulses and an analytical gate construction method reducing memory requirements.

Findings

Achieved average gate fidelities of 99.99% for fSim gates.

Realized CZ and CNOT gates with fidelities above 99.9%.

Provided an analytical decomposition method for gate construction.

Abstract

We present a gradient-based method to construct high-fidelity, two-qubit quantum gates in a system consisting of two transmon qubits coupled via a tunable coupler. In particular, we focus on single flux quantum (SFQ) pulses as a promising and scalable alternative to traditional control schemes that use microwave electronics. We develop a continuous embedding scheme to optimize these discrete pulses, taking advantage of auto-differentiation of our model. This approach allows us to achieve fSim-type gates with average gate fidelities on the order of 99.99% and CZ and CNOT gates with fidelities above 99.9%. Furthermore, we provide an alternative semi-analytical construction of these gates via an exact decomposition using a pair of fSim gates which leads to the reduction in memory required to store the associated pulse sequences.