Practical implications of SFQ-based two-qubit gates

TL;DR

This paper analyzes SFQ-based two-qubit gates in superconducting quantum computers, addressing design challenges like leakage, and demonstrates that with optimal control and architecture, high-fidelity gates comparable to microwave systems are achievable, enhancing scalability.

Contribution

It provides the first thorough analysis of SFQ-based two-qubit gates, proposing control and architecture solutions to mitigate leakage and achieve high fidelity.

Findings

SFQ-based two-qubit gates exhibit high leakage to non-computational states.

Optimal control methods can suppress leakage and improve gate fidelity.

SFQ-based gates can match microwave-based systems in fidelity and speed.

Abstract

Scalability of today's superconducting quantum computers is limited due to the huge costs of generating/routing microwave control pulses per qubit from room temperature. One active research area in both industry and academia is to push the classical controllers to the dilution refrigerator in order to increase the scalability of quantum computers. Superconducting Single Flux Quantum (SFQ) is a classical logic technology with low power consumption and ultra-high speed, and thus is a promising candidate for in-fridge classical controllers with maximized scalability. Prior work has demonstrated high-fidelity SFQ-based single-qubit gates. However, little research has been done on SFQ-based multi-qubit gates, which are necessary to realize SFQ-based universal quantum computing. In this paper, we present the first thorough analysis of SFQ-based two-qubit gates. Our observations show that SFQ-based two-qubit gates tend to have high leakage to qubit non-computational subspace, which presents severe design challenges. We show that despite these challenges, we can realize gates with high fidelity by carefully designing optimal control methods and qubit architectures. We develop optimal control methods that suppress leakage, and also investigate various qubit architectures that reduce the leakage. After carefully engineering our SFQ-friendly quantum system, we show that it can achieve similar gate fidelity and gate time to microwave-based quantum systems. The promising results of this paper show that (1) SFQ-based universal quantum computation is both feasible and effective; and (2) SFQ is a promising approach in designing classical controller for quantum machines because it can increase the scalability while preserving gate fidelity and performance.