Analysis of parametrically driven exchange-type (iSWAP) and two-photon (bSWAP) interactions between superconducting qubits

TL;DR

This paper investigates parametrically driven exchange and two-photon interactions in superconducting qubits, deriving analytic models and experimentally validating the gates' performance, highlighting scalability and fidelity considerations.

Contribution

It introduces a theoretical framework for iSWAP and bSWAP gates in superconducting qubits and compares their speed and scalability through experiments and simulations.

Findings

bSWAP is slower than iSWAP but offers better scalability.

Experimental results confirm theoretical predictions.

Higher transmon levels and dissipation effects are included in simulations.

Abstract

A current bottleneck for quantum computation is the realization of high-fidelity two-qubit quantum operations between two and more quantum bits in arrays of coupled qubits. Gates based on parametrically driven tunable couplers offer a convenient method to entangle multiple qubits by selectively activating different interaction terms in the effective Hamiltonian. Here, we study theoretically and experimentally a superconducting qubit setup with two transmon qubits connected via a capacitively coupled tunable bus. We develop a time-dependent Schrieffer-Wolff transformation and derive analytic expressions for exchange-interaction gates swapping excitations between the qubits (iSWAP) and for two-photon gates creating and annihilating simultaneous two-qubit excitations (bSWAP). We find that the bSWAP gate is generally slower than the more commonly used iSWAP gate, but features favorable scalability properties with less severe frequency crowding effects, which typically degrade the fidelity in multi-qubit setups. Our theoretical results are backed by experimental measurements as well as exact numerical simulations including the effects of higher transmon levels and dissipation.