Single qubit gates in frequency-crowded transmon systems

TL;DR

This paper demonstrates that fast, high-fidelity single-qubit gates are achievable in frequency-crowded transmon systems using two-quadrature control and analytical pulse shaping, overcoming spectral crowding challenges.

Contribution

It introduces a novel pulse control method combining two-quadrature control and Magnus expansion-derived pulse shapes for efficient qubit addressing in crowded spectra.

Findings

Fast single-qubit gates with high fidelity are possible.

Two-quadrature control outperforms DRAG alone.

Leakage state anharmonicity does not limit gate speed.

Abstract

Recent experimental work on superconducting transmon qubits in 3D cavities show that their coherence times are increased by an order of magnitude compared to their 2D cavity counterparts. However to take advantage of these coherence times while scaling up the number of qubits it is advantageous to address individual qubits which are all coupled to the same 3D cavity fields. The challenge in controlling this system comes from spectral crowding, where leakage transition of qubits are close to computational transitions in other. Here it is shown that fast pulses are possible which address single qubits using two quadrature control of the pulse envelope while the DRAG method alone only gives marginal improvements over the conventional Gaussian pulse shape. On the other hand, a first order result using the Magnus expansion gives a fast analytical pulse shape which gives a high fidelity gate for a specific gate time, up to a phase factor on the second qubit. Further numerical analysis corroborates these results and yields to even faster gates, showing that leakage state anharmonicity does not provide a fundamental quantum speed limit.