Ultracoherent superconducting cavity-based multiqudit platform with error-resilient control

TL;DR

This paper demonstrates a superconducting cavity-based multiqudit system with ultra-long coherence times and high-fidelity control, enabling advanced quantum information processing with error mitigation techniques.

Contribution

The authors design a multimode SRF cavity system with optimized transmon coupling, achieving record coherence times and high-fidelity state preparation and entanglement.

Findings

Single-photon lifetimes of 20.6 ms and 15.6 ms for two cavity modes.

Fock states up to N=20 prepared with over 95% fidelity.

Two-mode entanglement with 99.9% fidelity after post-selection.

Abstract

Superconducting radio-frequency (SRF) cavities offer a promising platform for quantum computing due to their long coherence times, yet integrating nonlinear elements like transmons for control often introduces additional loss. We report a multimode quantum system based on a 2-cell elliptical shaped SRF cavity, comprising two cavity modes weakly coupled to an ancillary transmon circuit, designed to preserve coherence while enabling efficient control of the cavity modes. We mitigate the detrimental effects of the transmon decoherence through careful design optimization that reduces transmon-cavity couplings and participation in the dielectric substrate and lossy interfaces, to achieve single-photon lifetimes of 20.6 ms and 15.6 ms for the two modes, and a pure dephasing time exceeding 40 ms. This marks an order-of-magnitude improvement over prior 3D multimode memories. Leveraging sideband interactions and novel error-resilient protocols, including measurement-based correction and post-selection, we achieve high-fidelity control over quantum states. This enables the preparation of Fock states up to $N = 20$ with fidelities exceeding 95%, the highest reported to date to the authors' knowledge, as well as two-mode entanglement with an estimated coherence-limited fidelities of 99.9% after post-selection. These results establish our platform as a robust foundation for quantum information processing, allowing for future extensions to high-dimensional qudit encodings.