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Published March 25, 2025 | Version Published
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Preserving Phase Coherence and Linearity in Cat Qubits with Exponential Bit-Flip Suppression

Creators

  • 1. ROR icon Amazon (United States)
  • 2. ROR icon California Institute of Technology

Abstract

Cat qubits, a type of bosonic qubit encoded in a harmonic oscillator, can exhibit an exponential noise bias against bit-flip errors with increasing mean photon number. Here, we focus on cat qubits stabilized by two-photon dissipation, where pairs of photons are added and removed from a harmonic oscillator by an auxiliary, lossy buffer mode. This process requires a large loss rate and strong nonlinearities of the buffer mode that must not degrade the coherence and linearity of the oscillator. In this work, we show how to overcome this challenge by coloring the loss environment of the buffer mode with a multipole filter and optimizing the circuit to take into account additional inductances in the buffer mode. Using these techniques, we achieve near-ideal enhancement of cat-qubit bit-flip times with increasing photon number, reaching over 0.1 s with a mean photon number of only 4. Concurrently, our cat qubit remains highly phase coherent, with phase-flip times corresponding to an effective lifetime of 𝑇1,eff≃70  μ⁢s, comparable with the bare oscillator lifetime. We achieve this performance even in the presence of an ancilla transmon, used for reading out the cat-qubit states, by engineering a tunable oscillator-ancilla dispersive coupling. Furthermore, the low nonlinearity of the harmonic oscillator mode allows us to perform pulsed cat-qubit stabilization, an important control primitive, where the stabilization can remain off for a significant fraction (e.g., two-thirds) of a 3  μ⁢s cycle without degrading bit-flip times. These advances are important for the realization of scalable error correction with cat qubits, where large noise bias and low phase-flip error rate enable the use of hardware-efficient outer error-correcting codes.

Copyright and License

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Acknowledgement

We thank Alex Retzker and Yufeng Ye for helpful comments on the manuscript and the staff from across the AWS Center for Quantum Computing that enabled this project. We also thank Fiona Harrison, Harry Atwater, David Tirrell, and Tom Rosenbaum at Caltech and Simone Severini, Bill Vass, James Hamilton, Nafea Bshara, and Peter DeSantis at AWS for their involvement and support of the research activities at the AWS Center for Quantum Computing.

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PhysRevX.15.011070.pdf

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2409.17556 (arXiv)

Funding

Amazon (United States)

Dates

Accepted
2025-02-12

Caltech Custom Metadata

Caltech groups
AWS Center for Quantum Computing, Institute for Quantum Information and Matter, Division of Engineering and Applied Science (EAS), Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published