Abstract
Cat qubits, for which logical and are coherent states of a harmonic mode, offer a promising route towards quantum error correction. Using dissipation to our advantage so that photon pairs of the harmonic mode are exchanged with single photons of its environment, it is possible to stabilize the logical states and exponentially increase the bit-flip time of the cat qubit with the photon number . A large two-photon dissipation rate ensures fast qubit manipulation and short error-correction cycles, which are instrumental to correct the remaining phase-flip errors in a repetition code of cat qubits. Here, we introduce and operate an autoparametric superconducting circuit that couples a mode containing the cat qubit to a lossy mode whose frequency is set at twice that of the cat mode. This passive coupling does not require a parametric pump, and it reaches a rate . With such a strong two-photon dissipation, bit-flip errors of the autoparametric cat qubit are prevented for a characteristic time up to 0.3 s with only a mild impact on phase-flip errors. In addition, we illustrate how the phase of a quantum superposition between and can be arbitrarily changed by driving the harmonic mode while keeping the engineered dissipation active.
- Received 13 July 2023
- Revised 9 February 2024
- Accepted 18 March 2024
DOI:https://doi.org/10.1103/PhysRevX.14.021019
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.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The fragility of quantum systems hinders the promise of practical quantum computing, but quantum error correction offers a promising way forward. The use of bosonic modes as quantum bits could considerably lower the number of qubits needed for such correction. One notable example is the cat qubit, based on quantum superpositions of two classical states of light. Amazingly, dissipation can be used to stabilize cat qubits by constantly removing entropy from the resonator, preserving quantum information. The stronger the engineered dissipation, the faster entropy is removed and the better the cat qubit is corrected. Our study builds upon prior methods by introducing an autoparametric superconducting circuit to increase the engineered dissipation rate.
The cat qubit stands out for its exponential suppression of bit-flip errors as the number of photons increases, at the modest cost of a linear increase of phase-flip error probability. To increase the dissipation rate—and hence stabilize such a qubit—we channel the dissipation through a second resonator at twice the qubit frequency. We demonstrate the expected exponential improvement of bit-flip time, extending its value by more than 4 orders of magnitude up to 0.3 s, while the phase-flip time decreases only by a factor of 6. Additionally, we demonstrate coherent control of this cat qubit, performing a fundamental quantum operation known as a “Z gate” with a fidelity of 96.5%.
This cat qubit could be the basis of a repetition code that corrects the remaining phase flip errors. With such a large dissipation rate, potentially more interesting qubits based on superpositions of four classical states of light could also be protected.
