Abstract
The heavy-fluxonium circuit is a promising building block for superconducting quantum processors due to its long relaxation and dephasing time at the flux-frustration point. However, the suppressed charge matrix elements and low transition frequency make it challenging to perform fast single-qubit gates using standard protocols. We report on new protocols for reset, fast coherent control, and readout that allow high-quality operation of the qubit with a 14 MHz transition frequency, an order of magnitude lower in energy than the ambient thermal energy scale. We utilize higher levels of the fluxonium to read out the qubit state and to initialize the qubit with 97% fidelity corresponding to cooling it to . Instead of using standard microwave pulses, we control the qubit only with fast-flux pulses, generating control fields much larger than the qubit frequency. We develop a universal set of gates based on nonadiabatic Landau-Zener transitions that act in 20–60 ns, less than the single-qubit Larmor period. We measure qubit coherence of for a fluxonium in a 2D architecture and realize single-qubit gates with an average gate fidelity of 99.8% as characterized by randomized benchmarking.
- Received 4 March 2020
- Revised 21 October 2020
- Accepted 2 December 2020
DOI:https://doi.org/10.1103/PhysRevX.11.011010
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
Superconducting circuits are a promising platform for quantum computing. The biggest challenge in advancing this technology remains the realization of a quantum bit (or qubit) that is sufficiently coherent and can be manipulated with high fidelity. A recently introduced qubit design called the “heavy fluxonium” has better coherence than comparable designs, though it is challenging to manipulate with standard microwave pulses. To use this device as a practical qubit, we develop protocols to initialize and read out the stored quantum state and perform high-fidelity quantum gates.
The heavy fluxonium design represents a qubit as a current flowing either clockwise or counterclockwise in a ring composed of an array of superconducting junctions. In our device, we operate the qubit at a much lower frequency than previous designs, which drastically reduces the environmental noise. We also develop a cooling protocol similar to laser cooling of atoms to initialize the qubit. Despite its low frequency, we are able to operate it at similar speeds to traditional superconducting qubits by manipulating it with direct magnetic flux pulses that allow quantum gates to be performed in a single qubit oscillation period.
Though already demonstrating among the best qubit coherences and gate fidelities, improvements in design and materials should lead to significant improvements. Further next steps are to build devices with multiple coupled circuits and extend our protocols to demonstrate high-fidelity logic operations between multiple qubits.
