[Paper Review] The Flux Qubit Revisited

Fei Yan, Simon Gustavsson+11 more|arXiv (Cornell University)|Aug 25, 2015

Quantum Information and Cryptography9 references6 citations

TL;DR

This paper revisits the superconducting flux qubit design, achieving a planar device with broad frequency tunability, strong anharmonicity, and $T_1 > 40\,\mu\text{s}$ across 22 qubits. By identifying photon shot noise in the readout resonator as the dominant $T_2$ limit and mitigating it via dynamical decoupling, the authors achieve $T_2 \approx 85\,\mu\text{s}$, approaching the $2T_1$ limit, thus establishing a key pathway to high-coherence, reproducible qubits.

ABSTRACT

The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). Here, we revisit the design and fabrication of the superconducting flux qubit, achieving a planar device with broad frequency tunability, strong anharmonicity, high reproducibility, and relaxation times in excess of $40\,\mu$s at its flux-insensitive point. Qubit relaxation times $T_1$ across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise, and 1/f flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in $T_2\approx 85\,\mu$s, approximately the $2T_1$ limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting $T_2$ in contemporary qubits based on transverse qubit-resonator interaction.

Motivation & Objective

• To improve the coherence and reproducibility of superconducting flux qubits for scalable quantum information processing.
• To identify and mitigate the dominant noise source limiting $T_2$ in transverse qubit-resonator architectures.
• To achieve consistent $T_1$ performance across 22 qubits using a unified noise model.
• To demonstrate that photon shot noise in the resonator is the primary cause of dephasing at the flux-insensitive point.

Proposed method

• Design and fabrication of a planar flux qubit with broad frequency tunability and high anharmonicity.
• Use of a single noise model combining resonator loss, ohmic charge noise, and 1/f flux noise to explain $T_1$ across 22 qubits.
• Identification of residual thermal photons in the readout resonator as the dominant source of $T_2$ dephasing.
• Implementation of a dynamical decoupling protocol to suppress photon shot noise and extend $T_2$.
• Measurement of $T_1$ and $T_2$ at the flux-insensitive point to assess coherence under controlled noise conditions.
• Systematic characterization of qubit relaxation and dephasing across multiple devices to ensure reproducibility.

Experimental results

Research questions

• RQ1 What noise sources dominate $T_1$ and $T_2$ in planar flux qubits at the flux-insensitive operating point?
• RQ2 Can a single noise model consistently explain $T_1$ across 22 independently fabricated qubits?
• RQ3 Is photon shot noise in the readout resonator the primary limit on $T_2$ in transverse qubit-resonator systems?
• RQ4 To what extent can dynamical decoupling mitigate photon shot noise to extend $T_2$?
• RQ5 Can $T_2$ be extended to approach the $2T_1$ limit in a reproducible, planar flux qubit architecture?

Key findings

• Qubit relaxation times $T_1$ exceed $40\,\mu\text{s}$ across 22 qubits, with consistent performance explained by a unified noise model.
• $T_1$ is well described by a model including resonator loss, ohmic charge noise, and 1/f flux noise, with the latter previously thought to affect only dephasing.
• Dephasing at the flux-insensitive point is dominated by residual thermal photons in the readout resonator, identified as the primary $T_2$ limit.
• Photon shot noise is mitigated using a dynamical decoupling protocol, resulting in $T_2 \approx 85\,\mu\text{s}$.
• The achieved $T_2$ value is approximately the $2T_1$ limit, indicating near-optimal coherence under current noise conditions.
• The results uniquely identify photon shot noise as a critical, previously underappreciated, limit on $T_2$ in transverse qubit-resonator systems.