Matthew Thibodeau

Readout-induced leakage of the fluxonium qubit

  1. Aayam Bista,
  2. Matthew Thibodeau,
  3. Ke Nie,
  4. Kaicheung Chow,
  5. Bryan K. Clark,
  6. and Angela Kou
Dispersive readout is widely used to perform high-fidelity measurement of superconducting qubits. Much work has been focused on the qubit readout fidelity, which depends on the achievable
signal-to-noise ratio and the qubit relaxation time. As groups have pushed to increase readout fidelity by increasing readout photon number, dispersive readout has been shown to strongly affect the post-measurement qubit state. Such effects hinder the effectiveness of quantum error correction, which requires measurements that both have high readout fidelity and are quantum non-demolition (QND). Here, we experimentally investigate non-QND effects in the fluxonium. We map out the state evolution of fluxonium qubits in the presence of resonator photons and observe that these photons induce transitions in the fluxonium both within and outside the qubit subspace. We numerically model our system and find that coupling the fluxonium-resonator system to an external spurious mode is necessary to explain observed non-QND effects.
arxiv pdf

The Floquet Fluxonium Molecule: Driving Down Dephasing in Coupled Superconducting Qubits

  1. Matthew Thibodeau,
  2. Angela Kou,
  3. and Bryan K. Clark
High-coherence qubits, which can store and manipulate quantum states for long times with low error rates, are necessary building blocks for quantum computers. We propose a superconducting
qubit architecture that uses a Floquet flux drive to modify the spectrum of a static fluxonium molecule. The computational eigenstates have two key properties: disjoint support to minimize bit flips, along with first- and second-order insensitivity to flux noise dephasing. The rates of the three main error types are estimated through numerical simulations, with predicted coherence times of approximately 50 ms in the computational subspace and erasure lifetimes of about 500 μs. We give a protocol for high-fidelity single qubit rotation gates via additional flux modulation on timescales of roughly 500 ns. Our results indicate that driven qubits are able to outperform some of their static counterparts.
arxiv pdf