Kaicheung Chow

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

Parametrically-controlled microwave-photonic interface for the fluxonium

  1. Ke Nie,
  2. Aayam Bista,
  3. Kaicheung Chow,
  4. Wolfgang Pfaff,
  5. and Angela Kou
Converting quantum information from stationary qubits to traveling photons enables both fast qubit initialization and efficient generation of flying qubits for redistribution of quantum
information. This conversion can be performed using cavity sideband transitions. In the fluxonium, however, direct cavity sideband transitions are forbidden due to parity symmetry. Here we circumvent this parity selection rule by using a three-wave mixing element to couple the fluxonium to a resonator. We experimentally demonstrate a scheme for interfacing the fluxonium with traveling photons through microwave-induced parametric conversion. We perform fast reset on the fluxonium qubit, initializing it with > 95% ground state population. We then implement controlled release and temporal shaping of a flying photon, useful for quantum state transfer and remote entanglement. The simplicity and flexibility of our demonstrated scheme enables fluxonium-based remote entanglement architectures.
arxiv pdf