Dario Rosenstock

Non-Markovian Relaxation Spectroscopy of Fluxonium Qubits

  1. Ze-Tong Zhuang,
  2. Dario Rosenstock,
  3. Bao-Jie Liu,
  4. Aaron Somoroff,
  5. Vladimir E. Manucharyan,
  6. and Chen Wang
Recent studies have shown that parasitic two-level systems (TLS) in superconducting qubits, which are a leading source of decoherence, can have relaxation times longer than the qubits
themselves. However, the standard techniques used to characterize qubit relaxation is only valid for measuring T1 under Markovian assumptions and could mask such non-Markovian behavior of the environment in practice. Here, we introduce two-timescale relaxometry, a technique to probe the qubit and environment relaxation simultaneously and efficiently. We apply it to high-coherence fluxonium qubits over a frequency range of 0.1-0.4 GHz, which reveals a discrete spectrum of TLS with millisecond lifetimes. Our analysis of the spectrum is consistent with a random distribution of TLS in the aluminum oxide tunnel barrier of the Josephson junction chain of the fluxonium with an average density and electric dipole similar to previous TLS studies at much higher frequencies. Our study suggests that investigating and mitigating TLS in the junction chain is crucial to the development of various types of noise-protected qubits in circuit QED.
arxiv pdf

Arbitrary controlled-phase gate on fluxonium qubits using differential ac-Stark shifts

  1. Haonan Xiong,
  2. Quentin Ficheux,
  3. Aaron Somoroff,
  4. Long B. Nguyen,
  5. Ebru Dogan,
  6. Dario Rosenstock,
  7. Chen Wang,
  8. Konstantin N. Nesterov,
  9. Maxim G. Vavilov,
  10. and Vladimir E. Manucharyan
Large scale quantum computing motivates the invention of two-qubit gate schemes that not only maximize the gate fidelity but also draw minimal resources. In the case of superconducting
qubits, the weak anharmonicity of transmons imposes profound constraints on the gate design, leading to increased complexity of devices and control protocols. Here we demonstrate a resource-efficient control over the interaction of strongly-anharmonic fluxonium qubits. Namely, applying an off-resonant drive to non-computational transitions in a pair of capacitively-coupled fluxoniums induces a ZZ-interaction due to unequal ac-Stark shifts of the computational levels. With a continuous choice of frequency and amplitude, the drive can either cancel the static ZZ-term or increase it by an order of magnitude to enable a controlled-phase (CP) gate with an arbitrary programmed phase shift. The cross-entropy benchmarking of these non-Clifford operations yields a sub 1% error, limited solely by incoherent processes. Our result demonstrates the advantages of strongly-anharmonic circuits over transmons in designing the next generation of quantum processors.
arxiv pdf