Nikitha Jain

Active isolation of transmon readout with a tunable inductive bridge

  1. Chunyang Ding,
  2. Chuyao Tong,
  3. Paul Varosy,
  4. Nikitha Jain,
  5. Ziyi Zhao,
  6. Eric I. Rosenthal,
  7. Konrad W. Lehnert,
  8. and David I. Schuster
Scalable superconducting quantum computation requires fast, high-fidelity readout. Currently, this is assisted through cryogenic amplification of the readout signal. In traditional
dispersive readout schemes, the amplifiers that are used to provide high signal-to-noise ratio also generate thermal noise that can decohere the qubits, demanding additional ferrite-based isolators to suppress this noise. Here, we integrate a chip-scale microwave switch, the tunable inductive bridge, into the readout chain of a fixed-frequency transmon. Flux-pulsing the switch, such that it transmits only during the readout, did not make a discernable difference to qubit coherence, residual excited-state population, or readout fidelity metrics. By injecting broadband noise that mimics amplifier thermal photons, we show that the switch suppresses the resulting qubit dephasing by more than 30~dB, consistent with its independently measured on-off ratio, establishing it as a potential chip-based integrated replacement for readout-chain isolators.
arxiv pdf

Benchmarking Single-Qubit Gates on a Noise-Biased Qubit Beyond the Fault-Tolerant Threshold

  1. Bingcheng Qing,
  2. Ahmed Hajr,
  3. Ke Wang,
  4. Gerwin Koolstra,
  5. Long B. Nguyen,
  6. Jordan Hines,
  7. Irwin Huang,
  8. Bibek Bhandari,
  9. Zahra Padramrazi,
  10. Larry Chen,
  11. Ziqi Kang,
  12. Christian Jünger,
  13. Noah Goss,
  14. Nikitha Jain,
  15. Hyunseong Kim,
  16. Kan-Heng Lee,
  17. Akel Hashim,
  18. Nicholas E. Frattini,
  19. Justin Dressel,
  20. Andrew N. Jordan,
  21. David I. Santiago,
  22. and Irfan Siddiqi
The ubiquitous noise in quantum system hinders the advancement of quantum information processing and has driven the emergence of different hardware-efficient quantum error correction
protocols. Among them, qubits with structured noise, especially with biased noise, are one of the most promising platform to achieve fault-tolerance due to the high error thresholds of quantum error correction codes tailored for them. Nevertheless, their quantum operations are challenging and the demonstration of their performance beyond the fault-tolerant threshold remain incomplete. Here, we leverage Schrödinger cat states in a scalable planar superconducting nonlinear oscillator to thoroughly characterize the high-fidelity single-qubit quantum operations with systematic quantum tomography and benchmarking tools, demonstrating the state-of-the-art performance of operations crossing the fault-tolerant threshold of the XZZX surface code. These results thus embody a transformative milestone in the exploration of quantum systems with structured error channels. Notably, our framework is extensible to other types of structured-noise systems, paving the way for systematic characterization and validation of novel quantum platforms with structured noise.
arxiv pdf