Rishi N. Patel

Preserving phase coherence and linearity in cat qubits with exponential bit-flip suppression

  1. Harald Putterman,
  2. Kyungjoo Noh,
  3. Rishi N. Patel,
  4. Gregory A. Peairs,
  5. Gregory S. MacCabe,
  6. Menyoung Lee,
  7. Shahriar Aghaeimeibodi,
  8. Connor T. Hann,
  9. Ignace Jarrige,
  10. Guillaume Marcaud,
  11. Yuan He,
  12. Hesam Moradinejad,
  13. John Clai Owens,
  14. Thomas Scaffidi,
  15. Patricio Arrangoiz-Arriola,
  16. Joe Iverson,
  17. Harry Levine,
  18. Fernando G.S.L. Brandão,
  19. Matthew H. Matheny,
  20. and Oskar Painter
Cat qubits, a type of bosonic qubit encoded in a harmonic oscillator, can exhibit an exponential noise bias against bit-flip errors with increasing mean photon number. Here, we focus
on cat qubits stabilized by two-photon dissipation, where pairs of photons are added and removed from a harmonic oscillator by an auxiliary, lossy buffer mode. This process requires a large loss rate and strong nonlinearities of the buffer mode that must not degrade the coherence and linearity of the oscillator. In this work, we show how to overcome this challenge by coloring the loss environment of the buffer mode with a multi-pole filter and optimizing the circuit to take into account additional inductances in the buffer mode. Using these techniques, we achieve near-ideal enhancement of cat-qubit bit-flip times with increasing photon number, reaching over 0.1 seconds with a mean photon number of only 4. Concurrently, our cat qubit remains highly phase coherent, with phase-flip times corresponding to an effective lifetime of T1,eff≃70 μs, comparable with the bare oscillator lifetime. We achieve this performance even in the presence of an ancilla transmon, used for reading out the cat qubit states, by engineering a tunable oscillator-ancilla dispersive coupling. Furthermore, the low nonlinearity of the harmonic oscillator mode allows us to perform pulsed cat-qubit stabilization, an important control primitive, where the stabilization can remain off for a significant fraction (e.g., two thirds) of a 3 μs cycle without degrading bit-flip times. These advances are important for the realization of scalable error-correction with cat qubits, where large noise bias and low phase-flip error rate enable the use of hardware-efficient outer error-correcting codes.
arxiv pdf

Loss channels affecting lithium niobate phononic crystal resonators at cryogenic temperature

  1. E. Alex Wollack,
  2. Agnetta Y. Cleland,
  3. Patricio Arrangoiz-Arriola,
  4. Timothy P. McKenna,
  5. Rachel G. Gruenke,
  6. Rishi N. Patel,
  7. Wentao Jiang,
  8. Christopher J. Sarabalis,
  9. and Amir H. Safavi-Naeini
We investigate the performance of microwave-frequency phononic crystal resonators fabricated on thin-film lithium niobate for integration with superconducting quantum circuits. For
different design geometries at millikelvin temperatures, we achieve mechanical internal quality factors Qi above 105−106 at high microwave drive power, corresponding to 5×106 phonons inside the resonator. By sweeping the defect size of resonators with identical mirror cell designs, we are able to indirectly observe signatures of the complete phononic bandgap via the resonators‘ internal quality factors. Examination of quality factors‘ temperature dependence shows how superconducting and two-level system (TLS) loss channels impact device performance. Finally, we observe an anomalous low-temperature frequency shift consistent with resonant TLS decay and find that material choice can help to mitigate these losses.
arxiv pdf

Cryogenic microwave-to-optical conversion using a triply-resonant lithium niobate on sapphire transducer

  1. Timothy P. McKenna,
  2. Jeremy D. Witmer,
  3. Rishi N. Patel,
  4. Wentao Jiang,
  5. Raphaël Van Laer,
  6. Patricio Arrangoiz-Arriola,
  7. E. Alex Wollack,
  8. Jason F. Herrmann,
  9. and Amir H. Safavi-Naeini
Quantum networks are likely to have a profound impact on the way we compute and communicate in the future. In order to wire together superconducting quantum processors over kilometer-scale
distances, we need transducers that can generate entanglement between the microwave and optical domains with high fidelity. We present an integrated electro-optic transducer that combines low-loss lithium niobate photonics with superconducting microwave resonators on a sapphire substrate. Our triply-resonant device operates in a dilution refrigerator and converts microwave photons to optical photons with an on-chip efficiency of 6.6×10−6 and a conversion bandwidth of 20 MHz. We discuss design trade-offs in this device, including strategies to manage acoustic loss, and outline ways to increase the conversion efficiency in the future.
arxiv pdf