Guillaume Marcaud

Improving the lifetime of aluminum-based superconducting qubits through atomic layer etching and deposition

  1. Neha Mahuli,
  2. Joaquin Minguzzi,
  3. Jiansong Gao,
  4. Rachel Resnick,
  5. Sandra Diez,
  6. Cosmic Raj,
  7. Guillaume Marcaud,
  8. Matthew Hunt,
  9. Loren Swenson,
  10. Jefferson Rose,
  11. Oskar Painter,
  12. and Ignace Jarrige
We present a dry surface treatment combining atomic layer etching and deposition (ALE and ALD) to mitigate dielectric loss in fully fabricated superconducting quantum devices formed
from aluminum thin films on silicon. The treatment, performed as a final processing step prior to device packaging, starts by conformally removing the native metal oxide and fabrication residues from the exposed surfaces through ALE before \textit{in situ} encapsulating the metal surfaces with a thin dielectric layer using ALD. We measure a two-fold reduction in loss attributed to two-level system (TLS) absorption in treated aluminum-based resonators and planar transmon qubits. Treated transmons with compact capacitor plates and gaps achieve median Q and T1 values of 3.69±0.42×106 and 196±22~μs, respectively. These improvements were found to be sustained over several months. We discuss how the combination of ALE and ALD reverses fabrication-induced surface damages to significantly and durably improve device performance via a reduction of the TLS defect density in the capacitive elements.
arxiv pdf

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