Shahriar Aghaeimeibodi

Broadband and high-precision two-level system loss measurement using superconducting multi-wave resonators

  1. Cliff Chen,
  2. Shahriar Aghaeimeibodi,
  3. Yuki Sato,
  4. Matthew H. Matheny,
  5. Oskar Painter,
  6. and Jiansong Gao
Two-level systems (TLS) are known to be a dominant source of dissipation and decoherence in superconducting qubits. Superconducting resonators provide a convenient way to study TLS-induced
loss due to easier design and fabrication in comparison to devices that include non-linear elements. However, accurately measuring TLS-induced loss in a resonator in the quantum regime is challenging due to low signal-to-noise ratio (SNR) and the temporal fluctuations of the TLS, leading to uncertainties of 30% or more. To address these limitations, we develop a multi-wave resonator device that extends the resonator length from a standard quarter-wave λ/4 to Nλ/4 where N=37 at 6GHz. This design provides two key advantages: the TLS-induced fluctuations are reduced by a factor of N‾‾√ due to spatial averaging over an increased number of independent TLS, and the measurement SNR for a given intra-resonator energy density improves by a factor of N‾‾√. The multi-wave resonator also has fundamental and harmonic resonances that allow one to study the frequency dependence of TLS-induced loss. In this work we fabricate both multi-wave and quarter-wave coplanar waveguide resonators formed from thin-film aluminum on a silicon substrate, and characterize their TLS properties at both 10mK and 200mK. Our results show that the power-dependent TLS-induced loss measured from both types of resonators agree well, with the multi-wave resonators achieving a five-fold reduction in measurement uncertainty due to TLS fluctuations, down to 5%. The Nλ/4 resonator also provides a measure of the fully unsaturated TLS-induced loss due to the improved measurement SNR at low intra-resonator energy densities. Finally, measurements across seven harmonic resonances of the Nλ/4 resonator between 4GHz – 6.5GHz reveals no frequency dependence in the TLS-induced loss over this range.
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