High-Coherence Kerr-cat qubit in 2D architecture

  1. Ahmed Hajr,
  2. Bingcheng Qing,
  3. Ke Wang,
  4. Gerwin Koolstra,
  5. Zahra Pedramrazi,
  6. Ziqi Kang,
  7. Larry Chen,
  8. Long B. Nguyen,
  9. Christian Junger,
  10. Noah Goss,
  11. Irwin Huang,
  12. Bibek Bhandari,
  13. Nicholas E. Frattini,
  14. Shruti Puri,
  15. Justin Dressel,
  16. Andrew Jordan,
  17. David Santiago,
  18. and Irfan Siddiqi
The Kerr-cat qubit is a bosonic qubit in which multi-photon Schrodinger cat states are stabilized by applying a two-photon drive to an oscillator with a Kerr nonlinearity. The suppressed
bit-flip rate with increasing cat size makes this qubit a promising candidate to implement quantum error correction codes tailored for noise-biased qubits. However, achieving strong light-matter interactions necessary for stabilizing and controlling this qubit has traditionally required strong microwave drives that heat the qubit and degrade its performance. In contrast, increasing the coupling to the drive port removes the need for strong drives at the expense of large Purcell decay. By integrating an effective band-block filter on-chip, we overcome this trade-off and realize a Kerr-cat qubit in a scalable 2D superconducting circuit with high coherence. This filter provides 30 dB of isolation at the qubit frequency with negligible attenuation at the frequencies required for stabilization and readout. We experimentally demonstrate quantum non-demolition readout fidelity of 99.6% for a cat with 8 photons. Also, to have high-fidelity universal control over this qubit, we combine fast Rabi oscillations with a new demonstration of the X(90) gate through phase modulation of the stabilization drive. Finally, the lifetime in this architecture is examined as a function of the cat size of up to 10 photons in the oscillator achieving a bit-flip time higher than 1 ms and only a linear decrease in the phase-flip time, in good agreement with the theoretical analysis of the circuit. Our qubit shows promise as a building block for fault-tolerant quantum processors with a small footprint.

Localization and reduction of superconducting quantum coherent circuit losses

  1. M. Virginia P. Altoé,
  2. Archan Banerjee,
  3. Cassidy Berk,
  4. Ahmed Hajr,
  5. Adam Schwartzberg,
  6. Chengyu Song,
  7. Mohammed Al Ghadeer,
  8. Shaul Aloni,
  9. Michael J. Elowson,
  10. John Mark Kreikebaum,
  11. Ed K. Wong,
  12. Sinead Griffin,
  13. Saleem Rao,
  14. Alexander Weber-Bargioni,
  15. Andrew M. Minor,
  16. David I. Santiago,
  17. Stefano Cabrini,
  18. Irfan Siddiqi,
  19. and D. Frank Ogletree
Quantum sensing and computation can be realized with superconducting microwave circuits. Qubits are engineered quantum systems of capacitors and inductors with non-linear Josephson
junctions. They operate in the single-excitation quantum regime, photons of 27μeV at 6.5 GHz. Quantum coherence is fundamentally limited by materials defects, in particular atomic-scale parasitic two-level systems (TLS) in amorphous dielectrics at circuit interfaces.[1] The electric fields driving oscillating charges in quantum circuits resonantly couple to TLS, producing phase noise and dissipation. We use coplanar niobium-on-silicon superconducting resonators to probe decoherence in quantum circuits. By selectively modifying interface dielectrics, we show that most TLS losses come from the silicon surface oxide, and most non-TLS losses are distributed throughout the niobium surface oxide. Through post-fabrication interface modification we reduced TLS losses by 85% and non-TLS losses by 72%, obtaining record single-photon resonator quality factors above 5 million and approaching a regime where non-TLS losses are dominant. [1]Müller, C., Cole, J. H. & Lisenfeld, J. Towards understanding two-level-systems in amorphous solids: insights from quantum circuits. Rep. Prog. Phys. 82, 124501 (2019)