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.

Scalable High-Performance Fluxonium Quantum Processor

  1. Long B. Nguyen,
  2. Gerwin Koolstra,
  3. Yosep Kim,
  4. Alexis Morvan,
  5. Trevor Chistolini,
  6. Shraddha Singh,
  7. Konstantin N. Nesterov,
  8. Christian Jünger,
  9. Larry Chen,
  10. Zahra Pedramrazi,
  11. Bradley K. Mitchell,
  12. John Mark Kreikebaum,
  13. Shruti Puri,
  14. David I. Santiago,
  15. and Irfan Siddiqi Singh
The technological development of hardware heading toward universal fault-tolerant quantum computation requires a large-scale processing unit with high performance. While fluxonium qubits
are promising with high coherence and large anharmonicity, their scalability has not been systematically explored. In this work, we propose a superconducting quantum information processor based on compact high-coherence fluxoniums with suppressed crosstalk, reduced design complexity, improved operational efficiency, high-fidelity gates, and resistance to parameter fluctuations. In this architecture, the qubits are readout dispersively using individual resonators connected to a common bus and manipulated via combined on-chip RF and DC control lines, both of which can be designed to have low crosstalk. A multi-path coupling approach enables exchange interactions between the high-coherence computational states and at the same time suppresses the spurious static ZZ rate, leading to fast and high-fidelity entangling gates. We numerically investigate the cross resonance controlled-NOT and the differential AC-Stark controlled-Z operations, revealing low gate error for qubit-qubit detuning bandwidth of up to 1 GHz. Our study on frequency crowding indicates high fabrication yield for quantum processors consisting of over thousands of qubits. In addition, we estimate low resource overhead to suppress logical error rate using the XZZX surface code. These results promise a scalable quantum architecture with high performance for the pursuit of universal quantum computation.