Joel Howard

Precision frequency tuning of tunable transmon qubits using alternating-bias assisted annealing

  1. Xiqiao Wang,
  2. Joel Howard,
  3. Eyob A. Sete,
  4. Greg Stiehl,
  5. Cameron Kopas,
  6. Stefano Poletto,
  7. Xian Wu,
  8. Mark Field,
  9. Nicholas Sharac,
  10. Christopher Eckberg,
  11. Hilal Cansizoglu,
  12. Raja Katta,
  13. Josh Mutus,
  14. Andrew Bestwick,
  15. Kameshwar Yadavalli,
  16. and David P. Pappas
Superconducting quantum processors are one of the leading platforms for realizing scalable fault-tolerant quantum computation (FTQC). The recent demonstration of post-fabrication tuning
of Josephson junctions using alternating-bias assisted annealing (ABAA) technique and a reduction in junction loss after ABAA illuminates a promising path towards precision tuning of qubit frequency while maintaining high coherence. Here, we demonstrate precision tuning of the maximum |0⟩→|1⟩ transition frequency (fmax01) of tunable transmon qubits by performing ABAA at room temperature using commercially available test equipment. We characterize the impact of junction relaxation and aging on resistance spread after tuning, and demonstrate a frequency equivalent tuning precision of 7.7 MHz (0.17%) based on targeted resistance tuning on hundreds of qubits, with a resistance tuning range up to 18.5%. Cryogenic measurements on tuned and untuned qubits show evidence of improved coherence after ABAA with no significant impact on tunability. Despite a small global offset, we show an empirical fmax01 tuning precision of 18.4 MHz by tuning a set of multi-qubit processors targeting their designed Hamiltonians. We experimentally characterize high-fidelity parametric resonance iSWAP gates on two ABAA-tuned 9-qubit processors with fidelity as high as 99.51±0.20%. On the best-performing device, we measured across the device a median fidelity of 99.22% and an average fidelity of 99.13±0.12%. Yield modeling analysis predicts high detuning-edge-yield using ABAA beyond the 1000-qubit scale. These results demonstrate the cutting-edge capability of frequency targeting using ABAA and open up a new avenue to systematically improving Hamiltonian targeting and optimization for scaling high-performance superconducting quantum processors.
arxiv pdf

Demonstrating two-qubit entangling gates at the quantum speed limit using superconducting qubits

  1. Joel Howard,
  2. Alexander Lidiak,
  3. Casey Jameson,
  4. Bora Basyildiz,
  5. Kyle Clark,
  6. Tongyu Zhao,
  7. Mustafa Bal,
  8. Junling Long,
  9. David P. Pappas,
  10. Meenakshi Singh,
  11. and Zhexuan Gong
The speed of elementary quantum gates, particularly two-qubit entangling gates, ultimately sets the limit on the speed at which quantum circuits can operate. In this work, we demonstrate
experimentally two-qubit entangling gates at nearly the fastest possible speed allowed by the physical interaction strength between two superconducting transmon qubits. We achieve this quantum speed limit by implementing experimental gates designed using a machine learning inspired optimal control method. Importantly, our method only requires the single-qubit drive strength to be moderately larger than the interaction strength to achieve an arbitrary entangling gate close to its analytical speed limit with high fidelity. Thus, the method is applicable to a variety of platforms including those with comparable single-qubit and two-qubit gate speeds, or those with always-on interactions.
arxiv pdf