J. Biesecker

Coherence-limited digital control of a superconducting qubit using a Josephson pulse generator at 3 K

  1. M. A. Castellanos-Beltran,
  2. A. J. Sirois,
  3. L. Howe,
  4. D. I. Olaya,
  5. J. Biesecker,
  6. S. P. Benz,
  7. and P. F. Hopkins
Compared to traditional semiconductor control electronics (TSCE) located at room temperature, cryogenic single flux quantum (SFQ) electronics can provide qubit measurement and control
alternatives that address critical issues related to scalability of cryogenic quantum processors. Single-qubit control and readout have been demonstrated recently using SFQ circuits coupled to superconducting qubits. Experiments where the SFQ electronics are co-located with the qubit have suffered from excess decoherence and loss due to quasiparticle poisoning of the qubit. A previous experiment by our group showed that moving the control electronics to the 3 K stage of the dilution refrigerator avoided this source of decoherence in a high-coherence 3D transmon geometry. In this paper, we also generate the pulses at the 3 K stage but have optimized the qubit design and control lines for scalable 2D transmon devices. We directly compare the qubit lifetime T1, coherence time T∗2 and gate fidelity when the qubit is controlled by the Josephson pulse generator (JPG) circuit versus the TSCE setup. We find agreement to within the daily fluctuations for T1 and T∗2, and agreement to within 10% for randomized benchmarking. We also performed interleaved randomized benchmarking on individual JPG gates demonstrating an average error per gate of 0.46% showing good agreement with what is expected based on the qubit coherence and higher-state leakage. These results are an order of magnitude improvement in gate fidelity over our previous work and demonstrate that a Josephson microwave source operated at 3 K is a promising component for scalable qubit control.
arxiv pdf

Digital control of a superconducting qubit using a Josephson pulse generator at 3 K

  1. L. Howe,
  2. M. Castellanos-Beltran,
  3. A. J. Sirois,
  4. D. Olaya,
  5. J. Biesecker,
  6. P. D. Dresselhaus,
  7. S. P. Benz,
  8. and P. F. Hopkins
Scaling of quantum computers to fault-tolerant levels relies critically on the integration of energy-efficient, stable, and reproducible qubit control and readout electronics. In comparisonto traditional semiconductor control electronics (TSCE) located at room temperature, the signals generated by Josephson junction (JJ) based rf sources benefit from small device sizes, low power dissipation, intrinsic calibration, superior reproducibility, and insensitivity to ambient fluctuations. Previous experiments to co-locate qubits and JJ-based control electronics resulted in quasiparticle poisoning of the qubit; degrading the qubit’s coherence and lifetime. In this paper, we digitally control a 0.01~K transmon qubit with pulses from a Josephson pulse generator (JPG) located at the 3~K stage of a dilution refrigerator. We directly compare the qubit lifetime T1, coherence time T∗2, and thermal occupation Pth when the qubit is controlled by the JPG circuit versus the TSCE setup. We find agreement to within the daily fluctuations on ±0.5 μs and ±2 μs for T1 and T∗2, respectively, and agreement to within the 1\% error for Pth. Additionally, we perform randomized benchmarking to measure an average JPG gate error of 2.1×10−2. In combination with a small device size (<25~mm2) and low on-chip power dissipation (≪100 μW), these results are an important step towards demonstrating the viability of using JJ-based control electronics located at temperature stages higher than the mixing chamber stage in highly-scaled superconducting quantum information systems.[/expand]
arxiv pdf