Junling Long

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

Overlap junctions for superconducting quantum electronics and amplifiers

  1. Mustafa Bal,
  2. Junling Long,
  3. Ruichen Zhao,
  4. Haozhi Wang,
  5. Sungoh Park,
  6. Corey Rae Harrington McRae,
  7. Tongyu Zhao,
  8. Russell E. Lake,
  9. Daniil Frolov,
  10. Roman Pilipenko,
  11. Silvia Zorzetti,
  12. Alexander Romanenko,
  13. and David P. Pappas
Due to their unique properties as lossless, nonlinear circuit elements, Josephson junctions lie at the heart of superconducting quantum information processing. Previously, we demonstrated
a two-layer, submicrometer-scale overlap junction fabrication process suitable for qubits with long coherence times. Here, we extend the overlap junction fabrication process to micrometer-scale junctions. This allows us to fabricate other superconducting quantum devices. For example, we demonstrate an overlap-junction-based Josephson parametric amplifier that uses only 2 layers. This efficient fabrication process yields frequency-tunable devices with negligible insertion loss and a gain of ~ 30 dB. Compared to other processes, the overlap junction allows for fabrication with minimal infrastructure, high yield, and state-of-the-art device performance.
arxiv pdf

Kinetic Inductance Traveling Wave Amplifiers For Multiplexed Qubit Readout

  1. Leonardo Ranzani,
  2. Mustafa Bal,
  3. Kin Chung Fong,
  4. Guilhem Ribeill,
  5. Xian Wu,
  6. Junling Long,
  7. Hsiang-Sheng Ku,
  8. Robert P. Erickson,
  9. David Pappas,
  10. and Thomas A. Ohki
We describe a kinetic inductance traveling-wave (KIT) amplifier suitable for superconducting quantum information measurements and characterize its wideband scattering and noise properties.
We use mechanical microwave switches to calibrate the four amplifier scattering parameters up to the device input and output connectors at the dilution refrigerator base temperature and a tunable temperature load to characterize the amplifier noise. Finally, we demonstrate the high fidelity simultaneous dispersive readout of two superconducting transmon qubits. The KIT amplifier provides low-noise amplification of both readout tones with readout fidelities in excess of 89% and negligible effect on qubit lifetime and coherence.
arxiv pdf