Transmon qubit readout fidelity at the threshold for quantum error correction without a quantum-limited amplifier

  1. Liangyu Chen,
  2. Hang-Xi Li,
  3. Yong Lu,
  4. Christopher W. Warren,
  5. Christian J. Križan,
  6. Sandoko Kosen,
  7. Marcus Rommel,
  8. Shahnawaz Ahmed,
  9. Amr Osman,
  10. Janka Biznárová,
  11. Anita Fadavi Roudsari,
  12. Benjamin Lienhard,
  13. Marco Caputo,
  14. Kestutis Grigoras,
  15. Leif Grönberg,
  16. Joonas Govenius,
  17. Anton Frisk Kockum,
  18. Per Delsing,
  19. Jonas Bylander,
  20. and Giovanna Tancredi
High-fidelity and rapid readout of a qubit state is key to quantum computing and communication, and it is a prerequisite for quantum error correction. We present a readout scheme for
superconducting qubits that combines two microwave techniques: applying a shelving technique to the qubit that effectively increases the energy-relaxation time, and a two-tone excitation of the readout resonator to distinguish among qubit populations in higher energy levels. Using a machine-learning algorithm to post-process the two-tone measurement results further improves the qubit-state assignment fidelity. We perform single-shot frequency-multiplexed qubit readout, with a 140ns readout time, and demonstrate 99.5% assignment fidelity for two-state readout and 96.9% for three-state readout – without using a quantum-limited amplifier.

Deep Neural Network Discrimination of Multiplexed Superconducting Qubit States

  1. Benjamin Lienhard,
  2. Antti Vepsäläinen,
  3. Luke C.G. Govia,
  4. Cole R. Hoffer,
  5. Jack Y. Qiu,
  6. Diego Ristè,
  7. Matthew Ware,
  8. David Kim,
  9. Roni Winik,
  10. Alexander Melville,
  11. Bethany Niedzielski,
  12. Jonilyn Yoder,
  13. Guilhem J. Ribeill,
  14. Thomas A. Ohki,
  15. Hari K. Krovi,
  16. Terry P. Orlando,
  17. Simon Gustavsson,
  18. and William D. Oliver
Demonstrating the quantum computational advantage will require high-fidelity control and readout of multi-qubit systems. As system size increases, multiplexed qubit readout becomes
a practical necessity to limit the growth of resource overhead. Many contemporary qubit-state discriminators presume single-qubit operating conditions or require considerable computational effort, limiting their potential extensibility. Here, we present multi-qubit readout using neural networks as state discriminators. We compare our approach to contemporary methods employed on a quantum device with five superconducting qubits and frequency-multiplexed readout. We find that fully-connected feedforward neural networks increase the qubit-state-assignment fidelity for our system. Relative to contemporary discriminators, the assignment error rate is reduced by up to 25 % due to the compensation of system-dependent nonidealities such as readout crosstalk which is reduced by up to one order of magnitude. Our work demonstrates a potentially extensible building block for high-fidelity readout relevant to both near-term devices and future fault-tolerant systems.

Microwave Package Design for Superconducting Quantum Processors

  1. Sihao Huang,
  2. Benjamin Lienhard,
  3. Greg Calusine,
  4. Antti Vepsäläinen,
  5. Jochen Braumüller,
  6. David K. Kim,
  7. Alexander J. Melville,
  8. Bethany M. Niedzielski,
  9. Jonilyn L. Yoder,
  10. Bharath Kannan,
  11. Terry P. Orlando,
  12. Simon Gustavsson,
  13. and William D. Oliver
Solid-state qubits with transition frequencies in the microwave regime, such as superconducting qubits, are at the forefront of quantum information processing. However, high-fidelity,
simultaneous control of superconducting qubits at even a moderate scale remains a challenge, partly due to the complexities of packaging these devices. Here, we present an approach to microwave package design focusing on material choices, signal line engineering, and spurious mode suppression. We describe design guidelines validated using simulations and measurements used to develop a 24-port microwave package. Analyzing the qubit environment reveals no spurious modes up to 11GHz. The material and geometric design choices enable the package to support qubits with lifetimes exceeding 350 {\mu}s. The microwave package design guidelines presented here address many issues relevant for near-term quantum processors.

Microwave Packaging for Superconducting Qubits

  1. Benjamin Lienhard,
  2. Jochen Braumüller,
  3. Wayne Woods,
  4. Danna Rosenberg,
  5. Greg Calusine,
  6. Steven Weber,
  7. Antti Vepsäläinen,
  8. Kevin O'Brien,
  9. Terry P. Orlando,
  10. Simon Gustavsson,
  11. and William D. Oliver
Over the past two decades, the performance of superconducting quantum circuits has tremendously improved. The progress of superconducting qubits enabled a new industry branch to emerge
from global technology enterprises to quantum computing startups. Here, an overview of superconducting quantum circuit microwave control is presented. Furthermore, we discuss one of the persistent engineering challenges in the field, how to control the electromagnetic environment of increasingly complex superconducting circuits such that they are simultaneously protected and efficiently controllable.