Coherent control of a symmetry-engineered multi-qubit dark state in waveguide quantum electrodynamics

  1. Maximilian Zanner,
  2. Tuure Orell,
  3. Christian M. F. Schneider,
  4. Romain Albert,
  5. Stefan Oleschko,
  6. Mathieu L. Juan,
  7. Matti Silveri,
  8. and Gerhard Kirchmair
Quantum information is typically encoded in the state of a qubit that is decoupled from the environment. In contrast, waveguide quantum electrodynamics studies qubits coupled to a mode
continuum, exposing them to a loss channel and causing quantum information to be lost before coherent operations can be performed. Here we restore coherence by realizing a dark state that exploits symmetry properties and interactions between four qubits. Dark states decouple from the waveguide and are thus a valuable resource for quantum information but also come with a challenge: they cannot be controlled by the waveguide drive. We overcome this problem by designing a drive that utilizes the symmetry properties of the collective state manifold allowing us to selectively drive both bright and dark states. The decay time of the dark state exceeds that of the waveguide-limited single qubit by more than two orders of magnitude. Spectroscopy on the second excitation manifold provides further insight into the level structure of the hybridized system. Our experiment paves the way for implementations of quantum many-body physics in waveguides and the realization of quantum information protocols using decoherence-free subspaces.

Efficient and low-backaction quantum measurement using a chip-scale detector

  1. Eric I. Rosenthal,
  2. Christian M. F. Schneider,
  3. Maxime Malnou,
  4. Ziyi Zhao,
  5. Felix Leditzky,
  6. Benjamin J. Chapman,
  7. Waltraut Wustmann,
  8. Xizheng Ma,
  9. Daniel A. Palken,
  10. Maximilian F. Zanner,
  11. Leila R. Vale,
  12. Gene C. Hilton,
  13. Jiansong Gao,
  14. Graeme Smith,
  15. Gerhard Kirchmair,
  16. and K. W. Lehnert
Superconducting qubits are a leading platform for scalable quantum computing and quantum error correction. One feature of this platform is the ability to perform projective measurements
orders of magnitude more quickly than qubit decoherence times. Such measurements are enabled by the use of quantum-limited parametric amplifiers in conjunction with ferrite circulators – magnetic devices which provide isolation from noise and decoherence due to amplifier backaction. Because these non-reciprocal elements have limited performance and are not easily integrated on-chip, it has been a longstanding goal to replace them with a scalable alternative. Here, we demonstrate a solution to this problem by using a superconducting switch to control the coupling between a qubit and amplifier. Doing so, we measure a transmon qubit using a single, chip-scale device to provide both parametric amplification and isolation from the bulk of amplifier backaction. This measurement is also fast, high fidelity, and has 70% efficiency, comparable to the best that has been reported in any superconducting qubit measurement. As such, this work constitutes a high-quality platform for the scalable measurement of superconducting qubits.