Julius Feigl

A high-fidelity two-qubit gate for multimode superconducting P-mon qubits

  1. Frederik Pfeiffer,
  2. Federico A. Roy,
  3. Niklas J. Glaser,
  4. Julius Feigl,
  5. Leon Koch,
  6. Kevin Kiener,
  7. Gleb Krylov,
  8. Johannes Schirk,
  9. Christian M. F. Schneider,
  10. Lasse Södergren,
  11. Florian Wallner,
  12. Max Werninghaus,
  13. Carlos A. Riofrío,
  14. and Stefan Filipp
To scale superconducting quantum processors, it is essential to achieve long coherence times while engineering interactions that do not introduce additional decoherence channels. In
superconducting qubit systems, this can be realized using multimode circuits that feature a protected qubit mode alongside a distinct mediator mode. Building on this concept, our recently developed P-mon qubit provides intrinsic protection against decoherence from the readout environment. We extend this approach to controlled two-qubit interactions, by exploiting the mediator modes of P-mons for on-demand coupling. Because direct interactions between the qubit modes are strongly suppressed, unwanted ZZ-type interactions are significantly reduced to below 3.6(5) kHz in the idle state. When tuning the coupled mediator modes on resonance, the cross-Kerr interaction between the qubit and the hybridized mediator modes leads to a qubit-state dependent frequency shift. By selectively addressing these transitions, we implement a 180 ns long CZ gate and determine a fidelity of 99.62(4) %. These results represent a significant step toward a scalable superconducting architecture that maintains high performance at scale.
arxiv pdf

Scalable Single-Step Generation of W States in 2D Superconducting Qubit Lattices

  1. João H. Romeiro,
  2. Federico A. Roy,
  3. Niklas Bruckmoser,
  4. Ivan Tsitsilin,
  5. Niklas J. Glaser,
  6. Christian M. F. Schneider,
  7. Gerhard B. P. Huber,
  8. Saya A. Schöbe,
  9. Johannes Schirk,
  10. Florian Wallner,
  11. Malay Singh,
  12. Julius Feigl,
  13. Leon Koch,
  14. Lasse Södergren,
  15. Max Werninghaus,
  16. and Stefan Filipp
The reliable generation of multi-qubit entanglement is a prerequisite for large-scale quantum information technologies. In particular, W states are a valuable resource owing to their
resilience under local loss or measurement. Nevertheless, preparing these states with sequential two-qubit gates often requires substantial time overhead. By contrast, engineered simultaneous interactions enable fast entanglement generation, even in qubit systems with limited nearest-neighbour connectivity. Here, we demonstrate a set of fast and robust operations for coherently distributing a single excitation across a lattice of arbitrary size, thereby directly generating W states from initial product states. In 2D lattices, the excitation propagates along both directions simultaneously, such that the total entanglement time scales only with the largest dimension. We exploit this property to prepare a six-qubit W state in a 3×2 superconducting lattice within 99 ns, achieving a tomographic fidelity of 83.9±1.0%. We then extend the protocol to create entanglement across chains of up to seven qubits, with the largest W state generated in 264 ns with a fidelity of 79.6±1.3%.
arxiv pdf