Gustav Andersson

Modular quantum processor with an all-to-all reconfigurable router

  1. Xuntao Wu,
  2. Haoxiong Yan,
  3. Gustav Andersson,
  4. Alexander Anferov,
  5. Ming-Han Chou,
  6. Christopher R. Conner,
  7. Joel Grebel,
  8. Yash J. Joshi,
  9. Shiheng Li,
  10. Jacob M. Miller,
  11. Rhys G. Povey,
  12. Hong Qiao,
  13. and Andrew N. Cleland
Superconducting qubits provide a promising approach to large-scale fault-tolerant quantum computing. However, qubit connectivity on a planar surface is typically restricted to only
a few neighboring qubits. Achieving longer-range and more flexible connectivity, which is particularly appealing in light of recent developments in error-correcting codes, however usually involves complex multi-layer packaging and external cabling, which is resource-intensive and can impose fidelity limitations. Here, we propose and realize a high-speed on-chip quantum processor that supports reconfigurable all-to-all coupling with a large on-off ratio. We implement the design in a four-node quantum processor, built with a modular design comprising a wiring substrate coupled to two separate qubit-bearing substrates, each including two single-qubit nodes. We use this device to demonstrate reconfigurable controlled-Z gates across all qubit pairs, with a benchmarked average fidelity of 96.00%±0.08% and best fidelity of 97.14%±0.07%, limited mainly by dephasing in the qubits. We also generate multi-qubit entanglement, distributed across the separate modules, demonstrating GHZ-3 and GHZ-4 states with fidelities of 88.15%±0.24% and 75.18%±0.11%, respectively. This approach promises efficient scaling to larger-scale quantum circuits, and offers a pathway for implementing quantum algorithms and error correction schemes that benefit from enhanced qubit connectivity.
arxiv pdf

Intermodulation spectroscopy and the nonlinear response of two-level systems in superconducting coplanar waveguide resonators

  1. Janka Biznárová,
  2. J. C. Rivera Hernández,
  3. Daniel Forchheimer,
  4. Jonas Bylander,
  5. David B. Haviland,
  6. and Gustav Andersson
Two-level system (TLS) loss is typically limiting the coherence of superconducting quantum circuits. The loss induced by TLS defects is nonlinear, resulting in quality factors with
a strong dependence on the circulating microwave power. We observe frequency mixing due to this nonlinearity by applying a two-tone drive to a coplanar waveguide resonator and measuring the intermodulation products using a multifrequency lock-in technique. This intermodulation spectroscopy method provides an efficient approach to characterizing TLS loss in superconducting circuits. Using harmonic balance reconstruction, we recover the nonlinear parameters of the device-TLS interaction, which are in good agreement with the standard tunnelling model for TLSs.
arxiv pdf

Broadband Bandpass Purcell Filter for Circuit Quantum Electrodynamics

  1. Haoxiong Yan,
  2. Xuntao Wu,
  3. Andrew Lingenfelter,
  4. Yash J. Joshi,
  5. Gustav Andersson,
  6. Christopher R. Conner,
  7. Ming-Han Chou,
  8. Joel Grebel,
  9. Jacob M. Miller,
  10. Rhys G. Povey,
  11. Hong Qiao,
  12. Aashish A. Clerk,
  13. and Andrew N. Cleland
In circuit quantum electrodynamics (QED), qubits are typically measured using dispersively-coupled readout resonators. Coupling between each readout resonator and its electrical environment
however reduces the qubit lifetime via the Purcell effect. Inserting a Purcell filter counters this effect while maintaining high readout fidelity, but reduces measurement bandwidth and thus limits multiplexing readout capacity. In this letter, we develop and implement a multi-stage bandpass Purcell filter that yields better qubit protection while simultaneously increasing measurement bandwidth and multiplexed capacity. We report on the experimental performance of our transmission-line–based implementation of this approach, a flexible design that can easily be integrated with current scaled-up, long coherence time superconducting quantum processors.
arxiv pdf

Multipartite entanglement in a microwave frequency comb

  1. Shan W. Jolin,
  2. Gustav Andersson,
  3. J. C. Rivera Hernández,
  4. Ingrid Strandberg,
  5. Fernando Quijandría,
  6. Joe Aumentado,
  7. Riccardo Borgani,
  8. Mats O. Tholén,
  9. and David B. Haviland
Significant progress has been made with multipartite entanglement of discrete qubits, but continuous variable systems may provide a more scalable path toward entanglement of large ensembles.
We demonstrate multipartite entanglement in a microwave frequency comb generated by a Josephson parametric amplifier subject to a bichromatic pump. We find 64 correlated modes in the transmission line using a multifrequency digital signal processing platform. Full inseparability is verified in a subset of seven modes. Our method can be expanded to generate even more entangled modes in the near future.
arxiv pdf

Nonexponential decay of a giant artificial atom

  1. Gustav Andersson,
  2. Baladitya Suri,
  3. Lingzhen Guo,
  4. Thomas Aref,
  5. and Per Delsing
In quantum optics, light-matter interaction has conventionally been studied using small atoms interacting with electromagnetic fields with wavelength several orders of magnitude larger
than the atomic dimensions. In contrast, here we experimentally demonstrate the vastly different giant atom regime, where an artificial atom interacts with acoustic fields with wavelength several orders of magnitude smaller than the atomic dimensions. This is achieved by coupling a superconducting qubit to surface acoustic waves at two points with separation on the order of 100 wavelengths. This approach is comparable to controlling the radiation of an atom by attaching it to an antenna. The slow velocity of sound leads to a significant internal time-delay for the field to propagate across the giant atom, giving rise to non-Markovian dynamics. We demonstrate the non-Markovian character of the giant atom in the frequency spectrum as well as nonexponential relaxation in the time domain.
arxiv pdf