Abigail Shearrow

Meter-scale quantum interconnects between large quantum processors

  1. Neereja M. Sundaresan,
  2. Nicholas Cotaj,
  3. Yadav Prasad Kandel,
  4. Eric Lewandowski,
  5. Christian Robert Longton,
  6. Yves Martin,
  7. Jae-woong Nah,
  8. Kathryn Pooley,
  9. Martin Sandberg,
  10. Michael Scanlon,
  11. Chi Xiong,
  12. Emanuil Sashev Yanev,
  13. David Abraham,
  14. Abhinandan Antony,
  15. Vincent Arena,
  16. Nicholas Bornman,
  17. Khanh Dang,
  18. Oliver Dial,
  19. Christopher Guarino,
  20. Gabriel Haspil,
  21. Jared Hertzberg,
  22. Kentaro Heya,
  23. Swetha Kamlapurkar,
  24. Michael Karschner,
  25. George Keefe,
  26. Eric Liniger,
  27. Laura Luo,
  28. Thomas G. McConkey,
  29. David Olson,
  30. Jason Orcutt,
  31. Dmitry Ovchinnikov,
  32. Timothy Phung,
  33. Sankeerth Rajalingam,
  34. Michael Selvanayagam,
  35. William Shanks,
  36. Dongbing Shao,
  37. Abigail Shearrow,
  38. Adam Stabile,
  39. Jiri Stehlik,
  40. Joseph Suttle,
  41. Jeng-Bang Yau,
  42. and Eric Zhang
Modular quantum architecture requires long-distance quantum interconnects to stitch together individual processor modules. Such interconnects, long-distance couplers or L-couplers,
must support coherent quantum operation over meter-scale distances and scale to multiple connections between any two modules. Here we demonstrate four L-couplers operating between two 120-qubit IBM Nighthawk R1 superconducting qubit processors. Each L-coupler joins two Nighthawk lattice qubits through two frequency-tunable L-qubits, two RF-SQUID based „i2m“ couplers, and 80 cm of superconducting aluminum coax. Each cable installs perpendicular to the processor surface, to enable denser connections, and is joined to the chip by an indium-to-indium room temperature pressure bond made after both modules were mounted. L-coupler modes near 5 GHz, with internal quality factors of 0.86 to 1.09e6, carry relay-based state transfer at a mean network benchmarking fidelity of 99.00%, with a best link of 99.29 +/- 0.07%. We then realize distant CZ gates between lattice qubits in separate modules at a mean fidelity of 94.75%, with a best link of 96.42 +/- 0.28%, and use them to generate entangled states across the two processors, including a demonstration of simultaneous Bell pair generation using two adjacent L-couplers. These four high-fidelity L-couplers establish a practical path to scalable quantum interconnects for modular quantum computing.
arxiv pdf

Quasiparticle Poisoning of Superconducting Qubits from Resonant Absorption of Pair-breaking Photons

  1. Chuan-Hong Liu,
  2. David C. Harrison,
  3. Shravan Patel,
  4. Christopher D. Wilen,
  5. Owen Rafferty,
  6. Abigail Shearrow,
  7. Andrew Ballard,
  8. Vito Iaia,
  9. Jaseung Ku,
  10. Britton L. T. Plourde,
  11. and Robert McDermott
The ideal superconductor provides a pristine environment for the delicate states of a quantum computer: because there is an energy gap to excitations, there are no spurious modes with
which the qubits can interact, causing irreversible decay of the quantum state. As a practical matter, however, there exists a high density of excitations out of the superconducting ground state even at ultralow temperature; these are known as quasiparticles. Observed quasiparticle densities are of order 1~μm−3, tens of orders of magnitude larger than the equilibrium density expected from theory. Nonequilibrium quasiparticles extract energy from the qubit mode and induce discrete changes in qubit offset charge, a potential source of dephasing. Here we show that a dominant mechanism for quasiparticle poisoning in superconducting qubits is direct absorption of high-energy photons at the qubit junction. We use a Josephson junction-based photon source to controllably dose qubit circuits with millimeter-wave radiation, and we use an interferometric quantum gate sequence to reconstruct the charge parity on the qubit island. We find that the structure of the qubit itself acts as a resonant antenna for millimeter-wave radiation, providing an efficient path for photons to generate quasiparticle excitations. A deep understanding of this physics will pave the way to realization of next-generation superconducting qubits that are robust against quasiparticle poisoning and could enable a new class of quantum sensors for dark matter detection.
arxiv pdf