J. Ku

Entangling interactions between artificial atoms mediated by a multimode left-handed superconducting ring resonator

  1. T. McBroom-Carroll,
  2. A. Schlabes,
  3. X. Xu,
  4. J. Ku,
  5. B. Cole,
  6. S. Indrajeet,
  7. M. D. LaHaye,
  8. M. H. Ansari,
  9. and B. L. T. Plourde
Superconducting metamaterial transmission lines implemented with lumped circuit elements can exhibit left-handed dispersion, where the group and phase velocity have opposite sign, in
a frequency range relevant for superconducting artificial atoms. Forming such a metamaterial transmission line into a ring and coupling it to qubits at different points around the ring results in a multimode bus resonator with a compact footprint. Using flux-tunable qubits, we characterize and theoretically model the variation in the coupling strength between the two qubits and each of the ring resonator modes. Although the qubits have negligible direct coupling between them, their interactions with the multimode ring resonator result in both a transverse exchange coupling and a higher order ZZ interaction between the qubits. As we vary the detuning between the qubits and their frequency relative to the ring resonator modes, we observe significant variations in both of these inter-qubit interactions, including zero crossings and changes of sign. The ability to modulate interaction terms such as the ZZ scale between zero and large values for small changes in qubit frequency provides a promising pathway for implementing entangling gates in a system capable of hosting many qubits.
arxiv pdf

Phonon downconversion to suppress correlated errors in superconducting qubits

  1. V. Iaia,
  2. J. Ku,
  3. A. Ballard,
  4. C. P. Larson,
  5. E. Yelton,
  6. C. H. Liu,
  7. S. Patel,
  8. R. McDermott,
  9. and B. L. T. Plourde
Quantum error correction can preserve quantum information in the presence of local errors; however, errors that are correlated across a qubit array are fatal. For superconducting qubits,
high-energy particle impacts due to background radioactivity or cosmic ray muons produce bursts of energetic phonons that travel throughout the substrate and create excitations out of the superconducting ground state, known as quasiparticles, which poison all qubits on the chip. Here we use thick normal metal reservoirs on the back side of the chip to promote rapid downconversion of phonons to sufficiently low energies where they can no longer poison qubits. We introduce a pump-probe scheme involving controlled injection of pair-breaking phonons into the qubit chips. We examine quasiparticle poisoning on chips with and without backside metallization and demonstrate a reduction in the flux of pair-breaking phonons by more than a factor of 20. In addition, we use a Ramsey interferometer scheme to simultaneously monitor quasiparticle parity on three qubits for each chip and observe a two-order of magnitude reduction in correlated poisoning due to ambient radiation. Our approach reduces correlated errors due to background radiation below the level necessary for fault-tolerant operation of a multiqubit array.
arxiv pdf