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@INPROCEEDINGS{Willsch:1014281,
      author       = {Willsch, Dennis and Rieger, Dennis and Pop, Ioan},
      title        = {{O}bservation of {J}osephson {H}armonics in {T}unnel
                      {J}unctions},
      reportid     = {FZJ-2023-03211},
      year         = {2023},
      abstract     = {Superconducting quantum processors have a long road ahead
                      to reach fault-tolerant quantum computing. One of the most
                      daunting challenges is taming the numerous microscopic
                      degrees of freedom ubiquitous in solid-state devices.
                      State-of-the-art technologies, including the world's largest
                      quantum processors, employ aluminum oxide (AlOx) tunnel
                      Josephson junctions (JJs) as sources of nonlinearity,
                      assuming an idealized pure sin(phi) current-phase relation
                      (CPR). However, this celebrated sin(phi) CPR is only
                      expected to occur in the unrealistic limit of vanishingly
                      low-transparency channels in the AlOx barrier. Here we show
                      that the standard CPR fails to describe the energy spectra
                      of transmon artificial atoms across various samples and
                      laboratories. Instead, a mesoscopic model of tunneling
                      through an inhomogeneous AlOx barrier predicts $\%-level$
                      contributions from higher Josephson harmonics.By including
                      these in the transmon Hamiltonian, we obtain orders of
                      magnitude better agreement between the computed and measured
                      energy spectra. The reality of Josephson harmonics
                      transforms qubit design and prompts a reevaluation of models
                      for quantum gates, parametric amplification and mixing,
                      Floquet qubits, protected Josephson Rhombus chains, etc.
                      Indeed, we show that engineered Josephson harmonics can
                      reduce the charge dispersion and the associated errors in
                      transmon qubits by an order of magnitude.},
      organization  = {IBM Qiskit Seminar, Online (USA)},
      subtyp        = {Invited},
      cin          = {JSC},
      cid          = {I:(DE-Juel1)JSC-20090406},
      pnm          = {5111 - Domain-Specific Simulation $\&$ Data Life Cycle Labs
                      (SDLs) and Research Groups (POF4-511) / BMBF 13N16149 -
                      QSolid (BMBF-13N16149)},
      pid          = {G:(DE-HGF)POF4-5111 / G:(DE-Juel1)BMBF-13N16149},
      typ          = {PUB:(DE-HGF)31},
      doi          = {10.34734/FZJ-2023-03211},
      url          = {https://juser.fz-juelich.de/record/1014281},
}