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@INPROCEEDINGS{Lyatti:1022157,
      author       = {Lyatti, Matvey and Gundareva, Irina and Röper, Torsten and
                      Grützmacher, Detlev and Schäpers, Thomas},
      title        = {{T}owards high-{T}c superconducting qubit with {TH}z plasma
                      frequency},
      reportid     = {FZJ-2024-01278},
      year         = {2023},
      abstract     = {Nowadays, superconducting quantum circuits are mainly based
                      on the tunnel junctions fabricated from low-temperature
                      (low-Tc) superconductors [1]. The superconducting tunnel
                      junctions can be considered as LC oscillators in which the
                      Josephson effect provides the nonlinearity required for
                      selective access to quantum levels. Quantum proces-sors
                      consisting of hundreds of qubits have already been
                      demonstrated. However, the quality factor of low-temperature
                      superconducting qubits is limited to a few million and a
                      little progress has been made in recent years. It is
                      therefore im-portant to explore new platforms for the
                      superconducting qubits where higher quality factors can be
                      achieved.Despite having large energy gaps, the cuprate
                      high-temperature (high-Tc) superconductors were out of the
                      quan-tum scene because of the d-wave symmetry of the order
                      parameter where the energy gap vanishes in nodal directions
                      [2]. However, there are numerous evidences that the d-wave
                      symmetry of the order parameter can be broken in na-noscale
                      devices made of cuprate superconductors. We systematically
                      studied the order parameter symmetry in the ul-tra-thin
                      YBa2Cu3O7-x (YBCO) films using Andreev reflection
                      spectroscopy and found that it is possible to achieve a
                      ful-ly-gaped state with an exponentially low number of
                      quasiparticles at low temperatures [3]. The nanoscale YBCO
                      film can be considered as a quantum-engineered
                      superconductor where the superconducting gap is controlled
                      by quantum size effects. These fully-gapped films can be
                      used for the quantum applications including quantum
                      computing. To prove this idea, we fabricated the ultra-thin
                      YBCO nanowires which demonstrated an abrupt switching from
                      the super-conducting to normal state due to the phase
                      slippage and a current hysteresis as shown in Figure 1 [4].
                      The behaviour of the phase-slip YBCO is very similar to that
                      of the underdamped Josephson junction. The plasma frequency
                      of the phase-slip YBCO nanowires is in the THz frequency
                      range resulting in the crossover between the
                      thermal-activated and quantum regimes at 12-13 K [5]. The
                      switching current distribution of the nanowires shows the
                      distinct peaks which is a manifestation of the quantized
                      energy levels. We found that the life time of the excited
                      state in YBCO nanowires probed with incoherent broadband THz
                      radiation from the thermal radiation source exceeds 20 msec
                      at a temperature of 5.4 K providing the quality factor of
                      order of 1010 which is 3-4 orders of magnitude higher than
                      that in the low-Tc Jo-sephson junctions. The absorption of a
                      single optical photon results in the switching of the
                      nanowire from the excited to the ground energy level which
                      is an independent evidence of low number of unpaired
                      quasiparticles in the nanoscale YBCO devices. Our findings
                      show that nanoscale high-Tc superconducting films and
                      devices made from these films are promising platforms for
                      quantum applications that in the future might outperform the
                      conventional superconducting qubits and superconducting
                      single-photon detectors fabricated from low-Tc
                      superconductors.[1] M. H. Devoret $\&$ R. J. Schoelkopf,
                      “Superconducting Circuits for Quantum Information: An
                      Outlook”, Science, vol. 339, pp. 1169-1174, 2013.[2] C.C.
                      Tsuei $\&$ J.R. Kirtley, “Pairing symmetry in cuprate
                      superconductors”, Rev. Mod. Phys., vol. 72, pp. 969-1016,
                      2000.[3] M. Lyatti, I. Gundareva, T. Röper, Z. Popovic, D.
                      Grützmacher, T. Schäpers, “Quantum size effects in
                      ultra-thin YBa2Cu3O7-x films”, arXiv:2304.03591, 2023.[4]
                      M. Lyatti, M. A. Wolff, A. Savenko, M. Kruth, S. Ferrari, U.
                      Poppe, W. Pernice, R. E. Dunin-Borkowski, and C. Schuck,
                      “Experi-mental evidence for hotspot and phase-slip
                      mechanisms of voltage switching in ultra-thin YBa2Cu3O7-x
                      nanowires”, Phys. Rev. B, vol. 98, 054505, 2018.[5] M.
                      Lyatti, M. A. Wolff, I. Gundareva, M. Kruth, S. Ferrari, R.
                      E. Dunin-Borkowski, C. Schuck, “Energy-level quantization
                      and single-photon control of phase slips in YBa2Cu3O7-x
                      nanowires. Nat Commun., vol. 11, 763, 2020.},
      month         = {Sep},
      date          = {2023-09-10},
      organization  = {15th International Workshop on High
                       Temperature Superconductors in High
                       Frequency Fields, Giardini Naxos
                       (Italy), 10 Sep 2023 - 13 Sep 2023},
      subtyp        = {After Call},
      cin          = {PGI-9},
      cid          = {I:(DE-Juel1)PGI-9-20110106},
      pnm          = {5222 - Exploratory Qubits (POF4-522)},
      pid          = {G:(DE-HGF)POF4-5222},
      typ          = {PUB:(DE-HGF)6},
      doi          = {10.34734/FZJ-2024-01278},
      url          = {https://juser.fz-juelich.de/record/1022157},
}