Towards high-Tc superconducting qubit with THz plasma frequency
Creators: name: Lyatti, Matvey
name: Gundareva, Irina
name: Röper, Torsten
name: Grützmacher, Detlev
name: Schäpers, Thomas
Publisher: name: Forschungzentrum Jülich
Year: 2023

description: 15th International Workshop on High Temperature Superconductors in High Frequency Fields, HTSHFF2023, Giardini Naxos, Italy, 10 Sep 2023 - 13 Sep 2023
description: 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 &amp; R. J. Schoelkopf, “Superconducting Circuits for Quantum Information: An Outlook”, Science, vol. 339, pp. 1169-1174, 2013.[2] C.C. Tsuei &amp; 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.