Voltage-Controlled Superconducting Quantum Bus

  1. L. Casparis,
  2. N. J. Pearson,
  3. A. Kringhøj,
  4. T. W. Larsen,
  5. F. Kuemmeth,
  6. J. Nygård,
  7. P. Krogstrup,
  8. K. D. Petersson,
  9. and C. M. Marcus
We demonstrate the ability of an epitaxial semiconductor-superconductor nanowire to serve as a field-effect switch to tune a superconducting cavity. Two superconducting gatemon qubits
are coupled to the cavity, which acts as a quantum bus. Using a gate voltage to control the superconducting switch yields up to a factor of 8 change in qubit-qubit coupling between the on and off states without detrimental effect on qubit coherence. High-bandwidth operation of the coupling switch on nanosecond timescales degrades qubit coherence.

Anharmonicity of a Gatemon Qubit with a Few-Mode Josephson Junction

  1. A. Kringhøj,
  2. L. Casparis,
  3. M. Hell,
  4. T. W. Larsen,
  5. F. Kuemmeth,
  6. M. Leijnse,
  7. K. Flensberg,
  8. P. Krogstrup,
  9. J. Nygård,
  10. K. D. Petersson,
  11. and C. M. Marcus
Coherent operation of gate-voltage-controlled hybrid transmon qubits (gatemons) based on semiconductor nanowires was recently demonstrated. Here we experimentally investigate the anharmonicity
in epitaxial InAs-Al Josephson junctions, a key parameter for their use as a qubit. Anharmonicity is found to be reduced by roughly a factor of two compared to conventional metallic junctions, and dependent on gate voltage. Experimental results are consistent with a theoretical model, indicating that Josephson coupling is mediated by a small number of highly transmitting modes in the semiconductor junction.