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.

Evolution of Nanowire Transmons and Their Quantum Coherence in Magnetic Field

  1. F. Luthi,
  2. T. Stavenga,
  3. O. W. Enzing,
  4. A. Bruno,
  5. C. Dickel,
  6. N. K. Langford,
  7. M. A. Rol,
  8. T. S. Jespersen,
  9. J. Nygard,
  10. P. Krogstrup,
  11. and L. DiCarlo
We present an experimental study of nanowire transmons at zero and applied in-plane magnetic field. With Josephson non-linearities provided by the nanowires, our qubits operate at higher
magnetic fields than standard transmons. Nanowire transmons exhibit coherence up to 70 mT, where the induced superconducting gap in the nanowire closes. We demonstrate that on-chip charge noise coupling to the Josephson energy plays a dominant role in the qubit dephasing. This takes the form of strongly-coupled two-level systems switching on 100 ms timescales and a more weakly coupled background producing 1/f noise. Several observations, including the field dependence of qubit energy relaxation and dephasing, are not fully understood, inviting further experimental investigation and theory. Using nanowires with a thinner superconducting shell will enable operation of these circuits up to 0.5 T, a regime relevant for topological quantum computation.

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.

A Semiconductor Nanowire-Based Superconducting Qubit

  1. T. W. Larsen,
  2. K. D. Petersson,
  3. F. Kuemmeth,
  4. T. S. Jespersen,
  5. P. Krogstrup,
  6. J. Nygard,
  7. and C. M. Marcus
We introduce a hybrid qubit based on a semiconductor nanowire with an epitaxially grown superconductor layer. Josephson energy of the transmon-like device („gatemon“) is
controlled by an electrostatic gate that depletes carriers in a semiconducting weak link region. Strong coupling to an on-chip microwave cavity and coherent qubit control via gate voltage pulses is demonstrated, yielding reasonably long relaxation times (0.8 {\mu}s) and dephasing times (1 {\mu}s), exceeding gate operation times by two orders of magnitude, in these first-generation devices. Because qubit control relies on voltages rather than fluxes, dissipation in resistive control lines is reduced, screening reduces crosstalk, and the absence of flux control allows operation in a magnetic field, relevant for topological quantum information.