M.E. Gershenson

A Parity-Protected Superconductor-Semiconductor Qubit

  1. T. W. Larsen,
  2. M.E. Gershenson,
  3. L. Casparis,
  4. A. Kringhøj,
  5. N. J. Pearson,
  6. R. P. G. McNeil,
  7. F. Kuemmeth,
  8. P. Krogstrup,
  9. K. D. Petersson,
  10. and C. M. Marcus
Coherence of superconducting qubits can be improved by implementing designs that protect the parity of Cooper pairs on superconducting islands. Here, we introduce a parity-protected
qubit based on voltage-controlled semiconductor nanowire Josephson junctions, taking advantage of the higher harmonic content in the energy-phase relation of few-channel junctions. A symmetric interferometer formed by two such junctions, gate-tuned into balance and frustrated by a half-quantum of applied flux, yields a cos(2{\phi}) Josephson element, reflecting coherent transport of pairs of Cooper pairs. We demonstrate that relaxation of the qubit can be suppressed ten-fold by tuning into the protected regime.
arxiv pdf

Microresonators fabricated from high-kinetic-inductance Aluminum films

  1. Wenyuan Zhang,
  2. K. Kalashnikov,
  3. Wen-Sen Lu,
  4. P. Kamenov,
  5. T. DiNapoli,
  6. and M.E. Gershenson
We have studied superconducting coplanar-waveguide (CPW) resonators fabricated from disordered (granular) films of Aluminum. Very high kinetic inductance of these films, inherent to
disordered materials, allows us to implement ultra-short (200 μm at a 5GHz resonance frequency) and high-impedance (up to 5 kΩ) half-wavelength resonators. We have shown that the intrinsic losses in these resonators at temperatures ≲250mK are limited by resonator coupling to two-level systems in the environment. The demonstrated internal quality factors are comparable with those for CPW resonators made of conventional superconductors. High kinetic inductance and well-understood losses make these disordered Aluminum resonators promising for a wide range of microwave applications which include kinetic inductance photon detectors and superconducting quantum circuits.
arxiv pdf

Josephson Metamaterial with a widely tunable positive/negative Kerr constant

  1. Wenyuan Zhang,
  2. W. Huang,
  3. M.E. Gershenson,
  4. and M.T. Bell
We report on the microwave characterization of a novel one-dimensional Josephson metamaterial composed of a chain of asymmetric superconducting quantum interference devices (SQUIDs)
with nearest-neighbor coupling through common Josephson junctions. This metamaterial demonstrates a strong Kerr nonlinearity, with a Kerr constant tunable over a wide range, from positive to negative values, by a magnetic flux threading the SQUIDs. The experimental results are in good agreement with the theory of nonlinear effects in Josephson chains. The metamaterial is very promising as an active medium for Josephson traveling-wave parametric amplifiers; its use facilitates phase matching in a four-wave mixing process for efficient parametric gain.
arxiv pdf

Spectroscopic Evidence of the Aharonov-Casher effect in a Cooper Pair Box

  1. M.T. Bell,
  2. W.Zhang,
  3. L.B. Ioffe,
  4. and M.E. Gershenson
We have observed the effect of the Aharonov-Casher (AC) interference on the spectrum of a superconducting system containing a symmetric Cooper pair box (CPB) and a large inductance.
By varying the charge ng induced on the CPB island, we observed oscillations of the device spectrum with the period Δng=2e. These oscillations are attributed to the charge-controlled AC interference between the fluxon tunneling processes in the CPB Josephson junctions. Total suppression of the tunneling (complete destructive interference) has been observed for the charge ng=e(2n+1). The CPB in this regime represents the 4π-periodic Josephson element, which can be used for the development of the parity-protected superconducting qubits.
arxiv pdf