U. Hübner

Tuning the energy gap of a flux qubit by AC-Zeeman shift

  1. G. Oelsner,
  2. E. Il'ichev,
  3. and U. Hübner
We propose to tune the minimal energy level splitting of a superconducting qubit by a microwave induced ac Zeeman shift. We experimentally investigate the usability of this approach
to overcome parameter spread induced by the micro fabrication of superconducting artificial quantum circuits. To do so, we dress the qubit by a strong tone, effectively shifting its energy levels. By a two-tone spectroscopy of this dressed system the shift of the qubit’s energy levels can be probed. A theoretical treatment allowed us to completely explain the observed experimental dependencies and reconstruct the influence of the strong driving to the dissipative dynamics of the qubit.
arxiv pdf

Detection of weak microwave fields with an underdamped Josephson junction

  1. G. Oelsner,
  2. C. K. Andersen,
  3. M. Rehák,
  4. M. Schmelz,
  5. S. Anders,
  6. M. Grajcar,
  7. U. Hübner,
  8. K. Mølmer,
  9. and E. Il'ichev
We have constructed a microwave detector based on the voltage switching of an underdamped Josephson junction, that is positioned at a current antinode of a {lambda}/4 coplanar waveguide
resonator. By measuring the switching current and the transmission through a waveguide capacitively coupled to the resonator at different drive frequencies and temperatures we are able to fully characterize the system and assess its detection efficiency and sensitivity. Testing the detector by applying a classical microwave field with the strength of a single photon yielded a sensitivity parameter of 0.5 in qualitative agreement with theoretical calculations.
arxiv pdf

Parametric amplification by coupled flux qubits

  1. M. Rehak,
  2. P. Neilinger,
  3. M. Grajcar,
  4. G. Oelsner,
  5. U. Hubner,
  6. E. Il'ichev,
  7. and H.-G. Meyer
We report the parametric amplification of a microwave signal in a Kerr medium formed from superconducting qubits. Two mutually coupled flux qubits, embedded in the current antinode
of a superconducting coplanar waveguide resonator, are used as a nonlinear element. Shared Josephson junctions provide the qubit-resonator coupling, resulting in a device with a measured gain of about 20 dB. We argue, that this arrangement represents a unit cell which can be straightforwardly extended to a quasi one-dimensional quantum metamaterial with a large tunable Kerr nonlinearity.
arxiv pdf

Dressed-state amplification by a superconducting qubit

  1. G. Oelsner,
  2. P. Macha,
  3. O. V. Astafiev,
  4. E. Il'ichev,
  5. M. Grajcar,
  6. U. Hübner,
  7. B. I. Ivanov,
  8. P. Neilinger,
  9. and H.-G. Meyer
We demonstrate amplification of a microwave signal by a strongly driven two-level system in a coplanar waveguide resonator. The effect known from optics as dressed-state lasing is observed
with a single quantum system formed by a persistent current (flux) qubit. The transmission through the resonator is enhanced when the Rabi frequency of the driven qubit is tuned into resonance with one of the resonator modes. Amplification as well as linewidth narrowing of a weak probe signal has been observed. The laser emission at the resonator’s fundamental mode has been studied by measuring the emission spectrum. We analyzed our system and found an excellent agreement between the experimental results and the theoretical predictions obtained in the dressed-state model.
arxiv pdf