K. Geerlings

Quantum back-action of variable-strength measurement

  1. M. Hatridge,
  2. S. Shankar,
  3. M. Mirrahimi,
  4. F. Schackert,
  5. K. Geerlings,
  6. T. Brecht,
  7. K. M. Sliwa,
  8. B. Abdo,
  9. L. Frunzio,
  10. S. M. Girvin,
  11. R. J. Schoelkopf,
  12. and M. H. Devoret
Measuring a quantum system can randomly perturb its state. The strength and nature of this back-action depends on the quantity which is measured. In a partial measurement performed
by an ideal apparatus, quantum physics predicts that the system remains in a pure state whose evolution can be tracked perfectly from the measurement record. We demonstrate this property using a superconducting qubit dispersively coupled to a cavity traversed by a microwave signal. The back-action on the qubit state of a single measurement of both signal quadratures is observed and shown to produce a stochastic operation whose action is determined by the measurement result. This accurate monitoring of a qubit state is an essential prerequisite for measurement-based feedback control of quantum systems.
arxiv pdf

Demonstrating a Driven Reset Protocol of a Superconducting Qubit

  1. K. Geerlings,
  2. Z. Leghtas,
  3. I. M. Pop,
  4. S. Shankar,
  5. L. Frunzio,
  6. R. J. Schoelkopf,
  7. M. Mirrahimi,
  8. and M. H. Devoret
Qubit reset is crucial at the start of and during quantum information algorithms. We present the experimental demonstration of a practical method to force qubits into their ground state,
based on driving certain qubit and cavity transitions. Our protocol, nicknamed DDROP (Double Drive Reset of Population) is tested on a superconducting transmon qubit in a 3D cavity. Using a new method for measuring population, we show that we can prepare the ground state with a fidelity of at least 99.5 % in less than 3 microseconds; faster times and higher fidelity are predicted upon parameter optimization.
arxiv pdf

Improving the Quality Factor of Microwave Compact Resonators by Optimizing their Geometrical Parameters

  1. K. Geerlings,
  2. S. Shankar,
  3. E. Edwards,
  4. L. Frunzio,
  5. R. J. Schoelkopf,
  6. and M. H. Devoret
Applications in quantum information processing and photon detectors are stimulating a race to produce the highest possible quality factor on-chip superconducting microwave resonators.
We have tested the surface-dominated loss hypothesis by systematically studying the role of geometrical parameters on the internal quality factors of compact resonators patterned in Nb on sapphire. Their single-photon internal quality factors were found to increase with the distance between capacitor fingers, the width of the capacitor fingers, and the impedance of the resonator. Quality factors were improved from 210,000 to 500,000 at T = 200 mK. All of these results are consistent with our starting hypothesis.
arxiv pdf