William F. Kindel

Generation and efficient measurement of single photons from fixed frequency superconducting qubits

  1. William F. Kindel,
  2. M. D. Schroer,
  3. and K. W. Lehnert
We demonstrate and evaluate an on-demand source of single itinerant microwave photons. Photons are generated using a highly coherent, fixed-frequency qubit-cavity system, and a protocol
where the microwave control field is far detuned from the photon emission frequency. By using a Josephson parametric amplifier (JPA), we perform efficient single-quadrature detection of the state emerging from the cavity. We characterize the imperfections of the photon generation and detection, including detection inefficiency and state infidelity caused by measurement backaction over a range of JPA gains from 17 to 33 dB. We observe that both detection efficiency and undesirable backaction increase with JPA gain. We find that the density matrix has its maximum single photon component ρ11=0.36±0.01 at 29 dB JPA gain. At this gain, backaction of the JPA creates cavity photon number fluctuations that we model as a thermal distribution with an average photon number n¯=0.041±0.003.
arxiv pdf

Controlling a mechanical oscillator with a tunable coherent feedback network

  1. Joseph Kerckhoff,
  2. Reed W. Andrews,
  3. H. S. Ku,
  4. William F. Kindel,
  5. Katarina Cicak,
  6. Raymond W. Simmonds,
  7. and K. W. Lehnert
We demonstrate a fully cryogenic microwave feedback network composed of modular superconducting devices interconnected by transmission lines and designed to control a mechanical oscillator
coupled to one of the devices. The network is partitioned into an electromechanical device and a dynamically tunable controller that coherently receives, processes and feeds back continuous microwave signals that modify the dynamics and readout of the mechanical state. While previous electromechanical systems represent some compromise between efficient control and efficient readout of the mechanical state, as set by the electromagnetic decay rate, this flexible controller yields a closed-loop network that can be dynamically and continuously tuned between both extremes much faster than the mechanical response time. We demonstrate that the microwave decay rate may be modulated by at least a factor of 10 at a rate greater than $10^4$ times the mechanical response rate.
arxiv pdf