Jesper Ilves

Photon-noise-tolerant dispersive readout of a superconducting qubit using a nonlinear Purcell filter

  1. Yoshiki Sunada,
  2. Kenshi Yuki,
  3. Zhiling Wang,
  4. Takeaki Miyamura,
  5. Jesper Ilves,
  6. Kohei Matsuura,
  7. Peter A. Spring,
  8. Shuhei Tamate,
  9. Shingo Kono,
  10. and Yasunobu Nakamura
Residual noise photons in a readout resonator become a major source of dephasing for a superconducting qubit when the resonator is optimized for a fast, high-fidelity dispersive readout.
Here, we propose and demonstrate a nonlinear Purcell filter that suppresses such an undesired dephasing process without sacrificing the readout performance. When a readout pulse is applied, the filter automatically reduces the effective linewidth of the readout resonator, increasing the sensitivity of the qubit to the input field. The noise tolerance of the device we fabricated is shown to be enhanced by a factor of three relative to a device with a linear filter. The measurement rate is enhanced by another factor of three by utilizing the bifurcation of the nonlinear filter. A readout fidelity of 99.4% and a QND fidelity of 99.2% are achieved using a 40-ns readout pulse. The nonlinear Purcell filter will be an effective tool for realizing a fast, high-fidelity readout without compromising the coherence time of the qubit.
arxiv pdf

Fast readout and reset of a superconducting qubit coupled to a resonator with an intrinsic Purcell filter

  1. Yoshiki Sunada,
  2. Shingo Kono,
  3. Jesper Ilves,
  4. Shuhei Tamate,
  5. Takanori Sugiyama,
  6. Yutaka Tabuchi,
  7. and Yasunobu Nakamura
Coupling a resonator to a superconducting qubit enables various operations on the qubit including dispersive readout and unconditional reset. The speed of these operations is limited
by the external decay rate of the resonator. However, increasing the decay rate also increases the rate of qubit decay via the resonator, limiting the qubit lifetime. Here, we demonstrate that the resonator-mediated qubit decay can be suppressed by utilizing the distributed-element, multi-mode nature of the resonator. We show that the suppression exceeds two orders of magnitude over a bandwidth of 600 MHz. We use this „intrinsic Purcell filter“ to demonstrate a 40-ns readout with 99.1% fidelity and a 100-ns reset with residual excitation of less than 1.7%.
arxiv pdf

On-demand generation and characterization of a microwave time-bin qubit

  1. Jesper Ilves,
  2. Shingo Kono,
  3. Yoshiki Sunada,
  4. Shota Yamazaki,
  5. Minkyu Kim,
  6. Kazuki Koshino,
  7. and Yasunobu Nakamura
Superconducting circuits offer a scalable platform for the construction of large-scale quantum networks where information can be encoded in multiple temporal modes of propagating microwaves.
Characterization of such microwave signals with a method extendable to an arbitrary number of temporal modes with a single detector and demonstration of their phase-robust nature are of great interest. Here we show the on-demand generation and Wigner tomography of a microwave time-bin qubit with superconducting circuit quantum electrodynamics architecture. We perform the tomography with a single heterodyne detector by dynamically changing the measurement quadrature with a phase-sensitive amplifier independently for the two temporal modes. By generating and measuring the qubits with hardware lacking a shared phase reference, we demonstrate conservation of phase information in each time-bin qubit generated.
arxiv pdf