Deterministic Quantum Communication Between Fixed-Frequency Superconducting Qubits via Broadband Resonators

  1. Takeaki Miyamura,
  2. Zhiling Wang,
  3. Kohei Matsuura,
  4. Yoshiki Sunada,
  5. Keika Sunada,
  6. Kenshi Yuki,
  7. Jesper Ilves,
  8. and Yasunobu Nakamura
Quantum communication between remote chips is essential for realizing large-scale superconducting quantum computers. For such communication, itinerant microwave photons propagating
through transmission lines offer a promising approach. However, demonstrations to date have relied on frequency-tunable circuit elements to compensate for fabrication-related parameter variations between sender and receiver devices, introducing control complexity and limiting scalability. In this work, we demonstrate deterministic quantum state transfer and remote entanglement generation between fixed-frequency superconducting qubits on separate chips. To compensate for the sender-receiver mismatch, we employ a frequency-tunable photon-generation technique which enables us to adjust the photon frequency without modifying circuit parameters. To enhance the frequency tunability, we implement broadband transfer resonators composed of two coupled coplanar-waveguide resonators, achieving a bandwidth of more than 100 MHz. This broadband design enables successful quantum communication across a 30-MHz range of photon frequencies between the remote qubits. Quantum process tomography reveals state transfer fidelities of around 78% and Bell-state fidelities of around 73% across the full frequency range. Our approach avoids the complexity of the control lines and noise channels, providing a flexible pathway toward scalable quantum networks.

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.