Observation and manipulation of quantum interference in a Kerr parametric oscillator

  1. Daisuke Iyama,
  2. Takahiko Kamiya,
  3. Shiori Fujii,
  4. Hiroto Mukai,
  5. Yu Zhou,
  6. Toshiaki Nagase,
  7. Akiyoshi Tomonaga,
  8. Rui Wang,
  9. Jiao-Jiao Xue,
  10. Shohei Watabe,
  11. Sangil Kwon,
  12. and Jaw-Shen Tsai
Quantum tunneling is the phenomenon that makes superconducting circuits „quantum“. Recently, there has been a renewed interest in using quantum tunneling in phase space
of a Kerr parametric oscillator as a resource for quantum information processing. Here, we report a direct observation of quantum interference induced by such tunneling in a planar superconducting circuit. We experimentally elucidate all essential properties of this quantum interference, such as mapping from Fock states to cat states, a temporal oscillation induced by the pump detuning, as well as its characteristic Rabi oscillations and Ramsey fringes. Finally, we perform gate operations as manipulations of the observed quantum interference. Our findings lay the groundwork for further studies on quantum properties of Kerr parametric oscillators and their use in quantum information technologies.

Active Initialization Experiment of Superconducting Qubit Using Quantum-circuit Refrigerator

  1. Teruaki Yoshioka,
  2. Hiroto Mukai,
  3. Akiyoshi Tomonaga,
  4. Shintaro Takada,
  5. Yuma Okazaki,
  6. Nobu-Hisa Kaneko,
  7. Shuji Nakamura,
  8. and Jaw-Shen Tsai
The initialization of superconducting qubits is one of the essential techniques for the realization of quantum computation. In previous research, initialization above 99% fidelity
has been achieved at 280 ns. Here, we demonstrate the rapid initialization of a superconducting qubit with a quantum-circuit refrigerator (QCR). Photon-assisted tunneling of quasiparticles in the QCR can temporally increase the relaxation time of photons inside the resonator and helps release energy from the qubit to the environment. Experiments using this protocol have shown that 99\% of initialization time is reduced to 180 ns. This initialization time depends strongly on the relaxation rate of the resonator, and faster initialization is possible by reducing the resistance of the QCR, which limits the ON/OFF ratio, and by strengthening the coupling between the QCR and the resonator.

Quasiparticle tunneling and 1/f charge noise in ultrastrongly coupled superconducting qubit and resonator

  1. Akiyoshi Tomonaga,
  2. Hiroto Mukai,
  3. Fumiki Yoshihara,
  4. and Jaw-Shen Tsai
We report an experimentally observed anomalous doubly split spectrum and its split-width fluctuation in an ultrastrongly coupled superconducting qubit and resonator. From an analysis
of Rabimodel and circuit model Hamiltonians, we found that the doubly split spectrum and split-width fluctuation are caused by discrete charge hops due to quasiparticle tunnelings and a continuous background charge fluctuation in islands of a flux qubit. During 70 hours in the spectrum measurement, split width fluctuates but the middle frequency of the split is constant. This result indicates that quasiparticles in our device seem mainly tunnel one particular junction. The background offsetcharge obtained from split width has the 1/f noise characteristic.

Generating time-domain linear cluster state by recycling superconducting qubits

  1. Shotaro Shirai,
  2. Yu Zhou,
  3. Keiichi Sakata,
  4. Hiroto Mukai,
  5. and Jaw-Shen Tsai
Cluster states, a type of highly entangled state, are essential resources for quantum information processing. Here we demonstrated the generation of a time-domain linear cluster state
(t-LCS) using a superconducting quantum circuit consisting of only two transmon qubits. By recycling the physical qubits, the t-LCS equivalent up to four physical qubits was validated by quantum state tomography with fidelity of 59%. We further confirmed the true generation of t-LCS by examining the expectation value of an entanglement witness. Our demonstrated protocol of t-LCS generation allows efficient use of physical qubits which could lead to resource-efficient execution of quantum circuits on large scale.