Daisuke Hoshi

Realisation of Protected Cat Qutrit via Engineered Quantum Tunnelling

  1. Sangil Kwon,
  2. Daisuke Hoshi,
  3. Toshiaki Nagase,
  4. Daichi Sugiyama,
  5. Hiroto Mukai,
  6. Kengo Takemura,
  7. Rintaro Kojima,
  8. Yu Zhou,
  9. Shohei Watabe,
  10. Fumiki Yoshihara,
  11. and Jaw-Shen Tsai
Engineering quantum tunnelling in phase space has emerged as a viable method for creating a protected qubit with biased-noise properties. A promising approach is to combine a Kerr nonlinearity
with multi-photon transitions, resulting in a system known as a Kerr parametric oscillator (KPO). In this work, we implement a three-photon KPO and explore its potential as a protected qutrit. We confirm quantum coherence by demonstrating three-photon Rabi oscillations and performing direct Wigner function measurements that reveal three-component cat-like states. We observe breathing-like dynamics in phase space, arising from exotic temporal interference between the qutrit and excited states. The frequency of this interference corresponds to the energy gap between the qutrit and excited manifolds, thereby providing an experimental hallmark of qutrit space protection. We also identify a higher-order pump term as the main mechanism suppressing photon occupation; mitigating this term is necessary to maximize protection. Our findings elucidate the basic quantum properties of the three-photon KPO and establish the first step toward its use as an alternative qutrit platform.
arxiv pdf

Entangling Schrödinger’s cat states by seeding a Bell state or swapping the cats

  1. Daisuke Hoshi,
  2. Toshiaki Nagase,
  3. Sangil Kwon,
  4. Daisuke Iyama,
  5. Takahiko Kamiya,
  6. Shiori Fujii,
  7. Hiroto Mukai,
  8. Shahnawaz Ahmed,
  9. Anton Frisk Kockum,
  10. Shohei Watabe,
  11. Fumiki Yoshihara,
  12. and Jaw-Shen Tsai
In quantum information processing, two primary research directions have emerged: one based on discrete variables (DV) and the other on the structure of quantum states in a continuous-variable
(CV) space. It is increasingly recognized that integrating these two approaches could unlock new potentials, overcoming the inherent limitations of each. Here, we show that such a DV-CV hybrid approach, applied to superconducting Kerr parametric oscillators (KPOs), enables us to entangle a pair of Schrödinger’s cat states by two straightforward methods. The first method involves the entanglement-preserving and deterministic conversion between Bell states in the Fock-state basis (DV encoding) and those in the cat-state basis (CV encoding). This method would allow us to construct quantum networks in the cat-state basis using conventional schemes originally developed for the Fock-state basis. In the second method, the iSWAP‾‾‾‾‾‾‾√ gate operation is implemented between two cat states following the procedure used for Fock-state encoding. This DV-like gate operation on CV encoding not only completes the demonstration of a universal quantum gate set in a KPO system but also enables faster and simpler gate operations compared to previous SWAP gate implementations on bosonic modes. Our work offers a simple yet powerful application of DV-CV hybridization while also highlighting the scalability of this planar KPO system.
arxiv pdf