RQC Seminar

308th RQC Seminar

• Speaker

  Dr. Sahel Ashhab
  ( NICT )

• Date

  13:00-14:00 (1:00pm-2:00pm), Thursday October 1, 2026

• Venue

  Hybrid( Zoom,
  Large Meeting Room, 2F, Welfare and Conference Building, Wako Campus / 和光地区 統合支援施設2階 大会議室 (C61))

• Title

  Using optimal control theory to design two-qubit gates for realistic superconducting circuits

• Inquiries

  yasunobu.nakamura[at]riken.jp

Abstract
Protocols for implementing two-qubit gates are typically developed using simple models of two-qubit systems. Realistic quantum processors differ from the simple models in several ways that complicate the implementation of the protocols in practice. For example, any realistic device suffers from noise and decoherence. Furthermore, some of the popular designs of superconducting qubits are weakly harmonic, which means that they have additional quantum states that often play a non-negligible role in the dynamics. Additionally, there are often practical constraints on the frequencies and strengths of the control signals. Using optimal control theory techniques, we have performed numerical calculations to optimize two-qubit gates for weakly anharmonic qubits. If we model each qubit as a few-level quantum system, and we ignore loss mechanisms, we obtain the surprising result that the higher energy levels speed up two-qubit gates. This result contradicts the intuitive picture in which higher energy levels are a source of leakage that leads to a slowing down of quantum gates. Including loss from higher energy levels in the theoretical model produces the intuitively expected slow-down effect. We also perform calculations to optimize few-qudit gates and investigate the minimum times needed to perform few-qudit operations. We find that while the operation time grows rapidly with the number of qudits, the dependence on qudit dimension is much weaker, which can be understood based on the stronger interactions between higher energy levels and parallel dynamics in many-level quantum systems.