title: Ultrafast quantum computation in ultrastrongly coupled circuit QED systems.
abstract: The latest technological progress of achieving the ultrastrong-coupling regime in circuit quantum electrodynamics (QED) systems has greatly promoted the developments of quantum physics, where novel quantum optics phenomena and potential computational benefits have been predicted. Here, we propose a scheme to accelerate the nontrivial two-qubit phase gate in a circuit QED system, where superconducting flux qubits are ultrastrongly coupled to a transmission line resonator (TLR), and two more TLRs are coupled to the ultrastrongly-coupled system for assistant. The nontrivial unconventional geometric phase gate between the two flux qubits is achieved based on close-loop displacements of the three-mode intracavity fields. Moreover, as there are three resonators contributing to the phase accumulation, the requirement of the coupling strength to realize the two-qubit gate can be reduced. Further reduction in the coupling strength to achieve a specific controlled-phase gate can be realized by adding more auxiliary resonators to the ultrastrongly-coupled system through superconducting quantum interference devices. We also present a study of our scheme with realistic parameters considering imperfect controls and noisy environment. Our scheme possesses the merits of ultrafastness and noise-tolerance due to the advantages of geometric phases.
authors.affiliation: College of Communications Engineering, PLA University of Science and Technology, Nanjing 210007, China.|Quantum Physics and Quantum Information Division, Beijing Computational Science Research Center, Beijing 100193, China.
authors.forename: Yimin
authors.initials: Y
authors.lastname: Wang
authors.affiliation: Pillar of Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Road, 487372, Singapore.
authors.forename: Chu
authors.initials: C
authors.lastname: Guo
authors.affiliation: Quantum Physics and Quantum Information Division, Beijing Computational Science Research Center, Beijing 100193, China.
authors.forename: Guo-Qiang
authors.initials: GQ
authors.lastname: Zhang
authors.affiliation: Center for Quantum Sciences and School of Physics, Northeast Normal University, Changchun 130024, China.
authors.forename: Gangcheng
authors.initials: G
authors.lastname: Wang
authors.affiliation: Pillar of Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Road, 487372, Singapore.
authors.forename: Chunfeng
authors.initials: C
authors.lastname: Wu