Observing pure effects of counter-rotating terms without ultrastrong coupling: A single photon can simultaneously excite two qubits

TL;DR

This paper proposes a superconducting circuit setup to observe how a single photon can simultaneously excite two qubits through counter-rotating terms, revealing effects beyond the rotating-wave approximation.

Contribution

It introduces a novel superconducting circuit approach to detect simultaneous qubit excitation by a single photon, emphasizing the role of counter-rotating terms and interference effects.

Findings

Single photon can excite two qubits via higher-order transitions.

Faster coherent dynamics due to involving only one intermediate state.

Controllable interference effects between transition paths.

Abstract

The coherent process that a single photon simultaneously excites two qubits has recently been theoretically predicted by [https://link.aps.org/doi/10.1103/PhysRevLett.117.043601 {Phys. Rev. Lett. 117, 043601 (2016)}]. We propose a different approach to observe a similar dynamical process based on a superconducting quantum circuit, where two coupled flux qubits longitudinally interact with the same resonator. We show that this simultaneous excitation of two qubits (assuming that the sum of their transition frequencies is close to the cavity frequency) is related to the counter-rotating terms in the dipole-dipole coupling between two qubits, and the standard rotating-wave approximation is not valid here. By numerically simulating the adiabatic Landau-Zener transition and Rabi-oscillation effects, we clearly verify that the energy of a single photon can excite two qubits via higher-order transitions induced by the longitudinal couplings and the counter-rotating terms. Compared with previous studies, the coherent dynamics in our system only involves one intermediate state and, thus, exhibits a much faster rate. We also find transition paths which can interfere. Finally, by discussing how to control the two longitudinal-coupling strengths, we find a method to observe both constructive and destructive interference phenomena in our system.