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

Abstract

The coherent process that a single photon simultaneously excites two qubits has recently been theoretically predicted by [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 longitudinalcoupling strengths, we find a method to observe both constructive and destructive interference phenomena in our system.

Broad-band negative refraction via simultaneous multi-electron transitions

“…where the superscripts over the arrows indicate the polarizations of the electric fields. In the following calculations, we only consider the transitions from the lower band to the upper band because of the rotating-wave approximation [60,61] and initial condition. However, it has been shown that the counter-rotating terms will essentially influence the quantum dynamics in combination with some specific initial states [60,62].…”

Phase-controlled phonon laser

“…Therefore, such a non-classical phonon laser provides the key source for future fundamental studies and applications of quantum or nonlinear acoustics, such as vacuum Casimir-Rabi splittings [67] , squeezed phonons [68,69], and bistable phonon emission [70]. If the RWA is not satisfied, the pure effects [71] of counter-rotating terms should be observed. It is worth noting that the thermal noise of mechanical oscillators is detrimental for the coherent single phonon processes, while the negativity of the mechanical oscillator's Wigner function can still be observed with the thermal phonon occupation = n 2 th [66], which is corresponding to a 100 MHz mechanical mode in 10 mK environment.…”

Spin squeezing by one-photon–two-atom excitation processes in atomic ensembles

“…One of the most interesting nonlinear optical effects predicted in the USC regime consists of the simultaneously excitation of two or more spatially separated atoms by a single photon [22][23][24]. This process is reversible, so that the atoms can return to a lower-energy state by collectively emitting one photon.…”