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arXiv:2111.13442 · Physical Review A

Ultrastrong coupling of a qubit with a nonlinear optical resonator

Fabio Mauceri, Alberto Mercurio, Salvatore Savasta, Omar Di Stefano

Published Feb 23, 2022• 1 citation
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Abstract

The authors study the quantum Rabi model in the ultrastrong regime in the presence of cavity nonlinearities and identify inconsistencies from various nonlinear Hamiltonians. To resolve the issues, the authors develop a microscopic theory and provide a consistent nonlinear-resonator quantum Rabi model satisfying the gauge principle.
DOI
10.1103/physreva.105.023719

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We examine a generalized Rabi model formed by two identical qubits interacting with a common oscillator mode. In the near-resonance configuration where the oscillator frequency is close to the transition frequency of the qubit, we determine the ground state of the model approximately by using a variational method and a transformation method. Both methods are shown to approximate the ground state accurately for a range of coupling strengths in the ultrastrong-coupling regime. In addition, the accuracy of the approximation can be further improved by using perturbation theory. We also characterize the ground-state entanglement between the two qubits by calculating the negativity.
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We present a circuit-QED scheme which makes it possible to reach the ultrastrong-coupling regime of a nondipolar interaction between a single qubit and a quantum resonator. We show that the system Hamiltonian is well approximated by a two-photon quantum Rabi model and propose a simple scattering experiment to probe its fundamental properties. In particular, we identify a driving scheme that reveals the change in selection rules characterizing the breakdown of the rotating-wave approximation and the transition from strong to ultrastrong two-photon interactions. Finally, we show that a frequency crowding in a narrow spectral region is observable in the output fluorescence spectrum as the coupling strength approaches the collapse point, paving the way to the direct observation of the onset of the spectral collapse in a solid-state device.