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arXiv:1901.04246 · Physical Review A

Collective radiance effects in the ultrastrong-coupling regime

Qian Bin, Xin‐You Lü, Tai-Shuang Yin, Yong Li, Ying Wu

Published Mar 5, 2019• 30 citations
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Abstract

We investigate the collective radiance characteristics of qubits in the ultrastrong-coupling regime, where the radiance witness is defined based on the resonator-qubit dressed basis. An ultrastrong hyperradiance effect is demonstrated when the dressed state of the system is resonantly driven. Interestingly, we show that, besides the resonator-qubit coupling strength, the parity-symmetry-breaking-induced cascade transition can significantly enhance the collective radiance of the qubits, which allows us to manipulate the transitions between subradiance, superradiance, and hyperradiance via adjusting the parity symmetry of the system with an external magnetic field. This work extends the collective radiance theory to the ultrastrong-coupling regime, and offers potential applications in the engineering of laser devices.

DOI
10.1103/physreva.99.033809

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We investigate the collective radiance characteristics of qubits in the ultrastrong-coupling regime, where the radiance witness is defined based on the resonator-qubit dressed basis. An ultrastrong hyperradiance effect is demonstrated when the dressed state of the system is resonantly driven. Interestingly, we show that, besides the resonator-qubit coupling strength, the parity-symmetry-breaking-induced cascade transition can significantly enhance the collective radiance of the qubits, which allows us to manipulate the transitions between subradiance, superradiance, and hyperradiance via adjusting the parity symmetry of the system with an external magnetic field. This work extends the collective radiance theory to the ultrastrong-coupling regime, and offers potential applications in the engineering of laser devices.
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Several recent experiments have reported a variety of new collective behaviors of two atoms in a cavity. An important experimental result was that the back-reaction in a high-quality cavity leads to subradiance rather than superradiance. Further investigation by changing the parameter domain has shown the possibility of hyperradiance. In this paper we present a model of hyperradiance which is based on the idea of vacuum Rabi splittings when the driving field is detuned from the atom or cavity resonance so that it hits one of the dressed states. We show that this hyperradiance is accompanied by significant nonclassicality of the output. Some aspects, but not nonclassicality, of the hyperradiance when the driving field is very weak can be understood semiclassically. Our mechanism of hyperradiance applies to atoms coupled symmetrically to the cavity mode, whereas in the work of M. Pleinert et al. [Optica 4, 779 (2017)] atoms have to be coupled almost asymmetrically to the cavity.
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We develop a rigorous theoretical framework for interaction-induced phenomena in the waveguide quantum electrodynamics (QED) driven by mechanical oscillations of the qubits. Specifically, we predict that the simplest setup of two qubits, harmonically trapped over an optical waveguide, enables the ultrastrong coupling regime of the quantum optomechanical interaction. Moreover, the combination of the inherent open nature of the system and the strong optomechanical coupling leads to emerging parity-time (PT) symmetry, quite unexpected for a purely quantum system without artificially engineered gain and loss. The PT phase transition drives long-living subradiant states, observable in the state-of-the-art waveguide QED setups.
In his seminal paper, Dicke demonstrated that a group of emitters in collective states is able to radiate with increased intensity in particular directions with modified decay rates, a phenomenon which he called superradiance. Along with its counterpart, subradiance, this effect has been studied in-depth for the last 60 years. Here, we report a new phenomenon in the collective behavior of coherently driven atoms. We show that the fundamental setup of two atoms coupled to a single-mode cavity can be distinctly exceeding the free-space superradiant behavior of two atoms, a completely new paradigm in the subject, which we call hyperradiance. The phenomenon is further accompanied by strong quantum fluctuations. Surprisingly, hyperradiance arises for atoms radiating out-of-phase, an alleged non-ideal condition, where one expects subradiance. We are able to explain the onset of hyperradiance in a transparent way by a photon cascade taking place among manifolds of Dicke states with different photon numbers under particular out-of-phase coupling conditions.
We demonstrate symmetry protected multiphoton bundle emission in the cavity QED system under the ultrastrong coupling regime. Our proposal only enables the super-Rabi oscillations with periodic generation of even correlated photons in the cavity, which is realized by combining the laser driven flip of qubit and the symmetry conserved transitions induced by Rabi interaction with parity symmetry. Combined with dissipation, only 2n-photon bundle emissions are allowed, due to the almost perfect suppression of bundle emissions with odd correlated photons. Meanwhile, the corresponding purities are significantly enhanced by the parity symmetry. This work extends multiphoton bundle emission to the ultrastrong coupling regime, and offers the prospect of exploring symmetry-protected multiphoton physics.