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Abstract We study the parity-time (PT) symmetry characteristics and the applications to nonlinear optics in an optical trimer system consisting of two indirectly coupled standing-mode micro-cavities and a two-level quantum emitter (QE) placed at the intersection of two cavities. We find this trimer system can exhibit analogical phenomena as those in typical $$\mathscr{PT}$$ 𝒫𝒯 -symmetric dimer systems composed of a passive cavity directly coupled to an active cavity. This system, whose $$\mathscr{PT}$$ 𝒫𝒯 symmetry is demonstrated by our analytic results, can be transformed between the $$\mathscr{PT}$$ 𝒫𝒯 -symmetric phase and the $$\mathscr{PT}$$ 𝒫𝒯 -broken phase by adjusting relevant system parameters. Then, with this system, we observe both the linear and nonlinear parts of the transmission field become remarkably enhanced and can further reach peak values around the $$\mathscr{PT}$$ 𝒫𝒯 breaking point. In addition, we show the negative correlation between the gain degree of the linear (nonlinear) transmission part and decay rate of the QE. This trimer proposal is feasible for experiments and may provide a promising platform for $$\mathscr{PT}$$ 𝒫𝒯 -symmetric optics of low-light levels. Moreover, novel phenomena arising from the QE-cavity-coupling induced nonlinearity gain could be explored to fabricate photonic devices and controllable nonlinear optical media for quantum information process and communication of photons.

We study the multistability of fixed points for a dimerized system of Rydberg atoms driven by two laser fields and trapped in a two-dimensional (2D) square lattice. For identical driving fields, the fixed points of this system exhibit stable uniform, unstable uniform, stable nonuniform, or oscillating nonuniform phases in the presence of a bistable region. For different driving fields, however, all (stable, unstable, or oscillating) phases become nonuniform instead, which then results in two islets isolated from the mainland of Rydberg excitation. We also show that a tristable region may stretch out from the (nonuniform) bistable region near the islets, indicating that a richer phase diagram can be attained by tuning the Rabi frequencies and/or detunings of the driving fields. Last but not least, the fixed points can adiabatically evolve from the islets to the mainland but can not inversely evolve from the mainland to the islets. [ABSTRACT FROM AUTHOR]

We study in theory the single-photon Rydberg excitation in an ensemble of two-level cold atoms at finite low temperatures. Rydberg population plotted against laser detuning may exhibit a nonlinear bistable behavior arising from the dipole–dipole interactions manifested as a van der Waals (vdW) potential. We find that Doppler broadening can result in a remarkable reduction of the bistable region, which may disappear as the atomic temperature increases to a certain value, of the order of tens to hundreds of μK. This special temperature, depending critically on the vdW potential, can be largely raised by partially eliminating the Doppler effect, when a double-photon excitation scheme is used to replace the single-photon excitation scheme. [ABSTRACT FROM AUTHOR]

We study a double-cavity optomechanical system with two movable mirrors acting as mechanical oscillators coupled via a common girder. The fluctuation parts of mechanical oscillations induced by vacuum noises exhibit a limited quantum synchronization owing to the Heisenberg uncertainty. Exerting periodic modulations on the driving lasers or mechanical oscillators, we find that it is viable to attain a 50% ∼ 75% enhancement on this quantum synchronization with appropriate modulation frequencies and amplitudes. A direct comparison shows that the optimal modulations on driving lasers are more favorable than the optimal modulations on mechanical oscillators in that the former could result in a greater enhancement than the latter. Our findings should be instructive for further studies on multi-cavity optomechanical systems to pursue a promising platform for quantum information processing. [ABSTRACT FROM AUTHOR]

We study the steady-state quantum correlations arising from the atom–field and interatomic interplays in two-level Rydberg atoms coherently driven by an external laser field. Three kinds of quantum correlations, i.e., atom–atom correlation, atom–field entanglement and photon–photon correlation, are simultaneously examined by considering dipole–dipole interactions (DDI) for pairwise Rydberg atoms. They are shown to be closely linked with single and double Rydberg excitations, which can be modulated to work in the blockade or antiblockade regime depending on the driving field frequency, the DDI strength and the Rydberg decay rate. As a result, we obtain strongly correlated atoms and highly antibunching photons (indispensable resources in applications of quantum information processing) intermediated with robust atom–field entanglement. [ABSTRACT FROM AUTHOR]