Exceptional points (EPs) are the non-Hermitian singularities in the parameters space of open physical systems, where the eigenenergies and associated eigenstates coalesce. EPs have demonstrated great ability of manipulating eigenstates in classical systems, but the effects and applications of EPs in quantum domain remain largely unexplored. Here we show that EPs can significantly enhance the quantum coherence of atom-cavity interaction in an extended configuration of microcavity featuring chiral EPs, where the introduction of an auxiliary lossy cavity offers multiple new degrees of freedom to tailor the photonic density of states (PhDOS) of the cavities. We find from PhDOS that the proposed model can simultaneously reduce the effective linewidth of cavities and enhance the effective coupling strength of atom-cavity interaction by over one order of magnitude, leading to the prominent Rabi oscillations and improved lifetime of dressed states even the total system dissipation is increased. Our proposal thus holds great promise for building highperformance quantum devices without demanding requirement of strong atom-cavity interaction and enriches the understanding of EPs in manipulating quantum states.
Exceptional points (EPs) are the non-Hermitian singularities in the parameters space of open physical systems, where the eigenenergies and associated eigenstates coalesce. EPs have demonstrated great ability of manipulating eigenstates in classical systems, but the effects and applications of EPs in quantum domain remain largely unexplored. Here we show that EPs can significantly enhance the quantum coherence of atom-cavity interaction in an extended configuration of microcavity featuring chiral EPs, where the introduction of an auxiliary lossy cavity offers multiple new degrees of freedom to tailor the photonic density of states (PhDOS) of the cavities. We find from PhDOS that the proposed model can simultaneously reduce the effective linewidth of cavities and enhance the effective coupling strength of atom-cavity interaction by over one order of magnitude, leading to the prominent Rabi oscillations and improved lifetime of dressed states even the total system dissipation is increased. Our proposal thus holds great promise for building highperformance quantum devices without demanding requirement of strong atom-cavity interaction and enriches the understanding of EPs in manipulating quantum states.
The ability to harness the ubiquitous dissipation in realistic quantum systems holds great promise for developing advanced quantum technologies. In particular, the presence of dissipation violates the fundamental premise of Hermiticity and results in the non-Hermitian singularities associated with coalesced eigenstates in parameter space known as exceptional points (EPs), providing a new strategy to manipulate quantum states in both dynamical evolution and stationary limit. In this work, we show that for a microcavity supporting chiral EPs, light-matter interaction between the cavity photons and a quantum emitter can be flexibly switched from coherent to fully dissipative by tuning the phase factor of a complex coupling rate, an additional degree of freedom offered by the chiral EPs of a microcavity. The tunable coherent and dissipative light-matter interactions are evidenced by the distinguishing quantum dynamics and optical spectra, where the dispersion of a coupled quantum system changes from level repulsion to level attraction. While dissipation is generally considered fatal for quantum effects and should be suppressed, here we find that the EP-enabled dissipative light-matter interaction shows a unique advantage in achieving high-efficiency single-photon emission in a weak-coupling regime through the non-Hermitian photon blockade, which originates from the anharmonicity in the losses instead of in the eigenenergy levels for the conventional photon blockade (CPB). A giant enhancement of more than 5 orders of magnitude is found for both photon correlations and intensity compared to that of CPB in coherent light-matter interaction. We expect our work to enrich the understanding of light-matter interaction in open photonic resonators and provide insight into the manipulation of quantum states and quantum correlations in a wide variety of quantum systems by utilizing the non-Hermicity of electromagnetic environments.
An open quantum system operated at the spectral singularities where dimensionality reduces, known as exceptional points (EPs), demonstrates distinguishing behavior from the Hermitian counterpart. Here, we present an analytical description of local density of states (LDOS) for microcavity featuring chiral EPs, and unveil the anomalous spontaneous emission dynamics from a quantum emitter (QE) due to the non-Lorentzian response of EPs. Specifically, we reveal that a squared Lorentzian term of LDOS contributed by chiral EPs can destructively interfere with the linear Lorentzian profile, resulting in the null Purcell enhancement to a QE with special transition frequency, which we call EP induced transparency. While for the case of constructive interference, the squared Lorentzian term can narrow the linewidth of Rabi splitting even below that of bare components, and thus significantly suppresses the decay of Rabi oscillation. Interestingly, we further find that an open microcavity with chiral EPs supports atom-photon bound states for population trapping and decay suppression in long-time dynamics. As applications, we demonstrate the advantages of microcavity operated at chiral EPs in achieving high-fidelity entanglement generation and high-efficiency single-photon generation. Our work unveils the exotic cavity quantum electrodynamics unique to chiral EPs, which opens the door for controlling light-matter interaction at the quantum level through non-Hermiticity, and holds great potential in building high-performance quantum-optics devices.
Atom-photon dressed states are a basic concept of quantum optics. Here, we demonstrate that the non-Hermiticity of an open cavity can be harnessed to form the dressed bound states (DBSs) and identify two types of DBSs, the vacancylike DBS and Friedrich-Wintgen DBS, in a microring resonator operating at a chiral exceptional point. With the analytical DBS conditions, we show that the vacancylike DBS occurs when an atom couples to the standing-wave mode that is a node of the photonic wave function and characterized by null spectral density at cavity resonance. However, the Friedrich-Wintgen DBS can be accessed by continuously tuning the system parameters, such as the atom-photon detuning, and evidenced by a vanishing Rabi peak in the emission spectrum, an unusual feature in the strong-coupling anticrossing. We also demonstrate the quantum-optics applications of the proposed DBSs. Our work exhibits quantum state control through non-Hermiticity of open quantum system and presents a clear physical picture of DBSs at chiral exceptional points, which holds great potential for building high-performance quantum devices for sensing, photon storage, and nonclassical light generation.
Exceptional points (EPs), which arose early from non-Hermitian physics, significantly amplify the system's response to minor perturbations, and they act as a useful concept to enhance measurement in metrology. In particular, such a metrological enhancement grows dramatically with the EP's order. However, the Langevin noises intrinsically existing in the non-Hermitian systems diminish this enhancement. In this study, we propose a protocol for quantum metrology with the construction of higher-order EPs (HOEPs) in an atom-cavity system through Hermitian magnon-photon interaction. The construction of HOEPs utilizes the atom-cavity non-Hermitian-like dynamical behavior but avoids the external Langevin noises via the Hermitian interaction. A general analysis is exhibited for the construction of arbitrary n th -order EP (EPn). As a demonstration of the superiority of these HOEPs in quantum metrology, we work out an EP3/4-based atomic sensor with sensitivity being orders of magnitude higher than that achievable in an EP2-based atomic sensor. We further unveil the mechanism behind the sensitivity enhancement from HOEPs. The experimental establishment for this proposal is suggested with potential candidates. This EP-based atomic sensor, taking advantage of the atom-light interface, offers new insight into quantum metrology with HOEPs.
Open photonic systems provide a versatile platform for non-Hermitian physics, enabling control over complex spectra, transport, and light-matter interactions. Exceptional points (EPs), at which eigenvalues and eigenvectors coalesce and the governing operator becomes defective, play a central role because they combine branch-point spectral topology, nonanalytic perturbative response, and controllable eigenstate conversion. This Review provides a unified framework for EP photonics by systematically distinguishing exceptional degeneracies according to the underlying operator, spectral variable, boundary conditions, and experimentally accessible observables. We discuss Hamiltonian EPs, absorbing EPs associated with scattering zeros, real-frequency scattering-matrix and Jones-matrix EPs, Bloch and Floquet EPs, and Liouvillian EPs in open quantum systems. We review their spectral topology, static and dynamical encircling, higher-order exceptional structures, and coexistence with bound states in the continuum, together with applications in sensing, lasing, coherent absorption, directional scattering, polarization and wavefront control, nonlinear optics, optical storage, nonreciprocal photonics, and quantum photonics. We also critically assess the current limitations, practical challenges, and future perspectives of EP-based photonic technologies, with particular attention to robustness, noise, scalability, and experimentally measurable performance.
Higher-order exceptional points (EPs) in non-Hermitian systems have attracted great interest due to their advantages in sensitive enhancement and distinct topological features. However, realization of such EPs is still a challenge because more fine-tuning parameters are generically required in quantum systems, compared to the second-order EP (EP2). Here, we propose a non-Hermitian three-mode optomechanical system in the blue-sideband regime for predicting the third-order EP (EP3). By deriving the pseudo-Hermitian condition for the proposed system, one cavity with loss and the other with gain must be required. Then we show that EP3 or EP2 can be observed when the mechanical resonator (MR) is neutral, loss, or gain. For the neutral MR, we find that two degenerate or two nondegenerate EP3s can be predicted by tuning system parameters in the parameter space, while four nondegenerate EP2s can be observed when the system parameters deviate from EP3s, which is distinguished from the previous study in the red-detuned optomechanical system. For the gain (loss) MR, we find that only two degenerate EP3s or EP2s can be predicted by tuning enhanced coupling strength. Our proposal provides a potential way to predict higher-order EPs or multiple EP2s and study multimode quantum squeezing around EPs using blue-detuned non-Hermitian optomechanical systems.