Cavity QED System Research Articles

Frequency-Resolved Purcell Effect for the Dissipative Generation of Steady-State Entanglement.

We report a driven-dissipative mechanism to generate stationary entangled W states among strongly interacting quantum emitters placed within a cavity. Driving the ensemble into the highest energy state-whether coherently or incoherently-enables a subsequent cavity-enhanced decay into an entangled steady state consisting of a single deexcitation shared coherently among all emitters, i.e., a W state, well known for its robustness against qubit loss. The nonharmonic energy structure of the interacting ensemble allows this transition to be resonantly selected by the cavity, while quenching subsequent off-resonant decays. Evidence of this purely dissipative mechanism should be observable in state-of-the-art cavity QED systems in the solid state, enabling new prospects for the scalable stabilization of quantum states in dissipative quantum platforms.

Emission spectrum and photon statistics in cavity-QED system under incoherent pumping

The emission spectrum is an important tool for studying the properties and interactions of matter. By comparing analytical and numerical solutions, we validate the applicability and limitations of the low-excitation approximation. Here we show that even with a moderate pumping rate, rich multiple peaks can be observed in the emission spectrum which is the result of excitation of higher dressed rungs. Moreover, we find distinct photon distributions in the steady state under different pumping and coupling conditions and the physical mechanisms behind these phenomena are also illustrated. Additionally, we also show that the interference between the emission channels of the emitter and cavity can result in asymmetry of the emission spectrum of the whole system even if the emitter is resonant with the cavity field. Our findings can help in understanding the nonlinear characteristics and photon statistics in the cavity-QED systems.

Fusion of atomic W-like states in cavity QED systems

Fusion of atomic W-like states in cavity QED systems

Cavity QED systems for steady-state sources of Wigner-negative light

We present a theoretical investigation of optical cavity quantum electrodynamics (cavity QED) systems, as described by the driven, open Jaynes–Cummings model and some of its variants, as potential sources of steady-state Wigner-negative light. We consider temporal modes in the continuous output field from the cavity and demonstrate pronounced negativity in their Wigner distributions for experimentally relevant parameter regimes. We consider models of both single and collective atomic spin systems, and find a rich structure of Wigner-distribution negativity as the spin size is varied. We also demonstrate an effective realization of all of the models considered using just a single 87Rb atom and based upon combinations of laser- and laser-plus-cavity-driven Raman transitions between magnetic sublevels in a single ground hyperfine state.

Control of the Purcell effect via unexcited atoms and exceptional points

We examine the possible control of the celebrated Purcell effect in cavity quantum electrodynamics. We demonstrate that the presence of an unexcited atom can significantly alter the Purcell decay depending on the strength of coupling of the unexcited atom with the cavity mode though the excited atom has to be weakly coupled for it to be in the Purcell regime. This cooperative behavior is distinct from the nonradiative nature of the singlet state which is an entangled state of the two-atom system. We present a physical interpretation for inhibition as due to interference between two polariton channels of decay. For specific values of the parameters, we bring out a connection to exceptional points in the cavity QED system as the unexcited atom and cavity mode can produce a second-order exceptional point. We further show how two unexcited atoms can create a third-order exceptional point leading to inhibition of the Purcell effect. We also discuss the case when the Purcell effect can be enhanced. Published by the American Physical Society 2024

Emergent Macroscopic Bistability Induced by a Single Superconducting Qubit

The photon blockade breakdown in a continuously driven cavity QED system has been proposed as a prime example for a first-order driven-dissipative quantum phase transition. However, the predicted scaling from a microscopic behavior—dominated by quantum fluctuations—to a macroscopic one—characterized by stable phases—and the associated exponents and phase diagram have not been observed so far. In this work we couple a single transmon qubit with a fixed coupling strength g to a superconducting cavity that is bandwidth κ tunable to controllably approach this thermodynamic limit. Even though the system remains microscopic, we observe its behavior becoming increasingly macroscopic as a function of g/κ. For the highest realized g/κ of approximately 287, the system switches with a characteristic timescale as long as 6 s between a bright coherent state with approximately 8×103 intracavity photons and the vacuum state. This exceeds the microscopic timescales by 6 orders of magnitude and approaches the perfect hysteresis expected between two macroscopic attractors in the thermodynamic limit. These findings and interpretation are qualitatively supported by neoclassical theory and large-scale quantum-jump Monte Carlo simulations. Besides shedding more light on driven-dissipative physics in the limit of strong light-matter coupling, this system might also find applications in quantum sensing and metrology. Published by the American Physical Society 2024

Quantum Coherent Feedback Control With Photons

The purpose of this paper is to study two-photon dynamics induced by the coherent feedback control of a cavity quantum electrodynamics (cavity-QED) system coupled to a waveguide. In this set-up, the two-level system in the cavity can work as a photon source, and the photon emitted into the waveguide can re-interact with the cavity-QED system many times after the transmission and reflection in the waveguide, during which the feedback can tune the number of the photons in and out of the cavity. We analyze the dynamics of two-photon processes in this coherent feedback network in two scenarios: the continuous mode coupling scheme and the discrete periodic mode coupling scheme between the waveguide and cavity. The difference of these coupling schemes is due to their relative scales and the number of semi-transparent mirrors for coupling. Specifically, in the continuous mode coupling scheme, the generation of two-photon states is influenced by the length of the feedback loop by the waveguide and the coupling strength between the waveguide and the cavity-QED system. By tuning the length of the waveguide and the coupling strength, we are able to generate two-photon states efficiently. In the discrete periodic mode coupling scheme, the Rabi oscillation in the cavity can be stabilized and there are no notable two-photon states in the waveguide.

Nonperturbative effects of deep strong light-matter interaction in a mesoscopic cavity-QED system

Nonperturbative effects of deep strong light-matter interaction in a mesoscopic cavity-QED system

A Multi-Qubit Quantum Gate Using the Zeno Effect

The Zeno effect, in which repeated observation freezes the dynamics of a quantum system, stands as an iconic oddity of quantum mechanics. When a measurement is unable to distinguish between states in a subspace, the dynamics within that subspace can be profoundly altered, leading to non-trivial behavior. Here we show that such a measurement can turn a non-interacting system with only single-qubit control into a two- or multi-qubit entangling gate, which we call a Zeno gate. The gate works by imparting a geometric phase on the system, conditioned on it lying within a particular nonlocal subspace. We derive simple closed-form expressions for the gate fidelity under a number of non-idealities and show that the gate is viable for implementation in circuit and cavity QED systems. More specifically, we illustrate the functioning of the gate via dispersive readout in both the Markovian and non-Markovian readout regimes, and derive conditions for longitudinal readout to ideally realize the gate.

Strong parametric dispersive shifts in a statically decoupled two-qubit cavity QED system

Strong parametric dispersive shifts in a statically decoupled two-qubit cavity QED system

Matrix product states and numerical mode decomposition for the analysis of gauge-invariant cavity quantum electrodynamics

There has been a problem of gauge ambiguities with the Rabi Hamiltonian due to the fact that it can be derived from two formally different but physically equivalent fundamental Hamiltonians. This problem has recently been resolved for models with a single quantized electromagnetic mode. In this paper, we mathematically and numerically verify this for multimode models. With this established, we combine the numerical methods, matrix product states (MPS) and numerical mode decomposition (NMD), for analyzing cavity QED systems. The MPS method is used to efficiently represent and time evolve a quantum state. However, since the coupling structure of the Rabi Hamiltonian is incompatible with MPS, it is numerically transformed into an equivalent Hamiltonian that has a chain coupling structure, which allows efficient application of MPS. The technique of NMD is used to extract the numerical electromagnetic modes of an arbitrary environment. As a proof of concept, this combined approach is demonstrated by analyzing one-dimensional cavity QED systems in various settings.

Electromagnetically induced transparency via localized surface plasmon mode-assisted hybrid cavity QED

In recent years, most studies have focused on the perfect absorption and high-efficiency quantum memory of the one-sided system, ignoring the characteristics of its optical switching contrast. Thus, the performance of all-optical switching and optical transistors is limited. Herein, we propose a localized surface plasmon (LSP) mode-assisted cavity QED system which consists of a Λ-shaped three-level quantum emitter (QE), a metal nanoparticle and a one-sided optical cavity with a fully reflected mirror. In this system, the QE coherently couples to the cavity and LSP mode respectively, which is manipulated by the control field. As a result, considerably high and stable switch contrast of 90% can be achievable due to the strong confined field of the LSP mode and perfect absorption of the optical medium. In addition, we obtain a power dependent effect between the control field and the transmitted frequency as a result of the converted dark state. We employ the Heisenberg–Langevin equation and numerical master equation formalisms to explain high switching, controllable output light and the dark state. Our system introduces an effective method to improve the performance of optical switches based on the one-sided system in quantum information storage and quantum communication.

Spontaneous Scattering of Raman Photons from Cavity-QED Systems in the Ultrastrong Coupling Regime.

We show that spontaneous Raman scattering of incident radiation can be observed in cavity-QED systems without external enhancement or coupling to any vibrational degree of freedom. Raman scattering processes can be evidenced as resonances in the emission spectrum, which become clearly visible as the cavity-QED system approaches the ultrastrong coupling regime. We provide a quantum mechanical description of the effect, and show that ultrastrong light-matter coupling is a necessary condition for the observation of Raman scattering. This effect, and its strong sensitivity to the system parameters, opens new avenues for the characterization of cavity QED setups and the generation of quantum states of light.

Inhibition of double excitation and strong quantum entanglement via engineered cavity interactions

We propose a method for realizing the inhibition of simultaneous excitation of two identical qubits in a single-mode cavity QED system, where two qubits are driven by a coherent field and coupled to a cavity with engineered qubit-cavity interactions. By suitably chosen placements of the qubits in the cavity and by adjusting the relative decay strengths of the qubits and cavity field, we close many unwanted excitation pathways. This leads to an inhibition of the doubly excited state. In addition, we show that these two qubits are strongly entangled over a broad regime of the system parameters. We show that a strong signature of nonexcitation of the doubly excited state is the bunching property of the cavity photons, which thus provides a possible measurement of this nonexcitation. We also present dynamical features of the inhibition of simultaneous excitation of two qubits. The proposal presented in this paper can be realized not only in traditional cavity QED, but also in noncavity topological photonics involving edge modes.

Nonlinear pumping induced multipartite entanglement in a hybrid magnon cavity QED system

We present a proposal to produce bipartite and tripartite entanglement in a hybrid magnon cavity QED system. Two macroscopic yttrium iron garnet (YIG) spheres are coupled to a single-mode microwave cavity, where the cavity photons are generated via a two-photon process induced by a strong pump field. Using mean-field theory, we show that nonlinear pumping can result in strong bipartite entanglement between the cavity photon and magnon under two conditions, i.e., ${\ensuremath{\delta}}_{c}{\ensuremath{\delta}}_{m}=2{g}^{2}$ and ${\ensuremath{\delta}}_{c}=\ensuremath{-}{\ensuremath{\delta}}_{m}$. For the latter one, we also show the possibility for producing bipartite entanglement between two magnon modes as well as tripartite entanglement among three modes. Combining these two conditions, we further derive a third condition, i.e., ${\ensuremath{\delta}}_{m}^{2}\ensuremath{-}{\ensuremath{\phi}}^{2}+2{g}^{2}=0$, where tripartite entanglement can be achieved when two magnon modes have different resonant frequencies.

Vibration induced transparency: Simulating an optomechanical system via the cavity QED setup with a movable atom

We simulate an optomechanical system via a cavity QED scenario with a movable atom and investigate its application in the tiny mass sensing. We find that the steady-state solution of the system exhibits a multiple stability behavior, which is similar to that in the optomechanical system. We explain this phenomenon by the opto-mechanical interaction term in the effective Hamiltonian. Due to the dressed states formed by the effective coupling between the vibration degree of the atom and the optical mode in the cavity, we observe a narrow transparent window in the output field. We utilize this vibration induced transparency phenomenon to perform the tiny mass sensing. We hope our study will broaden the application of the cavity QED system to quantum technologies.

Entangling two cavity modes and squeezing magnon mode via parametric down-conversion

A scheme to entangle two cavity modes and squeeze magnon mode in a magnon–cavity QED system is presented, where the two microwave cavity modes are coupled to a massive yttrium iron garnet sphere. The nonlinearity used in our system originates from parametric down-conversion. By using the mean field approximation and employing experimentally feasible parameters, we indicate that the entanglement between squeezed cavity mode and magnon mode can be transferred to the other cavity mode and magnon mode, and then the two cavity modes get entangled. Meanwhile, the magnon mode is squeezed in our QED system. Furthermore, we show that it is a good way to enhance entanglement and squeezing by increasing the nonlinear gain. Our results denote that magnon–cavity QED system is a powerful platform for studying macroscopic quantum phenomena, which illustrates a new approach to photon–photon entanglement and magnon squeezing.

Enhancement of the two-photon blockade effect via Van der Waals interaction

We theoretically investigate the influence of the Van der Waals interaction on the two-photon blockade phenomenon with the corresponding photon correlation functions g(2)(0) > 1 and g(3)(0) < 1 in a two-atom cavity-QED system, where two three-level ladder-type atoms are coherently driven by a pumping field and a coupling field simultaneously. Choosing a specific frequency of the coupling field, we show the energy splitting phenomenon caused by electromagnetically induced transparency. Correspondingly, the two-photon blockade phenomenon can be achieved near the two-photon resonant frequency. Using the Van der Waals interaction between the Rydberg states of the two atoms, we also show that the two-photon blockade can be improved when two atoms radiate in-phase or out-of-phase. As a result, two photons leak from the cavity simultaneously, but the third photon is blockaded. These results presented in this study hold potential applications in manipulating photon states and generating nonclassical light sources.

W states fusion based on the resonant interactions in cavity QED system

W states fusion based on the resonant interactions in cavity QED system

Fano effect induced giant and robust enhancement of photon correlations in cavity QED systems.

The Fano effect arising from the interference between two dissipation channels of the radiation continuum enables tuning of the photon statistics. Understanding the role of the Fano effect and exploiting it to achieve strong photon correlations are of both fundamental and applied significance. We present an analytical description of Fano-enhanced photon correlations based on cavity quantum electrodynamics to show that the Fano effect in atom-cavity systems can improve the degree of antibunching by over four orders of magnitude. The enhancement factors and the optimal conditions are explicitly given, and found to relate to the Fano parameter q. Remarkably, the Fano enhancement manifests robustness against the decoherence processes and can survive in the weak coupling regime. We expect our work to provide insight to tuning the photon statistics through the Fano effect, which offers a new, to the best of our knowledge, route to enhance the photon correlations, as well as the possibility of generating nonclassical light in a wider diversity of systems without the need of a strong light-matter interaction.