Atom-cavity Coupling Strength Research Articles

Critical optical-hysteresis regime and optical-bistability transformation

In this paper, we study the optical-hysteresis regime in a driven-dissipative Bose-Hubbard dimer under a symmetric configuration and analyze the classical optical bistability with the Gross-Pitaevskii mean-field approach. We find that the critical point of the optical-hysteresis regime can be determined by the classical optical-bistability transformation predicted with a steady state equation, and the critical tunneling rate is given by analyzing the bistable threshold points. In addition, we apply the same approach to a single nonlinear microcavity coupled with an atom and the critical atom-cavity coupling strength of the optical-hysteresis regime is also obtained. Finally, in order to illustrate that the critical point could be determined by the classical optical-bistability transformation, we analyze the Liouvillian gap and find it opens at the critical point of the optical-hysteresis regime. This work clarifies the relation between optical hysteresis and classical optical bistability, which provides theoretical references for the modulation of optical hysteresis in experiment.

Time-Varying Engineered Reservoir for the Improved Estimation of Atom-Cavity Coupling Strength

In this paper, we consider the application of quantum reservoir engineering in quantum metrology. More precisely, we are concerned with a system setup where a sequence of atoms constructing the “time-varying” quantum reservoir interact, in turn, with the trapped field in a cavity through the Jaynes–Cummings Hamiltonian. In particular, we were able to manipulate the initial states of reservoir atoms in order to enhance estimation precision regarding the coupling strength between each atom and the cavity (the coupling strength between each atom and the cavity was assumed to be identical). The novelty of this work lies in alternately preparing the atoms at two different states in a pairwise manner, such that the cavity could converge into a squeezed state with photonic loss to the environment taken into account. The control scheme proposed here thus leads to higher precision compared to the previous work where reservoir atoms were initialized at the same state, which drove the cavity to a coherent state. Detailed theoretical analysis and numerical simulations are also provided. In addition, this system setup and the associated control scheme are easily implemented for quantum metrology, since no entanglement is required for the preparation of atom states, and the final cavity state can stay steady.

Trapping of single atom and precise control of its coupling strength in micro-optical cavity

Cavity quantum electrodynamic system with strongly coupled single atoms provides a good platform for studying quantum information processing, quantum simulation, quantum network, and distributed quantum computing. Cooling and trapping single atoms is a crucial technique in the quantum technology. At present, in a high-finesse cavity with finite space, cooling and trapping single atoms is a big challenge, even though it is a mature technique for free space. Great efforts have been made to cool and trap single atoms inside a cavity, and for a trapped atom its lifetime has reached as long as tens of seconds. Developing a more flexible method of cooling and trapping single atoms in a cavity is still essential for a strongly coupled cavity quantum electrodynamic system. In this work, we demonstrate experimentally that a single cesium atom in a cavity can be trapped by utilizing a single optical tweezer settled in cavity mode, and its lifetime is (2.60 ± 0.18) s. The experiment is carried out in a Fabry-Perot cavity, which is assembled by two concave mirrors each with a curvature radius of 100 mm, and cavity length of 335 μm. The concave surfaces are highly reflective, and the cavity has a finesse of 6.1 × 10<sup>4</sup>. The 1080 nm optical tweezer with a waist of 2 μm is formed by an achromatic lens group with a numerical aperture of 0.4. At first, the precooled atomic assemble released from the magneto-optical trap (MOT) is transferred into cavity mode by an optical dipole trap with a waist of 36 μm. Then, one of the successfully transferred atoms is captured by the optical tweezer with the aid of cavity cooling mechanism. A blue detuned cavity locking laser is used as a standing-wave optical trap along the cavity axis. The signal of successfully trapped one atom is obtained by recording transmission of the cavity that will decrease owing to the strong coupling induced vacuum Rabi splitting. Finally, we demonstrate the precise manipulation of the atom-cavity coupling strength, which is achieved by scanning the position of the trapped atom step by step by using a high-precision translation stage. The system realized in this work can be used to study the dynamics of single atom-photon interactions with adjustable coupling strength. In addition, the mechanism adopted in this work is compatible with constructing tweezer arrays inside cavity mode, and thus possesses more flexibility and great potentials in cavity-based quantum entanglement and quantum simulation.

Tunable magnomechanically induced transparency and fast-slow light in a hybrid cavity magnomechanical system

We theoretically explore the tunability of magnomechanically induced transparency (MMIT) phenomenon and fast-slow light effect in a hybrid cavity magnomechanical system in which a high-quality yttrium iron garnet (YIG) sphere and an atomic ensemble are placed inside a microwave cavity. In the probe output spectrum, we can observe magnon-induced transparency (MIT) and MMIT due to the photon-magnon and phonon-magnon couplings. We further investigate the effect of atomic ensemble on the absorption spectrum. The results show that better transparency can be obtained by choosing appropriate atomic ensemble parameters. We give an explicit explanation for the mechanism of the Fano resonance phenomenon. Moreover, we discuss phenomena of slow-light propagation. The maximum group delay increases significantly with the increasing atom–cavity coupling strength, and the conversion between slow light and fast light can also be achieved by adjusting the atom–cavity coupling strength. These results may have potential applications for quantum information processing and high precision measurements.

Manipulation of nonreciprocal unconventional photon blockade in a cavity-driven system composed of an asymmetrical cavity and two atoms with weak dipole–dipole interaction

We present a work of manipulating collective unconventional photon blockade (UCPB) and nonreciprocal UCPB (NUCPB) in a cavity-driven system composed of an asymmetrical single-mode cavity and two interacting identical two-level atoms (TLAs). When the atoms do not interact directly, the frequency and intensity restrictions of collective UCPB can be specified, and a giant NUCPB exists due to the splitting of optimal atom–cavity coupling strength in proper parameter regime. However, if a weak atom–atom interaction which provides a new and feeble quantum interference pathway to UCPB is taken into account, two restrictions of UCPB are combined complexly, which are rigorous to be matched simultaneously. Due to the push-and-pull effect induced by weak dipole–dipole interaction, the UCPB regime is compressed more or less. NUCPB is improved as a higher contrast is present when the two complex UCPB restrictions are matched, while it is suppressed when the restrictions are mismatched. In general, whether NUCPB is suppressed or promoted depends on its working parameters. Our findings show a prospective access to produce giant quantum nonreciprocity by a couple of weakly interacting atoms.

Superradiant phase transitions in one-dimensional correlated Fermi gases with cavity-induced umklapp scattering

The phase transitions of one dimensional correlated Fermi gases in a transversely driven optical cavity, under the umklapp condition that the cavity wavenumber equals two times of Fermi wavenumber, are studied with the bosonization and renormalization group (RG) techniques. The bosonization of Fermi fields gives rise to an all-to-all sine-Gordon (SG) model due to the cavity-assisted non-local interactions, where the Bose fields at any two spatial points are coupled. The superradiant phase transition is then linked to the Kosterlitz-Thouless (KT) phase transition of the all-to-all SG model. The nesting effect, in which the superradiant phase transition can be triggered by infinitely small atom-cavity coupling strength, is preserved for any non-attractive local interactions. The phase transition occurs at finite critical coupling strength for attractive local interactions. Nevertheless, the critical dimension of the KT phase transition is also 2 like that in an ordinary local SG model. Our work provides an analytical framework for understanding the phase transitions in correlated intra-cavity Fermi gases.

Giant nonreciprocal unconventional photon blockade with a single atom in an asymmetric cavity

Inspired by the advantage of unconventional photon blockade (UCPB), we propose a simple cavity-driven quantum electrodynamics model composed of a single two-level atom embedded in an asymmetrical Fabry-Perot cavity. We derive the necessary matching conditions of frequency and intensity required for unconventional photon blockade. In the asymmetric cavity, the optimal atom-cavity coupling strength to get UCPB is related to the driving direction, which causes the equal-time second-order correlation function ${g}^{(2)}(0)$ of the cavity field depending on the driving direction, too. Due to the direction dependent effect, the output photons can be bunched in one direction incident while they are antibunched in the opposite direction incident, which is the so-called nonreciprocal UCPB (NUCPB). A giant NUCPB can be obtained under the appropriate parameter regime, where the output photons are fully antibunching in the opposite driving case. Our findings show a prospective path to producing giant quantum nonreciprocity.

Atom-modulated dynamic optical hysteresis in driven-dissipative systems

We introduce a two-level atom into a cavity as an ancilla to control the hysteresis of the system where the cavity is described by a driven-dissipative nonlinear Kerr model. We find that the dynamic optical hysteresis can be modulated by changing the atom-cavity coupling strength, and the nonlinearity induced by atom-cavity coupling weakens the Kerr nonlinearity in the weak coupling regime. This leads to the change in the critical point corresponding to the decrease in the hysteresis area. The physics behind the modulation is closely related to the effect of the atom-cavity coupling on the discontinuity of the first nonzero Liouvillian eigenvalue. A relative deviation of the metastable state and target steady state is defined to characterize the relation between the hysteresis and adiabaticity of the system. Taking only two Liouvillian eigenvectors into account in the composite system, we derive an expression for the dependence of the hysteresis area on the sweeping speed and discuss its feature in the slow sweeping limit.

Tunable fast–slow light conversion based on optomechanically induced absorption in a hybrid atom–optomechanical system

The fast and slow light effects of a transmitted probe light are researched in a hybrid atom–cavity optomechanical system, which is composed of an optical cavity, a mechanical resonator, and an atomic ensemble. The results show that the fast light generated in this hybrid system can be easily converted into slow light and vice versa, and that this fast–slow light conversion depends on a critical power of the control light, which corresponds to the fact that the probe light can be completely absorbed. Interestingly, this critical power can be modulated by changing the cavity coupling parameter or the atom–cavity coupling strength. Therefore, the fast–slow light conversion can be easily controlled.

Generation and distribution of atomic entanglement in coupled-cavity arrays

We study the dynamics of entanglement in a one-dimensional coupled-cavity array, with each cavity containing a two-level atom, via the Jaynes-Cummings-Hubbard (JCH) Hamiltonian in the single-excitation sector. The model features a rich variety of dynamical regimes that can be harnessed for entanglement control. The protocol is based on setting an excited atom above the ground state and further letting it evolve following the natural dynamics of the Hamiltonian. Here we focus on the concurrence between pairs of atoms and its relation to atom-field correlations and the involved free-field modes. We show that the extension and distribution pattern of pairwise entanglement can be manipulated through a judicious tuning of the atom-cavity coupling strength. By also including static noise in the cavity frequencies, we explore the onset of Anderson localization and its interplay with the atomic trapping known to take place in the strong-hopping regime. Remarkably, we find that the stronger the disorder is, the higher are the atom-field correlations, whereas the atomic concurrence responds nonmonotonically. Overall, our work offers a comprehensive account of the machinery of the single-excitation JCH Hamiltonian and contributes to the design of hybrid light-matter quantum networks.

Magnon-induced optical high-order sideband generation in hybrid atom-cavity optomagnonical system.

The nonlinearity of magnons plays an important role in the study of an optomagnonical system. Here in this paper, we focus on the high-order sideband and frequency comb generation characteristics in the atom coupled optomagnonical resonator. We find that the atom-cavity coupling strength is related to the nonlinear coefficients, and the efficiency of sidebands generation could be reinforced by tuning the polarization of magnons. Besides, we show that the generation of the sidebands could be suppressed under the large dissipation condition. This study provides a novel way to engineer the low-threshold high-order sidebands in hybrid optical microcavities.

Interference-Induced Photon Antibunching in a Cavity QED Setup with a Three-Level Atom

Interference-induced photon antibunching is investigated for a three-level system coupled to a cavity mode, in which the level scheme is of a lambda type. One of the transitions is weakly coupled to the cavity mode while the other is driven by a control field. Without the control field, i.e., in the conventional cavity quantum electrodynamics (QED) scheme in the weak coupling regime, photon antibunching can be achieved by carefully choosing the coupling strength and the detuning. This condition can be alleviated by tuning the strength and the frequency of the control field. The condition for the optimal control field that yields photon blockade is derived, and the effects of changing various parameters are investigated. For a given a set of dissipation rates, the atom-cavity coupling strength g needs to be higher than a certain threshold value, although the whole system still remains in the weak coupling regime. A numerical method for finding the optimal parameter regime is proposed.

Tunable optical second-order sideband effects in a parity-time symmetric optomechanical system

We theoretically investigate the optical second-order sideband generation (OSSG) in an optical parity-time (PT) symmetric system, which consists of a passive cavity trapping the atomic ensemble and an active cavity. It is found that near the exceptional point (EP), the efficiency of the OSSG increases sharply not only for the blue probe-pump detuning resonant case but also for the red one. Using experimentally achievable parameters, we study the effect of the atomic ensemble on the efficiency of the OSSG. The numerical results show that the efficiency of the OSSG is 30% higher than that of the first-order sideband, which is realized easily by simultaneously modulating the atom-cavity coupling strength and detuning. Moreover, the efficiency of the OSSG can also be tuned effectively by the pump power, and the efficiency is robust when the pump power is strong enough. This study may have some guidance for modulating the nonlinear optical properties and controlling light propagation, which may stimulate further applications in optical communications.

Improving photon antibunching with two dipole-coupled atoms in whispering-gallery-mode microresonators

In the two-atom or multiatom system, the atoms can interact with each other through exchange of virtual photon. This kind of energy exchange is often referred as the dipole-dipole interaction (DDI). Here we consider this DDI system consisting of a pair of two-level atoms strongly coupled with a bimodal whispering-gallery-mode (WGM) microresonator which is driven by an external laser field. Our aim is to explore the photon correlation characteristics of the proposed architecture using realistic experimental parameter values. We compare in detail the quality of photon antibunching (i.e., the smallness of the second-order correlation function) from three involved configurations in cavity quantum electrodynamics (QED): (i) only one two-level atom, (ii) two far apart two-level atoms without DDI, and (iii) two DDI (dipole-coupled) two-level atoms are respectively coupled to the driven WGM microresonator through the evanescent field. We clearly show that the DDI between both atoms can distinctly enhance the photon antibunching even in the weak-coupling regime in configuration (iii) with feature-rich line shapes. We also find that the photon antibunching can be modulated by properly adjusting the atom-cavity coupling strength. In addition, we display that this strong photon antibunching is robust against the cooperative atomic decay. Our DDI-based cavity QED scheme may provide an alternative way to the construction of integrated on-chip single-photon sources.

Quantum metrology enhanced by coherence-induced driving in a cavity-QED setup

We propose a quantum metrology scheme in a cavity QED setup to achieve the Heisenberg limit. In our scheme, a series of identical two-level atoms randomly pass through and interact with a dissipative single-mode cavity. Different from the entanglement based Heisenberg limit metrology scheme, we do not need to prepare the atomic entangled states before they enter into the cavity. We show that the initial atomic coherence will induce an effective driving to the cavity field, whose steady state is an incoherent superposition of orthogonal states, with the superposition probabilities being dependent on the atom-cavity coupling strength. By measuring the average photon number of the cavity in the steady state, we demonstrate that the root-mean-square of the fluctuation of the atom-cavity coupling strength is proportional to $1/N_c^2$ ($N_c$ is the effective atom number interacting with the photon in the cavity during its lifetime). It implies that we have achieved the Heisenberg limit in our quantum metrology process. We also discuss the experimental feasibility of our theoretical proposal. Our findings may find potential applications in quantum metrology technology.

Quantum router modulated by the dipole-dipole interaction in a X-shaped coupled cavity array

We theoretically investigate the single-photon router using two dipole-dipole interaction (DDI) atoms embedded in a multichannel coupled cavity arrays (CCAs). When the atoms are symmetrically coupled to the cavity, the DDI influences both the height and position of the two peaks and gives rise to a shift in the position of the resonance. It is also shown that the effects of DDI are similar to the positive detuning. On the other hand, for the atom-cavity asymmetric coupling case, it is found that, Fano-line shapes are revealed evidently in the scattering spectra by adjusting the interatomic DDI. Moreover, Fano resonance can also be controlled by modulating the atom-cavity coupling strength and the detuning in the presence of DDI. Finally, in the incident channel the total reflection of the single photon is obtained based on the coherent resonance and the existence of the photonic bound states. Based on these characteristics, it is a feasible scheme to generate a Fano-typed router and it could be useful for designing the single-photon devices.

Atom-assisted second-order sideband generation in an optomechanical system with atom-cavity-resonator coupling

Second-order sideband generation (SSG) is investigated in an optomechanical system with atom-cavity-resonator coupling. We focus mainly on the effect of the atomic ensemble trapped in the cavity on the SSG. The numerical results show that the second-order sideband is induced not only for the blue probe-pump detuning resonant case but also for the red one while coupling the atoms with the cavity. Moreover, it is found that the efficiency of the SSG can be significantly enhanced while increasing the atom-cavity coupling strength. More interestingly, the SSG is also sensitive to the atom-pump detuning. It is demonstrated that when optimizing this detuning and the atom-cavity coupling strength together, a robust second-order sideband can be induced and the efficiency of the SSG even surpasses the one of the first-order sideband generation. In addition, the efficiency of the SSG can also be tuned effectively by the decay rate of the atom.

Four-wave mixing response in a hybrid atom-optomechanical system

We study the four-wave mixing (FWM) effect in a hybrid atom-optomechanical system, which is composed of an ensemble of two-level atoms trapped inside an optical cavity and a moveable cavity mirror serving as a mechanical resonator. It is shown that the FWM signal in this hybrid system can be significantly enhanced due to the coupling between the optical cavity and the atom ensemble. The larger the atom-cavity coupling strength is, the stronger the FWM will be. More interestingly, the FWM signal can be effectively modulated by the atom-pump detuning. It is demonstrated that, just by tuning this detuning to be positive or negative, the FWM signal can be easily switched on or off for the zero probe-pump detuning case. The obtained results may have potential applications in all-optical wavelength convertors, all-optical switches, or other optical devices.

The Dynamical Casimir Effect in Squeezed Vacuum State

The dynamical Casimir effect (DCE) tells that the photons can be generated in the vacuum cavity. We here show how the squeezing of the cavity mode and the coupling between the atoms and cavity influence the DCE. We show that the atoms can be served as probe to detect the DCE. Our results show that the photon number increases with the increasing of the photonic squeezing but decreases with the atom-cavity coupling. The excitation number of the atom increases with both of the increasing of the squeezing and the atom-cavity coupling strength.

Deterministic quantum controlled-PHASE gates based on non-Markovian environments

We study the realization of the quantum controlled-PHASE gate in an atom-cavity system beyond the Markovian approximation. The general description of the dynamics for the atom-cavity system without any approximation is presented. When the spectral density of the reservoir has the Lorentz form, by making use of the memory backflow from the reservoir, we can always construct the deterministic quantum controlled-PHASE gate between a photon and an atom, no matter the atom-cavity coupling strength is weak or strong. While, the phase shift in the output pulse hinders the implementation of quantum controlled-PHASE gates in the sub-Ohmic, Ohmic or super-Ohmic reservoirs.