Ensemble Of Two-level Atoms Research Articles

Peculiarities of the interaction-induced modifications of the decay of different Zeeman sublevels of an atom excited in isotropic environment

On the basis of a consistent quantum microscopic approach, we consider the decay of a multilevel atom excited in an ensemble of resonant two-level atoms. We show that the decay rate of different Zeeman sublevels of the excited state can be different, unlike in the case of a free atom. This effect is observed even when the environment is isotropic. Furthermore, we find that different irreducible components of the density matrix of initially excited multilevel atoms also differ in terms of decay rate.

Optical attenuation without absorption

We consider a coherent state of light propagating through an ensemble of two-level atoms where all the atoms are initially in their ground state. In ordinary absorption, the transition of atoms to their excited state along with the absorption of a photon will remove energy from the beam and attenuate the signal. Here we show that post-selecting on those cases in which none of the atoms made a transition to the excited state can give even more attenuation than would normally occur due to absorption. The same process can also produce amplification when there is a sufficiently strong interaction between the photons and the atoms.

Mott polaritons in cavity-coupled quantum materials

We show that strong electron–electron interactions in quantum materials can give rise to electronic transitions that couple strongly to cavity fields, and collective enhancement of these interactions can result in ultrastrong effective coupling strengths. As a paradigmatic example we consider a Fermi–Hubbard model coupled to a single-mode cavity and find that resonant electron-cavity interactions result in the formation of a quasi-continuum of polariton branches. The vacuum Rabi splitting of the two outermost branches is collectively enhanced and scales with , where L is the number of electronic sites, and the maximal achievable value for geff is determined by the volume of the unit cell of the crystal. We find that geff for existing quantum materials can by far exceed the width of the first excited Hubbard band. This effect can be experimentally observed via measurements of the optical conductivity and does not require ultrastrong coupling on the single-electron level. Quantum correlations in the electronic ground state as well as the microscopic nature of the light–matter interaction enhance the collective light–matter interaction compared to an ensemble of independent two-level atoms interacting with a cavity mode.

Powerful harmonic charging in a quantum battery.

We consider a harmonic charging field as an energy charger for the quantum battery, which consists of an ensemble of two-level atoms. The charging of noninteracting atoms is completely fulfilled, which exhibits a substantial improvement over previous static charging fields. Involving the repulsive interactions of atoms, the fully charging is achieved with shorter charged period over the noninteracting case, yielding an advantage for the charging. Excluding the charging field, a quantum phase transition is induced by the attractive atom-atom interactions, and the interacting atoms become degenerate in the ground state. We find that the degenerate states play a negative role in the charging due to the gapless energies. The atoms with strong attractive interactions can not be charged completely, which is accompanied by a drop of the maximum stored energy.

Theory of Bose condensation of light via laser cooling of atoms.

A Bose-Einstein condensate (BEC) is a quantum phase of matter achieved at low temperatures. Photons, one of the most prominent species of bosons, do not typically condense due to the lack of a particle number conservation. We recently described a photon thermalization mechanism which gives rise to a grand canonical ensemble of light with effective photon number conservation between a subsystem and a particle reservoir. This mechanism occurs during Doppler laser cooling of atoms where the atoms serve as a temperature reservoir while the cooling laser photons serve as a particle reservoir. In contrast to typical discussions of BEC, our system is better treated with a controlled chemical potential rather than a controlled particle number, and is subject to energy-dependent loss. Here, we address the question of the possibility of a BEC of photons in this laser cooling photon thermalization scenario and theoretically demonstrate that a Bose condensation of photons can be realized by cooling an ensemble of two-level atoms (realizable with alkaline-earth atoms) inside a Fabry-Pérot cavity.

Atomic quadrature squeezing and quantum state transfer in a hybrid atom–optomechanical cavity with two Duffing mechanical oscillators

In this paper, we investigate theoretically the quantum state transfer in a laser driven hybrid optomechanical cavity with two Duffing-like anharmonic movable end mirrors containing an ensemble of identical two-level trapped atoms. The quantum state transfer from the Bogoliubov modes of the two anharmonic oscillators to the atomic mode results in the atomic quadrature squeezing beyond the standard quantum limit of 3 dB which can be controlled by both the optomechanical and atom-field coupling strengths. Interestingly, the generated atomic squeezing can be made robust against the noise sources by means of the Duffing anharmonicity. Moreover, the results reveal that the presence of the Duffing anharmonicity provides the possibility of transferring strongly squeezed states between the two mechanical oscillators in a short operating time and with a high fidelity.

Controlled bistability of cold Rydberg excitation with Doppler broadening

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.

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.

Study on energy extraction assisted with quantum correlated coherence in bath

Based on a hybrid model of a single-mode microcavity system plus an ensemble of two-level atoms (TLAs), we investigate the effect of quantum correlated coherence (QCC) [Tan K C, <i>et al.</i> 2016 Phys. Rev. A 94, 022329] of bath on the dynamic behaviors of system. The dynamic equations of system for a general bath with QCC have been derived. With the help of the GHZ-like state with QCC and its reference state, the role of QCC as a thermodynamic resource has been clearly shown where QCC could be used to enhance the system's energy. Meanwhile, combining with the analytical and numerical simulation methods, the influences of effective temperature of <inline-formula><tex-math id="M3">\begin{document}$ GHZ $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20181525_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20181525_M3.png"/></alternatives></inline-formula>-like bath and the coupling strength between the system and the bath on the energy effect of QCC have been studied. We find that the energy contribution of QCC to the cavity field relies not only on the effective temperature of bath but also on the coupling strength. That is completely different from the case of traditional thermal bath where the energy captured by the cavity from the bath only depends on the bath temperature, i.e., the thermal distribution of TLAs. Moreover, several interesting phenomena, in the paper, have been shown: 1) the higher of the effective temperature of bath, the larger of the cavity's energy extracted from the QCC of bath; 2) under the fixed effective temperature of bath, the smaller of the coupling strength the larger of the maximal extractable energy from QCC of bath; 3) there exists the trade-off between the cavity's energy and the capability of cavity capturing the energy of TLAs entering the cavity, i.e., the cavity's energy extracted from each TLA crossing the cavity always decreases as the energy of cavity increases; 4) the energy contribution of QCC of bath to cavity is beyond the one of the thermal distribution of TLAs in bath, and it could become more prominent when the coupling strength is taken the smaller value, which also means that in the case of weak coupling strength it is the QCC of bath not the thermal distribution of bath dominating the cavity's energy. Thus, the QCC of bath could be viewed as a kind of high quality thermodynamic resource. It has the potential applications in the design of a quantum engine with high output power or efficiency, and the enhancement of charging speed of quantum battery. Our investigation is beneficial to the further understanding of quantum coherence in quantum thermodynamic regime.

Large Fluctuations in Two-Level Systems With Stimulated Emission

We consider a system of $N$ identical independent Markov processes, each taking values 0 or 1. The system describes a stochastic dynamics of an ensemble of two-level atoms. The atoms are exposed to a photon flux. Under the photon flux action, every atom changes its state with some intensities either from its ground state (state 0) to the excited state (state 1) or from the excited state to the ground state (the stimulated emission). The atom can also change the state from the excited to the ground one spontaneously. We study rare events when the big cumulative emission occurs on the fixed time interval $[0,T]$. To this end we apply the large deviation theory which allows one to make asymptotic analysis as $N\to\infty$.

Фазово-модулированный двухкомпонентный импульс в диспергирующей среде

Abstract A theory of optical two-component pulse of self-induced transparency in a dispersive medium has been developed. Using the generalized version of the perturbative expansion method, the system of material equations for an ensemble of two-level atoms and the wave equation in a dispersive medium are reduced to coupled nonlinear Schrödinger equations. A solution in the form of a phase-modulated two-component $$0\pi $$ pulse is obtained. The pulse components oscillate at the sum and difference frequencies in the vicinity of the carrier-wave frequency. Explicit analytical expressions for the parameters and formation conditions of a nonlinear wave, which depend on the dispersive properties of the medium, are presented. Known results are obtained for a particular case.

Dicke Quantum Phase Transition for a Bose-Einstein Condensate in a Two-Mode Optical Cavity

In this paper we mainly discuss the ground state properties of the two-mode Dicke model, which is realized in an ensemble of two-level atoms interacting simultaneously with two quantized cavity fields. We reveal rich phase diagrams and discover the second-order quantum phase transition from the normal phase to the superradiant phase by means of the spin-coherent-state variational method. While the critical phase transition point can be shifted by the detuning of the cavity mode or the atom-field coupling imbalance parameter. The collective atom-field coupling imbalance parameter can make the phase transition point symmetrically shift left or right in the resonance or non-resonance. If the two collective atom-photon coupling strengths are not equal in the resonance, the system may be in different phases, while the phases occupied are completely symmetrical. When one of the coupling constants vanishes or two couplings are equal, the ground-states’s properties and related QPT reduce to that of a standard or an ordinary Dicke model.

Purcell-Dicke enhancement of the Casimir-Polder potential

The emission by an initially completely inverted ensemble of two-level atoms in the long-wavelength regime is simultaneously enhanced by both collective effects (Dicke effect) and dielectric environments (Purcell effect), thus giving rise to a combined Purcell–Dicke effect. We study this effect by treating the ensemble of N atoms as a single effective N + 1-level “Dicke atom” which couples to the environment-assisted quantum electrodynamic field. We find that an environment can indeed alter the superradiant emission dynamics, as exemplified using a perfectly conducting plate. As the emission acquires an additional anisotropy in the presence of the plate, we find an associated resonant Casimir-Polder potential for the atom that is collectively enhanced and that exhibits a superradiant burst in its dynamics. An additional tuneability of the effect is introduced by applying an external driving laser field.

Localization Transition for Light Scattering by Cold Atoms in an External Magnetic Field.

We establish a localization phase diagram for light in a random three-dimensional (3D) ensemble of motionless two-level atoms with a threefold degenerate upper level, in a strong static magnetic field. Localized modes appear in a narrow spectral band when the number density of atoms ρ exceeds a critical value ρ_{c}≃0.1k_{0}^{3}, where k_{0} is the wave number of light in the free space. A critical exponent of the localization transition taking place upon varying the frequency of light at a constant ρ>ρ_{c} is estimated to be ν=1.57±0.07. This classifies the transition as an Anderson localization transition of 3D orthogonal universality class.

Pump-probe response sensitive to atomic excitation in a hybrid optomechanical system

We study the pump-probe response in an optomechanical cavity accommodating an ensemble of two-level atoms interacting with a cavity mode, which is pumped by a strong driving field and probed by a weak signal field. The atomic excitation is examined without taking any approximations and may be in any regimes ranging from the low limit to the high limit for a given collective coupling constant between the atomic ensemble and the cavity mode. This then results in different absorption and dispersion spectra (such as optomechanically induced transparency, Fano-resonance, normal-mode splitting) when the excitation transfer from cavity mode to atomic ensemble is controlled to change from ineffective to effective. This indicates that a hybrid optomechanical response is indeed determined by the excitation of the atomic ensemble embedded in the optomechanical cavity. It is also of interest that the output signal component and the inside atomic polarization show either synchronous or asynchronous behaviors depending on how the atom-field detuning is chosen, i.e. equal to the mechanical resonator frequency or its negative. Such features may open up an effective avenue in coherently manipulating the internal atomic polarization via the external signal field, and vice versa.

Probing nonclassicality in an optically driven cavity with two atomic ensembles

The possibility of observing nonclassical features in a physical system comprised of a cavity with two ensembles of two-level atoms has been investigated by considering different configurations of the ensembles with respect to the Node and Antinode of the cavity field under the framework of open quantum systems. The study reveals the strong presence of nonclassical characters in the physical system by establishing the existence of many facets of nonclassicality, such as the sub-Poissonian boson statistics and squeezing in single modes, intermodal squeezing, intermodal entanglement, antibunching, and steering. The effect of a number of parameters, characterizing the physical system, on the different aspects of nonclassicality are also investigated. Specifically, it is observed that the depth of the nonclassicality witnessing parameters can be enhanced by externally driving one of the ensembles with an optical field. The work is done in the presence of open system effects, in particular, use is made of Langevin equations along with a suitable perturbative technique.

Ground-state cooling of rotating mirror in double-Laguerre-Gaussian-cavity with atomic ensemble.

A scheme is proposed to cool a rotating mirror close to its ground state in a double-Laguerre-Gaussian-cavity optomechanical system, where an auxiliary cavity and a two-level atomic ensemble simultaneously couple to the original optomechanical cavity. By choosing parameters reasonably, we find that the cooling process of the rotating mirror can be strengthened greatly while the heating process can be suppressed effectively. We show that the proposed ground-state cooling scheme can work well no matter whether in the weak or strong coupling regime for the atomic ensemble and original cavity. Compared with previous related schemes, our scheme works in the unresolved sideband regime with fewer strict limitations for the auxiliary systems.

Enhancing optical nonreciprocity by an atomic ensemble in two coupled cavities

We study the optical nonreciprocal propagation in an optical molecule of two coupled cavities with one of them interacting with a two-level atomic ensemble. The effect of increasing the number of atoms on the optical isolation ratio of the system is studied. We demonstrate that the significant nonlinearity supplied by the coupling of the atomic ensemble with the cavity leads to the realization of greatly-enhanced optical nonreciprocity compared with the case of single atom.

Dynamical theory of photon superradiative emission by nanoscale system of Bose-condensed magnons

We have shown the possibility of non-Dicke superradiance for non-ideal magnon Bose-Einstein condensate (BEC) in a broadband frequency bath. Here, it is found that all the stored energy in the system of Bose-condensed magnons can be irradiated into a short pulse with a time delay caused by the strong frequency modulation of magnons due to direct inter-particle interactions in the Bose-condensed state. The last mechanism radically distinguishes this effect from the well-known Dicke superradiance of two-level atomic ensemble where the delay is connected with enhancement of the inter-atomic correlations due to exchange by virtual photons. In our case, the superradiance is the consequence of Bose-condensation in the coherent state where the particles are coupled by direct interaction. We have discussed the conditions for observation of this effect for Bose-condensed magnons in a solid-state sample with a spatial size smaller comparing with the wavelength of the emitted field. In general, we had shown that this kind of superradiance can proceed in samples with ferromagnetic type interaction. As for the antiferromagnetic ones, the effect of magnon superradiance takes place without delay.

Bound state in the continuum by spatially separated ensembles of atoms in a coupled-cavity array

We present an analytic solution of bound states in the continuum (BICs) for photons and atoms in a one-dimensional coupled cavity array. These bound states are formed by two ensembles of twolevel atoms confined in separated cavities of the array. We show that in the regime where the photon hopping rate between neighboring cavities is high compared with the collective Rabi frequency, the BIC corresponds to a subradiant collective atomic state in which the two ensembles of atoms are strongly entangled. On the other hand in the low photon hopping rate regime, the BIC behaves a quantum cavity in which photons can be trapped between the two ensemble of atoms.