Collection Of Two-level Atoms Research Articles

Macroscopic entanglement between ferrimagnetic magnons and atoms via crossed optical cavities.

We consider a two-dimensional opto-magnomechanical (OMM) system including two optical cavity modes, a magnon mode, a phonon mode, and a collection of two-level atoms. We show how the stationary entanglement between two-level atoms and magnons can be achieved. The presence of two optical cavities leads the atom-magnon entanglement to be achieved in a wide parameter regime. Furthermore, it is shown that one optical cavity can get entangled with magnons, phonons, and the other optical cavity. The entanglement is robust against thermal noise. The work may find applications in building hybrid quantum networks and quantum information processing.

A comparison of Boltzmann and Gibbs definitions of microcanonical entropy for small systems

Two different definitions of entropy, S = k ln W, in the microcanonical ensemble have been competing for over 100 years. The Boltzmann/Planck definition is that W is the number of states accessible to the system at its energy E (also called the surface entropy). The Gibbs/Hertz definition is that W is the number of states of the system up to the energy E (also called the volume entropy). These two definitions agree for large systems but differ by terms of order N−1 for small systems, where N is the number of particles in the system. For three analytical examples (a generalized classical Hamiltonian, identical quantum harmonic oscillators, and the spinless quantum ideal gas), neither the Boltzmann/Planck entropy nor heat capacity is extensive because it is always proportional to N − 1 rather than N, but the Gibbs/Hertz entropy is extensive and, in addition, gives thermodynamic quantities, which are in remarkable agreement with canonical ensemble calculations for systems of even a few particles. In a fourth example, a collection of two-level atoms, the Boltzmann/Planck entropy is in somewhat better agreement with canonical ensemble results. Similar model systems show that temperature changes when two subsystems come to thermal equilibrium are in better agreement with expectations for the Gibbs/Hertz temperature than for the Boltzmann/Planck temperature, except when the density of states is decreasing. I conclude that the Gibbs/Hertz entropy is more useful than the Boltzmann/Planck entropy for comparing microcanonical simulations with canonical molecular dynamics simulations of small systems.

Influence of Virtual Photon Process on the Generation of Squeezed Light from Atoms in an Optical Cavity

We show that a collection of two-level atoms in an optical cavity beyond the rotating wave approximation and in the dispersive-adiabatic and non-dispersive adiabatic regime constitutes a nonlinear medium and is capable of generating squeezed state of light. It is found that squeezing produced in the non-dispersive adiabatic regime is significantly high compared to that produced in the dispersive-adiabatic limit. On the other hand, we also show that it could be possible to observe the Dicke superradiant quantum phase transition in the dispersive-adiabatic regime where the Ã2 term is negligible. Such a system can be an essential component of a larger quantum-communication system.

Geometric phase, entanglement, and quantum Fisher information near the saturation point

Considering a collection of two-level atoms in the presence of a saturating monochromatic, steady-state field, we investigate the geometric phase (GP) of an arbitrary medium’s atom. We find that it is possible to detect the saturation of the atomic response by the GP computation. This is an interesting result, because we can predict the collective behaviour of the atomic system—i.e., the saturation of the optical response of the medium- by investigating the GP of a single medium’s atom, described as a qubit. Moreover, we find that the plot of the atomic GP in terms of the detuning Δ is very similar to the absorption spectrum of the medium. In addition, it is shown that when the intensity of the driving laser field tends to saturation intensity, the qubit approaches maximum correlation with its environment described by the driving field and other qubits in the atomic system. Furthermore, we find that the behaviour of the entanglement is very analogous to that of the GP and the absorption coefficient. Besides, we adopt the atom to estimate the decoherence parameter by using the quantum Fisher information (QFI), an important measure of the information content of quantum states. Interestingly, we find that when the atomic system approaches its saturation point, the QFI decays with increasing the laser intensity, and therefore the parameter estimation becomes more inaccurate.

Influence of an absorbing dielectric background on bistable response of a dense collection of two-level atoms

The stationary problem of light interaction with a dense collection of two-level atoms embedded in a dielectric is treated semiclassically. The effect of dielectric absorption on the phenomenon of intrinsic optical bistability is discussed in terms of the complex local-field enhancement factor. The calculations accounting for propagation effects (nonlocal regime) show that even low dielectric absorption results in the elimination of response instabilities.

Spin squeezing and entanglement via hole-burning in atomic coherent states

We study the generation of spin squeezing via the hole burning of selected Dicke states out of an atomic coherent state prepared for a collection of N two-level atoms or ions. The atoms or ions of the atomic coherent state are not entangled, but the removal of one or more Dicke states generates entanglement, and spin squeezing occurs for some ranges of the relevant parameters. Spin squeezing in a collection of two-level atoms or ions is of importance for precision spectroscopy.

Spin squeezing via ladder operations on an atomic coherent state

We study the generation of spin squeezing by the repeated action of the angular momentum (Dicke) lowering operator on an atomic coherent state prepared for a collection of $N$ two-level atoms or ions. The atoms or ions of the atomic coherent state are not entangled, but the action of the lowering operator (or similarly of the raising operator) generates entanglement among the atoms, and spin squeezing occurs for some ranges of the relevant parameter. Spin squeezing in a collection of two-level atoms or ions is of importance for precision spectroscopy. We discuss methods by which our spin-squeezed states could be generated in the contexts of cavity QED and trapped ions.

Interference effects in a model of two-channel photon absorption by a collection of two-level atoms

I study a model of photon absorption by a collection of two-level atoms interacting resonantly with two degenerate modes of the quantized electromagnetic field. The two fields provide two channels by which the atoms may be excited and the interference between these channels gives rise to nontrivial steady states, the trivial state just being the two-mode vacuum. The model studied is soluble.

Optical solitons in a few-cycle regime: Breakdown of slow-envelope approximation

The propagation of few-cycles optical pulses in a collection of two-level atoms is investigated beyond the traditional slowly varying envelope approximation. It is shown that in certain conditions, a femtosecond pulse can evolve into a stable spatiotemporal optical soliton.

Degenerate four-wave mixing in a dense two-level system embedded in a dielectric medium

The nonlinear absorption and reflection spectra of a degenerate four-wave mixing (DFWM) in a dense collection of two-level atoms embedded in a linear dielectric medium are investigated theoretically in terms of the slowly varying envelope approximation and the undepleted pump approximation. The numerical results show that the linear dielectric host is beneficial to the DFWM reflection spectra of a dense two-level atom system embedded in a linear dielectric medium.

Amplification of the intracavity field in a strongly coupled atom-cavity system driven by a strong resonant pulse

We investigate the behaviour of the intracavity field in a strongly coupled atom–cavity system (a collection of two-level atoms inside a high-Q cavity) driven by a short strong external resonant pulse. A new effect is predicted, the amplification of the intracavity field appears if the atomic number density in a cavity exceeds the instability threshold which is proportional to the square of the strong driving field. The amplification gain is proportional to the amplitude of the strong driving field. The amplitude of the intracavity field in the vicinity of a critical point above the threshold is time-dependent modulated with harmonics of half of the Rabi frequency of the driving field.

A dense collection of two-level atoms embedded in a dielectric medium in a ring cavity

The hysteresis behavior of a dense collection of two-level atoms embedded in a linear dielectric medium in an optical cavity is investigated theoretically in terms of a parametric formulation method under the thin-sample approximation. The relation of the output intensity to the input intensity is obtained in an explicit formula. The numerical results show that the hysteresis behavior and the cavity transmission are different from those in vacuum because of the local-field enhancement and the local cooperative decay effect.

Nonlinear optical response of a dense two-level atom system embedded in a linear dielectric medium

The nonlinear optical (local) response of a dense collection of two-level atoms embedded in a linear dielectric medium is investigated theoretically. The nonlinear response of absorption and dispersion of two-level atoms associated with a weak probe field in the presence of a strong pump field is calculated in terms of the linear response theory. The numerical results show that the absorption and dispersion spectra are different from those in vacua because of the local-field effect enhancement and the local cooperative effect.

Macroscopic field superpositions from collective interactions

We study the generation of both coherent superpositions and mixtures of field states through the interaction of an initially coherent field state with a collection of two-level atoms. The proposed scheme to build up a given superposition is based on effective dispersive interactions which arise from both the large detuning limit and the strong-field limit. The coherent superposition is obtained by interacting with a classical field after the cluster leaves the cavity followed by a measurement of the atomic population.

Quantized fields in a nonlinear dielectric medium: A microscopic approach

Theories which have been used to describe the quantized electromagnetic field interacting with a nonlinear dielectric medium are either phenomenological or derived by quantizing the macroscopic Maxwell equations. Here we take a different approach and derive a Hamiltonian describing interacting fields from one which contains both field and matter degrees of freedom. The medium is modelled as a collection of two-level atoms, and these interact with the electromagnetic field. The atoms are grouped into effective spins and the Holstein- Primakoff representation of the spin operators is used to expand them in one over the total spin. When the lowest-order term is combined with the free atomic and field Hamiltonians, a theory of noninteracting polaritons results. When higher-order terms are expressed in terms of polariton operators, standard nonlinear optical interactions emerge.

Spontaneous excitation of atoms near a phase conjugating mirror

It is shown by means of a semiclassical theory that the ground state of a localized collection of two-level atoms is unstable when the atomic sample is sufficiently close to a phase-conjugating mirror with high gain. Any small disturbance causes the atoms to become excited and begin fluorescing, the fluorescence taking the form of a sequence of superradiant pulses. Experiments to test this prediction seem possible. >

Squeezing of Light by Coherent Attenuation

We study the interaction of light with an amplifier or attenuator which can be approximated by a collection of two-level atoms which are prepared in identical atomic coherent states, giving a non-zero coherence in the initial ensemble-averaged atomic density matrix. It is found that light in an initial coherent state can be squeezed if the system is used in the attenuator mode, but not if it is used as an amplifier.

Propagation of modulated optical fields through saturable-absorbing media: a general theory of modulation spectroscopy

The propagation of a weakly modulated light beam through a nonlinear material is treated. The optical field representing such a beam consists of a strong carrier-frequency component and two weak, symmetrically displaced sidebands that combine to form a field, which may be purely amplitude modulated (AM), frequency modulated (FM), or some combination of these two modulation forms. It is shown that, for any optical nonlinearity, two modulation forms exist that have the property that the form of modulation is invariant under propagation of the beam. If a modulation form other than one of these natural modes is injected into the nonlinear medium, the modulation form will change as the beam propagates, asymptotically approaching the natural mode that experiences the lower attenuation. Explicit expressions for these natural modes are presented for the case in which the nonlinear medium can be modeled as a collection of two-level atoms. For the special case of an on-resonance pump beam, the natural modes correspond to pure amplitude modulation and pure frequency modulation. This formalism provides a general description of saturation spectroscopy for both AM and FM fields. Formulas are derived for the rate at which a pure FM beam is converted to an AM beam owing to its interaction with a two-level atomic medium. We also consider the variation that is due to propagation of the depth of modulation of an AM wave interacting resonantly with two-level atoms. The formalism predicts the existence of spectral features whose shape depends sensitively on the internal relaxation processes of the material. Experimental spectra are presented for ruby, alexandrite, and fluorescein in glass and are interpreted.

Quantum statistics of light after one-photon interaction with matter

The time development of the normally ordered generating functional of a light beam interacting linearly with a collection of two-level atoms is determined exactly. Rigorous conclusions on changes of the statistical properties of the beam are drawn. It is shown that the field after the interaction appears to be the superposition of a thermal field and of a field statistically similar to the initial one.

Coherent radiation and two-level electric dipole transitions, a quantitative classical analogy

Abstract The familiar qualitative notion that a collection of two-level atoms interacting with a radiation field behaves like a classical pendulum is made precise.