Initial Atomic Coherence Research Articles

Quantum properties of correlated emission laser with dephasing and phase fluctuation

We consider a two-photon laser in a cascade configuration coupled to an ordinary vacuum reservoir via a partially transmitting mirror. The impacts of the dephasing and the phase fluctuation on the entanglement, two-mode squeezing, and the intensity of the intracavity radiation are studied at steady-state. It so happens that the initial atomic coherence is the cause for the quantum properties of the cavity radiation even in the presence of the decoherence. We show that the dephasing rates and the phase fluctuations hurt the amount of squeezing and entanglement of the intracavity radiation. However, the dephasing rate leads to an increase in the intensity of the cavity radiation. Moreover, the phase fluctuation degrades the intensity of the intracavity radiation near to the initial maximum atomic coherence and it enhances otherwise.

Nonclassical atomic system dynamics time-dependently interacts with finite entangled pair coherent parametric converter cavity fields

We consider a time-dependent model that describes a qubit time-dependently interacting with a cavity containing finite entangled pair coherent parametric converter fields. The dynamics of some quantum phenomena, as: phase space information, quantum entanglement and squeezing, are explored by atomic Husimi function, atomic Wehrl entropy, variance, and entropy squeezing. The influences of the unitary qubit-cavity interaction, the difference between the two-mode photon numbers, the initial atomic coherence, and the time-dependent qubit location are investigated. It is found that the regularity, the amplitudes and the frequency of the quantum phenomena can be controlled by the physical parameters. For the initial atomic pure state, the qubit-cavity entanglement, the qubit phase space information, and atomic squeezing can be generated strongly compared to those of the initial atomic mixed state. The time-dependent location parameters enhance the generated quantum phenomena, and their effect can be enhanced by the parameters of the two-mode photon numbers and the initial atomic coherence.

Entanglement of atoms induced by thermal noise in the presence of initial atomic coherence

In this article, author investigated the dynamics of entanglement of two dipole-coupled natural or artificial two-level atoms (qubits) interacting nonresonantly with the intensive one-mode cavity thermal field. Author found an exact solution of the quantum Liouville equation for the full density matrix of the system two atoms + field mode for a coherent initial state of atoms in the dressed states representation. The full system density matrix is used to calculate the two-atom reduced density matrix and to calculate the quantitative criterion for atom-atom entanglement ‒ negativity. The results of computer simulation of the time dependence of negativity showed that in the case of a model with nonresonant interaction, the presence of initial atomic coherence leads to a significant decrease in the maximum degree of atomic entanglement, in contrast to the model with resonant interaction of atoms and a field. For the resonance model, the initial atomic coherence greatly enhances the degree of atomic entanglement.

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.

Rabi oscillations produced by adiabatic pulse due to initial atomic coherence.

If an electromagnetic pulse is detuned from atomic transition frequency by amount Δ>1/τ, where τ is the turn-on time of the pulse, then atomic population adiabatically follows the pulse intensity without causing Rabi oscillations. Here we show that, if initially, the atom has nonzero coherence, then the adiabatic pulse yields Rabi oscillations of atomic population ρaa(t), and we obtain analytical solutions for ρaa(t). Our findings can be useful for achieving generation of coherent light in the backward direction in the QASER scheme in which modulation of the coupling between light and atoms is produced by Rabi oscillations. Initial coherence can be created by sending a short resonant pulse into the medium followed by a long adiabatic pulse, which leads to the light amplification in the backward direction.

Entanglement between two qubits induced by thermal field

We have investigated the influence of dipole-dipole interaction and initial atomic coherence on atomic entanglement dynamics of two qubits. We have considered a model, in which only one atom couples to a quantum electromagnetic field in a cavity, since one of them can move around the cavity. We have shown that the entanglement arises for all pure atomic state even when both atoms are initially in the excited states. We have also derived that degree of entanglement is enhanced in the presence of the atomic coherence.

Influence of Stark shift on entanglement of two atoms with degenerate two-photon transitions for entangled and disentangled initial states

Considering two-atom degenerate two-photon Jaynes–Cummings model, we investigate the atom–atom entanglement when (a) atoms and field are prepared initially in the entangled atom-field W- or GHZ state, (b) two atoms are prepared initially in the coherent disentangled state and the cavity field is prepared in a few-photon Fock state, and study the effect of the Stark shift on entanglement. The results show that the atom–atom negativity evolves periodically with time and the periods are affected by the Stark shift and the degree of the initial atomic coherence. Considering different initial states we derived that the atom–atom entanglement can be greatly increased or decreased due to the presence of the Stark shift. In addition, the entanglement sudden death effect vanishes due to the presence of Stark shift for some initial states.

Atomic focusing by quantum fields: Entanglement properties

The coherent manipulation of the atomic matter waves is of great interest both in science and technology. In order to study how an atom optic device alters the coherence of an atomic beam, we consider the quantum lens proposed by Averbukh et al [1] to show the discrete nature of the electromagnetic field. We extend the analysis of this quantum lens to the study of another essentially quantum property present in the focusing process, i.e., the atom-field entanglement, and show how the initial atomic coherence and purity are affected by the entanglement. The dynamics of this process is obtained in closed form. We calculate the beam quality factor and the trace of the square of the reduced density matrix as a function of the average photon number in order to analyze the coherence and purity of the atomic beam during the focusing process.

Entanglement between qubits due to the atomic coherence

We have investigated the influence of dipole-dipole interaction and initial atomic coherence on the dynamics of two-qubit entanglement. We have considered a model, in which only one atom couples to a quantum electromagnetic field in a finite-Q cavity, since one of them can freely move outside the cavity. We have shown the possibility of disappearance of the entanglement sudden death effect and stabilization of atomic entanglement in the presence of the dipole interaction of qubits.

Coherence-enhanced entanglement between two atoms at high temperature

This paper studies the entanglement properties in a system of two dipole—dipole coupled two-level atoms resonantly interacting with a single-mode thermal field. The results show that, when the temperature of the cavity is high enough (corresponding to the large value of the mean photon number), the entanglement is greatly enhanced due to the initial atomic coherence. These results are helpful for controlling the atomic entanglement by changing the initial parameters of the system.

Polarization evolution of a few-cycle ultrashort laser pulse propagating in a degenerate three-level medium

The propagation of an arbitrary polarized few-cycle ultrashort laser pulse in a degenerate three-level medium is investigated by using an iterative predictor-corrector finite-difference time-domain method. It is found that the polarization evolution of the ultrashort laser pulse is dependent not only on the initial atomic coherence of the medium but also on the polarization condition of the incident laser pulse. When the initial effective area is equal to $2\ensuremath{\pi}$, complete linear-to-circular and circular-to-linear polarization conversion of few-cycle ultrashort laser pulses can be achieved due to the quantum interference effects between the two different transition paths.

Atomic coherence control on the entanglement of two atoms in two-photon processes

Considering two identical two-level atoms interacting with a single-mode thermal field through two-photon processes, this paper studies the atomic coherence control on the entanglement between two two-level atoms, and finds that the entanglement is greatly enhanced due to the initial atomic coherence. The results show that the entanglement can be manipulated by changing the initial parameters of the system, such as the superposition coefficients and the relative phases of the initial atomic coherent state and the mean photon number of the cavity field.

Coherence-enhanced and -controlled entanglement of two atoms in a single-mode thermal field

We have investigated the entanglement of two identical two-level atoms in a single-mode thermal reservoir when these two atoms are initially prepared in coherent states. It is discovered that the dynamical entanglement is greatly enhanced due to initial atomic coherence as compared with the case when the atoms are initially prepared in incoherent states. It is very valuable that we can manipulate the atomic entanglement by changing the relative phases and amplitudes of the polarized atoms.

ATOMIC DIPOLE SQUEEZING IN THE DEGENERATE Λ QUANTUM BEAT THREE-LEVEL SYSTEM

By using the full quantum theory, we investigate the time evolution of the atomic dipole squeezing parameters of a Λ-type three-level atom interacting with a single-mode coherent optical field, and study the influence of the initial coherent-field intensity, the initial atomic coherence, the initial populations and energy splitting of the two lower atomic levels on the atomic dipole squeezing. The influence of a classical external driving field coupling to the atom on the atomic dipole squeezing is also explored at the end of the paper.

Two-Photon Micromaser with Initial Atomic Coherence

We investigate the quantum dynamics of a two-photon micromaser pumpedby atoms injected in the superposition state of the upper andintermediate levels. We simulate a master equation governing the systemby the Monte Carlo wavefunction approach and analyse the steady-statebehaviour as a function of the atomic transit time. The atomiccoherence can effectively enhance the intensity and sub-Poissonian ofthe cavity field as compared with the atomic mixture. It is alsodiscovered that the phase of the cavity field can be shifted byadjusting the detuning between the atom and field. This result showsthat it is possible to manipulate the phase of the cavity field bydetuning, due to atomic coherence.

Generation of higher-order atomic dipole squeezing in a high-Q micromaser cavity. (VI). Atomic damping by a squeezed vacuum

In our previous serial works, we investigated the generation of higher-order atomic dipole squeezing (HOADS) in a high-Q micromaser cavity. In this paper, we continue to study HOADS in this system but consider the effect of atomic damping by a squeezed vacuum in the absence of an external field. The corresponding master equation is introduced and its solutions for an initial SU(2) coherent state of the atom are presented. In the absence of the atomic damping, we have shown that stable and permanent HOADS can be generated by properly preparing the atom in certain SU(2) coherent state. We demonstrate the destructive effect of atomic damping on HOADS by changing the initial atomic coherence or the average number of the reservoir photons. The relation between HOADS and second-order atomic dipole squeezing (SOADS) is discussed. It is found that the atomic damping has more destructive effects on SOADS than on HOADS.

COHERENT TWO-PHOTON PULSE PROPAGATION THROUGH A COHERENTLY PREPARED MEDIUM

Propagation of pulses through a medium comprising of coherently prepared two-level atoms undergoing two-photon transition has been studied using Maxwell–Bloch equations. The solution of the electromagnetic field envelopes has been obtained under various conditions and the effect of initial atomic coherence is clearly brought out in these solutions.

Dressed-state approach to population trapping in the Jaynes-Cummings model

The phenomenon of atomic population trapping in the Jaynes-Cummings model is analysed from a dressed-state point of view. A general condition for the occurrence of partial or total trapping from an arbitrary, pure initial atom-field state is obtained in the form of a bound to the variation of the atomic inversion. More generally, it is found that in the presence of initial atomic or atom-field coherence the population dynamics is governed not by the field's initial photon distribution, but by a ‘weighted dressedness’ distribution characterizing the joint atom-field state. In particular, individual revivals in the inversion can be analytically described to good approximation in terms of that distribution, even in the limit of large population trapping. This result is obtained through a generalization of the Poisson Summation Formula method for an analytical description of revivals developed by Fleischhauer and Schleich (1993, Phys. Rev A, 47, 4258).

EVOLUTION OF DISTANCE BETWEEN DENSITY OPERATORS IN A NON-RESONANT INTERACTION SYSTEM

By considering the detuning and the initial atomic coherence,a general expression of distance between density operators of arbitrary initial states (reference states) is obtained. The emphasize is put on the influence of detuning on the evolution in the interaction of a two-level atom of arbitrary initial states with the coherent field. The results show that the distance between density operators is obviously dependent on the value of detuning.

Nonclassical states in cavity with injected atoms

It is found that displacement and squeezing of arbitrary initial states of the field can be produced in the model of a lossless cavity when a large number of two-level atoms is injected, which are prepared in a superposition state coupled to the single mode via a multi-photon transition under the weak atom-field interaction. The dependence of the field density matrix elements on the number of injected atoms indicates that due to the same initial atomic coherence the emission of individual atoms is a co-operative process.