Atomic Density Matrix Research Articles

Estimating phase parameters of a three-level system interacting with two classical monochromatic fields in simultaneous and individual metrological strategies

Recently, the Hilbert–Schmidt speed, as a special class of quantum statistical speed, has been reported to improve interferometric phase in a single-parameter quantum estimation. Here, we test this concept in the multiparameter scenario where two laser phases are estimated in a theoretical model consisting of a three-level atom interacting with two classical monochromatic fields. When the atom is initially prepared in the lower bare state, taking into account the detuning parameters, we extract an exact analytical solution of the atomic density matrix in the case of a two-photon resonant transition. Further, we compare the performance of laser phase parameter estimation in individual and simultaneous metrological strategies and explore the role of quantum coherence in improving the efficiency of unknown multiphase shift estimation protocols. The obtained results show that the Hilbert–Schmidt speed detects the lower bound on the statistical estimation error as well as the optimal estimation regions, where its maximal corresponds to the maximal quantum Fisher information; further, the performance of simultaneous multiparameter estimation with individual estimation inevitably depends on the detuning parameters of the three-level atom. Aside from the quantum entanglement, the quantum coherence is also a crucial resource to improve the accuracy of a metrological protocol.

On the statistical background of quantum mechanics: generalities and a concrete example

We revisit our description of randomness in quantum processes that began in collaboration of Jean Ginibre. The calculations were performed on a worked example: the fluorescence of a single two-level atom pumped by a resonant laser field. This pump laser is described classically (by a function, not an operator). Our aim is first to built a Kolmogorov-type equation (K-equation) for the atomic state, so that the two parameters θ, φ that define this density matrix are random functions of time, therefore the atomic density matrix is a random density matrix. Such an approach, initiated for gas kinetics, was not yet applied to quantum phenomena, whereas it is especially tailored to very quick events well separated (in time) like the quantum jumps observed in spontaneous emission of photons by an atom. Here, we try to clarify the basis of our statistical approach leading to the K-equation below, and we present the main results deduced from it. We explain finally that our approach can be interpreted in terms of Everett’s theory of many-worlds, because at every emission a new history begins for the atom, with two nonoverlapping wave functions.

Optically polarized selective transmission of a fractional vector vortex beam by the polarized atoms with external magnetic fields.

We investigate the role of external magnetic fields and linearly polarized pump light, especially when their directions are parallel or vertical, on the propagation of the fractional vector vortex beams (FVVBs) through a polarized atomic system. Herein, the different configurations of external magnetic fields lead to various optically polarized selective transmissions of FVVBs with different fractional topological charge α caused by the polarized atoms, which is theoretically demonstrated by the atomic density matrix visualization analysis and experimentally explored by Cesium atom vapor. Meanwhile, we find that the FVVBs-atom interaction is a vectorial process due to the different optical vector polarized states. In this interaction process, the atomic optically polarized selection property provides potential for the realization of the magnetic compass based on warm atoms. For the FVVBs, due to the rotational asymmetry of the intensity distribution, we can observe some transmitted light spots with unequal energy. Compared with the integer vector vortex beam, it is possible to obtain a more precise magnetic field direction by fitting the different "petal" spots of the FVVBs.

Scaled opposite-spin atomic-orbital based algebraic diagrammatic construction scheme for the polarization propagator with asymptotic linear-scaling effort: Theory and implementation.

A novel local approach for the quantum-chemical computation of excited states is presented, where the concept of the atomic-orbital formulation of the second-order Møller-Plesset energy expression is extended to the second-order algebraic diagrammatic construction scheme by virtue of the Laplace transform. The scaled opposite-spin second-order algebraic diagrammatic construction method with Cholesky decomposed densities and density-fitting, or CDD-DF-SOS-ADC(2) for short, exploits the sparsity of the two-electron repulsion integrals, the atomic ground-state density matrix, and the atomic transition density matrix to drastically reduce the computational effort. By using a local density-fitting approximation, it is shown that asymptotically linear scaling can be achieved for linear carboxylic acids. For electron-dense systems, sub-cubic scaling can be achieved if the excitation is local, and hence the transition density is sparse. Furthermore, the memory footprint and accuracy of the CDD-DF-SOS-ADC(2) method are explored in detail.

Optical reconstruction of the collective density matrix of a qutrit

Reliable tomography of a quantum state of atoms in room-temperature vapors offers interesting applications in quantum-information science. To step toward the applications, here, we theoretical investigate a technique of reconstruction of a collective density matrix of atoms in a state of a total angular momentum equal to unity. By studying transformation of density matrix due to magnetic-field pulses and deriving explicit relations between optical signals and specific density-matrix elements, we demonstrate full reconstruction of quantum states. Numerical simulations allow us to demonstrate that the reconstruction fidelity of pure and mixed states may exceed 0.95 and that the technique is robust against noise and uncertainties.

Free Energy Profiles of Proton Transfer Reactions: Density Functional Benchmark from Biased Ab Initio Dynamics.

By coupling an enhanced sampling algorithm with an orbital-localized variant of Car-Parrinello molecular dynamics, the so-called atomic centered density matrix propagation model, we reconstruct the free energy profiles along reaction pathways using different density functional approximations (DFAs) ranging from locals to hybrids. In particular, we compare the computed free energy barrier height of proton transfer (PT) reactions to those obtained by a more traditional static approach, based on the intrinsic reaction coordinate (IRC), for two case systems, namely malonaldehyde and formic acid dimer. The obtained results show that both the IRC profiles and the potentials of mean force, derived from biased dynamic trajectories, are very sensitive to the density functional approximation applied. More precisely, we observe that, with the notable exception of M06-L, local density functionals always strongly underestimate the reaction barrier heights. More generally, we find that also the shape of the free energy profile is very sensitive to the density functional choice, thus highlighting the effect, often neglected, that the choice of DFA has also in the case of dynamics simulations.

Itô – Stratonovich dilemma in the problem of laser cooling of atoms: limits of applicability of the semiclassical approximation

We report a comparative analysis of the limits of laser cooling of atoms in light fields both on the basis of a numerical solution of the quantum kinetic equation for the atomic density matrix, which makes it possible to accurately take into account quantum recoil effects during the interaction of atoms with field photons, and within the framework of the semiclassical approach using various forms of the Fokker – Plank equation. The analysis allows one to outline the limits of applicability of the semiclassical approach, as well as to choose an unambiguous form for introducing the diffusion contribution to the Fokker – Planck equation in the semiclassical description of atomic kinetics.

Laser cooling limits in fields with a polarisation gradient of atoms with different recoil energies

Based on the numerical solution of the quantum kinetic equation for the atomic density matrix, which makes it possible to accurately take into account the recoil effects in the interaction of atoms with field photons, we have studied the limits of laser cooling of atoms using closed optical transitions characterised by different recoil parameters (the ratio of the recoil energy to the natural linewidth). It is shown that for optical transitions with an insufficiently small recoil parameter, the polarisation effects, which lead to the possibility of sub-Doppler laser cooling, lose their efficiency and do not ensure an attainment of the temperature below the Doppler limit. The analysis performed allows one to outline the boundaries of the sub-Doppler theory of laser cooling of atoms.

Collective radiation effects in rubidium vapor beyond the yoked superradiance

It is known that being pumped with infrared light an ensemble of Rb atoms can demonstrate an upconversion and re-emit blue light at 420 nm. In pulsed regime this radiation can be enhanced by collective effects, which is called yoked superradiance. Here we demonstrate that not only the blue-light emission is affected by collective effects, but also the red light scattering. This becomes possible due to the coupling between different non-diagonal elements of the atomic density matrix. We develop a simple self-consistent model of the coupled atom-field system and numerically simulate its dynamics. We test the sensitivity of blue- and red-light pulse profiles on the density of Rb atoms and the background gas pressure.

Ultracold Lattice Gas Automata for Single Trapped Ion Interacting with a Laser Field

AbstractIn the same context of lattice gauge theory, a notion of cellular automaton (CA) is considered as a description of the underlying dynamics of fields evaluated on a discrete lattice. The explorative approach of this work is initiated by taking the automata evolution as an elementary description of fields dynamics on one side and pivotal role of locality on other side to connect the notion of states and transformations to density operators in quantum electrodynamics. In order to parameterize the definition of proposed model in the context of lattice field theory the nonlinear interaction is introduced with neighboring cells. In this paper, analytical and explicit expression of atomic density matrix for single trapped ion interacting with laser field based on cellular automata dynamics are discussed. The definition is further explored by introducing some probabilistic theories as the theoretical framework then it is concluded that the graph of cellular automata in continuum time limit of the CA dynamics leads to Maxwell–Bloch equation.

Quantum scheme for N-level atom interacting with a two two-level atom: Atomic Fisher information and entropy squeezing

In this work, we introduce a model of a two-atom interacting with a multi-level atom governed by su(2) Lie algebra in the presence of external classical field. The influence of the classical field on the system is discussed in detail for certain values for the classical terms. The atomic density matrix of the proposed system is obtained. The dynamical behavior of the atomic Fisher information as an indicator of the nonlocal correlation between a two-atom and su(2) field is discussed. Moreover, we examine the effect of classical field on the evolution of entropy squeezing and the geometric phase induced between the initial and final state of the proposed system. The results outlined some important phenomena as sudden death and sudden birth of entanglement in presence of the classical terms is observed through the dynamics of atomic Fisher information in the presence of classical field for the large number of levels.

Some features of the nonlocal correlation and geometric phase of the quantum system in two-mode nondegenerate entangled states

We propose a quantum two-level system interacting with a two-mode nondegenerate entangled states (TMNESs). The dynamical properties of entropy squeezing component is investigated based on atomic density matrix in the absence and presence of classical field effect. Moreover, the dynamical properties of the nonlocal correlation between the atom and TMNESs are studied and compared with geometric phase. We examine the influence of the initial interaction conditions and classical field on the evolution of nonlocal correlation, linear entropy and geometric phase. Furthermore, we discuss the link between the geometric phase, field purity and nonlocal correlation during the time dynamics. Finally, the results clarified that a geometric phase and entanglement are strongly affected by the classical field and detuning parameter.

Kinetic Equation for the Density Matrix of Atoms in the Field of a Broadband One-Photon Packet Taking into Account the Non-Wiener Dynamics

We have derived the quantum master kinetic equation for the density matrix of an atomic system in the field of a narrow-beam one-photon broadband packet in the conditions when the latter plays the role of a thermostat. This equation takes into account the Stark interaction of atoms with the broadband field of surroundings of the system, which are in the state with zero photon number density, as well as the interaction of broadband fields at the atomic system.

Continued-fraction representation of the Kraus map for non-Markovian reservoir damping

Quantum dissipation is studied for a discrete system that linearly interacts with a reservoir of harmonic oscillators at thermal equilibrium. Initial correlations between system and reservoir are assumed to be absent. The dissipative dynamics as determined by the unitary evolution of system and reservoir is described by a Kraus map consisting of an infinite number of matrices. For all Laplace-transformed Kraus matrices exact solutions are constructed in terms of continued fractions that depend on the pair correlation functions of the reservoir. By performing factorizations in the Kraus map a perturbation theory is set up that conserves in arbitrary perturbative order both positivity and probability of the density matrix. The latter is determined by an integral equation for a bitemporal matrix and a finite hierarchy for Kraus matrices. In the lowest perturbative order this hierarchy reduces to one equation for one Kraus matrix. Its solution is given by a continued fraction of a much simpler structure as compared to the non-perturbative case. In the lowest perturbative order our non-Markovian evolution equations are applied to the damped Jaynes–Cummings model. From the solution for the atomic density matrix it is found that the atom may remain in the state of maximum entropy for a significant time span that depends on the initial energy of the radiation field.

Studying the possibility of deep laser cooling of 24Mg atoms in an optical lattice: Two-level quantum model

The possible deep laser cooling of 24Mg atoms in a deep optical lattice in the presence of an additional pumping field resonant to the narrow 3s3s1S0 → 3s3p3P1 (λ = 457 nm) optical transition is studied. Two quantum models of the laser cooling of atoms in the optical trap are compared. One is based on the direct numerical solution to the kinetic quantum equation for an atomic density matrix; it considers both optical pumping and quantum recoil effects during interaction between the atoms and field photons. The second, simplified model is based on decomposing the states of the atoms over the levels of vibration in the optical trap and analyzing the evolution of these states. The comparison allows derivation of optical field parameters (pumping field intensity and detuning) that ensure cooling of the atoms to minimal energies. The conditions for fast laser cooling in an optical trap are found.

Optimization of stabilization regimes in atomic clocks, based on the effect of coherent population trapping

Using the atomic density matrix formalism, we investigate the dynamic regime of formation of the error signal for resonances of coherent population trapping excited in a three-level Λ-system by a bichromatic field. The optimal parameters of harmonic modulation used in the frequency locked loop are determined with allowance for low-frequency noise.

Broadband radiation transport in an optically dense gas in the presence of an RF field

The theory of transport of fluctuating double-frequency optical radiation in a gas of three-level atoms in the presence of rf radiation forming a closed excitation circuit has been generalized. The theory is based on the quantum kinetic equations for the atomic density matrix, which are solved together with the wave equations for classical electromagnetic fields. After averaging over the radiation fluctuation ensemble, a transition is made to equations for atomic-field correlation functions and transport equations for the spectral density of fields.

Role of coherence in transport through engineered atomic spin devices

We give a further step in the quantum mechanical description of engineered atomic spin structures by deriving a master equation of the Redfield type that governs the dynamics of the atomic spin density matrix. By generalizing this approach to charge-specific density matrices, we are able to describe magnetic transport quantities, such as the average inelastic current and the shot noise, accessible by tunneling spectroscopy. Our method suitably describes moderate lead-atom coupling regimes where quantum coherence effects cannot be disregarded. We contrast our approach with the existing descriptions in terms of rate equations and show examples where coherence effects are crucial to understand the physics of spin-polarized tunnel current through spin structures.

Nuclei-selected atomic-orbital response-theory formulation for the calculation of NMR shielding tensors using density-fitting

An atomic orbital density matrix based response formulation of the nuclei-selected approach of Beer, Kussmann, and Ochsenfeld [J. Chem. Phys. 134, 074102 (2011)] to calculate nuclear magnetic resonance (NMR) shielding tensors has been developed and implemented into LSDalton allowing for a simultaneous solution of the response equations, which significantly improves the performance. The response formulation to calculate nuclei-selected NMR shielding tensors can be used together with the density-fitting approximation that allows efficient calculation of Coulomb integrals. It is shown that using density-fitting does not lead to a significant loss in accuracy for both the nuclei-selected and the conventional ways to calculate NMR shielding constants and should thus be used for applications with LSDalton.

Theory of propagation of spectrum and correlations of radiation in optically dense gas in the case of the closed excitation contour

This work is devoted to generalization of the semi-classical theory of interaction of broadband laser radiation with the atomic gas at the room temperature in the cell in the case of the closed excitation contour. The atomic density matrix equations and spectrum and correlations transport equations have been derived for excitation by fluctuating field with Gaussian statistics. It is shown that the spatial oscillations of radiation intensity and atomic density matrix can be excited. It was found that such medium can serve as a filter of incoherent part of the radiation.