We examine angular distribution of the probability of correlated fluorescence photon emission from a linear chain of identical equidistant two-level atoms. We selectively excite one of the atoms by a resonant laser field. The atoms are coupled to each other via the dipole-dipole interaction and collective spontaneous emission. Our attention is focused on the simultaneous observation of correlated pairs of photons. It is found that the interference between the emitting atoms can result in a highly directional emission of photon pairs. These pairs of photons exhibit strong correlations and their emission is highly concentrated into specific directions. We demonstrate the crucial role of the selective coherent excitation in such a geometrical configuration. Shifting the driving field from an atom located at one end of the chain to the other causes the radiation pattern to flip to the opposite half of the detection plane. Furthermore, we find that atomic systems in which only an atom situated at a particular position within the linear chain is driven by a laser field can radiate correlated twin photons in directions along which the radiation of single photons is significantly reduced. Alternatively, superbunching in the emitted photon statistics preferentially occurs in directions of negligible or vanishing single-photon emission. The effect of superbunching strengthens as more emitters are added to the chain. Depending on the number of atoms and the position of the driven atom within the chain, the strongly correlated pairs of photons can be emitted into few well-defined directions.
We examine angular distribution of the probability of correlated fluorescence photon emission from a linear chain of identical equidistant two-level atoms. We selectively excite one of the atoms by a resonant laser field. The atoms are coupled to each other via the dipole-dipole interaction and collective spontaneous emission. Our attention is focused on the simultaneous observation of correlated pairs of photons. It is found that the interference between the emitting atoms can result in a highly directional emission of photon pairs. These pairs of photons exhibit strong correlations and their emission is highly concentrated into specific directions. We demonstrate the crucial role of the selective coherent excitation in such a geometrical configuration. Shifting the driving field from an atom located at one end of the chain to the other causes the radiation pattern to flip to the opposite half of the detection plane. Furthermore, we find that atomic systems in which only an atom situated at a particular position within the linear chain is driven by a laser field can radiate correlated twin photons in directions along which the radiation of single photons is significantly reduced. Alternatively, superbunching in the emitted photon statistics preferentially occurs in directions of negligible or vanishing single-photon emission. The effect of superbunching strengthens as more emitters are added to the chain. Depending on the number of atoms and the position of the driven atom within the chain, the strongly correlated pairs of photons can be emitted into few well-defined directions.
The probability of correlated emission of fluorescent photons as a function of detection directions has been investigated. The model system comprises identical two-level atoms arranged in the form of a line. A weak laser field resonantly excites only one of the atoms in the line. Two interaction mechanisms, namely, the vacuum-induced dipole–dipole interaction and the collective spontaneous emission couple the system of atoms. The aim is to observe the emission of a set of photon twins synchronized in time. It is seen that strongly directional emission of pairs of photons can take place due to the interference between the emitters. These highly correlated pairs of photons can be observed in very precise geometric directions. The observation is made based on two different detection procedures. It is found that the superradiant photons always tend to be bunched along the same direction.
We examine angular distribution of the correlations of fluorescence photons emitted from a linear chain of coupled two-level atoms. Attention is focused on the simultaneous emission of correlated pairs of photons. It is found that the collective behaviour of the atoms can result in a strongly directional emission of correlated pairs of photons. The number of directions and the degree of the correlation between the photons depends strongly on whether the atoms in the chain are equally or unequally spaced. In the case of unequally spaced atoms anticorrelated pairs of photons are emitted into specific directions. For equally spaced atoms, the emitted pairs of photons are strongly correlated and highly focused into specific directions. Depending on the number of atoms, the strongly correlated pairs of photons can be emitted into well-defined single or two directions.
In recent years, the radiative properties of atomic systems have been a hot topic in the research fields of quantum optics and quantum information. With the continuous development of nanophotonics, quantum antennas have become an important model for studying atomic radiation. In order to investigate these phenomena in depth, we investigate a system composed of two two-level atoms, and study the two-photon emission phenomenon of diatomic system under conditions of driving directional tunable laser field, interatomic dipole-dipole interaction, and spontaneous emission coherence. In this study, we diagonalize the atomic Hamiltonian to obtain the eigenvalues and entangled states of the system (symmetric and asymmetric states of two atoms), and use the rotating wave approximation to rotate the system into the laser frame. The evolution of the system is characterized mainly by the evolution of symmetric and asymmetric state, as well as the evolution of coherent terms. In our studies it is found that for identical atoms, certain laser directions and geometric configurations can exclusively drive the superradiant and subradiant states of atoms, which can enhance the first-order interference effect of the atoms and markedly increase the probability of two-photon emission in a specific detection direction. When the superradiant state of the atom is solely driven, there will be no coupling between the superradiant state and subradiant state, resulting in a correlation function angular distribution that is symmetric along the direction perpendicular to atomic axis. Further adjusting the laser direction causes the atomic interference patterns to shift, and the system will exhibit two-photon emission characteristics on one side or both sides. For nonidentical atomic systems, due to detuning between the two atoms, the laser cannot drive the superradiant state or subradiant state individually, and the influence of changing the laser direction on the coupling strength diminishes with the increase of detuning between the atoms. When the laser is in resonance with one of the atoms, due to the atomic interactions, the other atom can achieve the strongest coherent effect without resonating with the laser. This research reveals that atomic detuning is crucial for the correlation values and angular distribution of the correlation function. By adjusting the atomic detuning and laser direction, the system can display highly directed one-sided two-photon emission characteristics. However, different dissipation rates will lead the probability of two-photon emission to decrease. Our studies can achieve highly directional two-photon emission on one side or both sides, which provides a theoretical basis for studying the two-photon emission of nanoantennas.
Fluorescent photon emission probability from a system of three atoms has been studied as a function of detection angles. The system under investigation consists of identical bi-level atoms forming an equilateral triangle. Only a single atom in the triangle is resonantly excited by a weak laser. The interaction mechanism among the atoms is the vacuum-mediated dipole–dipole interaction and the collective decay process. The focus is to analyse the photon pairs simultaneously radiated by the system in the far-field domain. The analysis has been done for various interatomic separations. It is seen that the interference between the photon emitters can lead to strong emission in certain directions. An intuitive understanding of the interesting directions for strong photon bunching is given. It is found that a reduction in the intensity of radiation leads to superradiance along specific directions.
Frequency-resolved quantum correlation of resonance fluorescence is investigated in a two-atom radiating system. In this quantum radiating system, only one atom is driven by a laser field, and the spontaneous transition of the undriven atom resonates with one of the Rabi sidebands of the driven atom. A single-mode empty cavity is applied to serve as a Lorentzian filter to output the superbunched fluorescent photon pairs when its frequency is tuned to halfway between the central peak and one of the side peaks. In the case of large filter width, two-photon correlation signal and its physical correspondence can be bridged analytically in our approach. It reveals that this superbunching effect turns out to be the constructive quantum interference between a pair of coupled two-photon cascaded transitions. Ulteriorly, it is the consequence of the modulations of the unfiltered dressed-state transition amplitudes by the filter. Our analytical formalism also shows that, although the dipole-dipole interaction is usually weak, the interatomic coherence caused by this weak perturbation can also play a crucial role in breaking through the superbunching limit obtained from a single two-level atom in the same parameter regime. In addition to being a treasurable quantum pump to probe into the target quantum system, it is also found in our investigation that this superbunched fluorescence can serve as a promising quantum response in detecting this weak perturbation in the interior of the quantum source. A general case is also considered when the two-atom radiating system is monitored by two filter-detector monitoring systems. It is found that this filtered strong quantum correlation can be maintained even though the two photons are spatially separated.
Sub- and superradiant dynamics of spontaneously decaying atoms are manifestations of collective many-body systems. We study the internal dynamics and the radiation properties of two atoms in free space. Interesting results are obtained when the atoms are separated by less than half a wavelength of the atomic transition, where the dipole-dipole interaction gives rise to new coherent effects, such as (a) coherence between two intermediate collective states, (b) oscillations in the two-photon correlation G(2), (c) emission of two photons by one atom, and (d) the loss of directional correlation. We compare the population dynamics during the two-photon emission process with the dynamics of single-photon emission in the cases of a $\ensuremath{\Lambda}$ and a V scheme. We compute the temporal correlation and angular correlation of two successively emitted photons using the G(2) for different values of atomic separation. We find antibunching when the atomic separation is a quarter wavelength $\ensuremath{\lambda}∕4$. Oscillations in the temporal correlation provide a useful feature for measuring subwavelength atomic separation. Strong directional correlation between two emitted photons is found for atomic separation larger than a wavelength. We also compare the directionality of a photon spontaneously emitted by the two atoms prepared in phased-symmetric and phased-antisymmetric entangled states ${\ensuremath{\mid}\ifmmode\pm\else\textpm\fi{}⟩}_{{\mathbf{k}}_{0}}={e}^{i{\mathbf{k}}_{0}∙{\mathbf{r}}_{1}}\ensuremath{\mid}{a}_{1},{b}_{2}⟩\ifmmode\pm\else\textpm\fi{}{e}^{i{\mathbf{k}}_{0}∙{\mathbf{r}}_{2}}\ensuremath{\mid}{b}_{1},{a}_{2}⟩$ by a laser pulse with wave vector k0. Photon emission is directionally suppressed along k0 for the phased-antisymmetric state. The directionality ceases for interatomic distances less than $\ensuremath{\lambda}∕2$.