The space-time connection between the Mollow cascaded transitions with time orderings and the spatially directional correlated emissions of Mollow photons is analyzed theoretically in a strong laser-dressed subwavelength two-atom antenna, which supports only two-level electric dipole transitions. It can be regarded as the most prototypical collective quantum radiator to exploit the space-time quantum characteristics of the Mollow physics of resonance fluorescence. By constructing the photon filtering dynamics from the dressed two-atom antenna to a pair of quantum sensors, the frequency-resolved spatial two-point two-time correlations of different Mollow spectral lines are examined analytically in terms of the quantum sensor modes. It is shown that a rich significance of spatial directionality lies in the time orderings of cascaded emissions between different Mollow spectral lines. This feature enables the time orderings of different Mollow spectral lines to be tailored geometrically and thus prepared deterministically to produce highly directional temporal bunching effects of the Mollow photons with giant nonclassicality only for a specific two-photon detection order. The dressed two-atom system is therefore able to operate as a highly directional atomic antenna that may provide versatile applications in the fields of atom-photon-based quantum precise measurement physics and Mollow spectroscopy.
The space-time connection between the Mollow cascaded transitions with time orderings and the spatially directional correlated emissions of Mollow photons is analyzed theoretically in a strong laser-dressed subwavelength two-atom antenna, which supports only two-level electric dipole transitions. It can be regarded as the most prototypical collective quantum radiator to exploit the space-time quantum characteristics of the Mollow physics of resonance fluorescence. By constructing the photon filtering dynamics from the dressed two-atom antenna to a pair of quantum sensors, the frequency-resolved spatial two-point two-time correlations of different Mollow spectral lines are examined analytically in terms of the quantum sensor modes. It is shown that a rich significance of spatial directionality lies in the time orderings of cascaded emissions between different Mollow spectral lines. This feature enables the time orderings of different Mollow spectral lines to be tailored geometrically and thus prepared deterministically to produce highly directional temporal bunching effects of the Mollow photons with giant nonclassicality only for a specific two-photon detection order. The dressed two-atom system is therefore able to operate as a highly directional atomic antenna that may provide versatile applications in the fields of atom-photon-based quantum precise measurement physics and Mollow spectroscopy.
By constructing a quantum filtering system to synthesize the spectral and spatial properties of stimulated radiation, the directionality of spectrally correlated nonclassical properties of resonance fluorescence from a three-body quantum antenna is investigated. The quantum antenna consists of three two-level quantum emitters, two of which are identical and strongly driven by an external laser field to radiate the collective resonance fluorescence with three distinct spectral components. The collective stimulated radiation is shaped by the remaining auxiliary emitter via its spontaneous emission, which is assumed to be resonant with the higher-frequency spectral component of the two identical emitters. Due to the presence of the auxiliary emitter, highly directional nonclassicality of the spectral correlations can be generated, which is the consequence of the internal interference of the source and the spatial interference of the radiation field. The electric dipole-dipole interaction between the undriven emitter and the dressed two-body radiating source plays a crucial role in breaking the rotational symmetry of nonclassical signals, which is embodied by various photon correlation functions with a frequency-resolved version for different spectral combinations. By correlating the two opposite sidebands, the frequency-resolved intensity-intensity correlation functions can signal highly directional nonclassicality, and the frequency-resolved two-mode entanglement along a specific emission direction can be tested by the frequency-resolved anomalous correlation function. While the directional nonclassical correlations between the central-frequency and the three-body collective higher-frequency components can be identified through the frequency-resolved intensity-field correlation function. These various frequency-resolved correlation functions characterize the spatial asymmetry of nonclassical properties of the collective stimulated radiation from different perspectives, including the particle properties and the quantum coherences of the filtered photons. When the three-body collective higher-frequency sidebands are correlated with the central band and the lower-frequency sideband, respectively, the schemes of using the auxiliary undriven emitter to generate optimal nonclassical correlations with well-defined directions are discussed analytically. In addition, the results obtained from the three-body quantum antenna and a dressed two-body quantum antenna are compared.
Frequency-resolved correlation (FRC) is investigated in fluorescent emission radiated from a $\mathrm{\ensuremath{\Lambda}}$-type atomic system by employing weak coupling regime between quantum emitter and cavities, in which each cavity substituting a Lorentzian filter is assumed to pass through only one fluorescent photon at a time so that the analytical discussions for time orderings can be carried out in a truncated Hilbert space with single-excitation state for each cavity mode. In the limit of bad cavity (large passband width of filter) and short delay, the conditional time ordering amplitudes are introduced with the help of conditioned dressed atom-photon states prepared by preselection to probe into the effect of preferential screening for time orderings of filter-detector systems. Meanwhile, the past quantum state formalism is also applied in FRC to further excavate their collective monitoring effect. Based on the obtained two-mode correlation signals, it is found that, in nonresonant two-photon cascaded processes, bunching and antibunching can appear in two opposite detection orderings, respectively, and can be switched. The physical origin is explored that the two different transition amplitudes of emitted photons from a common spectral band of resonance fluorescence are responsible for opposite time orderings, thus, the imbalance between two complementary time orderings can be enhanced, which is impossible to achieve in resonance fluorescence of two-level atoms. This mechanism is embodied in the conditional time ordering amplitudes produced by this detection theory of cavity modes.
Abstract Frequency-resolved photon statistics of resonance fluorescence generated from a two-level system driven by a strong laser field and a weak laser field with equal frequencies are studied. The frequency resolution of fluorescent radiation is described by quantum filtering dynamics, which is simulated theoretically by two single-mode quantum optical cavities with tunable frequencies to scan the incident fluorescent radiation. By calculating the two-photon intensity–intensity correlation functions in terms of the cavity modes, we demonstrate that two-color strong correlations of resonance fluorescence can be generated not only between the opposite sidebands, but also between the central band and one of the sidebands: although both sidebands are broadened due to the perturbation of the weak laser field on the strong-field dressed atom. We emphasize that these properties are in contrast to the conventional case of the standard single-atom Mollow triplet. Moreover, if the resonance frequencies of the two filtering cavities are tuned appropriately, broadband two-color strong correlations are predicted, and the physical origin is revealed from the perspective of quantum interference of photon emission dynamics. This can be considered as a feasible scheme for the design of broadband non-classical light sources, and may be beneficial to the quantum precise detection of atomic and molecular dynamics via quantum optical spectroscopy.
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.
Multifold wave-particle quantum correlations are studied in strongly correlated three-photon emissions from the Mollow triplet via frequency engineering. The nonclassicality and the non-Gaussianity of the filtered field are discussed by correlating intensity signal and correlated balanced homodyne signals. Due to the non-Gaussian fluctuations in the Mollow triplet, new forms of the criterion of nonclassicality for non-Gaussian radiation are proposed by introducing intensity-dual quadrature correlation functions, which contain the information about strongly correlated three-photon emissions of the Mollow triplet. In addition, the time-dependent dynamics of non-Gaussian fluctuations of the filtered field is studied, which displays conspicuous asymmetry. Physically, the asymmetrical evolution of non-Gaussian fluctuations can be attributed to the different transition dynamics of the laser-dressed quantum emitter revealed by the past quantum state and conditional quantum state. Compared with the conventional three-photon intensity correlations that unilaterally reflect the particle properties of radiation, the multifold wave-particle correlation functions we proposed may convey more information about wave-particle duality of radiation, such as the quantum coherence of photon triplet and "which-path" in cascaded photon emissions in atomic systems.
Mollow physics in the two-photon regime shows interesting features such as versatile generation of highly correlated photon pairs, timely ordered and energy tunable. Here, we calculate analytically the power spectral density $S(\ensuremath{\omega})$ and the two-photon correlations ${g}^{(2)}(\ensuremath{\tau})$, which are essential to discuss and study such phenomena in the resonant-driven dressed-state regime. It is shown that there exist upper and lower bounds of the ${g}^{(2)}(\ensuremath{\tau})$ function for certain spectrally selected photon pairs. Recently reported experiments agree with the presented theory and thereby it is shown that the resonant-driven four-level system is an interesting source for steerable quantum light in quantum cascade setups. We furthermore discuss the unlikeliness of observing antibunching for the delay time $\ensuremath{\tau}=0$ in the exciton-biexciton correlation functions in such experiments, since antibunching stems from a coherent and in-phase superposition of different photon-emission events. Due to the occurring laser photon scattering, this coherent superposition state is easily disturbed and leads to correlation functions of g(2)(0)=1.