Some coherence properties of the light emitted from a two-level atom which is excited by a monochromatic coherent field are investigated. It is shown that the elastically scattered light is coherent, and that this compoment of the emitted field can be separated from the total field by narrow-band filtering at the driving frequency. These results are then applied to the problem of interference between the fluorescent and the elastically scattered field. An interference experiment is analyzed in which the detection of one emitted photon signifies the beginning of a Rabi cycle and the detection of a subsequent photon fixes the instant in the Rabi cycle when the interference is detected. It is shown that the visibility of the interference is near zero at the time when the fluorescence is maximum.
Some coherence properties of the light emitted from a two-level atom which is excited by a monochromatic coherent field are investigated. It is shown that the elastically scattered light is coherent, and that this compoment of the emitted field can be separated from the total field by narrow-band filtering at the driving frequency. These results are then applied to the problem of interference between the fluorescent and the elastically scattered field. An interference experiment is analyzed in which the detection of one emitted photon signifies the beginning of a Rabi cycle and the detection of a subsequent photon fixes the instant in the Rabi cycle when the interference is detected. It is shown that the visibility of the interference is near zero at the time when the fluorescence is maximum.
It has been predicted that when fluorescent photons emitted from an atom in the process of resonance fluorescence interfere with a portion of the coherent driving field, the visibility of the resulting interference exhibits nonclassical features and is zero at the moment when the atom is fully excited and when the fluorescent intensity is greatest.1 We show theoretically that a similar conclusion applies to interference between the fluorescent photons and the elastically scattered part of the driving field, which can be separated out with narrow band filters in practice. This involves the calculation of certain fourth order, three-time correlation functions that have not previously been studied. A possible application of the theory to an interference experiment will be discussed.
Resonance fluorescence of a two-level emitter displays persistently anti-bunching irrespective of the excitation intensity, but inherits the driving laser’s linewidth under weak monochromatic excitation. These properties are commonly explained in terms of two disjoined pictures, i.e., the emitter’s single photon saturation or passively scattering light. Here, we propose a unified model that treats all fluorescence photons as spontaneous emission, one at a time, and can explain simultaneously both the spectral and correlation properties of the emission. We theoretically derive the excitation power dependencies, measurable at the single-photon incidence level, of the first-order coherence of the whole resonance fluorescence and super-bunching of the spectrally filtered, followed by experimental confirmation on a semiconductor quantum dot micro-pillar device. Furthermore, our model explains peculiar coincidence bunching observed in phase-dependent two-photon interference experiments. Our work provides an intuitive understanding of coherent light-matter interaction and may stimulate new applications. Standard descriptions of resonance fluorescence consider either absorption and re-emission in a single-photon picture or passive scattering. This study shows that resonance fluorescence can be described by a pure quantum state of the emitter and photon, revealing a coherent superposition that challenges the conventional view of spontaneous emission as an incoherent process.
The phenomenon of resonance fluorescence provides an interesting manifestation of the quantum theory of light and is a “real world” application of the material of the Chapters 8 and 9. In this process, a two-level atom is typically driven by a resonant continuous-wave laser field and the spectral and quantum statistical properties of the fluorescent light emitted by the atom are measured. Experimentally this can be achieved by scattering a laser off a collimated atomic beam such that the directions of the laser beam, atomic beam, and detector axis are mutually perpendicular as shown in Fig. 10.1. If the driving field is monochromatic, then at low excitation intensity the atom absorbs a photon at the excitation frequency and reemits it at the same frequency as a consequence of conservation of energy. The spectral width of the fluorescent light is therefore very narrow. The situation, however, is considerably more complicated when the excitation intensity increases and the Rabi frequency associated with the driving field becomes comparable to, or larger than, the atomic linewidth. At such intensity levels, the Rabi oscillations show up as a modulation of the quantum dipole moment and sidebands start emerging in the spectrum of the emitted radiation. This so-called dynamic Stark splitting is an interesting feature of the atom–field interaction. In addition to that, the fluorescent light exhibits certain nonclassical properties including photon antibunching and squeezing. In this chapter, we develop a theory of resonance fluorescence to explain these phenomena.
We study the correlation between photons in the fluorescence light scattered from an atom driven by an intense bichromatic field having one strong and one weak component, with or without the use of frequency filters in front of the photodetectors. Depending on the filters used and on the spectral components selected, we predict signals corresponding either to bunching or to antibunching, and modulations at the driving field Rabi frequencies as well as at their sum and difference. In contrast with the case of monochromatic driving, the antibunching can occur between photons emitted into different (as well as the same) lines of the fluorescence spectrum.
The calculation is given of the spectral distribution of resonance fluorescence from an atom that is driven near a j=21 to j=21 transition by a monochromatic, x-polarized electric field and that is subject to radiation damping only. In accordance with Mollow's results for a two-level system, the power spectrum of the scattered radiation corresponding to the x component of the dipole moment contains a coherent part for all driving-field intensities. The coherent part is absent in the fluorescence radiation corresponding to the y and z components of the dipole moment. For weak driving fields all power scattered by these latter components is present in a narrow Lorentzian. Consistent with energy conservation, the width of the Lorentzian corresponds to the probability of excitation of the atom out of its ground level due to the presence of the driving field. For strong driving fields the power spectra corresponding to the x component and to the y or z component mainly differ in the heights and widths of their central incoherent band and their Rabi-precession-frequency-shifted side bands.
Abstract The interaction of light with a single two-level emitter is the most fundamental process in quantum optics, and is key to many quantum applications. As a distinctive feature, two photons are never detected simultaneously in the light scattered by the emitter. This is commonly interpreted by saying that a single two-level quantum emitter can only absorb and emit single photons. However, it has been theoretically proposed that the photon anticorrelations can be thought of as arising from quantum interference between two possible two-photon scattering amplitudes, which one refers to as coherent and incoherent. This picture is in stark contrast to the aforementioned one, in that it assumes that the atom has two different mechanisms at its disposal to scatter two photons at the same time. Here we experimentally validate the interference picture by showing that, when spectrally rejecting only the coherent component of the fluorescence light of a single two-level atom, the remaining light consists of photon pairs that have been simultaneously scattered by the atom. Our results offer fundamental insights into the quantum-mechanical interaction between light and matter and open up novel approaches for the generation of highly non-classical light fields enabling, for example, Fourier-limited photon-pair sources that approach the theoretical limit in brightness.