The incoherent part of the Mollow spectrum for resonance fluorescence has a subnatural linewidth in the weak-field limit. We show that this is due to squeezing of the fluctuations in the induced atomic dipole. The reduced linewidth persists for driving field intensities of approximately 12% of the saturation intensity, where Rabi sidebands begin to appear. We find a similar linewidth narrowing in the transmitted and fluorescent spectra for a single atom in a weakly driven resonant cavity. In this system single-quantum frequency splitting can produce a two-peaked spectrum. Both peaks have a narrowed linewidth. The spectrum of the transmitted light shows a second nonclassical effect that is due to squeezing. A spectral hole may appear at line center, giving a two-peaked spectrum even when there is no frequency splitting.
The incoherent part of the Mollow spectrum for resonance fluorescence has a subnatural linewidth in the weak-field limit. We show that this is due to squeezing of the fluctuations in the induced atomic dipole. The reduced linewidth persists for driving field intensities of approximately 12% of the saturation intensity, where Rabi sidebands begin to appear. We find a similar linewidth narrowing in the transmitted and fluorescent spectra for a single atom in a weakly driven resonant cavity. In this system single-quantum frequency splitting can produce a two-peaked spectrum. Both peaks have a narrowed linewidth. The spectrum of the transmitted light shows a second nonclassical effect that is due to squeezing. A spectral hole may appear at line center, giving a two-peaked spectrum even when there is no frequency splitting.
We calculate the spontaneous-emission spectrum and the spectrum of weakly driven fluorescence for a two-level atom coupled to a resonant-cavity mode. For strong atom-cavity coupling the spectra split into two peaks that can have subnatural linewidths. If the cavity linewidth is negligible, the spontaneous-emission spectrum has half the radiative linewidth of the atom; the spectrum of weakly driven fluorescence shows an additional 36% squeezing-induced narrowing. These effects can be observed using coupled-field and collective-polarization oscillators excited in a cavity containing N two-level atoms.
We consider the interaction of a two-level atom inside an optical parametric oscillator. In the weak-driving-field limit, we essentially have an atom-cavity system driven by the occasional pair of correlated photons, or weakly squeezed light. We find that we may have holes, or dips, in the spectrum of the fluorescent and transmitted light. This occurs even in the strong-coupling limit when we find holes in the vacuum-Rabi doublet. Also, spectra with a subnatural linewidth may occur. These effects disappear for larger driving fields, unlike the spectral narrowing obtained in resonance fluorescence in a squeezed vacuum; here it is important that the squeezing parameter N tends to zero so that the system interacts with only one correlated pair of photons at a time. We show that a previous explanation for spectral narrowing and spectral holes for incoherent scattering is not applicable in the present case, and propose an alternative explanation. We attribute these anomalous effects to quantum interference in the two-photon scattering of the system.
In his original work on the spectrum of resonance fluorescence, Mollow noted that, at weak excitation, one of the three Lorentzian components comprising the incoherent spectrum has negative weight.1 He commented that, as a consequence, for |ω − ω 0 | → ∞ the spectrum falls to zero as (ω − ω0)−4 rather than (ω − ω0)−2. In fact, the incoherent spectrum for weak excitation is a Lorentzian squared rather than a Lorentzian. The half-width is ( 2 − 1 ) 1 / 2 ( γ / 2 ) ≈ 0.64 ( γ / 2 ) , showing significant narrowing over the natural width γ/2. Although this result has been rederived a number of times, nothing appears to have been said about its physical significance. Recently it has been recognized that the fluorescence from a two-level atom exhibits squeezing.2 For weak excitation, photon antibunching in resonance fluorescence results from the self-homodyning of incoherent fluorescence from squeezed polarization fluctuations with coherent Rayleigh scattering. We show that the noted subnatural linewidth is also a manifestation of squeezing. A relationship exists between the incoherent optical spectrum and the spectrum of squeezing. The optical spectrum is formed from the sum of squeezing spectra for field quadratures in phase and out of phase with the Rayleigh scattering. The reduced linewidth results because inphase fluctuations are squeezed and contribute to the incoherent spectrum with negative weight.
The fluorescence spectrum of a strongly driven two-level atom located inside an optical cavity damped by a narrow-bandwidth squeezed vacuum is studied. We use a dressed atom model approach, first applied to squeezed vacuum problems by Yeoman and Barnett, to derive the master equation of the system and discuss the role of the cavity and the squeezed vacuum in the narrowing of the spectral lines and the population trapping effect. We find that in the presence of a single-mode cavity the effect of squeezing on the fluorescence spectrum is more evident in the linewidths of the Rabi sidebands rather than in the linewidth of the central component. Even in the absence of squeezing, the cavity can reduce the linewidth of the central component almost to zero, whereas the Rabi sidebands can be narrowed only to some finite value. In the presence of a two-mode cavity and a two-mode squeezed vacuum the signature of squeezing is evident in the linewidths of all spectral lines. We also establish that the narrowing of the spectral lines is very sensitive to the detuning of the driving field from the atomic resonance. Moreover, we find that the population trapping effect, predicted for the broadband squeezed vacuum case, may appear in a narrow-bandwidth case only if the input squeezed modes are perfectly matched to the cavity modes and if there is non-zero squeezing at the Rabi sidebands.
Summary form only given. We consider a single two-level atom inside a resonant optical cavity, with cavity loss rate K, atomic fluorescence rate into noncavity modes A, and atom-field coupling g. For large atom cavity coupling (g>A, K) and weak driving fields, the fluorescence from this system exhibits a splitting of the spectral line, known as vacuum-Rabi splitting. When the cavity loss rate is the largest rate in the problem (K>A, g), the fluorescent spectrum is Lorentzian with a cavity-enhanced width. We examine the regime where A>g>K, and find that the spectrum exhibits a hole at line center as a result of quantum interference effects. This effect is formally identical to the phenomenon of electromagnetically induced transparency (EIT) in multilevel atomic systems. We discuss the relationship of these spectral holes to the presence of squeezing in the intracavity field, and present results also for the emission spectra, and incoherent spectra in transmission and fluorescence.
We consider a two-level atom driven by an ideal degenerate optical parametric oscillator. For squeezing bandwidths of the order of the natural width $\ensuremath{\gamma},$ and smaller, we report remarkable spectral profiles for the scattered light. We also consider a system where the atom is driven by arbitrarily correlated light of arbitrary bandwidth. Employing analytic and numerical solutions, we find different spectral features depending on the classical or nonclassical nature of the driving field. The spectral linewidth can be both subnatural and narrower than the incident field linewidth, an effect that does not require nonclassical light. However, nonclassical light is essential for the appearance of a narrow hole at line center. It is a unique signature of the quantum nature of the field, and for ideal squeezing, the fluorescence at line center vanishes.