Squeezing and subnatural linewidth in the Mollow spectrum

Abstract

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.