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Physical Review Letters

Observation of linewidth narrowing due to coherent stabilization of quantum fluctuations

Daniel J. Gauthier, Yifu Zhu, THOMAS W. MOSSBERG

Published May 13, 1991• 96 citations
Publisher

Abstract

Quantum fluctuations of an optical transition moment are observed to be suppressed by strong coherent excitation of a weak auxiliary transition. The stabilization is manifest through the appearance of subnatural linewidths in the resonance fluorescence spectrum and is due to coherent mixing of atomic states. Our results are in quantitative agreement with the predictions of L. M. Narducci et al. [Phys. Rev. A 42, 1630 (1990)].

DOI
10.1103/physrevlett.66.2460

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Observation of subnatural fluorescence linewidths due to coherent stabilization of quantum fluctuations

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Recently, Narducci et al.1 studied V-configuration three-level atoms and predicted that quantum fluctuations associated with a "strong" optical transition driven by a coherent field can be stabilized through excitation of a coupled "weak" transition. This stabilization is manifest in a narrowing, to subnatural values, of the fluorescence linewidths associated with the strong transition as the weak transition is driven more and more strongly. We have experimentally investigated this problem and provide the first demonstration of the effect. In the experiment, a beam of Ba atoms was intercepted by two copropagating cw laser beams. One of the beams, of Rabi frequency Ω1, was tuned into resonance with the strong, 553.5 nm, 1S0→3P1 transition, while the other beam, of Rabi frequency Ω2, was tuned into resonance with the weak, 791.1 nm, 1S0→3P1 transition. The fluorescence spectrum of the strong transition was recorded for the case when Ω1≈35 MHz and Q2≈45 MHz and, it was found that the spectrum was three-peaked. Most interestingly, the measured linewidth (FWHM) for the central peak was 16 MHz, or 2 MHz less than the 18 MHz natural width of the strong transition even before the substantial instrumental broadening (≈8 MHz) of the peak was deconvolved.

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We study how the spectral properties of resonance fluorescence propagate through a two-atom system. Within the weak-driving-field approximation we find that, as we go from one atom to the next, the power spectrum exhibits both subnatural linewidth narrowing and large asymmetries while the noise spectrum of the squeezed quadrature narrows but remains otherwise unchanged. Analytical results for the observed spectral features of the fluorescence are provided and their origin is thoroughly discussed.

arXiv:physics/9902020•Physical Review A•May 16, 2000•3 citations
Quantum fluctuations of an optical transition moment are observed to be suppressed by strong coherent excitation of a weak auxiliary transition. The stabilization is manifest through the appearance of subnatural linewidths in the resonance fluorescence spectrum and is due to coherent mixing of atomic states. Our results are in quantitative agreement with the predictions of L. M. Narducci et al. [Phys. Rev. A 42, 1630 (1990)].
Recently, Narducci et al.1 studied V-configuration three-level atoms and predicted that quantum fluctuations associated with a "strong" optical transition driven by a coherent field can be stabilized through excitation of a coupled "weak" transition. This stabilization is manifest in a narrowing, to subnatural values, of the fluorescence linewidths associated with the strong transition as the weak transition is driven more and more strongly. We have experimentally investigated this problem and provide the first demonstration of the effect. In the experiment, a beam of Ba atoms was intercepted by two copropagating cw laser beams. One of the beams, of Rabi frequency Ω1, was tuned into resonance with the strong, 553.5 nm, 1S0→3P1 transition, while the other beam, of Rabi frequency Ω2, was tuned into resonance with the weak, 791.1 nm, 1S0→3P1 transition. The fluorescence spectrum of the strong transition was recorded for the case when Ω1≈35 MHz and Q2≈45 MHz and, it was found that the spectrum was three-peaked. Most interestingly, the measured linewidth (FWHM) for the central peak was 16 MHz, or 2 MHz less than the 18 MHz natural width of the strong transition even before the substantial instrumental broadening (≈8 MHz) of the peak was deconvolved.
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 effect of the laser linewidth on the resonance fluorescence spectrum of a two-level atom is revisited. The novel spectral features, such as hole-burning and dispersive profiles at line centre of the fluorescence spectrum are predicted when the laser linewidth is much greater than its intensity. The unique features result from quantum interference between different dressed-state transition channels. c # 1999 Elsevier Science B.V. PACS : 42.50.Hz; 32.80.-t Keywords: Resonance fluorescence; Laser linewidth; Quantum interference The study of resonance fluorescence spectra has provided much fundamental insight into the subject of atom1light interactions. It is well known that for weak laser field excitation, the spectrum exhibits a Lorentzian lineshape, whereas it develops into the Mollow triplet for strong field excitation [1]. The latter is a direct signature of stimulated emissions and absorptions of the atom during the time interval for one spontaneous decay. Recently, considera...
Resonance fluorescence has played a major role in quantum optics with predictions and later experimental confirmation of nonclassical features of its emitted light such as antibunching or squeezing. In the Rayleigh regime where most of the light originates from the scattering of photons with subnatural linewidth, antibunching would appear to coexist with sharp spectral lines. Here, we demonstrate that this simultaneous observation of subnatural linewidth and antibunching is not possible with simple resonant excitation. Using an epitaxial quantum dot for the two-level system, we independently confirm the single-photon character and subnatural linewidth by demonstrating antibunching in a Hanbury Brown and Twiss type setup and using high-resolution spectroscopy, respectively. However, when filtering the coherently scattered photons with filter bandwidths on the order of the homogeneous linewidth of the excited state of the two-level system, the antibunching dip vanishes in the correlation measurement. Our observation is explained by antibunching originating from photon-interferences between the coherent scattering and a weak incoherent signal in a skewed squeezed state. This prefigures schemes to achieve simultaneous subnatural linewidth and antibunched emission.
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.
We study how the spectral properties of resonance fluorescence propagate through a two-atom system. Within the weak-driving-field approximation we find that, as we go from one atom to the next, the power spectrum exhibits both subnatural linewidth narrowing and large asymmetries while the noise spectrum of the squeezed quadrature narrows but remains otherwise unchanged. Analytical results for the observed spectral features of the fluorescence are provided and their origin is thoroughly discussed.