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Europhysics Letters (EPL)

Spectral Correlations within the Fluorescence Triplet

Gerard Nienhuis

Published Jan 20, 1993• 10 citations
Publisher

Abstract

Spectral correlations of a light source are measured by setting the frequencies of two narrow filters at two frequencies within the spectrum of the source, and detecting the correlations between the two outputs. In the case of the fluorescence triplet of a two-level atom, correlations arise when the two filter frequencies are equal, and also when they are positioned symmetrically at opposite sides of the central driving frequency.

DOI
10.1209/0295-5075/21/3/006

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Spectral correlations of a light source are measured by setting the frequencies of two narrow filters at two frequencies within the spectrum of the source, and detecting the correlations between the two outputs. In the case of the fluorescence triplet of a two-level atom, correlations arise when the two filter frequencies are equal, and also when they are positioned symmetrically at opposite sides of the central driving frequency.
We study the photon correlations between the output of two spectral filters set within the fluorescence triplet of a two-state atom. The time uncertainty arising from the spectral resolution of the filters implies a possible interference between opposite orders of emission, contributing to the same detection order. Furthermore, the fluorescent emission is a quantum-mechanical process, and successive emissions in different components do not commute. The correlation functions are affected both by the memory time of the filters and by the noncommutativity of successive emissions. When the two filter bandwidths are larger than the widths of the components, this only modifies the short-time behavior of the correlation functions between two photons from different spectral components. For narrow filters, the entire correlation function is dominated by memory-time effects. Positive (bunching) correlations arise when the two filters are set at the same frequency, and also when they are positioned symmetrically at opposite sides of the driving frequency.
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.
Frequency-filtered photon correlations have been proven to be extremely useful in grasping how the detection process alters photon statistics. Harnessing the spectral correlations also permits refinement of the emission and unraveling of previously hidden strong correlations in a plethora of quantum-optical systems under continuous-wave excitation. In this work, we investigate such correlations for time-dependent excitation and develop a methodology to compute efficiently time-integrated correlations, which are at the heart of the photon-counting theory, and subsequently apply it to analyze the photon emission of pulsed systems. By combining this formalism with the —which facilitates frequency-resolved correlations—we demonstrate how spectral filtering enhances single-photon purity and suppresses multiphoton noise in time-bin-encoded quantum states. Specifically, filtering the central spectral peak of a dynamically driven two-level system boosts temporal coherence and improves the fidelity of time-bin entanglement preparation, even under conditions favoring multiphoton emission. These results establish spectral filtering as a critical tool for tailoring photon statistics in pulsed quantum light sources.
We study the two-time correlation function of spectrally filtered light from a quantum mechanical light source. The bunching and anti-bunching properties of the photo-statistics of a field display the quantum nature of the source. We study whether these properties are affected by the presence of filters that are modelled by dielectric slabs. We quantize the radiation field in the presence of the filters and express the correlation function in terms of source operators. As a quantum source we take the resonance fluorescence of a driven two-level atom. We find that the spectral correlations depend on the linewidth of the filters. When the pass time of the filter is much longer than the time scale on which the source evolves, the presence of the filter can conceal the quantum nature of the source. However, quantum mechanical correlations between opposite side bands survive even for very narrow filters.
We consider a special kind of correlation measurements in single-atom resonance fluorescence, which includes detection of a spectrally filtered photon followed by detection of an unfiltered photon. We relate the stationary normalized intensity correlation function measured in such a scheme to the reduced atomic state following detection of the filtered photon. In the limit of well separated spectral lines of the resonance fluorescence spectrum, we obtain the analytic expression for the reduced atomic state. We show that at resonance excitation, this state is inverted when the filter is tuned in between the central line and any of the sidebands. This effect is interpreted as a result of quantum interference between the complementary atomic radiative transitions giving rise to the detected photon. We propose to observe this effect in measuring the equal-time field correlation function of the filtered and unfiltered photons as a function of the filter resonance frequency.
A theoretical investigation of spectrally filtered broadband light fields is undertaken. More specifically, the balanced homodyne correlation measurement scheme is extended using spectral filters. The technique provides a way to spectrally resolve correlation functions, making it possible to measure the squeezing spectrum. Two-level resonance fluorescence is included as a concrete example of this, providing some predictions concerning the parameters of the setup for potential experiments. Furthermore, spectral filtering is compared to electronically filtering the current generated in the photodetectors. This is of interest since filtering the current is a classical operation, whereas having optical filters in the path of the radiation is a quantum one.