”Ghost Spectroscopy“: No Ghosts, but real-world spectroscopy with classical correlated photons in the spirit of quantum sensing


Ghost Imaging (GI) is a quantum imaging modality which exploits photon correlation for the image construction, whereas one photon of a correlated pair interacts with the object to be imaged and the experimentally determined correlation with the second photon yields the image. Therefore it is called correlated two-photon imaging. The intensity autocorrelation or second order correlation is thus transferred into a spatial image of the object, the ghost image.

This technique has great potential to provide robust imaging solutions in the presence of severe environment perturbations, and has overtime been demonstrated both in spatial and temporal domains. Since it was first demonstrated, numerous ghost imaging developments have been reported regarding new configurations with improved correlation protocols, new light sources, new detection techniques, and the exploration of potential application areas. Concurrently, various ghost modalities have been developed which can be regarded as respective analogies.

In a recently published Physical Review Applied article the Semiconductor Optics Group at Technische Universität Darmstadt succeeded in transferring the concept of the classical spatial correlation into the spectral wavelength domain, thus realizing for the first time a “ghost spectroscopy“ experiment with classical thermal photons. The Technical University Darmstadt researchers experimentally showed by utilizing two-photon-absorption interferometry that spectrally broadband near-infrared amplified spontaneous emission light emitted by a quantum-dot superluminescent diode (SLD) exhibits classical wavelength-wavelength (λ-λ) correlations. In the spirit of the famous Hanbury-Brown & Twiss experiment, this light exhibits a spectral second order correlation coefficient of two indicating spectral photon bunching, one of the key requirements of “ghost spectroscopy“.

After the demonstration of the prerequisite of spectral wavelength-wavelength correlations, the researchers proved the applicability of the phenomenon in a real-world absorption spectroscopy experiment with liquid chloroform. They succeeded in reconstructing a ghost spectrum which clearly exhibits the characteristic absorption feature, the “finger prints” of the absorption band of trichloromethane (chloroform) at 1214 nm.

This first demonstration of ghost spectroscopy with classical thermal light in analogy to GI closes a gap in the experimental photon correlation modalities. Until now, no ghost experiment with classical light exploiting thermal sources had been realized. One of the reason for this lacking realization of GS with thermal light are the challenges having an extremely high time resolution for the measurements of intensity correlations of spectrally broad-band light emitted by SLDs. Only by exploiting interferometric Two-Photon Absorption (TPA) detection it has been possible to access the ultra-short correlation time scales in the 10 femtosecond range. The second challenge has been the difficulty to find a light source, emitting broad-band light and exhibiting the requested wavelength correlations to enable “ghost spectroscopy“, here the semiconductor-based superluminescent diode.

Prof. Wolfgang Elsäßer, and his colleagues of the Semiconductor Optics Group expect that the highlights of the realized innovative scheme in the spirit of the ingredients of ghost spectroscopy with classical light with particular emphasis on the conceived source and detection schemes will further stimulate new applications of ghost modalities. They are convinced that the depicted and exploited analogy between ghost imaging and ghost spectroscopy will further fertilize the field, thus allowing to develop an even deeper understanding of the experimental scheme and ghost protocols. This will open avenues for perspectives and dissemination of the ghost modality idea in entering further real-world applications in Chemistry, Physics and Engineering.

Advances in Engineering-spectroscopy with classical correlated photons in the spirit of quantum sensing


Patrick Janassek, Sébastien Blumenstein, and Wolfgang Elsäßer. Ghost Spectroscopy with Classical Thermal Light Emitted by a Super Luminescent Diode.  Physical Review Applied 9, 021001 (2018).

Go To Physical Review Applied

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