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Terahertz Quantum Sensing

Kutas, M., Haase, B., Bickert, P., Riexinger, F., Molter, D., Freymann, G. von

Published Jan 1, 2019• 51 citations
OpenAlex

Abstract

Quantum sensing is highly attractive for accessing spectral regions in which the detection of photons is technically challenging: sample information is gained in the spectral region of interest and transferred via biphoton correlations into another spectral range, for which highly sensitive detectors are available. This is especially beneficial for terahertz radiation, where no semiconductor detectors are available and coherent detection schemes or cryogenically cooled bolometers have to be employed. Here, we report on the first demonstration of quantum sensing in the terahertz frequency range in which the terahertz photons interact with a sample in free space and information about the sample thickness is obtained by the detection of visible photons. As a first demonstration, we show layer thickness measurements with terahertz photons based on biphoton interference. As non-destructive layer thickness measurements are of high industrial relevance, our experiments might be seen as a first step towards industrial quantum-sensing applications.

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Quantum sensing is highly attractive for accessing spectral regions in which the detection of photons is technically challenging: sample information is gained in the spectral region of interest and transferred via biphoton correlations into another spectral range, for which highly sensitive detectors are available. This is especially beneficial for terahertz radiation, where no semiconductor detectors are available and coherent detection schemes or cryogenically cooled bolometers have to be employed. Here, we report on the first demonstration of quantum sensing in the terahertz frequency range in which the terahertz photons interact with a sample in free space and information about the sample thickness is obtained by the detection of visible photons. As a first demonstration, we show layer thickness measurements with terahertz photons based on biphoton interference. As non-destructive layer thickness measurements are of high industrial relevance, our experiments might be seen as a first step towards industrial quantum-sensing applications.
Quantum sensing is highly attractive for accessing spectral regions in which the detection of photons is technically challenging: Sample information is gained in the spectral region of interest and transferred via biphoton correlations into another spectral range, for which highly sensitive detectors are available. This is especially beneficial for terahertz radiation, where no semiconductor detectors are available and coherent detection schemes or cryogenically cooled bolometers have to be used. Here, we report on the first demonstration of quantum sensing in the terahertz frequency range in which the terahertz photons interact with a sample in free space and information about the sample thickness is obtained by the detection of visible photons. As a first demonstration, we show layer thickness measurements with terahertz photons based on biphoton interference. As nondestructive layer thickness measurements are of high industrial relevance, our experiments might be seen as a first step toward industrial quantum sensing applications.
In quantum sensing experiments, photons in the spectral region of interest interact with a sample and their properties are transferred via biphoton-correlation to another spectral region, where widely available and highly developed detectors can be used. Especially spectral regions where the photon detection is quite difficult can benefit from this novel technique. We report on the first demonstration of quantum sensing in the terahertz frequency range, in which the terahertz photons interact with a sample in free space determining the thickness of polytetrafluoroethylene plates by only detecting visible photons [1].
Although terahertz radiation has moved into the focus of technical applications, e.g. layer-thickness measurement, the generation and detection of radiation in this spectral range remains challenging as detectors are either cooled or expensive due to the laser systems they are based on. Thus, the generation and detection of terahertz radiation continues to be of great scientific interest. A novel quantum-optical concept offers the possibility to detect only visible light containing the measurement information gained in a different wavelength range [1]. This represents an extremely attractive alternative for the terahertz frequency range, since generation can be done with visible continuous-wave lasers while detection can be achieved with sophisticated and low-cost silicon-based detectors. We report on first demonstrations of this measurement technique in the terahertz frequency range.
Modern terahertz detectors enable direct measurement of the electric field, providing simultaneous access to amplitude and phase information of the terahertz radiation. However, conventional systems are often based on single-pixel detectors, limiting most imaging applications to slow raster-scanning techniques. This work presents the demonstration of a novel approach for amplitude- and phase-sensitive imaging in the terahertz range using quantum imaging with undetected photons. By decoupling the sample measurement in the terahertz range from the detection of the information in the visible, this technique enables the use of cost-effective, highly developed multi-pixel optical components. Central achievements are the observation of quantum interference with terahertz photons and its application to material characterization and molecular concentration analysis. Furthermore, image information was transferred from the terahertz to the visible spectral range, achieving a spatial resolution close to the terahertz wavelength. This approach thus provides direct access to amplitude, phase, and spectral information for imaging applications in the terahertz range using a multi-pixel camera.
Generation and Detection of terahertz radiation still remain a technological challenge, especially when compared to the visible or near infrared spectral range. For this reason, approaches are continuously pursued to transfer the terahertz radiation to other frequency ranges after interaction with the sample in order to utilize the advantages of the advanced detector technology. Novel quantum-optical concepts offer a further possibility to realize this transfer. This makes them an attractive alternative for the terahertz frequency range, as they only require cost-effective components such as visible continuous wave lasers and silicon-based detectors. We report on the first demonstration of imaging with undetected photons in the terahertz frequency range.
Using nonlinear interferometers is highly attractive for accessing spectral regions in which the photon detection is technically challenging. Especially for terahertz radiation this is a long-lasting problem, as their photon energy is in the range of a few meV. Therefore, the properties of terahertz photons after interaction with a sample are transferred to visible photons, whose detection can be realized by widely available imaging sensors. We demonstrated this concept with terahertz photons propagating in free space, determining the thickness of polytetrafluoroethylene pla tes by only detecting visible photons. We utilized a nonlinear interferometer with one periodically poled lithium niobate crystal driven by a 660-nm pump source. In the crystal, pairs of visible signal and correlated terahertz photons are created and separated into different paths of the interferometer afterwards. The terahertz photons pass the sample and gain information that can be transferred to the visible photons. To detect the signal photons with an uncooled sCMOS camera, the pump photons are filtered out by narrowband volume Bragg gratings. As the signal photons are mainly generated by down - and up-conversion of thermal photons besidesspontaneous parametric down-conversion, the frequency-angular spectra show nonlinear interference in the Stokes and anti-Stokes regions. This interference can be used to assess information of coatings that are mainly transparent in the terahertz frequency range. Establishing on this first demonstration of a nonlinear interferomete r with terahertz photons, we improved the visibility by a factor of 3 by modifying the experimental setup, helping this concept on its way towards industrial applications.