Abstract-Terahertz quantum sensing

 Björn Haase, Daniel Molter, Felix Riexinger, Mirco Kutas, Patricia Bickert,

https://advances.sciencemag.org/content/6/11/eaaz8065

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.

Abstract-Terahertz Quantum Sensing

Mirco Kutas, Björn Haase, Patricia Bickert, Felix Riexinger, Daniel Molter, Georg von Freymann

https://arxiv.org/abs/1909.06855

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 entanglement into another spectral range, for which highly sensitive detectors are available. This is especially beneficial for terahertz radiation, as the corresponding photon energy lies in the range of a few meV - an energy 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. A nonlinear single-crystal interferometer setup with a periodically poled lithium niobate crystal (PPLN) and a 660 nm pump source is used, generating visible (signal) photons and associated (idler) photons in the terahertz frequency range. Separation from the pump photons and detection of the visible signal photons is achieved by using highly efficient and narrowband volume Bragg gratings and an uncooled scientific complementary metal-oxide-semiconductor (sCMOS) camera. The acquired frequency-angular spectra show quantum interference in the Stokes as well as the Anti-Stokes part of collinear forward generation caused by spontaneous parametric down-conversion (SPDC) and down-conversion as well as up-conversion of thermal photons. The information encoded in the quantum interference can be used to determine the thickness of coatings or functional layers that are mainly transparent in the terahertz spectral range. As a first demonstration, we show layer thickness measurements with terahertz photons based on induced coherence without induced emission.

Abstract-Spontaneous parametric down-conversion of photons at 660 nm to the terahertz and sub-terahertz frequency range

Björn Haase, Mirco Kutas, Felix Riexinger, Patricia Bickert, Andreas Keil, Daniel Molter, Michael Bortz, and Georg von Freymannark

Fig. 1 Phase-matching schemes. The periodic poling of the nonlinear crystal leads to a k-vector component kΛ, which can have two directions. This yields phase-matched terahertz generation in forward and backward direction. Pump (p), signal (s) and idler (THz) wave vectors have to be considered as non-collinear in down- (sd) and up-conversion (su).

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-5-7458

We report on spontaneous parametric down-conversion (SPDC) in periodically poled lithium niobate (PPLN) using 660 nm pump wavelength and the type 0 phase-matching condition to the terahertz and even sub-terahertz frequency range. Detection of the frequency-shifted signal photons is achieved by using highly efficient and narrowband volume Bragg gratings and an uncooled sCMOS camera. The acquired frequency-angular spectrum shows backward and forward generation of terahertz and sub-terahertz photons by SPDC, as well as up-conversion and higher order quasi phase-matching (QPM). The frequency-angular spectrum is theoretically calculated using a Monte-Carlo integration scheme showing a high agreement with the measurement. This work is one important step toward quantum sensing and imaging in the terahertz and sub-terahertz frequency range.

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