Nonlinear spectroscopy and microscopy techniques are ubiquitous in a wide range of applications across physics biology. However, these usually rely on high-powered pulsed laser systems. A promising alternative is to exploit entangled two-photon absorption (ETPA), which can lead tens orders magnitude lower incident fluxes than conventional schemes. the role different degrees freedom ETPA was unc...

Absorption lines from molecular hydrogen ($\rm H_2$) in the spectra of background sources are a powerful probe physical conditions intervening cold neutral medium. At high redshift, $z>2$, $\rm H_2$ conveniently shifted optical domain, allowing use ground-based telescopes to perform high-resolution spectroscopy, which is essential for proper analysis gas. We describe recent observational progre...