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Optical spectroscopy is a powerful technique that underpins fundamental research and applications alike.
It can be used to measure the chemical composition of a tested sample; to assess the safety of foods, air, or water; and to reveal the interaction of molecules in a complex living system (1–7).
With the continuous development of ultrashort laser sources, an alternative paradigm for spectroscopy has developed, whereby spectral signatures are obtained by the Fourier transform of the electric field of a probing laser pulse (8–12).
This time-resolved electric field spectroscopy (from here on, time-domain spectroscopy—TDS—for brevity) provides additional information unavailable to standard spectroscopic approaches, such as time-of-flight longitudinal localization of chemical species, a higher sensitivity to the interaction between the environment and the investigated molecule, and an unparalleled sensitivity in living samples (12). TDS was first pioneered in the terahertz spectral region (THz-TDS),
where femtosecond pulses are sufficiently short to properly sample and resolve the temporal oscillations of the electric field of a carrier-envelope–phase stable and broadband (single-cycle) pulse generated by the optical rectification (OR) of a short optical pump pulse. With the development of ultrafast sources, it became possible to extend the spectral domain of TDS to cover the whole infrared region, reaching even to visible wavelengths
The ability to sample such a broadband spectrum is key to the unparalleled specificity of TDS
Here, we show experimental results of a quantum-enhanced TDS. Using two-mode squeezed states, i.e., quantum-correlated fields generated by parametric down-conversion in a second-order nonlinear crystal (27), we were able to record a THz electric field with a noise of half the standard quantum limit.
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