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Twin Beams for Sensitivity Enhancement in Time-Domain Spectroscopy

Dionysis Adamou, Lennart Hirsch, Taylor Shields, Seungjin Yoon, Adetunmise C. Dada, Jonathan M R Weaver, Daniele Faccio, Marco Peccianti, Lucia Caspani, Matteo Clerici

Published Jan 1, 2025• 0 citations
OpenAlex

Abstract

We show that quantum correlations in ultrafast twin beams suppress noise and double the sensitivity of time-resolved THz field detection. Our results highlight the potential of quantum resources to enhance time-domain spectroscopy via electro-optic sampling.

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The time-resolved detection of mid- to far-infrared electric fields absorbed and emitted by molecules is among the most sensitive spectroscopic approaches and has the potential to transform sensing in fields such as security screening, quality control, and medical diagnostics. However, the sensitivity of the standard detection approach, which relies on encoding the far-infrared electric field into amplitude modulation of a visible or near-infrared probe laser pulse, is limited by the shot noise of the latter. This constraint cannot be overcome without using a quantum resource. Here, we show that this constraint can be overcome using a two-mode squeezed state. Quantum-correlated ultrashort pulses, generated by parametric down-conversion, enhance the sensitivity of far-infrared detection beyond the classical limit, achieving a twofold reduction in measured noise. This advancement paves the way for further development of ultrafast quantum metrology, moving toward quantum-enhanced time-resolved electric field spectroscopy with sensitivities beyond the standard quantum limit.
Time-domain spectroscopy (TDS) is among the most sensitive techniques used to study the properties of materials, structures and biological samples [1]. It is usually implemented using electro-optical sampling in X(2) crystals, where the polarization state of a short probe pulse is modified by the local electric field of the radiation to be measured. Such a polarization change is measured by a balanced detector as the delay between the probe pulse and the radiation under measurement varies, allowing the electric field of the latter to be sampled in the time domain.
Employing electro-optic sampling (EOS) with ultrashort probe pulses, recent experiments showed direct measurements of quantum vacuum fields and their correlations on subcycle timescales. Here, we propose a quantum-enhanced EOS where bright photon-number entangled twin beams are used to derive conditioned nonclassical probes. In the case of the quantum vacuum, this leads to a sixfold improvement in the signal-to-noise ratio over the classically probed EOS. In addition, engineering of the conditioning protocol yields a reliable way to extract higher-order moments of the quantum noise distribution and robust discrimination of the input quantum states, for instance, a vacuum and a few-photon cat state. These improvements open a viable route toward robust tomography of quantum fields in space-time, an equivalent of homodyne detection in energy-momentum space, and the possibility of precise experiments in real-space quantum electrodynamics.