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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Quantum-Enhanced Time-Domain Sampling of THz Fields

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

Published Jun 23, 2025• 0 citations

Abstract

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.

DOI: 10.1109/cleo/europe-eqec65582.2025.11109148

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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.

Electro-optical sampling of Terahertz fields with ultrashort pulsed probes is a well-established approach for directly measuring the electric field of THz radiation. This technique usually relies on balanced detection to record the optical phase shift brought by THz-induced birefringence. The sensitivity of electro-optical sampling is, therefore, limited by the shot noise of the probe pulse, and improvements could be achieved using quantum metrology approaches using, e.g., NOON states for Heisenberg-limited phase estimation. We report on our experiments on THz electro-optical sampling using single-photon detectors and a weak squeezed vacuum field as the optical probe. Our approach achieves field sensitivity limited by the probe state statistical properties using phase-locked single-photon detectors and paves the way for further studies targeting quantum-enhanced THz sensing.

Following recent progress in the experimental application of electro-optic sampling to the detection of the quantum fluctuations of the electromagnetic-field ground state and ultrabroadband squeezed states on a subcycle scale, we propose an approach to elevate broadband electro-optic sampling from a spectroscopic method to a full quantum tomography scheme, able to reconstruct a free-space quantum state directly in the time domain. By combining two recently developed methods to theoretically describe quantum electro-optic sampling, we analytically relate the photon-count probability distribution of the electro-optic signal to a transformed phase-space quasiprobability distribution of the sampled quantum state as a function of the time delay between the sampled midinfrared pulsed state and an ultrabroadband near-infrared probe pulse. We catalog and analyze sources of noise and show that in quantum electro-optic sampling with an ultrabroadband probe pulse one can expect to observe thermalization due to entanglement breaking. Mitigation of the thermalization noise enables a tomographic reconstruction of broadband quantum states while granting access to its dynamics on a subcycle scale. Published by the American Physical Society 2024

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.