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PhotonicsViews

Quantum spectroscopy using entangled photons

Wojciech S. Góra

Published Jan 13, 2025• 0 citations
Publisher

Abstract

Abstract The field of quantum optics is undergoing massive growth, which is enabling a host of new applications within the field. The quantum nature of light allows us to push the limits of various techniques including, but not limited to, spectroscopy, interferometry, sensing and imaging. At the heart of these is a need for a reliable source of entangled photons. The quantum revolution is underpinned by ultrashort pulse duration lasers, and especially fiber lasers. Ultrafast fiber lasers provide reliable ultrashort pulse durations at an unprecedented price point.

DOI
10.1002/phvs.202500005

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Abstract The field of quantum optics is undergoing massive growth, which is enabling a host of new applications within the field. The quantum nature of light allows us to push the limits of various techniques including, but not limited to, spectroscopy, interferometry, sensing and imaging. At the heart of these is a need for a reliable source of entangled photons. The quantum revolution is underpinned by ultrashort pulse duration lasers, and especially fiber lasers. Ultrafast fiber lasers provide reliable ultrashort pulse durations at an unprecedented price point.
Conspectus The enhanced interest in quantum-related phenomena has provided new opportunities for chemists to push the limits of detection and analysis of chemical processes. As some have called this the second quantum revolution, a time has come to apply the rules learned from previous research in quantum phenomena toward new methods and technologies important to chemists. While there has been great interest recently in quantum information science (QIS), the quest to understand how nonclassical states of light interact with matter has been ongoing for more than two decades. Our entry into this field started around this time with the use of materials to produce nonclassical states of light. Here, the process of multiphoton absorption led to photon-number squeezed states of light, where the photon statistics are sub-Poissonian. In addition to the great interest in generating squeezed states of light, there was also interest in the formation of entangled states of light. While much of the effort is still in foundational physics, there are numerous new avenues as to how quantum entanglement can be applied to spectroscopy, imaging, and sensing. These opportunities could have a large impact on the chemical community for a broad spectrum of applications. In this Account, we discuss the use of entangled (or quantum) light for spectroscopy as well as applications in microscopy and interferometry. The potential benefits of the use of quantum light are discussed in detail. From the first experiments in porphyrin dendrimer systems by Dr. Dong-Ik Lee in our group to the measurements of the entangled two photon absorption cross sections of biological systems such as flavoproteins, the usefulness of entangled light for spectroscopy has been illustrated. These early measurements led the way to more advanced measurements of the unique characteristics of both entangled light and the entangled photon absorption cross-section, which provides new control knobs for manipulating excited states in molecules. The first reports of fluorescence-induced entangled processes were in organic chromophores where the entangled photon cross-section was measured. These results would later have widespread impact in applications such as entangled two-photon microscopy. From our design, construction and implementation of a quantum entangled photon excited microscope, important imaging capabilities were achieved at an unprecedented low excitation intensity of 10 7 photons/s, which is 6 orders of magnitude lower than the excitation level for the classical two-photon image. New reports have also illustrated an advantage of nonclassical light in Raman imaging as well. From a standpoint of more precise measurements, the use of entangled photons in quantum interferometry may offer new opportunities for chemistry research. Experiments that combine molecular spectroscopy and quantum interferometry, by utilizing the correlations of entangled photons in a Hong–Ou–Mandel (HOM) interferometer, have been carried out. The initial experiment showed that the HOM signal is sensitive to the presence of a resonant organic sample placed in one arm of the interferometer. In addition, parameters such as the dephasing time have been obtained with the opportunity for even more advanced phenomenology in the future.
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Ultrafast laser-based micromanufacturing is essential for fabricating high-precision quantum devices such as quantum gates, photonic circuits, and waveguides. Besides the area of micromachining, recent applications of laser beams also include quantum imaging and sensing. The laser beams have been characterized for their wavelength and pulse durations. The techniques include homemade setups of intensity autocorrelation, frequency-resolved optical gating (FROG), and spectral phase interferometry for direct electric-field reconstruction (SPIDER) besides the commercial spectrographs. Beams obtained from non-collinear optical parametric amplifier (NOPA) capable of delivering tunable ultrafast pulses are also described. The dark core beams through Hermite Gaussian (HG) modes and thermal effects are also generated. All these beams are capable of generating polarization-entangled photons by the nonlinear-optical phenomenon of spontaneous parametric down-conversion (SPDC). After describing the construction of the entangled photon source (EPS), an interferometric setup for imaging based on extracting the phase and intensity of an object is discussed. The application areas of the EPS for quantum imaging, multi-photon applications and adaptive imaging are also highlighted. We propose that quantum imaging can overcome the existing challenges of real-time feedback during micromanufacturing and help prevent the wastage of resources.