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Conference on Lasers and Electro-Optics

Enhanced Two-photon Absorption Fluorescence of Fluorescein Biomarkers Using Squeezed Light Excitation

Tian Li, Fu Li, Charles Altuzarra, Anton Classen, G. S. Agarwal

Published Jan 1, 2020• 1 citation
Publisher

Abstract

We demonstrate that by using squeezed light for two-photon absorption (TPA) of fluorescein biomarkers, a fluorescence enhancement of nearly 50 is achieved as compared to conventional TPA using coherent light.
DOI
10.1364/cleo_at.2020.jth3n.5

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Two-photon absorption (TPA) is of fundamental importance in super-resolution imaging and spectroscopy. Its nonlinear character allows for the prospect of using quantum resources, such as entanglement, to improve measurement precision or to gain new information on, e.g., ultrafast molecular dynamics. Here, we establish the metrological properties of nonclassical squeezed light sources for precision measurements of TPA cross sections. We find that the Cram\'er-Rao bound does not provide a fundamental limit for the precision achievable with squeezed states in the limit of very small cross sections. Considering the most relevant measurement strategies---namely, photon-counting and quadrature measurements---we determine the quantum advantage provided by squeezed states as compared to coherent states. We find that squeezed states outperform the precision achievable by coherent states when performing quadrature measurements, which provide improved scaling of the Fisher information with respect to the mean photon number $\ensuremath{\sim}{n}^{4}$. Due to the interplay of the incoherent nature and the nonlinearity of the TPA process, unusual scaling can also be obtained with coherent states, which feature an $\ensuremath{\sim}{n}^{3}$ scaling in both quadrature and photon-counting measurements.
Abstract Recent theoretical and experimental advances have shown that squeezed states of light can be engineered to enhance the resolution of optical measurements. Extending such quantum-enhanced techniques to nonlinear optical processes has therefore attracted considerable interest. In particular, two-photon absorption (TPA), which has wide-ranging applications in physics, chemistry, and biology, has emerged as a promising platform for exploiting these quantum advantages. In this work, we investigate whether and how correlations between light modes can be used to improve optical resolution in TPA. We analyze non-degenerate TPA transmission measurements using two-mode squeezed light and compare different measurement strategies. We find that intensity correlation measurements offer better performance in terms of the achievable estimation error compared to normalized intensity correlations or measurements of the noise reduction factor (NRF). The achievable enhancement from intensity and NRF measurements are degraded by competing photon losses. In contrast, the normalized intensity correlation approach remains robust to loss, albeit at the cost of forfeiting the scaling enhancement at large photon numbers achievable with nonclassical light fields. Thus, we establish a trade-off between robustness to loss and the achievable quantum advantage. We further compute the corresponding Cramér–Rao bound, which sets the ultimate lower limit on the achievable estimation error for the photon-counting-based measurement strategies investigated in this work.