@phdthesis{13263,
  author = {Kristen Parzuchowski},
  title = {Setting Experimental Bounds on Entangled Two-Photon Absorption Cross Sections},
  abstract = {<p>Two-photon absorption (2PA) is widely used in microscopy for deep, sub-cellular imaging.<br />
However, the efficiency of 2PA is limited by the properties of both the absorber and the excitation<br />
light. Entangled photon pairs produced via spontaneous parametric down-conversion (SPDC) exhibit<br />
correlations in energy, time and space that may improve the excitation efficiency relative to<br />
a classical laser. The most significant improvement is expected at low photon flux where isolated<br />
pairs interact with the absorber. In this regime, the rate of the entangled two-photon absorption<br />
(E2PA) process scales linearly with photon flux and the E2PA cross section (σ<sub>E</sub>).</p>

<p>Despite over a decade of publications claiming to measure huge σ<sub>E</sub> that suggest a quantum<br />
advantage exists of up to 10 orders of magnitude, in this thesis I will show strong evidence that<br />
σ<sub>E</sub> are several orders of magnitude lower than previously reported. First, we provide relevant<br />
background information on nonlinear and quantum optics. Next, we discuss theoretical descriptions<br />
of σ<em><sub>E</sub></em> and review the large body of experimental work in the field. Afterwards, we discuss the four<br />
experiments we designed to measure E2PA.</p>

<p>In the first and third experiments, we measure SPDC transmittance through samples of two-photon<br />
absorbers in room-temperature liquids. In our second experiment, we collect fluorescence<br />
from samples excited with SPDC. Despite the high sensitivity of the techniques, we could not<br />
resolve a signal in any of the measurements. We set upper bounds on the σ<em><sub>E</sub></em> of eight independent<br />
absorbers that are up to five orders of magnitude lower than previously published σ<em><sub>E</sub></em>.</p>

<p>The third experiment also served as a classical 2PA (C2PA) measurement system. We made<br />
one-to-one comparisons between E2PA and C2PA to bound the quantum advantage. We derived<br />
absolute C2PA cross sections that closely agreed with values already reported in literature. In the<br />
fourth experiment, we designed a toluene-filled hollow-core-fiber platform for 2PA measurements. We measured C2PA down to 20 nanowatts, and expect to make further improvements. This<br />
platform is at least 4-fold more sensitive than a standard cuvette-based technique and thus is ideal<br />
for the next generation of E2PA measurements.</p>
},
  year = {2023},
  journal = {Department of Physics},
  volume = {Ph.D.},
  pages = {171},
  month = {2023/03},
  publisher = {University of Colorado},
  address = {Boulder},
}