# A Liquid-Core Fiber Platform for Classical and Entangled Two-Photon Absorption Measurements

## TL;DR

A new fiber platform improves two-photon absorption measurements and tests for quantum advantages in photon interactions.

## Contribution

A liquid-core fiber platform enables low-power two-photon absorption measurements and first waveguide-based entangled two-photon absorption attempt.

## Key findings

- The platform achieves a 45-fold improvement in classical two-photon absorption measurements at low laser power.
- No evidence of entangled two-photon absorption was found, setting an upper bound on its cross-section.
- The platform allows measurements in a regime where quantum advantages might otherwise be unobservable.

## Abstract

We introduce a toluene-filled fiber platform for two-photon
absorption
measurements. By confining both the light and molecular sample inside
the 5 μm hollow core of the fiber, we increase the distance
over which the nonlinear light–matter interaction occurs. With
only a 7.3 nL excitation volume, we measure classical two-photon absorption
(C2PA) at an average laser power as low as 1.75 nW, which is a 45-fold
improvement over a conventional free-space technique. We use this
platform to attempt to measure entangled two-photon absorption (E2PA),
a process with a limited regime where the quantum advantage is large.
This regime arises due to a crossover from linear to quadratic scaling
with photon flux as photon flux is increased. Recently, several teams
of researchers have reported that E2PA cross-sections are much smaller
than previously claimed. As a result, the linear scaling dominates
at photon fluxes so low that it is extremely difficult or impossible
to measure using conventional free-space techniques. In this report,
we implement the first E2PA measurement using a waveguide. We see
no evidence of E2PA, and we set an upper bound on the cross-section
consistent with these recent reports.