  Entangled photons provide transformative new paths in the fields of
communication, sensing, and computing. However, when entangled photons
propagate through a complex medium such as a biological tissue or a turbulent
atmosphere, their correlations are scrambled. Using wavefront shaping to
compensate for the scattering and retrieve the two-photon correlations is
challenging due to the low signal-to-noise ratio of the two-photon signal.
While previous works partly addressed this challenge by using feedback from a
strong classical laser beam that co-propagates with the entangled photons, such
methods frequently depend on assumptions about the complex medium, limiting the
applicability of quantum wavefront shaping. In this work, we propose and
demonstrate a new feedback mechanism that is inspired by Klyshko's advanced
wave picture, in which the classical laser beam counter-propagates with one of
the entangled photons and co-propagates with the other. The new Klyshko
feedback allows compensation of scattering in thick samples and even in
situations where each photon propagates through a different scattering medium.
Since the advanced wave picture applies whenever optical reciprocity is valid,
such Klyshko optimization can be utilized across a wide range of
configurations, offering a robust and alignment-free setup. We therefore
believe this protocol will open the door for real-world applications of quantum
wavefront shaping.
