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reproduce landmark experiments in single- and biphoton quantum optics, most notably the famous Hong- Ou-Mandel experiment( Fig. 1( e)). Compared to the original experiment, the beam splitter synthesized through the complex medium was entirely controllable, albeit inherently lossy. Consequently, researchers observed not only the classic Hong- Ou-Mandel dip( bunching, shown by the red curve in Fig. 1( e)) but also its generalization to antibunching( green curve), a phenomenon that cannot be observed using a conventional balanced beam splitter.
TOWARDS APPLICATIONS While early experiments were conducted in disordered layers of nanoparticles( e. g. white paint), it was soon realized that the large number of modes and the impossibility of measuring them all made quantum experiments extremely challenging. Consequently, the community shifted its focus toward phase screens( typically used to emulate atmospheric turbulence or thin biological tissues) and multimode fibers. For the latter, thanks to their finite number of modes, limited numerical aperture, and near absence of backscattering, these systems proved to be much more practical platforms for developing proof-of-principle experiments and conducting quantum information studies.
Transitioning from simple beam splitters to more complex programmable circuits has enabled researchers to continue exploiting disorder as a resource. Multimode fibers were essentially transformed into microscopic quantum lab-on-a-chip platforms, enabling the simulation of advanced quantum optics experiments. Demonstrations include
Figure 2. Applications of wavefront shaping in complex media for non-classical light.( a) Exploiting mode mixing in a multimode fiber( MMF) via wavefront shaping enables the implementation of arbitrary linear circuits. Here, spatial light modulators( SLMs) are programmed to emulate two-photon coherent absorption, accurately reproducing the expected two-photon survival rate as a function of the phase φ.( BS: Beam splitter; LTBS: Lossy Transmission Beam splitter).( Ref.[ 6 ])( b) Wavefront shaping applied to the pump laser or spontaneous parametric down-conversion( SPDC) output compensates for thin scattering media( e. g. atmospheric turbulence or biological tissue). The initial two-photon speckle is focused into a sharp peak in correlation space, restoring strong quantum correlations( Refs.[ 7,8 ]).( c) In imaging, photon-pair quantum correlations can be used as a feedback metric for adaptive optics. This approach corrects microscope aberrations, successfully restoring clear images of biological targets( Ref.[ 9 ]).( d) As a key step for quantum networks, entanglement swapping between distant users( Ada and Hedy) is achieved by controlling photon propagation from independent sources through an MMF. The reconstructed density matrix confirms the success of the protocol( Ref.[ 10 ]).
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