FOCUS
WAVEFRONT control
wavefront of the pump beam or directly that of the photon pairs, spatial correlations and even quantum entanglement can be preserved after propagation through thin scattering layers [ 7,8 ] or multimode fibers [ 12 ], offering interesting prospects for both free-space and fiber-based quantum communications( Fig. 2( b)). Furthermore, fueled by the development of camera-based multimode coincidence detection, wavefront shaping has become increasingly integrated with quantum imaging and sensing techniques, showing, for example, potential for increasing imaging depth in biological tissues( Fig. 2( c)). Most recently, the combination of efficient multiphoton sources and highly efficient superconducting nanowire single-photon detectors( SNSPDs) has enabled the demonstration of elementary quantum networks, in which nodes are connected by multimode fibers that act simultaneously as transmission channels and computational gates between independent photons( Fig. 2( d)).
FROM CLASSICAL TO NON-CLASSICAL WAVEFRONT SHAPING Today, more than a dozen research groups worldwide are exploring the still-emerging field of quantum optics in complex media, each group bringing a distinct disciplinary background [ 17 ]. While many of these efforts are driven by practical applications— such as preserving non-classical states under real-world conditions in free-space, underwater, or multimode-fiber communication channels— others continue to investigate more fundamental aspects of the interaction between disorder and quantum properties. Notable recent examples, which do not involve wavefront shaping, include the observation of coherent backscattering for photon pairs [ 13 ] and the violation of Bell inequalities [ 14 ].
Wavefront shaping itself, when applied to non-classical states, can also raise interesting questions. For example, until very recently, wavefront shaping techniques for manipulating non-classical states were essentially transposed from classical optics. These approaches assumed that the optimal strategy for structuring the optical field was completely independent of the state- classical or non-classical- populating it.
However, recent results demonstrate that optimal light shaping can fundamentally depend on the state ' s quantum properties. For instance, the wavefront patterns required to refocus entangled photon pairs through a thin scattering medium differ entirely from those used for classical light [ 15 ], influenced by the presence of entanglement( Fig. 3). This non-classical wavefront shaping
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Looking forward, wavefront shaping may no longer be viewed simply as a tool to control non-classical light in complex media. Instead, the quantum properties of light could be harnessed to enhance the performance of wavefront shaping itself. Combining these two physical concepts would allow us to surpass the fundamental limits of classical light shaping in scattering environments, such as by extending penetration depths in biological tissues or enabling advanced optical computing and communication protocols.
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