FOCUS
WAVEFRONT control
UNTANGLING THE QUANTUM SPECKLE
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Hugo DEFIENNE 1, *, Sylvain GIGAN 2
1
Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, F-75005 Paris, France
2
Laboratoire Kastler Brossel, ENS-Université PSL, CNRS, Sorbonne Université, Collège de France, 24 rue Lhomond, 75005 Paris, France * hugo. defienne @ gmail. com
https:// doi. org / 10.1051 / photon / 202614038
This is an Open Access article distributed under the terms of the Creative Commons Attribution License( https:// creativecommons. org / licenses / by / 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The propagation of non-classical light through disordered and complex optical media has long been regarded as a detrimental process, destroying fragile quantum correlations and entanglement. However, recent advances in wavefront shaping have challenged this view. In this article, we explore how spatial light modulators, combined with advanced non-classical sources and detection technologies, now enable us not only to preserve quantum properties through complex media, but also to actively harness disorder for quantum imaging, sensing, and information processing.
Conventional quantum optics requires exquisite control and alignment of the system through which the light propagates: lenses, cube beam splitters, polarizers, and waveplates. A common aspect of essentially all quantum optics experiments is therefore to carefully design the optical system, minimize losses, and carefully control the modal structure of the light. Consequently, propagating quantum states through a disordered or complex medium— be it a layer of white paint, strong atmospheric turbulence, or a multimode fiber— that mixes light in an unpredictable way across modes appears, at first sight, to be a very bad idea. In this article, we will cover why there is still some interesting physics at the intersection between these two fields, and how the emergence of wavefront shaping has turned this area from a niche topic into a rich field with strong application potential.
NON-CLASSICAL LIGHT MEETS COMPLEX MEDIA The propagation of coherent light through a complex medium gives rise to a very distinctive intensity pattern: the speckle. It results from a complex and seemingly unpredictable interference process between all the optical waves scattered by the inhomogeneities of the medium. Extensively studied, it acts as a fingerprint of both the properties of the disorder and those of the incident light, while also exhibiting universal statistical features, such as its intensity distribution or the coherent backscattering cone.
A primary question that arose in the early 2000s was what happens when a quantum state propagates through such a medium: do quantum features, such as squeezing, correlations, and entanglement, disappear or survive, and if they survive, do they remain accessible? Theoretically, since scattering in the optical domain is mostly elastic, a complex medium effectively performs a complex local unitary transformation on the incoming state, which should preserve its quantum properties. However, other real-world effects such as absorption, collection losses, or the temporal dynamics of the medium can induce decoherence and smear these non-classical features. To study this experimentally, a significant practical obstacle arises: the disorder spreads the light over a large number
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