Spatially entangled photon pairs from lithium niobate nonlocal metasurfaces

Abstract

Metasurfaces consisting of nanoscale structures are underpinning new physical principles for the creation and shaping of quantum states of light. Multiphoton states that are entangled in spatial or angular domains are an essential resource for many quantum applications; however, their production traditionally relies on bulky nonlinear crystals. We predict and demonstrate experimentally the generation of spatially entangled photon pairs through spontaneous parametric down-conversion from a metasurface incorporating a nonlinear thin film of lithium niobate covered by a silica meta-grating. We measure the correlations of photon pairs and identify their spatial antibunching through violation of the classical Cauchy-Schwarz inequality, witnessing the presence of multimode entanglement. Simultaneously, the photon-pair rate is strongly enhanced by 450 times as compared to unpatterned films because of high-quality-factor resonances. These results pave the way to miniaturization of various quantum devices by incorporating ultrathin metasurfaces functioning as room temperature sources of quantum-entangled photons.

Lithium Niobate Meta-Optics

“…In particular, the concept of etchless LiNbO 3 metasurfaces, namely, periodic structures realized onto otherwise unstructured films of LiNbO 3 , has been introduced ,, to avoid the complex etching process of LiNbO 3 , while granting access to highly delocalized resonances with high Q -factors. This concept has been recently tested experimentally in x -cut etchless LiNbO 3 metasurfaces, demonstrating experimental quality factors of Q = 455 and an over 50× enhancement of SHG in comparison to the bare LiNbO 3 film . We note that as the BIC modes consist of transversely propagating waves, they are strongly affected by the finite transverse size and edges of the metasurfaces.…”

“…The gray line is the transmittance spectrum of the metasurface exhibiting a resonance. Panels c and d are reprinted with permission from ref . Copyright 2022 The Authors .…”

“…The approaches typically used for integrated or bulk optics can only be partially applied to metasurfaces, which are open resonators with absorption. To date, among the various tools that are being developed, quantum–classical correspondence between SPDC and SFG was used to assess the first experimental results ,,, and suggest new designs. , A more rigorous approach is based on the Green’s function method for constructing two-photon amplitudes, which was shown to be able to calculate efficiency and directionality of photon-pair generation from a single spherical nanoresonator . But this method is hardly applicable to metasurfaces as it requires a rigorous description of the modal decomposition in terms of multipoles.…”

“…In addition, the trigonal crystal structure of LiNbO 3 with 3 m = C 3 v symmetry leads to nonvanishing values of the diagonal coefficients of the χ (2) tensor, with the d 33 component (associated with the c -axis) being the dominant one. , This is a particularly appealing property for nonlinear and electro-optics applications since it fosters the interaction of electric fields with the same polarization. This could be beneficial for realizing sources of entangled photons having identical polarization states via spontaneous parametric down-conversion (SPDC) in a metasurface (see Figure , middle). , The possibility to cut the material along a cleavage plane of choice, hence, defining a specific orientation of the crystalline axes, provides a further degree of freedom to engineer nonlinear optical processes in LiNbO 3 -based platforms.…”

“…This could be beneficial for realizing sources of entangled photons having identical polarization states via spontaneous parametric down- conversion (SPDC) in a metasurface (see Figure 1, middle). 41,42 The possibility to cut the material along a cleavage plane of choice, hence, defining a specific orientation of the crystalline axes, provides a further degree of freedom to engineer nonlinear optical processes in LiNbO 3 -based platforms.…”