Metamaterials constructed from deep subwavelength building blocks have been used to demonstrate phenomena ranging from negative refractive index and ε-near-zero to cloaking, emulations of general relativity, and superresolution imaging. More recently, metamaterials have been suggested as a new platform for quantum optics. We present the use of a dielectric metasurface to generate entanglement between the spin and orbital angular momentum of photons. We demonstrate the generation of the four Bell states on a single photon by using the geometric phase that arises from the photonic spin-orbit interaction and subsequently show nonlocal correlations between two photons that interacted with the metasurface. Our results show that metamaterials are suitable for the generation and manipulation of entangled photon states, introducing the area of quantum optics metamaterials.

The robust generation and propagation of multiphoton quantum states are crucial for applications in quantum information, computing, and communications. Although photons are intrinsically well isolated from the thermal environment, scaling to large quantum optical devices is still limited by scattering loss and other errors arising from random fabrication imperfections. The recent discoveries regarding topological phases have introduced avenues to construct quantum systems that are protected against scattering and imperfections. We experimentally demonstrate topological protection of biphoton states, the building block for quantum information systems. We provide clear evidence of the robustness of the spatial features and the propagation constant of biphoton states generated within a nanophotonics lattice with nontrivial topology and propose a concrete path to build robust entangled states for quantum gates.

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Multi-photon absorption processes have a nonlinear dependence on the amplitude of the incident optical field i.e. the number of photons. However, multi-photon absorption is generally weak and multi-photon events occur with extremely low probability. Consequently, it is extremely challenging to engineer quantum nonlinear devices that operate at the single photon level and the majority of quantum technologies have to rely on single photon interactions. Here, we demonstrate experimentally and theoretically that exploiting coherent absorption of N = 2 N00N states makes it possible to enhance the number of two-photon states that are absorbed. An absorbing metasurface placed inside a Sagnac-style interferometer into which we inject an N = 2 N00N state, exhibits twophoton absorption with 40.5% efficiency, close to the theoretical maximum. This high probability of simultaneous absorption of two photons holds the promise for applications in fields that require multi-photon upconversion but are hindered by high peak intensities. arXiv:1709.03428v2 [quant-ph]

The field of quantum information has been growing fast over the past decade. In particular, optical quantum computation, based on the concepts of KLM 1 and cluster states 2 , has witnessed experimental realizations of larger and more complex systems in terms of photon number 3 . Quantum optical systems, which offer long coherence times and easy manipulation of single qubits and photons, allow us to probe quantum properties of the light itself 4 and of the physical systems around it 5 . Recently, a linear scheme for quantum computing, relying on the bosonic nature of particles, has been proposed 6 and realized experimentally with photons 4,7,8 . The ability to efficiently measure superpositions of quantum states consisting of several photons is essential to the characterization of such systems and computation units.In fact, the entire field of quantum information completely relies on the ability to recover quantum states from measurements. However, the characterization of quantum states requires many measurements, and often necessitates complicated measurements schemes; for example, characterizing m qubits requires 2 2 m measurements. Here, we utilize structure, inherent to physically interesting quantum states of light, in order to reduce the complexity in the recovery of a