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Solid-state quantum devices use quantum entanglement for various quantum technologies, such as quantum computation, encryption, communication, and sensing. Solid-state platforms for quantum photonics include single molecules, individual defects in crystals, and semiconductor quantum dots, which have enabled coherent quantum control and read-out of single spins (stationary quantum bits) and generation of indistinguishable single photons (flying quantum bits) and their entanglement. In the past six years, new opportunities have arisen with the emergence of 2D layered van der Waals materials. These materials offer a highly attractive quantum photonic platform that provides maximum versatility, ultra-high light-matter interaction efficiency, and novel opportunities to engineer quantum states. In this Review, we discuss the recent progress in the field of two-dimensional layered materials towards coherent quantum photonic devices. We focus on the current state-of-the-art and summarize the fundamental properties and current challenges. Finally, we provide an outlook for future prospects in this rapidly advancing field.

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Coherent generation of indistinguishable single photons is crucial for many quantum communication and processing protocols. Solid-state realizations of two-level atomic transitions or three-level spin-Λ systems offer significant advantages over their atomic counterparts for this purpose, albeit decoherence can arise due to environmental couplings. One popular approach to mitigate dephasing is to operate in the weak-excitation limit, where excited-state population is minimal and coherently scattered photons dominate over incoherent emission. Here we probe the coherence of photons produced using two-level and spin-Λ solid-state systems. We observe that the coupling of the atomiclike transitions to the vibronic transitions of the crystal lattice is independent of the driving strength, even for detuned excitation using the spin-Λ configuration. We apply a polaron master equation to capture the non-Markovian dynamics of the vibrational manifolds. These results provide insight into the fundamental limitations to photon coherence from solid-state quantum emitters. arXiv:1904.05103v3 [cond-mat.mes-hall]

Photon-mediated interactions between atoms can arise via coupling to a common electromagnetic mode or by quantum interference. Here, we probe the role of coherence in cooperative emission arising from two distant but indistinguishable solid-state emitters because of path erasure. The primary signature of cooperative emission, the emergence of “bunching” at zero delay in an intensity correlation experiment, is used to characterize the indistinguishability of the emitters, their dephasing, and the degree of correlation in the joint system that can be coherently controlled. In a stark departure from a pair of uncorrelated emitters, in Hong-Ou-Mandel–type interference measurements, we observe photon statistics from a pair of indistinguishable emitters resembling that of a weak coherent state from an attenuated laser. Our experiments establish techniques to control and characterize cooperative behavior between matter qubits using the full quantum optics toolbox, a key step toward realizing large-scale quantum photonic networks.